Views of the National Parks

Welcome to the Views of the National Parks glossary. Many of the terms used throughout the program are explained here. Navigate to a term by scrolling through the list, or jumping to a specific section using the alphabetical index below. Feel free to contact us if you find a term missing from this list that you believe needs to be added. Thanks!

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Aa - Pronounced "ah-ah", this is a Hawaiian term for lava flows that have a rough, rubbly surface; aa is composed of broken lava blocks called clinkers. Source: U.S. Geological Survey

Ablation - The losses in glacial budget are products of ablation. In most glaciers, melting constitutes the majority of ablation, but evaporation, sublimation (direct conversion of ice to water vapor), wind erosion, and calving also contribute to net loss. Source: Katie KellerLynn

Ablation Zone - The ablation zone is that part of a glacier's surface over which ablation (wastage, primarily melting) exceeds accumulation each year. Source: Sharp (1988)

Abolition - Abolition is the act of abolishing—to end the observance or effect of—as in the abolition of slavery. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Abrasion - Abrasion is the mechanical wearing, grinding, scraping, or rubbing away (or down) of a rock surface by friction and impact, in this case by rocks and rock fragments frozen in a glacier. Source: Katie KellerLynn

Accretion - Accretion is a theory of continental growth by the addition of successive terrances to the craton. Also, the term is used for the process by which the Earth grew from a small nucleus by the gradual addition of solid bodies such as meteorites, asteroids, or planetesimals, formerly revolving about the Sun in independent orbits, but eventually drawn by gravitation to Eath and incorporated with it. Source: Bates and Jackson (1984)

Accumulation - Accumulation is the addition of ice and snow to a glacier. This occurs through a variety of processes including precipitation and wind deposition. Source: Katie KellerLynn

Accumulation Zone - The part of a glacier's surface over which more snow is deposited than is ablated (wasted away) each year is called the accumulation zone. Source: Sharp (1988)

Acid Rain - Coal contains varying amounts of sulfur, which upon burning, are converted to sulfur dioxide. This latter compound forms sulfurous acid by combining with water, and eventually the sulfurous acid is oxidized by the atmosphere to sulfuric acid, which gets rained out onto the land downwind of coal-burning installations. Source: Skinner and Porter (1995)

Active Margin - An active margin is another name for a convergent plate boundary, that is, a boundary between two segments of Earth’s lithosphere (plates) that are moving toward each other. It is synonymous with subduction zone. Source: Katie KellerLynn

Advance - An advance is an increase in the length of a glacier compared to a previous point in time. As ice in a glacier is always moving forward, its terminus advances when less ice is lost at the terminus as a result of melting or calving than reaches the terminus. Source: Bruce Molnia

Alfisols - Soils with a subsoil accumulation of silicate clay that are moderately weathered (have a high base saturation). Source: Kohnke and Franzmeier (1995)

Algae - A diverse group of organisms that survive in different types of habitats. From the dry desert, to the Arctic Circle, to boiling springs these organisms have found a way to extract enough from their environment to live in even the harshest surroundings. They range in size from microscopic to meters in length and in complexity from single-celled to complex organisms that would rival even large plants. Though these organisms may look like the true, "higher," plants, they are anything but, since they do not have roots or true stems and leaves. Source: University of Florida and Florida Dept. of Environmental Protection Web site

Algal Mat - Various species of algae can grow in dense mats at the bottoms of nutrient enriched lakes, spring fed systems, or intertidal areas. These mats produce gasses during photosynthesis that often causes the mats to rise to the surface. At the surface, winds pile the algal mats against shorelines or in navigation channels; these mats can be several acres in size. Source: University of Florida and Florida Dept. of Environmental Protection Web site

Alien Species - With respect to a particular ecosystem, any species—including seeds, eggs, spores, or other biological material capable of propagating that species—that is not native to that ecosystem, is an alien or exotic species. Source: National Park Service; Executive Order 13112–Invasive Species

Alpine Region - The term alpine is loosely to describe high elevations and cold climates. Alpine is capitalized when it refers specifically to the European Alps. Ecologically speaking, alpine describes the mountainous regions lying between timberline and snowline, and is said of the climate, flora, relief, or ecology. Source: Bates and Jackson (1987)

Altitude - The height of an object above a reference level, especially above sea level or above the earth's surface. Source: Jan Gillespie

Amberat - Made by packrats, amberats are urine-covered piles of collected vegetation, bones, sticks, and other items that have hardened over time, giving a glossy, yellow appearance. Paleoecologists examine these deposits to see what plants and animals were around in the past. Source: Katie KellerLynn

Ammonite - Ammonites are an extinct group of marine animals (subclass Ammonoidea) in the phylum Mollusca and class Cephalopoda. Their closest living relative is probably not the modern Nautilus, which they resemble, but rather the Subclass Coleoidea (e.g., octopus, squid, and cuttlefish). Their fossil shells take the form of flat spirals (though there are some rarer helically spiraled and non-spiraled forms, called heteromorphs) and are responsible for the animals’ name as they somewhat resemble a tightly coiled ram’s horn (the god Ammon was commonly depicted as a man with ram’s horns). Source: Wikipedia

Amniotic Egg - An amniotic egg is one that contains a thin membrane that forms a closed sac about an embryo or fetus of a reptile, bird, or mammal. The sac also contain amniotic fluid. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Amphibian - An amphibian is an organisms that can leave the water for extended periods of time, but still is required to return to the water to survive. Frogs, toads, salamanders, and newts are all types of amphibians. Source: Linda Lutz-Ryan

Anadromous - The term anadromous refers to species that ascend rivers from the sea for breeding. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Anaerobic - "Anaerobic" describes the ability to live, occur, or exist in the absence of free oxygen. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Anastomosing Stream - The channel pattern of a braided stream is said to be anastomosing, meaning branching and recombining. Source: Katie KellerLynn

Andesite - Andesite is a dark-colored, fine-grained extrusive igneous (volcanic) rock. Magma that produces andesite commonly erupts from stratovolcanoes as thick lava flows, some reaching several kilometers in length. This magma also can generate strong explosive eruptions to form pyroclastic flows and surges, and enormous eruption columns. Source: U.S. Geological Survey; Katie KellerLynn

Anhydrite - Anhydrite is a mineral, anhydrous calcium sulfate (CaSO4). It occurs in evaporite beds and readily alters to gypsum. Source: Bates and Jackson (1984)

Animal - A multi-cellular organism with eukaryotic cells (have membrane-bound organelles and a defined nucleus). Animals are one of the kingdoms of life and have tremendous diversity. They are different from plants and fungi because they are heterotrophs (requiring external sources of nitrogen and carbon for metabolism) and lack cells with cellulose walls, chlorophyll, and the capacity of photosynthesis. Source: National Park Service; Katie KellerLynn

Anoxic - "Anoxic" means greatly deficient in oxygen. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Aquatic - Aquatic is a term used to describe species that live in water or use water bodies as their primary habitat (e.g., rivers, streams, lakes, bays, and oceans). Source: National Park Service; Katie KellerLynn

Aquifer - An aquifer is a body of rock that is sufficiently permeable to conduct groundwater and to yield economically significant quantities of water to wells and springs. Source: Bates and Jackson (1984)

Arch - A natural arch is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame. Source: Natural Arch Information

Archaea - Archaea, meaning "old ones,” is a major division of living organisms. Although the exact phylogeny of the groups is uncertain, Archaea, Eukaryota, and Bacteria are the fundamental classifications in what is called the three-domain system. Like bacteria, Archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as Monera in the alternative five-kingdom taxonomy. They were originally described in extreme environments, but have since been found in all types of habitats. Source: Wikipedia

Archaeology - Archaeology is the scientific study of material remains of human life and activities (e.g., bones, artifacts, monuments). Source: Katie KellerLynn

Archean Eon - The Archean is the lowest (oldest) eon of the Standard Global Geochronometric Scale, below the Proterozic Eon. The lower boundary has not been defined; the upper boundary has been established geochronometrically at 2,500 million years ago. Source: Neuendorf et al. (2005)

Archipelago - Archipelago is a sea or area in a sea that contain numerous islands; also, the island group itself. Source: Bates and Jackson (1984)

Arête - A French term referring to the bones in a fish backbone, an arête is a jagged, narrow ridge that separates two adjacent glacier valleys or cirques. The ridge frequently resembles the blade of a serrated knife. Source: Bruce Molnia

Arthropod - An arthropod (e.g., insects, arachnids, and crustaceans) is a member of the phylum (Arthropoda) of invertebrate animals that have a segmented body and jointed appendages, a usually chitinous exoskeleton molted at intervals, and a dorsal anterior brain connected to a ventral chain ganglia. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Artillery - Artillery is any large-caliber weapon, such as a cannon, howitzer, or missile launcher. Also, the crews that operate artillery and the branch of an army that specializes in the use of such weapons are called artillery. Source: Kathryn Wright

Ash - Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 0.1 inches (2 mm) in diameter, which is slightly larger than the size of a pinhead. Source: U.S. Geological Survey

Asthenosphere - The asthenosphere is a division in the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 62 miles (100 km) and in some regions extends as deep as 435 miles (700 km). The rock within this zone is easily deformed. Source: Lutgens and Tarbuck (1992)

Augite - Augite is a dark mineral of the pyroxene group. It is an essential constituent of many basic igneous rocks. Source: Bates and Jackson (1984)


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Back Arc - The region adjacent to a subduction-related volcanic arc is called the back arc; it is on the side opposite the trench and subducting plate. Source: Bates and Jackson (1987)

Back-arc Basin - A basin floored by oceanic crust formed by seafloor spreading on the opposite side of a volcanic arc from an oceanic trench. Source: Neuendorf et al. (2005)

Bacteria - Bacteria is a prokaryotic round, spiral, or rod-shaped single-celled microorganism that may lack cell walls or is often aggregated into colonies or motile by means of flagella. Bacteria typically lives in soil, water, organic matter, or the bodies of plants and animals. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Banded Iron Formation - Banded iron formation is a chemical sedimentary rock, typically thin bedded or thinly laminated, containing at least 15% iron of sedimentary origin; it shows marked banding, generally of iron-rich minerals and chert or fine-grained quartz. Most iron formation is of Precambrian age. Source: Neuendorf et al. (2005)

Barnacle - Barnacles are marine crustaceans with feathery appendages for gathering food; they are free-swimming as larvae but become permanently fixed (as to rocks, boat hulls, or whales) as adults. Source: Katie KellerLynn

Barren Zone - An area of fresh, vegetation-free bedrock around the margin of a retreating glacier that documents the recent loss of ice represents the barren zone. Source: Bruce Molnia

Basal Flow - A number of mechanisms are at work as a glacier slips and slides on its bed. A thin layer of water (even a few millimeters thick) can significantly reduce friction between a glacier and its bed causing slippage. If the temperature of glacial ice is warm enough, a glacier will move over and around small irregularities in its bed by thawing and refreezing as pressure increases and decreases around obstacles. This is called pressure melting. Under cold conditions, glacial ice will flow around large obstacles without actually melting. This is called enhanced basal creep. Source: Katie KellerLynn

Basal Slide - see "Basal Flow"

Basal Slip - see "Basal Flow"

Basalt - Basalt is a hard, black volcanic rock with low silica content and low viscosity. Therefore, basaltic lava can flow quickly, and easily spread more than 30 miles (20 km) from a vent from which it flows. Source: U.S. Geological Survey

Base Saturation - Base saturation is the fraction of cation exchange capacity occupied by base cations (Na+, K+, Mg2+, Ca2+). The remainder of cation exchange capacity is occupied by acidic cations (H+, Al3+). The percentage of base saturation [(sum exchangeable bases)/CEC) × 100] is directly related to pH. High pH causes Al3+ to be precipitated as Al(OH)3. Low pH causes Al3+ to replace bases, which are leached out. At a pH 7, base saturation is 100%; at a pH of 4, base saturation is 0%. Source: Plattsburgh State University of New York

Basin - The term basin is widely applied: a lake basin, a glacially formed basin, a groundwater basin, a shallow depression on the sea floor, a circular depression on the Moon's surface. In general, a basin is a depressed area with no outlet. Source: Bates and Jackson (1987); Katie KellerLynn

Batholith - A large, generally discordant (contact not parallel to bedding of country rock) mass of igneous rock formed at great depths that has more than 40 square miles (100 km2) of surface exposure and no known floor. Source: Katie KellerLynn

Battery - A battery is a grouping of artillery pieces (e.g., a set of warship guns); the term also applies to an army artillery unit, corresponding to a company or infantry. Source: Kathryn Wright

Bedrock - The solid rock below any soil, gravel, or other superficial material. Source: National Park Service

Bennettitaleans - Bennettitaleans, also called cycadeoids, are primitive plants (gymnosperms) that resemble cycads but have different methods of reproduction. Bennettitaleans lived throughout the Mesozoic Era. Examples of bennettitaleans include Williamsonia (Jurassic through end Cretaceous), Williamsoniella (Jurassic through end Cretaceous), and Zamites (Triassic). Source: Enchanted Learning

Bentonite - Bentonite is soft clay or claystone formed by the chemical alteration of glassy volcanic ash in contact with water. Source: Neuendorf et al. (2005)

Bergschrund - A German term, a bergschrund is a single large crevasse or series of sub-parallel crevasses that develop at the head of a glacier; also, the location where ice pulls away from the bedrock wall of the cirque against which it accumulated. In winter the crevasse fills with snow. In spring or summer it reopens. Source: Bruce Molnia

Bergy Seltzer - Bergy seltzer is the sound made as air bubbles, formed at many atmospheres of pressure, are released during the melting of glacier ice. It sounds like crackling or sizzling similar to that made by champagne, seltzer water, or Rice Krispies, but louder. Source: Bruce Molnia

Bioaccumulation - Bioaccumulation is an increase in the concentration of a chemical in an organism over time, compared to the chemical's concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted. Source: Oregon State University

Biologist - See "Biology."

Biology - The branch of science that studies life and its various forms and processes. A person who studies biology is a biologist. Source: National Park Service

Biostratigraphic Unit - A group of rock strata that is identified by its fossil content. Source: Linda Lutz-Ryan

Biotite - Biotite is a common rock-forming mineral of the mica group. Source: Bates and Jackson (1984)

Biped - Bipeds are two-footed animals; bipedal is the adjective that describes them. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Bird - Members of the animal kingdom that are warm-blooded, lay eggs, have feathers covering the body, and have arms modified into wings. Source: National Park Service

Block Faulting - "Block faulting" is an impercise term typically used in reference to high-angle faulting (generally normal faulting) in which the crust is broken into separate blocks that move relative to one another. Typically, surfaces of adjacent blocks end up with different elevations or tilts. Source: Neuendorf et al. (2005)

Block Lava - Block lava consists of angular blocks; it is similar to aa but the fragments are more regular in shape, somewhat smaller and less vesicular. Source: Bates and Jackson (1987)

Blowdown - A blowdown is an instance of trees being blown down by the wind. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Bomb - A bomb is a pyroclast that was ejected while viscous and received its rounded shape while in flight. It is larger than 64 mm in size, and may be vesicular to hollow inside. The actual shape of form varies greatly and is used in descriptive classifications (e.g., rotational bomb, spindle bomb). Source: Bates and Jackson (1987)

Brachiopod - A brachiopod is any of the phylum (Brachiopoda) of marine invertebrates with bivalve shells within which is a pair of arms bearing tentacles by which currents of water is made to bring microscopic food to the mouth. Brachiopods have two valves, and this gives them a superficial resemblance to bivalved mollusks, but bivalves are symmetrical and brachiopods are asymmetrical. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Fortey (1991)

Braided Stream - Complex networks of branches that continuously separate and reunite characterize braided streams. Streams braid when they have a much greater sediment load than they can carry. Source: Bruce Molnia

Breaker Zone - The breaker zone is the portion of the surf zone where shallow-water waves over-steepen and break. Source: Pinet (1992)

Breccia - Breccia is a coarse-grained, clastic sedimentary rock composed of angular rock fragments fixed in a matrix. Source: Linda Lutz-Ryan; Katie KellerLynn

Brecciated - "Brecciated" describes a rock that has been crushed or broken into angular fragments. Source: Katie KellerLynn

Bronze - Bronze is an alloy (mixture of two or more metals) of copper and tin and sometimes other elements. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Brush Mastication - A mechanical fuel reduction technique that uses a tracked vehicle to shred brush and small diameter trees. This technique is used to reduce ladder fuels, provide defensible space for firefighters in the event of a wildfire, and create prescribed burn unit boundaries. Source: National Park Service

Bryozoan - Bryozoans (of the phylum Bryozoa) are aquatic, mostly marine, invertebrate animals that reproduce by budding and usually form permanently attached branched or mossy colonies. Source: Katie KellerLynn

Buoyant Force - In nature and the science of nature, called physics, buoyancy is an upward force on an object immersed in a fluid (i.e., a liquid or a gas); this buoyant force enables an object to float or at least to appear lighter. Buoyancy is important for many vehicles such as boats, balloons, and airships (e.g., the Hindenburg). In caves, the buoyant force of water aids in keeping ceilings of flooded passages from collapsing. Source: Wikipedia and Katie KellerLynn


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Calcium Carbonate - Calcium carbonate (CaCO3) is solid occurring in nature chiefly as the minerals calcite and aragonite. Source: Bates and Jackson (1984)

Calcium Phosphate - Calcium phosphate is the name given to a family of minerals containing calcium and phosphate ions. Calcium phosphate is an important raw material for the production of phosphoric acid and fertilizers. Source: Wikipedia

Caldera - A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. Source: U.S. Geological Survey

Calving - Calving is the process by which pieces of ice break away from the terminus of a glacier that ends in a body of water or from the edge of a floating ice shelf that ends in the ocean. Once they enter the water, the pieces are called icebergs. Source: Bruce Molnia

Cambrian - The Cambrian is the earliest period of the Paleozoic Era, thought to have covered the span of time between 542 and 488.3 million years ago; also, the corresponding system of rocks. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Capillary rise - During capillary rise, water held loosely in soil against the force of gravity, is drawn up from supplies lower in the soil profile by capillary action, in much the same way as kerosene moves up the wick in a lamp. Capillary water travels through the soil in response to pressure gradients and is the main source of water for plants. Source: Bradshaw and Weaver (1993)

Carbon - An abundant element that is a basic building block in the cells of all known life. Source: Linda Lutz-Ryan

Carbonate Rock - A carbonate rock consist chiefly of carbonate minerals (i.e., calcium, magnesium, or iron) such as limestone, dolomite, or carbonatite. Specifically, carbonate rocks are sedimentary rock composed of more than 50% carbonate minerals by weight. Source: Neuendorf et al. (2005)

Carbonation - Carbonation is an activity of chemical weathering. It is a chemical reaction of carbonic acid in rainwater, soil water, and groundwater with minerals. Carbonation most strongly affects carbonate minerals and rocks, such as limestone and marble. Source: Katie KellerLynn

Carbonization - Carbonization is the reduction of an organism's remains to a film of carbon. Source: Linda Lutz-Ryan

Carnivore - Carnivores belong to the order (Carnivora) of typically flesh-eating mammals that includes dogs, foxes, bears, raccoons, and cats. Sometimes the definition is expanded to include other animals besides mammals, for instance, raptors and snakes. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); National Park Service

Carnivorous Plants - Rather than digesting their prey, mostly insects, as animals do, carnivorous plants dissolve the exoskeleton and absorb nitrogen molecules because they live in nitrogen-deficient soils. Source: Katie KellerLynn

Catenas - Catenas are sequences of changing soil profiles as a function of slope and drainage. They develop on slopes under uniform conditions of climate and parent material, and are, therefore, useful in studying slope as a soil-forming factor. Source: Jenny (1941)

Cave Blister - Cave blisters are hemispherical, bulged deposits (speleothems) filled with clay, sand, or a mineral substance such as gypsum or opal. Blisters are usually found attached to coatings, crusts, coralloids, flowstone, dripstone, or cave walls. Source: Hill and Forti (1997)

Cave Crust - Cave crusts are speleothems that cover cave walls, ceilings, and floor sediments. They can occur inconspicuously over many miles of cave passages. Alternately, they can appear as conspicuous crystal linings that can turn drab cave passages into sparkling chambers. Source: Hill and Forti (1997)

Ceanothus - An American plant (e.g., vine, shrub, or small tree) of the buckhorn family of the genus Ceanothus. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Celsius - Celsius relates to the international thermometric scale on which the interval between the triple point—the condition of temperature and pressure under which the gaseous, liquid, and sold phases of a substance can exist in equilibrium—of water and the boiling point of water is divided into 99.99 degrees with 0.01° representing the triple point and 100° the boiling point. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Cenozoic - Beginning 65.5 million years ago, the Cenozoic Era is referred to as the "age of mammals" during which time mammals flourished and became dominant. This is the most recent geologic era. Source: Linda Lutz-Ryan; Katie KellerLynn

Cephalopod - Cephalopods are any of a class (Cephalopoda) of marine mollusks including the squids, cuttlefishes, and octopuses that move by expelling water from a tubular siphon under the head and that have a group of muscular usually sucker-bearing arms around the front of the head, highly developed eyes, and usually a sac containing ink which is ejected for defense or concealment. "Cephalo" means head; "pod" means foot. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Champsosaurs - Champsosaurs belong are a group of crocodile-like reptiles that diverged (separated) from the main line of diapsid reptiles during the early Cretaceous Period. They in freshwater rivers and swamps; most Champsosaurs were fairly small, reaching only about 5 feet (1.5 m) in length, but some specimens were more than 10 feet (3 m). Champsosaurs fed on fish, snails, mollusks, and turtles. The great width of the Champsosaur skull between their eyes, which provided a large area to which the jaw muscles could be attached, shows that their jaws were very powerful by, providing. They probably swam the same way as modern crocodiles and marine iguanas (i.e., by lateral undulations of their sinuous bodies and tails, tucking their legs tightly against their bodies for more streamlined movement). Source:

Channel - A channel is the deepest portion of a stream, bay, or straight. Source: Bates and Jackson (1984)

Chaparral - Chaparral is an ecological community composed of shrubby plants adapted to dry summers and moist winters that occurs especially in southern California. Source: Merriam-Webster’s Collegiate Dictionary (11th ed.)

Chatter Marks - Chatter marks are the rounded cracks that form when rocks embedded in the glacier continually chip away at the bedrock over which it flows. Source: Katie KellerLynn

Chitin - Chitin is a polysaccharide (C8H13NO5) and is one of the main components in the cell walls of fungi and the exoskeletons of insects and other arthropods. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Wikipedia

Chronosequence - A chronosequence is a sequence of related soils that differ from one another in certain properties primarily as a result of time as a soil-forming factor. Source: Agriculture and Agri-Food Canada

Cinder Cone - A cinder cone is a steep, conical hill of volcanic fragments that accumulate around and downwind from a vent. Source: U.S. Geological Survey

Cirque - A cirque is a steep-walled, gentle-floored, semicircular topographic hollow created by glacial excavation high in mountainous areas. Source: Katie KellerLynn

Civil War - Between 1861 and 1865, the (American) Civil War took place between the Union and Confederacy. Source: Kathryn Wright

Clade - A clade is a group of biological taxa (as species) that includes all decendents of one common ancestor. Source: Merriam-Webster'sCollegiate Dictionary (11th ed.)

Clastic - The term "clastic" describes a rock or sediment composed principally of fragments derived from preexisting rocks or minerals and transported some distance from their places of origin. Source: Bates and Jackson (1984)

Clay - Clay is an earthy, extremely fine-grained sediment or soft rock composed primarily of clay-size (diameter less than 1/256 mm [4 microns]) or colloidal (easily suspended because of high surface area) particles, having high plasticity. Source: Bates and Jackson (1984)

Claystone - Claystone is hardened clay (by pressure, cementation, or heat) having the texture and composition of shale but lacking its fine lamination or fissility. Source: Bates and Jackson (1984)

Climate - Climate is the characteristic weather of a region, particularly as regarding precipitation and temperature, averaged over some significant interval of time. A person who studies climate is known as a climatologist. Source: Katie KellerLynn

Climatologist - see "Climate"

Coastal Plain - A coastal plain is a low, generally broad plain that has its margin on an oceanic shore and its strata either horizontal or very gently sloping toward the water, and that generally represent a strip of recently prograded or emerged sea floor, e.g., the coastal plain of the southeastern United States extending for 4,827 miles (3,000 km) from New Jersey to Texas. Source: Bates and Jackson (1987)

Coccoliths - Coccoliths are individual plates of calcium carbonate formed by coccolithophores (single-celled algae) which are arranged around them in a coccosphere. The coccoliths are typically dispersed following death and breakup of the coccosphere, but some species shed them continually. Coccoliths sink through the water column to form an important part of deep-sea sediments. They were probably not common before the Jurassic. Source: Wikipedia; Neuendorf et al. (2005)

Col - A col is a high, narrow, sharp-edged pass or depression in a mountain range, generally across a ridge or through a divide, or between two adjacent peaks. In particular, cols refer to deep passes formed by the headward erosion and intersection of two cirques, as in the French Alps. Source: Bates and Jackson (1987)

Collagen - Collagen is any of a group of fibrous proteins that occur in vertebrates as the chief constituent of connective tissue or in bones; collagen yields gelatin and glue. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Colloidal - Most colloid chemists restrict the term “colloidal” to particles that have diameters from 1 to 100 millimicrons (mµ), but soil scientists set the upper limit as high as 1,000; 2,000; or even 5,000µ (= 5µ = 0.005 mm). Colloidal clay particles are of vast importance because they control to a great extent the physical and chemical behavior of soils. Water permeability, aeration, horizon development, swelling and shrinking, and the development of soil structure depend on the amount and kind of colloidal clay in the soil. Likewise, the growth of plants is affected by the soil colloids because they are the storehouse for many important nutrient elements. Source: Jenny (1941)

Colluvium - Colluvium is a general term applied to any loose, heterogeneous, and incoherent mass of soil material or rock fragments deposited by rainwash, sheetwash, or slow continuous downslope creep, usually collecting at the base of gentle slopes or hillsides. By contrast, alluvium is deposited by a stream or other body of running water. Source: Bates and Jackson (1984)

Colonialism - Colonialism is the quality or state of being colonial. In particular, Colonialism relates to the original 13 colonies of the United States. Source: Merriam-Websters’s Collegiate Dictionary (11th ed.)

Colonnade - With reference to columnar jointing, a colonnade is the lower zone that has thicker and better-formed columns than the upper zone. With reference to architecture, a colonnade is a series of columns set at regular intervals and usually supporting the base of a roof structure. Source: Bates and Jackson (1987); Merriam-Webster's Collegiate Dictionary (11th ed.)

Columnar Basalt - As lava cools, it contracts, causing tension in basaltic rock. Eventually, the tension is released by the creation of fractures, which join other cracks to form a hexagonal pattern. These surface cracks deepen vertically to form the columns called columnar basalt. Source: National Park Service

Community - A collection of all life (plants, animals, and other organisms) that interacts in a particular area. Source: National Park Service

Competition - The active demand or struggle between two or more species for some environmental resource in short supply (e.g., food or space). Source: National Park Service; Merriam-Webster's Collegiate Dictionary (11th ed.)

Composite Volcano - Composite volcanoes, also called stratovolcnoes, are steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows. They are usually constructed over a period of tens to hundreds of thousands of years. Stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. Source: U.S. Geological Survey

Confederate States - Confederate states are the 11 southern states succeeding from the United States in 1860 and 1861. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Conglomerate - A rock made up of rounded pebbles and cobbles in a matrix. Source: Linda Lutz-Ryan

Conodont - Conodonts are small, disjunct fossil elements assigned to the order Conodontophorida; they are commonly tooth-like in form but not function; they are produced by small marine animals of uncertain affinity. Source: Bates and Jackson (1984)

Contact - Geologically speaking, a contact is the surface between two types or ages of rocks. Source: Katie KellerLynn

Continental Drift - Proposed by the German meteorologist Alfred Wegener in 1912, the concept of continental drift has been superseded by the theory of plate tectonics. Wegner theorized that large plates of continental crust moved freely across a substratum of oceanic crust. Now plate tectonics shows that the Earth's crust is broken into interlocking plates with both continental and oceanic crust that move in relation to one another, causing seismic and tectonic activity at their boundaries. Source: Katie KellerLynn

Continental Shelf - The continental shelf is that part of a continent’s margin between the shoreline and continental slope (or, when no noticeable continental slope is apparent, at a depth of 656 feet [200 m]). It is characterized by its very gentle slope of 1°. Source: Katie KellerLynn

Coprolite - Coprolite is fossilized dung. Source: Linda Lutz-Ryan

Core - The central part of Earth is the core, beginning at a depth of about 4,666 miles (2,900 km), probably consisting of iron-nickel alloy; it is divisible into an outer core that may be liquid and an inner core about 2,092 miles (1,300 km) in radius that may be solid. Source: Bates and Jackson (1984)

Crater - A crater is a basin-like, rimmed structure at the top or on the flanks of a volcanic cone; it is form by explosion or collapse. Source: Bates and Jackson (1984)

Craton - The craton is the part of Earth's crust that has attained stability and has been little deformed for a long time. Some national parks that host portions of stable craton are Voyageurs, Isle Royale, and Grand Teton. Source: Katie KellerLynn

Cretaceous - The Cretaceous is the final period of the Mesozoic Era thought to have covered the span of time between 145.5 and 65.5 million years ago; also, the corresponding rocks. It is named after the Latin word of English chalk ("creta") because of the English chalk beds of this age. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Crevasse - A crevasse (crack) forms in response to differential stresses caused by glacial flow. A crevasse may form singly or in a series on the surface of a moving glacier. They range in shape from linear to arcuate and in length from feet to miles. Their orientation may be in any direction with respect to the glacier's flow. The deepest crevasses may exceed 100 feet (30 m). Source: Bruce Molnia

Crinoid - Crinoids are abundant fossils from Ordovician to Tertiary time; they are still abundant today, although a little less common in shallow water sites than they were during the Paleozoic and Mesozoic. The great majority of the group have long stalks, which are anchored to the sea bottom. The main part of the animal consists of a cup (or calyx) to which the stalk is attached at its upper end, and from the top of the calyx stretch arms, which are five in number, or more usually a multiple of five. Source: Fortey (1991)

Cross-Shore Current - Water motions perpendicular (onshore and offshore) to the coast are referred to as cross-shore currents. Source: Katie KellerLynn

Crust - The crust is the outermost, thinnest layer of Earth. It represents less than 0.1% of Earth's total volume. It lies above the Mohorovičić discontinuity and is less dense than the mantle rocks below. Source: Bates and Jackson (1984)

Crystal - A crystal is a homogeneous, solid body of a chemical element, compound, or mixture, having a regular repeating atomic arrangement that may be outwardly expressed by plane faces. Source: Bates and Jackson (1984)

Cuspate Foreland - Cuspate forelands are the largest seaward-projecting points of beach material built by wave action along an open coast. Source: Katie KellerLynn

Cyanobacteria - Cyanobacteria are a phylum of bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae, even though it is now known that they are not directly related to any of the other algal groups, which are all eukaryotes. Nonetheless, the description is still sometimes used to reflect their appearance and ecological role. Fossil traces of cyanobacteria have been found from around 3.8 billion years ago, but recent evidence has sparked controversy over this assertion. As soon they evolved, they became the dominant metabolism for producing fixed carbon in the form of sugars from carbon dioxide. Cyanobacteria are now one of the largest and most important groups of bacteria on Earth. Source: Wikipedia

Cycads - Cycads are an ancient group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious (having male reproductive organs in one individual and female in another) plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms or ferns, but are unrelated to either, belonging to the order Cycadales. They flourished especially during the Jurassic and are represented by four surviving families of palm-like tropical plants. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Wikipedia


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Dacite - Dacite lava is most often light gray, but can be dark gray to black. It is one of the most common rock types associated with enormous Plinian-style eruptions. Source: U.S. Geological Survey

Debris - see "Sediment"

Debris Cone - Debris cones are typically cone-shaped mounds of debris-covered ice with thick enough sediment cover to protect the ice from melting. Source: Bruce Molnia

Decapod - Decapods are any of the order (Decapoda) or crustaceans (e.g., shrimp, lobsters, and crabs). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Decay - Decay is the decomposition of organic matter. Source: Linda Lutz-Ryan

Denitrification - Denitrification is the conversion of nitrates into gaseous nitrogen and nitrous oxide. Source:

Density - The density of a substance is defined as its mass per unit volume. That is, a substance of mass m and volume V has a density ρ given by ρ = m / V. Source: Petrucci and Harwood (1993)

Deposition - Deposition is the laying down of rock-forming material by any natural agent (e.g., water, wind, ice), such as the mechanical settling of sediment from suspension in water. Source: Bates and Jackson (1984)

Devonian - The Devonian is a period of the Paleozoic Era (after the Silurian and before the Mississippian) thought to have covered the span of time between 416 and 385.3 million years ago; also, the corresponding system of rocks. It is named after Devonshire, England, where rocks of this age were first studied. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Diatom - Diatoms are a major group of eukaryotic algae and are one of the most common types of phytoplankton. Most diatoms are unicellular, though some form chains or simple colonies. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silicate. These walls show a wide diversity in form, some quite beautiful and ornate. Although diatoms are known from the Jurassic, they first become abundant in the Cretaceous, ranging to the present. Source: Wikipedia; Neuendorf et al. (2005)

Diatreme - A diatreme is a breccia-filled volcanic pipe that was formed by gaseous explosion. Source: Bates and Jackson (1984)

Diffusion - Diffusion is the migration or intermingling of molecules of different substances (gases, solids, or liquids) as a result of random molecular motion caused by thermal agitation, and in dissolved substances move from a region of higher to one of lower concentration. Source: Merriam-Webster’s Collegiate Dictionary (11th ed.); Petrucci and Howard (1993)

Dike - A dike is a tabular igneous intrusion that cuts across the bedding or foliation of preexisting country rocks. Source: Bates and Jackson (1987)

Diorite - Diorite is a group of plutonic rocks intermediate in composition between acidic and basic, characteristically composed of hornblende, oligoclase or andesine, pyroxene, and sometimes a little quartz. Diorite is the approximate intrusive equivalent of andesite. Diorite grades into monzonite with an increase in the alkali feldspar content. Source: Bates and Jackson (1984)

Discharge - Discharge is the rate of streamflow at a given instant in terms of volume per unit time. Source: Katie KellerLynn

Dissolution - Dissolution is the act or process of dissolving. Dissolution, which is so important to the origin of limestone caves, is the process of dissolving rocks into a homogenous solution, as when an acidic solution (e.g., carbonic acid derived from rainwater containing carbon dioxide) dissolves the calcium carbonate in limestone. Source: Bates and Jackson (1984); Neuendorf et al. (2005)

DNA - DNA stands for deoxyribonucleic acid—the chemical blueprints that dictate what an organism is. Typically DNA is found in the nucleus of each cell of an organism. It can be used to identify organisms and relationships between organisms. Source: National Park Service

Dome - A dome is an uplift of anticlinal structure, circular or elliptical in outline, in which the rocks dip gently away in all directions. A dome may be small (e.g., the Gulf Coast salt domes) or many kilometers across, as in the type structure, the Nashville Dome of Tennessee. Source: Bates and Jackson (1984)

Downcutting - The gradual wearing away of the land surface below a stream bed. The stream initially removes the upper layers of the land and works its way downward into the underlying layers over time. Source: Jan Gillespie

Downwasting - Downwasting is the thinning of a glacier due to the melting of ice. This loss of thickness may occur in both moving and stagnant ice. Source: Bruce Molnia

Dredge - With respect to human activities along coasts, to dredge is to deepen a waterway. It also is a machine with a metal collar and collecting bag that is dragged along the bottom of a body of water to sample rock, sediment, or bottom organisms. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Pinet (1992)

Drift - Drift is a collective term used to describe all types of glacier sedimentary deposits, regardless of the size or amount of sorting. The term includes all sediment that is transported by a glacier, deposited directly by a glacier, and deposited indirectly by running water that originated from a glacier. Source: Bruce Molnia

Drumlin - Drumlins are elongated ridges of glacial sediment sculpted by ice moving over the bed of a glacier. Generally, the down-glacier end of a drumlin is oval or rounded and the up-glacier end tapers. The shape is often compared to an inverted, blunt-ended canoe. Source: Bruce Molnia

Dugout - A dugout is either a shelter dug in a hillside or one dug into the ground and roofed with sod. Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Pinet (1992)


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Earth - Earth is the planet we call home. It revolves around the Sun and is the third planet out from it. Source: National Park Service

Earthquake - An earthquake is a sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated strain. Source: Bates and Jackson (1984)

Ecosystem - An area where communities of species (plants, animals, and other organisms) interact with one another and the surrounding environment (water, sunlight, soil). Source: National Park Service

Ecotone - Ecotone is a transition area between two adjacent ecological communities. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Effusive Eruption - An eruption dominated by the outpouring of lava onto the ground is an effusive eruption. This is in contrast to the violent fragmentation of magma by explosive eruptions. Source: U.S. Geological Survey

El Niño Event - Spanish for "the child," literally "the Christ child," because of the appearance of the flow at Christmas time; an El Niño is is an irregularly recurring flow of unusually warm surface waters from the Pacific Ocean toward and along the western coast of South America that prevents upwelling of nutrient-rich cold deep water and disrupts typical regional and global weather patterns. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Element - An element is a substance made up of only one kind of atom. Elements cannot be broken down by chemical means into simpler substances. Source: Linda Lutz-Ryan

Eluviation - The physical transport of particles in suspension is termed eluviation (from the Latin ex or e, meaning out, and lavere, meaning wash). Source: Bradshaw and Weaver (1993)

End, Glacier - see "Terminus"

Endemic - A species that is confined to a specific place or location is referred to an endemic. The term is used to describe rare or endangered species because their small native ranges make them susceptible to extinction. Source: National Park Service

Environment - The sum total of all external conditions that may act upon an organism or biotic community, which may influence its development or existence, is its environment. Source: Linda Lutz-Ryan

Eocene - An epoch in the Cenozoic Era between 55.8 and 33.9 million years ago; also, the corresponding worldwide series of rocks. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Epilimnion - The epilimnion is the water layer overlying the thermocline of a lake. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Equilibrium Line - The equilibrium line is the boundary between areas of gain and loss on a glacier's surface during one year. It is where accumulation equals ablation, and the net balance is zero. Source: Katie KellerLynn

Erosion - Erosion is the wearing away of soil and rock by weathering, mass wasting, and the action of streams, glaciers, waves, wind, and groundwater. Source: Bates and Jackson (1984)

Erratic - A rock of unspecified shape and size, transported a significant distance from its origin by a glacier or iceberg and deposited when the ice melts. Erratics range from pebble-size to larger than a house and are of a different composition than the bedrock or sediment upon which they are deposited. Source: Bruce Molnia

Eruption - Eruption is the ejection of volcanic materials (lava, pyroclasts, and volcanic gases) onto Earth's surface, either from a central vent or from a fissure or group of fissures. Source: Bates and Jackson (1984)

Eruption Cloud - Clouds of tephra and gases that form downwind from erupting volcanoes are referred to as eruption clouds. Source: U.S. Geological Survey

Eruption Column - Vertical pillars of tephra and gases that rise directly above a vent are eruption columns. Source: U.S. Geological Survey

Esker - An esker is a meandering, water-deposited, generally steep-sided sediment ridge that forms within a subglacial or englacial stream channel. Its floor can be bedrock, sediment, or ice. Generally composed of stratified sand and gravel, eskers can range from feet to miles in length and may exceed 100 feet (30 m) in height. Subsequent melting of the glacier exposes the deposit. Source: Bruce Molnia

Estuary - An estuary is a partially enclosed body of water where freshwater is mixed with saltwater. Source: Pinet (1992)

Euhedral - Geologists describe the shape of mineral grains as euhedral when the grain is completely bounded by its own rational (natural) faces and growth has not been interfered with by adjacent grains. Source: Bates and Jackson (1984)

Eustacy - Eustacy refers to fluctuations in worldwide sea level caused by changes in the quantity of seawater available. The greatest changes are caused by water being added to or removed from glaciers. Source: Bruce Molnia

Evaporation - The phenomenon of evaporation of a liquid can be understood using the fact that some molecules in the liquid are more energetic than others. Some of the faster-moving molecules in the liquid penetrate the surface and leave the liquid even at temperatures well below the boiling point. The molecules that escape the liquid by evaporation are those that have sufficient energy to overcome the attractive forces of the molecules in the liquid phase. Consequently, the molecules left behind in the liquid phase have a lower kinetic energy, causing the temperature of the liquid to decrease. Hence, evaporation is a cooling process. For example, an alcohol-soaked cloth if often placed on a patient's feverish head to cool and comfort. Source: Serway (1992)

Evaporite - Evaporites are rocks that result from the evaporation of saline water. Examples include anhydrite, rock salt, and various nitrates and borates. Most evaporites are derived from bodies of seawater, though saline lakes such as the Great Salt Lake in Utah may also be important sources. Source: Katie KellerLynn

Evaporite Rock - See "Evaporite" Source:

Evapotranspiration - Evapotranspiration is the total transfer of liquid water to water vapor at Earth’s surface: evaporation plus transpiration. Source: Bradshaw and Weaver (1993)

Evolution - The theory that life on Earth has developed gradually from a few simple organisms to many complex organisms. Source: Bates and Jackson (1984)

Exotic - A reference to something that is foreign to the current place being discussed. When looking at species, refers to species that originate from other communities. Opposite of a native species. Source: National Park Service

Experiment - To test a natural law a scientist designs a controlled situation—an experiment—to see if conclusions deduced from the natural law agree with experimental results. Source: Petrucci and Harwood (1993)

Explosive Eruption - An explosive eruption is characterized by the energetic ejection of pyroclastic material. Source: Bates and Jackson (1987)

Extant - A species that still exists is referred to as extant; it is the opposite of extinct. Source: Linda Lutz-Ryan

Extinct - Any species that is no longer alive today is referred to as extinct. Source: Linda Lutz-Ryan

Extirpation - To extirpate a species is to remove, exterminate, or eradicate it from an area. For example, overhunting and trapping has caused the loss of many of the large predators in Grand Canyon National Park. Source: National Park Service

Extrusive Igneous Rock - Extrusive rocks have been erupted onto the surface of the Earth; they include lava flows and pyroclastic material such as volcanic ash. Source: Bates and Jackson (1987)


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Fact - In science, a fact is an observation that has been repeatedly confirmed and for all practical purposes is accepted as "true." Truth is science, however, is never final, and what is accepted as a fact today may be modified or even discarded at some point in the future. Source: National Academy of Sciences (1999)

Fahrenheit - Fahrenheit relates to the international thermometric scale on which, under standard atmospheric pressure, the boiling point of water is at 212 degrees above zero of the scale, the freezing point is at 32 degrees above zero, and the zero point approximates the temperature produced by mixing equal quantities by weight of snow and common salt. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Family - A group of related plants or animals forming a taxonomic category ranking above genus and below order and usually comprising several or many genera. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Fault - A fault is a break in a rock mass along which displacement (movement) has occurred parallel to the fracture. Source: Katie KellerLynn

Fauna - Fauna is used in reference to animals. Derived from Roman mythology, Fauna was the sister of Faunus, the god of animals. Source: National Park Service

Feldspar - Feldspars are the most widespread of any mineral group and constitute 60% of Earth's crust. They occur in all types of rocks. They are white and gray to pink and have a hardness of 6 on Moh's hardness scale. Source: Bates and Jackson (1984)

Felsic - Felsic is a mnemonic adjective derived from feldspar + lenad (feldspathoid) + silica + c, and applied to an igneous rock having abundant light-colored minerals; also applied to those minerals (quartz, feldspars, feldspathoids, muscovite) as a group. It is the compliment of mafic. Source: Bates and Jackson (1984)

Feral - The term feral is used to describe free-living plants or animals, living under natural selection pressures, descended from domesticated ancestors. Source: Office of Technology Assessment, "Harmful Non-Indigenous Species in the United States"

Fins - Fins are narrow sandstone ridges, or walls, that are eroding along parallel joints. Cracks between closely spaced vertical joints become enlarged by chemical and physical weathering as water seeps down, dissolving iron-oxide cement and loosening sand grains. Frost action, with its alternate freezing, expanding, and thawing, also breaks up sandstone. Loose sediment is removed by wind and runoff. The joint spaces gradually enlarge into narrow valleys that separate the narrow wall, called fins. Source: Harris and Tuttle (1990)

Firn - Firn is an intermediate stage in the transformation of snow to glacier ice. Snow becomes firn when it has been compressed so that no pore spaces remain between flakes or crystals, a process that takes less than a year. Source: Bruce Molnia

Firn Line - The firn line marks the transition across a glacier, from edge to edge, between exposed glacier ice (below) and the snow-covered surface of a glacier (above). During the summer melt season, this line migrates up-glacier. At the end of the melt season the firn line separates the accumulation zone from the ablation zone. Source: Bruce Molnia

Fissure - A fissure is an elongate fracture (crack) at the surface of Earth from which lava erupts. Source: U.S. Geological Survey

Fjord - Fjords are glacially eroded or modified U-shaped valleys that extend below sea level and connect to the ocean. They are filled with seawater and depths may exceed 1,000 feet (305 m) below sea level. The largest Alaskan fiords are more than 100 miles (160 km) long and more than 5 miles (8 km) wide. Source: Bruce Molnia

Flatiron - A flatiron is a rock formation shaped like the flat face of an iron. Source: Katie KellerLynn

Flood Basalt - see "Plateau Basalt"

Fold - A fold is a bend, foliation, cleavage, or other planar feature in rocks. A fold is usually a product of deformation but the definition does not specify manner of origin. Source: Katie KellerLynn

Foliation - With respect to glaciers, foliation is the layering or banding that develops during the process of transformation of snow to glacier ice. Individual layers, called folia, are visible because of differences in crystal or grain size, alternation of clear ice and bubbly ice, or because of entrained sediment. Also, foliation is a general term for a planar arrangement of textural or structural features in rocks. Source: Bruce Molnia; Bates and Jackson (1987)

Foraminifera - Formaminitera are any protozoan belonging to the subcalss Sarcodina, order Foraminiferida, characterized by the presence of a test of one to many chambers composed of secreted calcite (rarely silica or aragonite) or of agglutinated particles. Most foraminifers are marine but freshwater forms are known. The first occurrence was in the Cambrian; they exist today. Source: Neuendorf et al. (2005)

Forbs - Forbs are herbs other than grass (See "Herb"). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Formation - A formation is a body of rock strata that consists dominantly of a certain lithologic type or combination of types. It is the fundamental lithostratigraphic unit. Formations may be combined into groups or subdivided into members. In reference to igneous or metamorphic rock, a formation is a lithologically distinct, mappable body of rock. Source: Bates and Jackson (1984)

Fossil - Fossils are remains or traces of past life found in geologic material. Source: Linda Lutz-Ryan

Fumarole - A fumarole is a vent from which volcanic gases escape into the atmosphere. Source: U.S. Geological Survey

Fungi - Fungi are multicellular organisms with eukaryotic cells (have membrane-bound organelles and a defined nucleus). Fungi are one of the kingdoms of life and are different from plants and animals because they decompose organic matter for energy. Source: National Park Service

Furrow - see "Gouge"


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Gabbro - Gabbros form a group of dark-colored, basic intrusive igneous rocks composed primarily of labradorite or bytownite and augite, with or without olivine and orthopyroxene; also, any member of that group. Gabbro is the approximate intrusive equivalent of basalt. Source: Bates and Jackson (1984)

Garlon - Garlon is a herbicide applied to tamarisk stumps to prevent any regrowth of this invasive species. Source: National Park Service

Garnet - Garnet is a mineral that has vitreous luster, no cleavage, and a variety of colors, dark red being characteristic. It is most commonly found in euhedral isometric crystals in metamorphic rocks. Garnet is used as a semiprecious stone (the birthstone for January) and as an abrasive. Source: Katie KellerLynn

Gas - With respect to the classification scheme based on the three states of matter (solid, liquid, gas), in a gas, distance between atoms or molecules are generally much greater than in a liquid. A gas always expands to fill its container. Source: Petrucci and Harwood (1993)

Gastropod - Gastropods (of the class Gastropoda) are mollusks (e.g., snails, slugs, clams, or squids) usually with a univalve shell, or no shell, and a distinct head bearing sensory organs. "Gasto" means stomach; "pod" means foot. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Gelifluction - The downslope movement of saturated soil is, in general, referred to as solifluction. In periglacial environments, the term gelifluction is preferred, that is, seasonal thawing (and flow) of soil above a frozen subsurface. Source: Katie KellerLynn

Genus - Genus is a category in the hierarchy of the classification of organisms. Genus has an intermediate rank between family and species. Source: Katie KellerLynn

Geodesy - Geodetic relates to geodesy, that is, the branch of applied mathematics concerned with (1) the determination of the size and shape of Earth, (2) the exact positions of points on Earth's surface, and (3) the description of variations in Earth's gravity field. Geodetic coordinates are usually expressed in terms of longitude and latitude. Source: Katie KellerLynn

Geologic Time Scale - The geologic time scale is a chronological arrangement of geologic events, commonly presented as a chart or table with the oldest event and time unit at the bottom and the youngest at the top. Source: Bates and Jackson (1984)

Geologist - see "Geology"

Geology - Geology is the study of planet Earth, including the materials of which it is made, the processes that act upon these materials, the products formed, and the history of the planet and its life-forms since its origin. A scientist who is trained in and works in any of the geological sciences is a geologist. Source: Bates and Jackson (1984)

Geomorphology - Geomorphology is the study of Earth’s landforms. The term geomorphic pertains to the form of Earth or its surface features. Source: Katie KellerLynn

Geophysicist - A geophysicist is a specialist in geophysics, that is, a scientist who studies the physical properties of Earth, or applies physical measurements to geological problems. Source: Bates and Jackson (1984)

Geosyncline - A geosyncline is a large, trough-like or basin-like downwarping of Earth's crust in which a thick succession of sedimentary and volcanic rocks accumulated. A geosyncline may form in part of the tectonic cycle in which orogeny follows. Source: Bates and Jackson (1984)

Geothermal Features - Geothermal features are the materials or landforms (e.g., hot springs, geysers, mud pots) formed by processes related to the heat of Earth's interior. Source: Katie KellerLynn

Geyser - Most geysers are hot springs that episodically erupt fountains of scalding water and steam. Such eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space. Source: U.S. Geological Survey

Geysermite - Sometimes referred to as a geyser stalagmite, geysermites are vent-shaped speleothems with thin-walled sides and crater-like, central holes, which are continuations of holes in the cave floor. These are so named because they are conic and hollow like hot-spring geysers and because they are believed to form identical to geysers. Source: Hill and Forti (1997)

Ginko - a gymnospermous dioecious (having male reproductive organs in one individual and female in another) tree (Ginkgo biloba) of eastern China that is widely grown as an ornamental or shade tree and has fan-shaped leaves and foul-smelling yellowish fleshy seed coats; also called maidenhair tree. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Glacial Lake - A glacial like is an accumulation of standing liquid water on, in, or under a glacier. Source: Bruce Molnia

Glacial Polish - Glacial polish is the smoothed surface of bedrock, which often shines in the sunlight, that is the result of fine abrasion from the sediment carried by a glacier. Source: Katie KellerLynn

Glacial Rebound - Masses of glacial ice depress Earth's surface. When the ice melts, the crust rebounds, which is referred to as glacial rebound. Source: National Park Service

Glacial Stream - A glacial stream is channelized accumulation of liquid water on, in, or under a glacier, moving under the influence of gravity. Source: Bruce Molnia

Glacier - A large, perennial accumulation of ice, snow, rock, sediment, and liquid water originating on land and moving downslope under the influence of its own weight and gravity. Glaciers are classified by their size, location, and thermal regime. Source: Bruce Molnia

Glacier Cave - A cave formed in or under a glacier, typically by running water, is a glacier cave. Steam or high heat flow also form glacier caves. Source: Bruce Molnia

Glacier Flour - Extremely fine sediment that results from the action of glaciers eroding rocks into silty particles is known as glacier flour. The particles are so fine that they remain suspended in water for a while before settling to the bottom. Once in water, it is called glacier milk. Source: Katie KellerLynn

Glacier Ice - Any ice that forms in or was part of a glacier, including land ice that is flowing or that shows evidence of having flowed, and glacier-derived ice floating in the sea, is considered glacier ice. Source: Bates and Jackson (1987)

Glacier Milk - see "Glacier Flour"

Glacier Mill - see "Moulin"

Glacier, Alpine - see "Glacier, Valley"

Glacier, Calving - A glacier with a terminus that ends in a body of water (river, lake, ocean) into which it calves icebergs is referred to as a calving glacier. Source: Bruce Molnia

Glacier, Cirque - A cirque glacier is a small glacier that forms within a cirque basin, generally high on the side of a mountain. Source: Bruce Molnia

Glacier, Cold - A cold glacier has a temperature below the pressure melting point of ice from top to bottom. In a cold glacier the glacier surface experiences very low winter temperatures, or low summer temperatures lead to negligible surface melting. Source: Katie KellerLynn

Glacier, Continental - see "Ice Sheet"

Glacier, Distributary - A tongue of glacier ice that flows away from the main trunk of a glacier is a distributary glacier. This may result from differences in the amount of melting, which changes the gradient of part of a glacier. Source: Bruce Molnia

Glacier, Hanging - A hanging glacier is a glacier that originates high on the wall of a glacier valley and descends only part of the way to the surface of the main glacier. Avalanching and icefalls are the mechanisms for ice and snow transfer to the valley floor below. Source: Bruce Molnia

Glacier, Outlet - The peripheral zone of an ice dome is often marked by a radiating pattern of outlet glaciers that extend way beyond the dome margin. They are offshoot glaciers extending from ice sheets or ice caps. Source: Katie KellerLynn

Glacier, Piedmont - A piedmont glacier is a fan or lobe-shaped glacier, located at the front of a mountain range. It forms when one or more valley glaciers flow from a confined valley onto a plain where it expands. The 30-mile (48-km) wide Malaspina is the largest one in Alaska. Source: Bruce Molnia

Glacier, Reconstituted - A reconstituted glacier forms below the terminus of a hanging glacier through accumulation and reconstitution by pressure melting (regelation) of ice blocks that have fallen or avalanched from the terminus of the hanging glacier. Source: Bruce Molnia

Glacier, Rock - Rock glaciers are distinctive from ice glaciers in that their movement is characterized by a large amount of embedded and overlying rock material. A rock glacier may be composed of (1) ice-cemented rock formed in talus that is subject to permafrost, (2) ice-cemented rock debris formed from avalanching snow and rock, or (3) rock debris that has a core of ice, either a debris-covered glacier or a remnant moraine. Source: Katie KellerLynn

Glacier, Tidewater - A glacier with a terminus that ends in a body of water influenced by tides, such as the ocean or a large lake, are referred to as tidewater glaciers. Typically, tidewater glaciers calve icebergs. Source: Bruce Molnia

Glacier, Valley - A glacier that flows for all or most of its length within the walls of a mountain valley is known as a valley glacier. Source: Bruce Molnia

Glacier, Warm - A warm glacier has a temperature at the pressure melting point of ice throughout. Glacier ice at the pressure melting point is in equilibrium with liquid water, so water can exist throughout warm glaciers all the way to the base. Source: Katie KellerLynn

Gneiss - Gneiss is a foliated rock formed by regional metamorphism, in which bands of granular minerals alternate with bands of flaky minerals. Gneiss is commonly feldspar- and quartz-rich, although this is not an essential factor in its definition. Source: Katie KellerLynn

Gorgonian - Gorgonians (of the order Gorgonacea) are any of the colonial, marine organisms (anthrozoans) usually with horning or branching axial skeletons, for example a sea fan. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Gouge - While being carried by a glacier, rocks and boulders abrade bedrock, forming gouges and grooves. Source: Katie KellerLynn

Graben - A graben is an elongate, relatively depressed crustal unit or block that is bounded by faults on its long sides. It is a structural form and may or may not be expressed in landforms such as a rift valley. Source: KellerLynn; Bates and Jackson (1984)

Gradient - Gradient is the slope of a stream, generally measured in feet per mile. Source: Lutgens and Tarbuck (1992)

Grain - A grain is a mineral or rock particle with a diameter of less than a few millimeters, such as a sand grain. It is a general term used to describe rocks, as in "fine-grained" or "coarse-grained." Source: Bates and Jackson (1984)

Granite - Granite is an intrusive igneous rock, meaning that it forms from magma that cools below Earth's surface. It has visible crystals of quartz (≤25%) and potassium and sodium-rich feldspar (>50%). Other common constituents of granite are mica and amphibole. Source: Katie KellerLynn

Graywacke - Graywacke is a layered gray rock made up of sand and mud eroded from volcanic sources. Source: Katie KellerLynn

Green Algae - Green algae are one type of algae (i.e., organisms that capture light energy through photosynthesis, converting inorganic substances into simple sugars) from which more complex plants emerged. The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. Almost all forms have chloroplasts which gives them a bright green color. Source: Wikipedia

Greenhouse Effect - The greenhouse effect causes the atmosphere to trap heat energy at Earth’s surface and within the atmosphere by absorbing and re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect Earth’s average global temperature would be -18°C, rather than the present 15°C. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that Earth’s average global temperature has already increased by 0.3°C to 0.6°C, since the beginning of this century, because of this enhancement. Source:

Greenhouse Gases - Greenhouse gases are responsible for the greenhouse effect. These gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFxClx); and tropospheric ozone (O3). Source:

Greenstone - Greenstone is a dark greenish-gray, fine-grained, weakly metamorphosed basic ingenous rock. Greenstone belt is a term applied to elongate or belt-like areas within Precambrian shields that are characterized by abundant greenstone. Source: Bates and Jackson (1987)

Groove - see "Gouge"

Groundwater - The part of subsurface water that is in the zone of saturation, including underground streams, is referred to as groundwater. Loosely, all subsurface water, as distinct from surface water, is called groundwater. Source: Bates and Jackson (1984)

Gypsum - Mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO4·2H2O. It is the most common sulfate mineral, occurring in many places in a variety of forms. Source: Jan Gillespie


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Habitat - The area an organism uses to live is its habitat, including the area used for foraging food and shelter. Source: National Park Service

Hadean Eon - Hadean is a suggested eon of the Precambrian, older than the oldest preserved rocks (>~3,800 million years). Source: Neuendorf et al. (2005)

Halite - Halite is native salt , a mineral (NaCl). It occurs in massive, granular, compact, or cubic-crystalline forms. Source: Bates and Jackson (1984)

Hanging Valley - A hanging valley is higher than the floor of the main valley. It is a former tributary glacier valley that is incised into the upper part of a U-shaped glacier valley. Hanging valley streams often enter the main valley as waterfalls. Source: Bruce Molnia

Harbor - With reference to coastal environments, a harbor is part of a body of water that is protected and deep enough to provide anchorage. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Herb - An herb is a seed-producing annual, biennial, or perennial plant that does not develop persistent woody tissue but dies down at the end of a growing season. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Herbaceous - "Herbaceous" describes herbs (See "Herb"). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Herbicide - The National Park Service uses the term herbicide to describe specific chemicals used to attack plants that are considered invasive species. Source: National Park Service

Herbivore - Organisms such as bighorn sheep, beaver, and many types of insects that feeds on plant matter are herbivores. Source: National Park Service

Herpetology - Herpetology is the study of amphibians and reptiles. Source: National Park Service

Hia C’ed O’odham - The Hia C'ed O'odhams (Sand People) lived in southern Arizona and the northern part of the Mexican state of Sonora. One of the few truly nomadic peoples in the United States, the Hia C'eds developed a hunting and gathering lifestyle specially adapted to that extremely dry part of the desert. Source: Encyclopedia of North American Indians

Historical Sciences - Historical sciences are concerned with reconstructing the past from evidence in the present. For instance, historical geology is a major branch of geology that is concerned with the evolution of Earth and its life-forms from its origin to the present day. This is not to be confused with the history of geology. Source: Katie KellerLynn; Neuendorf et al. (2005)

Holocene Epoch - The Holocene Epoch is the segment of geologic time in which we are living. It began 10,000 years ago and continues to the present. Source: Katie KellerLynn

Holotype - The specimen used by scientists to describe the characteristics of a newly discovered species is the holotype. Source: Linda Lutz-Ryan

Homogeneous - Uniform in structure or composition throughout. Source: Jan Gillespie

Horn - A pointed, mountain peak, typically pyramidal in shape, bounded by the walls of three or more cirques is a horn. Headward erosion cuts prominent faces and ridges into the peak. Source: Bruce Molnia

Hot Spot - A hot spot is a volcanic center that is 161 to 322 miles (100 to 200 km) across and persistent for at least a few tens of millions of years. It is thought to be the surface expression of a persistent rising plume of hot material rising from Earth's mantle. Source: Bates and Jackson (1984)

Humidity - Humidity is the concentration of water vapor in the air. Water vapor gets into the air by evaporation—a process by which fast-moving, liquid molecules manage to escape from the liquid and pass into the vapor above. Because molecules in a vapor move randomly in all directions, some of the gas molecules in the vapor will also move back into the liquid. When the number of evaporating molecules (going from liquid to gas) equals the number of condensing ones (going from gas to liquid), the vapor is “saturated.” Meteorologists (and speleologists) use the term relative humidity when they are discussing saturated and “undersaturated” air. Relative humidity does not refer to a specific amount of water vapor in the air; rather it refers to the ratio of water vapor that is present at a given temperature to the maximum possible amount that the air could hold at that same temperature. Relative humidity can be changed in two ways: by addition of water vapor or by change of temperature. Source: Skinner and Porter (1995) and Katie KellerLynn

Humus - Humus is the generally dark, more or less stable part of the organic matter of soil, so well decomposed that the original sources cannot be identified. The term is sometimes used incorrectly for the total organic matter of the soil, including relatively undecomposed material. Source: Bates and Jackson (1984)

Hydrology - The science that deals with global water (both liquid and solid), its properties, circulation, and distribution on and under Earth's surface and in the atmosphere. Source: Bates and Jackson (1984)

Hypothesis - A hypothesis is a testable statement about the natural world that can be used to build more complex inferenes and explanations. If a hypothesis survives testing by experiments, it is referred to as a theory. Source: Kennedy et al. (1998); Petrucci and Harwood (1993)


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Ice Age - The "Ice Age" consists of a series of climatic changes that took place during the Pleistocene Epoch (1.8 million years ago to 10,000 year ago). In actuality, many ice ages, separated by warm periods, occurred during the Pleistocene Epoch. Source: Katie KellerLynn

Ice Cap - An ice cap is a dome-shaped accumulation of glacier ice and perennial snow that completely covers a mountainous area or island, so that no peaks or nunataks poke through. Source: Bruce Molnia

Ice Dome - An ice dome forms on the surface of an inland ice sheet. It is a mass of ice with a rounded, gently sloping dome. Source: Katie KellerLynn

Ice Sheet - Ice sheets are huge glaciers, that is, they are thick, sub-continental to continental-scale accumulations of glacier ice and perennial snow that spread from a center of accumulation, typically in all directions. Source: Bruce Molnia

Ice Shelf - An ice shelf is the floating terminus of a glacier, typically formed when a terrestrial glacier flows into a deep water basin, such as in Antarctica and the Canadian Arctic. Source: Bruce Molnia

Ice Sizzle - see "Bergy Seltzer"

Ice Stream - An ice stream is a current of ice in an ice sheet or ice cap that flows faster than the surrounding ice. Usually ice streams are flowing to the ocean or to an ice shelf and are not constrained by exposed rock. Source: Bates and Jackson (1987)

Iceberg - An iceberg is a large, massive piece of floating glacier ice of any shape that has detached (calved) from the front of a glacier into a body of water. Most of the mass of an iceberg is below water level. Icebergs may reach lengths of many tens of kilometers Source: Katie KellerLynn

Ice-Dammed Lake - Ice-dammed lakes exist because ice dams restrict their water from flowing. Sometimes these lakes form because an advancing glacier had blocked a valley. Source: Bruce Molnia

Icefall - Where ice flows over a bed with a very steep gradient, typically at a higher rate than both above and below, icefalls form. As a result, the glacier surface is fractured and heavily crevassed. In a river system, a waterfall is the equivalent form. Source: Bruce Molnia

Icefield - An icefield is a continuous accumulation of snow and glacier ice that completely fills a mountain basin or covers a low-relief mountain plateau to a substantial depth. When the thickness become great enough, tongues of ice overflow the basins or plateaus as valley glaciers. Source: Bruce Molnia

Ice-Marginal Lake - A lake that is located adjacent to the terminus of a glacier is an ice-marginal lake. Typically, these lakes form in bedrock basins scoured by glaciers. They enlarge as the glacier retreats. Sometimes they are dammed by an end or recessional moraine. Source: Bruce Molnia

Igneous - Igneous describes a rock or mineral that solidified from molten or partly molten material (i.e., from magma). The term igneous is also applied to processes related to the formation of such rocks; that is, intrusions formed in preexisting rock (below the Earth’s surface), and extrusions formed by relatively viscous lava being emitted onto Earth’s surface. Igneous rocks constitute one of the three main classes into which rocks are divided, the others being sedimentary and metamorphic. Source: Katie KellerLynn

Illuviation - The redeposition of materials in soil following transport in water is illuviation. The term comes from the Latin il, meaning in, and lavere, meaning to wash. Source: Bradshaw and Weaver (1993)

Ilmenite - Ilmenite is an iron-black, opaque rhombohedral mineral (FeTiO3). It is the principal ore of titanium. Ilmenite is a common accessory mineral in basic igneous rocks and also is concentrated in mineral sands. Source: Katie KellerLynn

Impact Structure - An impact structure is a generally circular or crater-like structure produced by impact (usually extraterrestrial) on a planetary surface. Source: Neuendorf et al. (2005)

Impression - Although the actual organism has decayed, the mark left behind by an organism's surface in layers of fine-grained sediments is called an impression. Source: Linda Lutz-Ryan

Index Fossil - An index fossil is a fossil that identifies and dates that strata or succession of strata in which it is found. The best index fossils include swimming or floating organisms that evolved rapidly and were distributed widely, such as graptolites and ammonites. The fossil need not necessarily be either confined to, or found throughout every part of, the strata for which it serves as an index. Source: Bates and Jackson (1987)

Indigenous - The word “indigenous” describes a person or object that originated in or has been produced, grown, lives, or occurs naturally in a particular region or environment. Native is a synonym for indigenous. Source: Merriam-Websters’s Collegiate Dictionary (11th ed.)

Infantry - The branch of an army made up of units trained to fight on foot is the infantry. Source: Kathryn Wright

Inference - The act of passing from statistical sample data to generalizations (as of the value of population parameters), usually with calculated degrees of certainty, is inference. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Inholding - An inholding is a parcel of privately owned land surrounded by public land, for example, inside the boundary of a national park. Source: Katie KellerLynn

Insecticide - Insecticide is the term used to describe specific chemicals used to attack insects that are considered invasive species. Source: National Park Service

Intermediate - Intermediate is said of an igneous rock that is transitional between basic and silicic (or between mafic and felsic), generally having a silica content of between 54% to 65%. Source: Bates and Jackson (1987)

Internal Flow - A glacier flows because it deforms in response to stress set up within its ice mass by the force of gravity. At any point within a glacier, the ice is subject to stress as a result of the weight of overlying ice (often hundreds of meters of it) and the surface slope of the glacier. Source: Katie KellerLynn

Intertidal Zone - The intertidal zone is the area of shoreline that lies between the highest normal high tide and the lowest normal low tide. Source: Katie KellerLynn

Intrusive Igneous Rock - The process of emplacement of magma in preexisting rock is called intrusion; therefore, an intrusive igneous rock is the rock mass so formed within the surrounding rock. Source: Bates and Jackson (1987)

Invasive Species - Invasive species can cause ecological and economic harm and be threats to human health. They are invasive because they are not native to the biotic community or ecosystem into which they spread. Source: National Park Service

Invertebrate - Invertebrates belong to a large branch of animals that do not have a spinal column. Some examples of invertebrates are insects and crustaceans. Source: National Park Service

Ion - An ion is an atom or group of atoms that carries a positive (cation) or negative (anion) electric charge as a result of having lost or gained one or more electrons. Source: Merriam-Webster’s Collegiate Dictionary (11th ed.)

Isostasy - Isostasy is the condition of equilibrium, comparable to floating, of the units of the lithosphere and asthenosphere. Crustal loading, as by ice, water, sediments, or volcanic flows, leads to isostatic depression or downwarping; removal of load, to isostatic upwarping (or rebound). Source: Neuendorf et al. (2005)

Isostatic Rebound - See "Isostasy."

Isotope - An atom of an element having the same atomic number (i.e., same number of protons in the nucleus) but differing mass number (i.e., the sum of protons and neutrons in the nucleus). Most elements have several isotopes; for example, carbon-12, carbon-13, and carbon-14 all have six protons. Source: Skinner and Porter (1995)


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Jokulhaup - Jokulhaup is an Icelandic term used to describe a glacier outburst flood resulting from the failure of a glacier-ice-dam, glacier-sediment-dam, or from the melting of glacier ice by a volcanic eruption. Source: Bruce Molnia

Jurassic - The Jurassic is the second period of the Mesozoic Era (after the Triassic and before the Cretaceous) thought to have covered the span of time between 199.6 and 145.5 million years ago; also, the corresponding system or rocks. It is named after the Jura Mountains between France and Switzerland, in which rocks of this age were first studied. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Juxtaposed - "Juxtaposed" is to place side by side. Source: Katie KellerLynn


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Kame - Kames are sand and gravel deposits formed by running water on stagnant or moving glacier ice. Kames form on flat or inclined ice, in holes, or in cracks. A kame terrace forms between the glacier and the adjacent land surface. Shapes include hills, mounds, knobs, hummocks, or ridges. Source: Bruce Molnia

Karst - Landforms produced primarily through the dissolving of rock, such as limestone, dolomite, marble, gypsum, and salt, are collectively known as karst. Features of karst landscapes include sinkholes, caves, large springs, dry valleys, and sinking streams. These landscapes are characterized by efficient flow of groundwater through conduits that become larger as the bedrock dissolves. Source: Veni et al. (2001)

Kettle - A kettle is a depression that forms in an outwash plain or other glacial deposit by the melting of an in situ block of glacier ice that was separated from the retreating glacier margin and subsequently buried by glacier sedimentation. As the buried ice melts, the depression enlarges. Source: Bruce Molnia

Keystone Species - A keystone species influences biological diversity and ecosystem function. If a keystone species is removed from an ecosystem many other species are affected. Source: Julie Johndreau, NPS


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Laboratory - A laboratory is a place equipped for experimental study in a science or for testing and analysis. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Laccolith - A laccolith is a massive, concordant (said of a contact between an igneous intrusion and the preexisting rock, which parallels the foliation or bedding of the latter) igneous body intruded between preexisting strata. Source: Bates and Jackson (1984)

Lagoon - A lagoon is a shallow body of water that does not receive significant freshwater or seawater inflow and is separated from the open ocean by a barrier island or coral reef. They are minimally affected by tides. Source: Katie KellerLynn

Lahar - Lahar is an Indonesian word for rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars also are referred to as volcanic mudflows or debris flows. Source: U.S. Geological Survey

Landslide - Landslide is a general term used for a wide variety of processes and landforms involving the downslope movement, under gravity, of masses of soil and rock material. A broad range of landslide morphology, rates, patterns of movement, and scale occurs. Types include rockfall, mudflow, slump, and many others. Source: Bates and Jackson (1984)

Lapilli - Lapilli are pyroclastics in the general size range of 0.08 to 2.5 inches (2 to 64 mm). Source: Bates and Jackson (1984)

Laramide Orogeny - The Laramide Orogeny was a time of deformation (e.g., mountain building and uplift), typically recorded in the eastern Rocky Mountains of the United States, whose several phases extended from late Cretaceous until the end of the Paleocene (approximately 80 to 50 million years ago). Source: Neuendorf et al. (2005)

Latitude - The angular distance north or south of the equator, measured in degrees, is latitude. Source: Linda Lutz-Ryan

Lava - The word for magma (molten rock) when it erupts onto Earth's surface is lava. It is from the Italian word for stream, which is derived from the verb "lavare" meaning to wash. Source: U.S. Geological Survey

Lava Dome - Lava domes are rounded, steep-sided mounds built by very viscous magma, usually either dacite or rhyolite. Source: U.S. Geological Survey

Lava Flow - Masses of molten rock that pour onto Earth's surface during an effusive eruption are lava flows. Both moving lava and the resulting solidified deposits are referred to as lava flows. Source: U.S. Geological Survey

Lava Fountain - A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is a lava fountain. Source: U.S. Geological Survey

Lava Tube - Natural conduits through which lava travels beneath the surface of a lava flow are lava tubes. They form by the crusting over of lava channels and pahoehoe flows. Source: U.S. Geological Survey

Law - see "Natural Law"

Leaching - Leaching is the process in which water removes and transports soil humus and inorganic nutrients in solution. Source:

Lewis and Clark Expedition - On February 28, 1803, President Thomas Jefferson won approval from Congress to fund a small expeditionary group, whose mission was to explore the uncharted West. Jefferson’s secretary, Meriwether Lewis, and Lewis’ friend, William Clark, led the expedition. Over the next four years, the team of explorers would travel thousands of miles, experiencing lands, rivers, and peoples. Source: Public Broadcasting Service (PBS)

Lichen - Lichen is any of a number of complex plant-like organisms made up of an alga and a fungus growing in symbiotic association on a solid surface (as a rock). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Ligament - Ligament is a tough fibrous band of tissue connecting the articular extremities of bones or supporting an organ in place. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Lignite - Lignite is a brownish black coal that is intermediate in coalification between peat and subbituminous coal. Source: Bates and Jackson (1984)

Limestone - Limestone is a sedimentary rock consisting chiefly of the mineral calcite (calcium carbonate, CaCO3). Limestone is the most important and widely distributed of carbonate rocks and is the consolidated equivalent of limy mud, calcareous sand, or shell fragments. Source: Bates and Jackson (1984)

Limy - An rock is "limy" if it contains a significant amout of lime or limestone. Source: Katie KellerLynn

Liquid - With respect to the classification scheme based on the three states of matter (solid, liquid, gas), in a liquid, the atoms and molecules are separated by greater distances than in a solid. Movement of the atoms or molecules gives a liquid its most distinctive property—the ability to flow, covering the bottom and assuming the shape of its container. Source: Petrucci and Harwood (1993)

Lithology - Lithology is the study of rocks, in particular their physical characteristics such as color, mineralogic composition, and grain size. Source: Katie KellerLynn

Lithosphere - The lithosphere is the rigid outer layer of Earth, including the crust and upper mantle. Source: Katie KellerLynn

Loam - Loam is a rich, permeable soil composed of a mixture of clay, silt, sand, and organic matter. Source: Bates and Jackson (1984)

Logistics - The National Park Service uses the term logistics for the process of gathering, managing, and moving equipment and people. Source: National Park Service

Longitude - Longitude is the angular distance east or west on Earth's surface, measured in degrees from the Prime Meridian, which passes through Greenwich, England. Source: Linda Lutz-Ryan

Longshore Current - A longshore current is a horizontal movement of water in the surf zone that runs parallel to the shoreline and is powered by breakers. Source: Pinet (1992)

Longshore Sediment Transport - Longshore sediment transport is the movement of sand in the surf zone parallel to the shoreline by longshore currents. Source: Katie KellerLynn

Louisiana Purchase - By a treaty signed on April 30, 1803, the United States purchased from France the Louisiana Territory, more than 800,000 square miles (2 million km2) of land extending from the Mississippi River to the Rocky Mountains. The price was 60 million francs, about $15 million; $11,250,000 was to be paid directly, with the balance to be covered by the assumption by the United States of French debts to American citizens. Source: Gateway New Orleans

Lycopod - Lycopods are club mosses, that is, any of an order Lycopodiales of primative vascular plants (as ground pine). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)


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Maar - A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Source: U.S. Geological Survey

Mafic - Mafic is a mnemonic adjective derived from magnesium + ferric + ic, and applied to an igneous rock composed chiefly of dark, iron- and magnesium-rich minerals; also said of those minerals. It is the compliment of felsic. Source: Bates and Jackson (1984)

Magma - Magma is molten or partially molten rock beneath Earth's surface. When magma erupts onto the surface, it is called lava. Source: U.S. Geological Survey

Magnesium - Magnesium is a light, silver-white, malleable, ductile metallic element that occurs abundantly in nature and is used in metallurgical and chemical processes, in photography, signaling, and pyrotechnics because of the intense white light it produces on burning, and in construction, especially in the form of light alloys. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Magnetite - Magnetite is a black, strongly magnetic, opaque mineral. It constitutes an important ore of iron. It is a very common, widely distributed accessory mineral in rocks of all kinds. It also occurs as a heavy mineral in sands. Source: Katie KellerLynn

Mammal - Mammals are the collection of vertebrate animals. Female mammals are capable of nursing their young with milk. Source: National Park Service

Mantle - The zone of Earth below the crust and above the core is the mantle. It is divided into the upper mantle and lower mantle, with a transition zone between. Source: Bates and Jackson (1984)

Marble - Marble is a metamorphic rock consisting predominantly of fine- to coarse-grained recrystallized calcite or dolomite. Source: Bates and Jackson (1984)

Marl - "Marl" is a term loosely applied to a variety of materials, mostly of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under marine conditions or especially freshwater conditions; specifically marls are earthy substances containing 35%-65% clay and 65%-35% carbonate. Source: Neuendorf et al. (2005)

Marsh - Marshes are water-saturated, poorly drained areas, intermittently or permanently water-covered, having aquatic and grass-like vegetation, essentially without the formation of peat. Source: Bates and Jackson (1987)

Mass Balance - Mass balance is a measure of the change in mass of a glacier at a certain point for a specific period of time. It is the balance between accumulation and ablation. Source: Bruce Molnia

Matrix - The matrix is the finer-grained material enclosing the larger grains in a sediment or sedimentary rock; also, the "groundmass" of an igneous rock. Source: Bates and Jackson (1984)

Meridian - The meridian is a great circle on the surface of the Earth passing through the poles. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Mesic - The term mesic refers to a habitat or plant, for example, that requires a moderate amount of moisture. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Mesozoic - The Mesozoic is an era of geologic time, from the end of the Paleozoic to the beginning of the Cenozoic, or from about 251.0 to 65.5 million years ago; also, the rocks that formed during that era. It includes the Triassic, Jurassic, and Cretaceous Periods. It is often referred to as the "Age of Reptiles." Source: International Commission on Stratigraphy (2003); Katie KellerLynn

Metadacite - Metadacite is a grayish-white, fine-grained, metamorphosed volcanic rock. Source: Katie KellerLynn

Metamorphic - Metamorphic describes rocks that have undergone metamorphism (i.e., change). Metamorphic rocks have been derived from preexisting rocks and have undergone chemical, structural, or mineralogical changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in Earth’s crust. Metamorphic rocks constitute one of the three main classes into which rocks are divided, the others being sedimentary and igneous. Source: Katie KellerLynn

Meteorite - A meteorite is any solid object from interplanetary space that has fallen to Earth's surface without being vaporized by the frictional heating during its passage through the atmosphere. Most meteorites are thought to be fragments of asteroids and to consist of primitive solid matter similar from which Earth originally formed. Source: Bates and Jackson (1984)

Midden - Packrats build a protective abode known as a midden. This midden is a fortress of tangled vegetation, bones, sticks, and other items that are held together by organic glue, the urine of the packrat. Source: National Park Service

Mineral - A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Source: Bates and Jackson (1984)

Miocene Epoch - The Miocene is the first epoch of the Neogene Period, which lasted for nearly 18 million years (5.3 to 23.0 million years ago). Source: International Commission on Stratigraphy (2003)

Mississippian - Terminology used primarily in North America, Mississippian time occurred between about 359.2 and 318.1 million years ago; also, the rocks that formed during that time. It is named after the Mississippi River valley, in which good exposures of rocks of this age exist. It is the approximate equivalent of the Lower Carboniferous in European usage. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Mohorovičić Discontinuity - The Mohorovičić discontinuity, or Moho for short, is the boundary separating the crust and the mantle. It is discernible by an increase in seismic velocity. Source: Lutgens and Tarbuck (1992)

Mollisols - Mollisols are soils with thick, dark surface horizons that are high in organic matter content. Source: Kohnke and Franzmeier (1995)

Mollusk - Mollusks are members of the large phylum (Mollusca) of invertebrate animals (e.g., snails, clams, or squids) with a soft unsegmented body usually enclosed in a calcareous shell. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Montane - Environments found in, or related to, mountainous regions are referred to as montane. Source: National Park Service

Moraine - Moraine is a general term for unstratified, unsorted deposits of sediment that form through the direct action of, or contact with, glacier ice. Many different varieties are recognized based on their position with respect to a glacier. Source: Bruce Molnia

Moraine, Ablation - An ablation moraine is an irregularly shaped layer or pile of glacier sediment formed by the melting of a block of stagnant ice. Ultimately, it sits on the former bed of the glacier. Source: Bruce Molnia

Moraine, End - see "Moraine, Terminal"

Moraine, Ground - A blanket of glacier till deposited on all of the surfaces over which a glacier moved is called ground moraine. Source: Bruce Molnia

Moraine, Lateral - A lateral moraine is an accumulation of glacial drift along the lateral margins of a valley glacier, remaining as a ridge or embankment upon glacier recession; also, the surficial accumulation of ablation debris on the margin of an existing valley glacier. Source: Sharp (1988)

Moraine, Medial - A medial moraine is a sediment ridge, located on a glacier’s exposed ice surface, away from its valley walls, extending down-glacier to the terminus. It forms by the joining of two lateral moraines when two glaciers merge. Source: Bruce Molnia

Moraine, Push - A push moraine is a ridge of rock debris shoved up along the edge of an advancing glacier. Source: Sharp (1988)

Moraine, Recessional - A recessional moraine is a ridge of glacial sediment that forms when the terminus of a retreating glacier remains at or near a single location for a period of time sufficient for a cross-valley accumulation to form. A series of such moraines represents a number of pauses during retreat. Source: Bruce Molnia

Moraine, Terminal - The terminal moraine is a cross-valley, ridge-like accumulation of glacial sediment that forms at the farthest point reached by the terminus of an advancing glacier. Both end moraines and terminal moraines form at the snout of the glacier, but end moraines have not been identified as the furthest point of glacier advance. Source: Bruce Molnia

Mortar - A muzzle-loading cannon for firing shells at low velocities, short ranges, and high trajectories is called a mortar. Because of their short, stubby appearance, they resemble mortars used with pestles to grind up substances like oats, corn, and rice. Source: Carlin Timmons

Moulin - A narrow, tubular chute or crevasse through which water enters a glacier from the surface is called a moulin. Occasionally, the lower end of a moulin may be exposed in the face of a glacier or at the edge of a stagnant block of ice. Source: Bruce Molnia

Mud Flat - A mud flat is a relatively level area of fine silt along a shore (as in a sheltered estuary) or around an island, alternately covered and uncovered by they tide, or covered by shallow water. Source: Bates and Jackson (1987)

Mud Pot - A mud pot is a type of hot spring containing boiling mud, usually sulfurous and often multicolored, as in a paint pot. Mud pots are commonly associated with geysers and other hot springs in volcanic areas, especially Yellowstone National Park. Source: Bates and Jackson (1984)

Mudflow - see "Lahar"

Mudstone - Mudstone is a fine-grained sedimentary rock whose original constituents were clays or muds. Source: Wikipedia


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Native Species - With respect to a particular ecosystem, a species is native if, other than as a result of an introduction, it has historically occurred or currently occurs in that ecosystem. Source: National Park Service; Executive Order 13112–Invasive Species

Natural Law - Applying the scientific method requires careful observations of natural phenomena. When enough observations have been made that a pattern begins to emerge, scientists then formulate a generalization (natural law) describing the phenomena. Source: Petrucci and Harwood (1993)

Nearshore Zone - The nearshore zone extends seaward or lakeward an indefinite but generally short distance from the shoreline, usually beyond the surf zone. Source: Katie KellerLynn

Névé - Névé is a French term meaning a mass of hardened snow at the source or head of a glacier; it refers to the overall snow cover that exists during the melting period and sometimes from one year to another. In general, névé and accumulation zone are equivalent. Source: Bates and Jackson (1987)

Niche - The portion of the environment which a species occupies, defined in terms of the conditions under which an organism can survive, and may be affected by the presence of other competing organisms. Source: The Paleontology Portal

Nitrate - Nitrate (NO3-) is the form of nitrogen commonly found in soil and used by plants for building amino acids, DNA, and proteins. It is commonly produced by the chemical modification of nitrite by specialized bacteria. Source:

Nitrogen Fixation - Nitrogen fixation is the conversion of nitrogen gas (N2) in the atmosphere to nitrogen-containing organic compounds in soil. Source: Kohnke and Franzmeier (1995)

Nitrous Oxide - Nitrous oxide (N2O) is the gas found in the atmosphere that contributes to the greenhouse effect. Sources for nitrous oxide include land-use conversion, fossil fuel combustion, biomass burning, and soil fertilization. Source:

Nocturnal - Nocturnal is the term used to describe organisms that are active primarily at night. The opposite of this is diurnal, that is, organisms that are active during the day. Source: National Park Service

NPS - NPS is an acronym for the National Park Service—a bureau under the United States Department of the Interior of the federal government mandated to preserve and protect many natural, historic, and cultural sites of our nation. Source: National Park Service

Nuée Ardente - Meaning "glowing cloud" in French, a nuée ardente is a swiftly flowing, turbulent, gaseous cloud, sometimes incandescent, erupted from a volcano and containing ash and other pyroclastics in its lower part. The lower part of a nuée ardente is comparable to an ash flow. Source: Bates and Jackson (1987)

Nunatak - Nunatak is a Greenlandic term used to describe a mountain peak or ridge that pokes through the surface of an icefield or glacier. It may separate adjacent valley glaciers. Source: Bruce Molnia


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Obelisk - An obelisk is an upright, four-sided pillar that gradually tapers as it rises and terminates in a pyramid. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Obligate Wetland Species - Wetland indicator categories estimate the probability of a plant being found on a site classified as a wetland. An obligate wetland species almost always occurs in wetlands, >99% of the time. Source: U.S. Fish and Wildlife Service

Observation - An observation is a record and description of a natural phenomena, often involving measurements with instruments. An observation that is repeatedly confirmed is a fact. Source: Katie KellerLynn

Observatory - An observatory is a building or place given over to or equipped for observation of natural phenomena (as in astronomy); also, an institution whose primary purpose is making such observations. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Obsidian - Obsidian is a black or dark-colored volcanic glass, usually of rhyolitic composition, characterized by conchoidal (smoothly curved surface) fracture. It has been used for making jewelry, arrowheads, and art objects. Source: Bates and Jackson (1984)

Ogive - Ogives are rhythmically repeated, light and dark bandings within a glacier or a swell-and-swale configuration on its surface, convexly curved up-glacier, that form below some icefalls. Source: Sharp (1988)

Oligocene - The Oligocene is a geologic epoch in the Cenozoic Era between about 33.9 and 23 million years ago occurring between the Eocene and the Miocene Epochs; also, the corresponding worldwide series of rocks. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Olivine - Olivine is a green or brown orthorhombic mineral commonly found in igneous rocks (e.g., gabbro and basalt). Source: Katie KellerLynn

Omnivore - Omnivores eat both plants and other animals. They tend to have some teeth adapted to grinding plants and some teeth adapted for tearing and cutting. A modern example of an omnivore is humans. Source: Julie Johndreau, National Park Service

Ordovician - The Ordovician is the second earliest period of the Paleozoic Era (after the Cambrian and before the Silurian) thought to have covered the span of time between about 488.3 and 443.7 million years ago; also, the corresponding system of rocks. It is named after the Celtic tribe called the Ordovices. Source: Bates and Jackson (1984)

Organelle - An organelle is a discrete structure of a cell having specialized functions. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. An organelle is to the cell what an organ is to the body. Source: Wikipedia

Organic Act - Among other purposes, the Organic Act—not an official short title but merely a popular name used for convenience; the act has no official short title—established the National Park Service. The National Park Service Organic Act (16 U.S.C. l 2 3, and 4), consists of the Act of Aug. 25 1916 (39 Stat. 535) and amendments thereto. Source: National Park Service

Organism - An organism is an independent form of life, ranging from single-celled bacteria to multi-cellular plants and animals. Source: National Park Service

Orogeny - Orogeny is the process of formation of mountains. Source: Bates and Jackson (1984)

Osmosis - Osmosis is the movement of a solvent (e.g., water) through a semipermeable membrane (e.g., as of a living cell) from a region of low solvent potential to a region of high solvent potential. Source: Wikipedia; Merriam-Webster's Collegiate Dicitionary (11th ed.)

Ostracods - Ostracods belong to the class Crustacea (i.e., crustaceans); they are sometimes referred to as “seed shrimp” because of their appearance. Ostracods are small typically around one mm in size, but vary between 0.2 to 30 mm; they are laterally compressed and protected by a bivalve-like, chitinous or calcareous valve or "shell." The hinge of the two valves is in the upper, dorsal region of the body. Scientists have identified some 50,000 extinct and extant species, which are grouped into several orders. Ostracods appeared in the Cambrian and continue today. Source: Wikipedia; Neuendorf et al. (2005)

Outcrop - An outcrop is the part of a geologic formation or structure that appears at the surface of the Earth. The term also is used as a verb meaning to appear exposed and visible at Earth's surface. Source: Bates and Jackson (1984)

Outwash Plain - An outwash plain is a broad, low-sloping, alluvial plain composed of glacially eroded, sorted sediment (termed outwash), that has been transported by meltwater. The alluvial plain begins at the foot of a glacier and may extend for miles. Typically, the sediment becomes finer-grained with increasing distance from the glacier terminus. In mountainous areas, outwash plains are referred to as valley trains. Source: Bruce Molnia; Katie KellerLynn

Oxidation - Oxidation is the addition of oxygen atoms to mineral compounds. It occurs when water containing dissolved oxygen percolates through cracks in a rock, when water contacts the rock below a soil, or when rock is exposed directly to rain. The dissolved oxygen reacts especially with minerals that contain iron, and forms insoluble red oxides (the “ferric” variety formed in rust, Fe2O3) or yellow-brown hydroxides, which may crumble when they dry out. These minerals often provide color to soils. The chemical reactions of oxidation weathering may be reversed in environments where oxygen is in short supply or absent. Source: Bradshaw and Weaver (1993)

Ozone Layer - The ozone layer is an atmospheric layer at heights of about 20 to 30 miles (32 to 48 km) that is normally characterized by high ozone (a very reactive form of oxygen) content which blocks most solar radiation from entry into the lower atmosphere. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)


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Pahoehoe - Pronounced "pa-hoy-hoy," pahoehoe is a Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface. Source: U.S. Geological Survey

Paleocene - The Paleocene is an interval of geologic time between about 65.5 and 55.8 million years ago. It is the first epoch in the Cenozoic Era, recently replacing the Tertiary in terminology by the International Commission on Stratigraphy. Source: International Commission on Stratigraphy (2003)

Paleontologist - see "Paleontology"

Paleontology - Paleontology is the study of past life on Earth, as represented by the fossils of plants, animals, and other organisms. A scientist who studies past life is known as a paleontologist. Source: Katie KellerLynn; Jan Gillespie

Paleozoic - The Paleozoic is an era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 542 to 251 million years ago; also, the rocks deposited during this time. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Pangaea - Pangaea is the supercontinent that existed from about 300 to about 200 million years ago and included most of the continental crust of Earth. The present continents were derived from it by fragmentation via an intermediate stage of Laurasia on the north and Gondwana on the south. Source: Bates and Jackson (1984)

Parent Material - Parent material is the geologic material from which soils form. Parent material is one of five soil-forming factors: climate, organisms, relief (topography), and parent material over time. Source: Kohnke and Franzmeier (1995)

Parent Rock - A parent rock is the rock from which sediments or other rocks are derived. Source: Katie KellerLynn

Passive Margin - Passive margin is another name for a divergent plate boundary, that is, a boundary between two segments of Earth’s lithosphere that are moving away from each other. Passive margins are formed through rifting and continental rupture. Source: Katie KellerLynn

Ped - Pick up a clod of soil, and you’ve picked up a ped, which is a naturally formed unit of soil structure, for example, a granule, block, crumb, or aggregate. Source: Bates and Jackson (1984); Katie KellerLynn

Pedon - A pedon is the smallest unit of volume of soil that represents or exemplifies all horizons of a soil profile. It is usually a horizontal, more or less hexagonal area of about one square meter, but may be larger. The term is part of the classification system of the National Cooperative Soil Survey. Source: Bates and Jackson (1984)

Pelecypods - Pelecypods are of the class Pelecypoda, more commonly called Bivalva. Pelecypods are mollusks that typically have two-part shells, with both parts being more or less symmetrical. The class has 30,000 species, including scallops, clams, oysters, and mussels. Pelecypods are exclusively aquatic; they include both marine and freshwater forms. “Pelecy” means hatchet; “pod” means foot. Source: Wikipedia

Pelycosaurs - The "pelycosaurs" are extinct members of the Synapsida, a major branch of the Amniota, or egg-laying tetrapods. The only currently living synapsids are the mammals. In many respects, the pelycosaurs are intermediate between the reptiles and mammals, and so they have commonly been referred to as "mammal-like reptiles." However, they are characterized by a single dermal opening in the skull permitting muscle attachment to the jaw, which means that the pelycosaurs are not reptiles because reptiles have two such openings in their skulls. It is believed that the pelycosaurs, like their living mammal relatives, were endothermic, which means that they maintained a constant internal body temperature. This is another characteristic that sets pelycosaurs apart from the reptiles. If this view is correct, then pelycosaurs are one of the earliest examples of endothermic animals. Source: University of California-Berkeley Web site

Pennsylvanian - Terminology used primarily in North America, Pennsylvanian time occurred between about 318.1 and 299 million years ago; also, the rocks that formed during that time. It is named after the state of Pennsylvania, in which the rocks of this age are widespread. It is the approximate equivalent of the Upper Carboniferous in European usage. Source: Bates and Jackson (1984); International Commission on Stratigraphy (2003)

Perissodactyles - The odd-toed ungulates or Perissodactyla are large to very large browsing and grazing mammals with relatively simple stomachs and a large middle toe. The members of the order fall into two groups: the suborder Hippomorpha, horses, which have only one toe and tend to be fast runners with long legs, and the suborder Ceratomorpha, which contains two families of slower-moving, thick-set animals with several functional toes: the tapirs and the rhinoceroses. The odd-toed ungulates arose in what is now North America in the late Paleocene Epoch (less than 10 million years ago). By the start of the Eocene Epoch, perissodactyles had diversified and spread to occupy several continents. Source: Wikipedia

Permafrost - Permafrost is any soil, subsoil, or other surficial deposit, or even bedrock, occurring in arctic, sub-arctic, and alpine regions at a variable depth beneath Earth’s surface in which a temperature below freezing has existed continuously for a long time (two years to tens of thousands of years). This definition is based exclusively on temperature, and disregards the texture, degree of compaction, water content, and lithologic character of the material. The thickness of permafrost ranges from more than 305 feet (1,000 m) in the north to 76 inches (30 cm) in the south; it underlies about one-fifth of the world’s land area. Source: Bates and Jackson (1987)

Permian - The Permian is the last period of the Paleozoic Era (after the Pennsylvanian [or Upper Carboniferous]) thought to have covered the span of time between about 299 and 251million years ago; also the corresponding system of rocks. It is named after the province of Perm, USSR, where rocks of this age were first studied. Source: Bates and Jackson (1984), International Commission on Stratigraphy (2003)

Permineralization - Permineralization is a type of fossilization where the original hard parts of an organism have additional mineral material deposited in their pore spaces. Source: Bates and Jackson (1984)

Pesticide - Pesticide is a general term used to describe chemicals used to control species considered invasive. Source: National Park Service

Petrifaction - Petrifaction is a process of fossilization whereby organic matter is converted into a stony substance by infiltration of water containing dissolved inorganic matter (e.g., calcium carbonate or silica) which replaces the original organic materials, sometimes retaining the structure. Source: Bates and Jackson (1984)

Petrified Wood - Wood that has been replaced by minerals, yet retains its original form and structure, is petrified wood. Source: Linda Lutz-Ryan

pH - In 1909 the Danish biochemist Søren Sørensen proposed the term pH to refer to the "potential of hydrogen ion." He defined pH as the "negative of the logarithm of [H+]". It is a measure of acidity and alkalinity of a solution that is a number on a scale on which a value of 7 represents neutrality; lower numbers indicate increasing acidity, and higher numbers increasing alkalinity. Source: Petrucci and Harwood (1993); Merriam-Webster's Collegiate Dictionary (11th ed.)

Phylogeny - Phylogeny is the evolutionary history of an organism. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Phylum - Phylum is a primary category in biological taxonomy that ranks above class and below kingdom. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Physiology - Physiology is the study of the role and function of organisms, or the parts of an organism. Source: National Park Service

Phytoplankton - see "Plankton"

Piedmont - The term piedmont refers to a feature lying or formed at the base of a mountain or mountain range. Source: Bates and Jackson (1984)

Pillow Lava - Pillow lava is pillow-shaped lava that forms when cooled under water. A thin and flexible crust quickly forms; pressure builds, and the crust breaks allowing more lava to ooze through the crust. This sequence continues until long thick deposits of lava harden on the seafloor. Source: National Park Service

Pit Crater - A pit crater is a circular-shaped crater formed by the sinking collapse of the ground. Source: U.S. Geological Survey

Placer - A placer is a deposit formed when heavy minerals are mechanically concentrated by currents, most commonly streams and waves. Placers are sources of gold, tin, platinum, diamonds, and other valuable minerals. Source: Lutgens and Tarbuck (1992)

Plankton - Plankton is the passively floating or weakly swimming animal (zooplankton) or plant (phytoplankton) life of a body of water; plankton is usually quite minute. Source: Katie KellerLynn

Plant - A plant is a multi-cellular organism with eukaryotic cells (have membrane-bound organelles and a defined nucleus). Plants are one of the kingdoms of life and are different from animals and fungi because they are able to transform sunlight into useable energy via photosynthesis. Source: National Park Service

Plate Boundary - A plate boundary is a zone of seismic and tectonic activity along the edges of lithospheric plates, presumed to indicate relative motion between them. Source: Bates and Jackson (1984)

Plate Tectonics - Plate tectonics is a theory of global tectonics in which the lithosphere is divided into a number of plates whose pattern of horizontal movement is that of torsionally rigid bodies that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries. Source: Bates and Jackson (1984)

Plateau Basalt - Also known as flood basalts, plateau basalts occur as a result of fissure eruptions of basaltic lava along elongated cracks in Earth's surface. Magma wells up through the crack and pours out on either side in effusive eruptions, forming extensive sheets of basaltic lava. Successive eruptions add layers of lava until a plateau is formed. The Columbia River Plateau in the northwestern region of the United States is an example of this type of basalt. Source: National Park Service

Pleistocene Epoch - The Pleistocene is an epoch of geologic time associated with ice ages. It occurred after the Pliocene Epoch (1.8 million years ago) but before the Holocene Epoch in which we now live. Source: Katie KellerLynn

Plinian Eruption - A Plinian eruption is an explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gas is released at a high velocity from a vent. Large volumes of pyroclastics and tall eruption columns are characteristic of this type of eruption. Source: Bates and Jackson (1984)

Pliocene - The Pliocene is an epoch of geologic time between about 5.3 and 1.8 million years ago. It is between the Miocene and Pleistocene. Source: International Commission on Stratigraphy (2003)

Plucking - The mechanical removal of pieces of rock from a bedrock face that is in contact with glacier ice. Blocks are quarried and prepared for removal by the freezing and thawing of water in cracks, joints, and fractures. The resulting pieces are frozen into the glacier ice and transported away. Source: Bruce Molnia

Pluton - A pluton is an igneous intrusion. Source: Bates and Jackson (1987)

Polar Region - Polar regions are areas suitable for glacial growth because of the low annual temperatures. These low temperatures are due to the low amounts of solar radiation that reach these regions. Cold temperatures mean that the snow that does accumulate does not melt very easily. This is important because these same cold temperatures also reduce the amount of snowfall in these areas. In fact, Antarctica is a cold desert, due to the lack of annual precipitation found there, but it has the world's largest glaciers. Source: Katie KellerLynn

Porosity - The percentage of bulk volume of a rock or soil that is occupied by interstices (spaces), whether isolated or connected is referred to as porosity. Source: Bates and Jackson (1984)

Potash - Potash is an impure form of potassium carbonate (K2CO3) mixed with other potassium salts. Potash has been used since antiquity in the manufacture of glass and soap, and as a fertilizer. Source: Wikipedia

Precambrian - All geologic time and its corresponding rocks before the beginning of the Paleozoic is referred to as the Precambrian. It is equivalent to about 90% of geologic time. Source: Bates and Jackson (1984)

Precipitation - Precipitation is the deposit of water on Earth from the atmosphere, such as hail, mist, rain, sleet, or snow. Source: Katie KellerLynn

Predator - A predator is an animal that preys upon other animals as food. Predators are not limited to mammals; they can be birds, insects, fish, and amphibians. Source: National Park Service

Pressure - Pressure is defined as a force per unit area, that is, a force divided by the area over which the force is distributed. Source: Petrucci and Harwood (1993)

Pressure Melting Point - Pressure melting point is the temperature at which ice can melt at a given pressure. Source: Skinner and Porter (1995)

Prokaryote - Prokaryotes (from Old Greek pro- meeaning before and karyon meaning nut, referring to the cell nucleus) are organisms without a cell nucleus or indeed any other membrane-bound organelles. In most cases, they are unicellular. They are distinct from eukaryotes organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between the structure of prokaryotes and eukaryotes is so great that it is considered to be the most important distinction among groups of organisms. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. Source: Wikipedia

Pyrite - Pyrite is a common yellow mineral (FeS2).Pyrite has a brilliant metallic luster and an absence of cleavage, and has been mistaken for gold (which is softer and heavier). It commonly crystallizes in cubes, octahedrons, or pyritohedrons. Pyrite is the most widespread and abundant of the sulfide minerals and occurs in all kinds of rocks. Source: Bates and Jackson (1984)

Pyroclast - A pyroclast is an individual particle ejected during a volcanic eruption. It is usually classified according to size. Source: Katie KellerLynn

Pyroclastic - The term pyroclastic pertains to clastic rock material formed by volcanic expulsion from a volcanic vent; also, pertaining to rock texture or explosive origin. In the plural, the term is used as a noun. Source: Bates and Jackson (1984)

Pyroclastic Flow - A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 62 miles per hour (100 kph) or more. The temperature within a pyroclastic flow may exceed 932°F (500°C). Source: U.S. Geological Survey

Pyroxene - Pyroxene is a group of common rock-forming minerals. It is characterized by short, stout crystals and good prismatic cleavage in two directions intersecting at angles of about 87 degrees and 93 degrees. Source: Bates and Jackson (1984)


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Quadrupedal - Quadrupeds are animals with four feet; "quadrupedal" describes these animals. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Quarry - A quarry is an excavation or open workings, usually for the extraction of stone. Source: Katie KellerLynn

Quartz - Quartz is crystalline silica (SiO2). It is the most common mineral after feldspar. Source: Katie KellerLynn

Quartzite - Quartzite is a sandstone consisting of quartz grains cemented by secondary silica. Source: Bates and Jackson (1984)

Quaternary - Traditionally the Quaternary Period consisted of the Pleistocene and Holocene Epochs and covered the time from 1.8 million years ago to the present. Recently instead of breaking the Cenozoic Era into the Tertiary and Quaternary, however, the International Commission on Stratigraphy breaks the Cenozoic into the Paleogene and the Neogene Periods. Hence, although still used, the terms Quaternary and Tertiary are no longer formally accepted terms for geologic time. Source: International Commission on Stratigraphy (2003)


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Radioactive Decay - Radioactive decay is the set of various processes by which unstable atomic nuclei emit subatomic particles (radiation). Source: Wikipedia

Radioactivity - Radioactivity is the property of some elements (e.g., uranium) or isotopes (e.g., carbon-14) of spontaneously emitting energetic particles (e.g., electrons or alpha particles) by the disintegration of their nuclei. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Radiolarians - Radiolarians (also radiolaria) are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions. They occur as plankton throughout the ocean, and their shells are important fossils found from the Cambrian onwards. Source: Wikipedia

Radiometric Dating - Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40. The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Source: Bates and Jackson (1984)

Rain Shadow - A rain shadow is an area of low atmospheric precipitation in the lee of a mountain range. Source: Bradshaw and Weaver (1993)

Range - The geographical area in which a species is found is considered its range. Source: National Park Service

Rangeland - Rangeland is land on which the native vegetation is predominantly grasses, grass-like plants, forbs, or shrubs. This land includes natural grasslands, savannas, shrub lands, most deserts, tundra, areas of alpine communities, coastal marshes, and wet meadows. Source: Natural Resources Conservation Service (2001)

Rapid - A rapid is the part of a river where the current is fast and the surface is usually broken by obstructions. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Reclamation - With respect to abandoned mine lands or over-used sites, reclamation refers to the processes by which a previous natural state is restored. Source: Katie KellerLynn

Recurrence Interval - Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given event. The recurrence interval (sometimes called the return period) is based on the probability that the given event will be equaled or exceeded in any given year. Source: U.S. Geological Survey

Red Algae - Red algae are any of a division (Rhodophyta) of chiefly marine algae that have predominantly red pigmentation. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Reduction - Reduction is a process in which weathered material loses oxygen from its chemical structure; it is most common where rock or soil is totally waterlogged by still water that does not contain dissolved oxygen. In these circumstances, chemical reaction removes oxygen from the ferric oxides to form more soluble greenish ferrous iron (FeO). The green iron mineral may later be altered if water circulation makes oxygen available. Soils subject to alternate waterlogging and aeration are often mottled green-orange because they contain a mixture of ferrous and ferric oxides. Source: Bradshaw and Weaver (1993)

Regiment - A military unit of ground troops consisting of at least two battalions, usually commanded by a colonel, is a regiment. Source: Kathryn Wright

Regression - Regression is retreat of the sea from land areas and the consequent evidence of such withdrawal. Source: Bates and Jackson (1984, 1987)

Relative Dating - Relative dating is the chronological ordering of features, fossils, or events with respect to the geologic time scale without reference to their absolute age. Source: Bates and Jackson (1984)

Remnant - With respect to glacial geology, a remnant is an isolated melting mass of ice that has become detached from its source and the remainder of the glacier. Some remnants cover many square miles. Source: Bruce Molnia

Remote Sensing - Remote sensing is the collection of information about an object by a recording device that is not in physical contact with it. The term is usually restricted to mean methods that record reflected or radiated electromagnetic energy, rather than methods that involve significant penetration into the Earth. The technique employs such devices as camera, infrared detectors, microwave frequency receivers, and radar systems. Source: Bates and Jackson (1984)

Reptile - Reptiles are animals that are vertebrates, breath air, and are covered in scales. This includes snakes and lizards. Source: National Park Service

Researcher - A person who follows the scientific method to answer a question. Source: National Park Service

Reservoir - A reservoir is an artificial lake where water is collected and stored for human use. Source: Katie KellerLynn

Restoration - The return of a habitat, community, or ecosystem back to the state it was before being modified. Passive restoration allows the natural ecosystem to rejuvenate without intervention after a disturbance. Active restoration requires human intervention (e.g., planting seeds and saplings, releasing native animals). Source: National Park Service

Retreat - A decrease in the length of a glacier compared to a previous point in time. As ice in a glacier is always moving forward, its terminus retreats when more ice is lost at the terminus to melting or calving than reaches the terminus. During retreat, ice in a glacier does not move back up-glacier. Source: Bruce Molnia

Rhyolite - Rhyolite can look very different, depending on how it erupts. Explosive eruptions of rhyolite create pumice, which is white and full of bubbles. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black. Rhyolite often erupts explosively because its high silica content results in extremely high viscosity, which hinders the escape of gases. Source: U.S. Geological Survey

Richter Scale - The Richter scale is a scale of earthquake magnitude based on the motion of a seismograph. Source: Lutgens and Tarbuck (1992)

Rift Zone - A region of crust where extension results in formation of an array of kinematically realted normal faults, along with associated grabens, half grabens, and horsts. Some active rift zones have associated volcanic activity. Some rift zones evolve into troughs filled by very thick sequences of sediment. Some are broad with distributed faults, whereas some are narrow with localized faulting. Source: Neuendorf et al. (2005)

Roche Moutonnée - An elongated, rounded, asymmetrical, bedrock knob produced by glacier erosion. It has a gentle slope on its up-glacier side, and a steep- to vertical-face on the down-glacier side. Source: Bruce Molnia

Rock - Rocks are consolidated mixtures of minerals. Source: Lutgens and Tarbuck (1992)

Rock Flour - see "Glacier Flour"

Rockslide - see "Landslide"

Rockweed - Rockweed is coarse brown algae, either free-floating or attached to rocks, growing in marine environments. Source: Katie KellerLynn

Runoff - Runoff is the part of precipitation appearing in surface streams. It is water that flows over the land rather than infiltrating into the ground. Source: Bates and Jackson (1987); Lutgens and Tarbuck (1992)


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Saline - Saline means consisting of or containing salt. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Salinity - Salinity is the total amount of dissolved salts in seawater or brackish water, measured by weight in parts per thousand. Source: Bates and Jackson (1984)

Sand - Sand is a rock fragment or detrital particle smaller than a granule and larger than a coarse silt grain, having a diameter in the range of 1/16 to 2 mm (62–2,000 microns or 0.0025–0.08 inches). Sand is somewhat rounded by abrasion during transport. Source: Bates and Jackson (1987)

Sand Spits - Sand spits are points, tongues, or embankments made of sand; they have one end attached to the mainland and the other terminating in open water. They are finger-like extensions of beaches. Source: Katie KellerLynn

Sandstone - Sandstone is a sedimentary rock composed mainly of feldspar and quartz and varies in color (in a similar way to sand), through grey, yellow, red, and white. Source: Wikipedia

Saturation - Saturation is the maximum possible content of water vapor in the atmosphere for a given temperature. Saturation also applies to the degree to which pores in a rock (or soil, in the case of water) contain oil, gas, or water, generally expressed in percent of total pore space. Source: Bates and Jackson (1984)

Schist - Schist is a type of medium-grade metamorphic rock, notable for the preponderance of lamellar minerals, including micas, chlorite, talc, hornblende, or graphite. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. Source: Wikipedia; Katie KellerLynn

Science - see "Scientific Method"

Scientific Method - What distinguishes science from other fields of study is the method that scientists use to acquire knowledge and the special significance of this knowledge. Scientific knowledge can be used to predict future events. The scientific method originated in the 17th century with people such as Galileo, Francis Bacon, and Isaac Newton. The key to the method is to make no initial assumptions, but rather to make careful observations of natural phenomena. The scientific method is the combination of observations, experimentation, and the formulation of laws, hypotheses, and theories. Source: Petrucci and Harwood (1993)

Sea Fan - see "Gorgonian"

Secede - To formally withdraw from membership of an organization, association, or alliance is to secede. Source: Kathryn Wright

Sediment - In general, sediment is solid fragmental material transported by wind, water, or ice, chemically precipitated from solution, or secreted by organisms, and that forms in layers in loose unconsolidated form, e.g., sand, mud, till. Source: Bates and Jackson (1984)

Sedimentary Rock - Sedimentary rocks result from the consolidation of sediment; for example, a clastic rock such as sandstone, a chemical rock such as rock salt, or an organic rock such as coal. Sedimentary rocks constitute one of the three main classes into which rocks are divided, the others being igneous and metamorphic. Source: Katie KellerLynn

Sedimentology - Sedimentology is the scientific study of sedimentary rocks and of the processes by which they were formed. Source: Bates and Jackson (1984)

Seismograph - A seismograph is an instrument that records earthquake waves. Source: Lutgens and Tarbuck (1992)

Seismology - The study of earthquakes and of the structure of Earth by both natural and artificially generated waves. Source: Bates and Jackson (1984)

Serac - A serac is a jagged pinnacle or tower of ice, located on the surface of a glacier, formed as a glacier flows down an icefall, or by the intersection of crevasses. Frequently, large areas of a glacier will be covered by seracs. Source: Bruce Molnia

Shale - Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery, and parallel to the often indistinguishable bedding planes. Non-fissile rocks of similar composition but made of particles smaller than 1/16 mm are mudstones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones. Source: Wikipedia

Shear Zone - A shear zone is a tabular region of rock that has been brecciated by many parallel fractures. Source: Neuendorf et al. (2005)

Shield Volcano - Shield volcanoes have broad, gentle slopes built by the eruption of fluid basalt lava. The largest volcanoes on Earth are shield volcanoes. The name comes from a perceived resemblance to the shape of a warrior's shield. Source: U.S. Geological Survey

Shoreline - The shoreline is the zone where the ocean is in contact with dry land. Source: Pinet (1992)

Siege - A siege is a military blockade of a city of fortified place in which the attackers intend to compel the inhabitants to surrender. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Silica - Silica is silicon dioxide (SiO2), occurring in crystalline, amorphous, and impure forms (as in quartz, opal, and sand respectively). Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Silicified - "Silicified" describes organic material that has undergone silicification, that is, a process of fossilization whereby the original organic components of an organism are replaced by silica such as quartz, chalcedony, or opal. Source: Neuendorf et al. (2005)

Sill - A sill is a tabular igneous intrusion that parallels the planar structure of the surrounding rock. Source: Bates and Jackson (1984)

Silt - Silt is a detrital particle finer than fine sand and coarser than clay, commonly in the range of 1/16 to 1/256 mm. Source: Bates and Jackson (1984)

Siltstone - Siltstone is a sedimentary rock that is intermediate in grain size between the coarser sandstone and the finer shale. As its name implies, it is primarily composed of silt, defined as grains smaller than 62 micrometers. Source: Wikipedia

Silurian - The Silurian is a period of the Paleozoic Era thought to have covered the span of time between about 443.7 and 416 million years ago; also, the corresponding system of rocks. The Silurian follows the Ordovician and precedes the Devonian. It is named after the Silures, a Celtic tribe. Source: Bates and Jackson (1984)

Silviculture - Silviculture is the branch of forestry dealing with the development and care of forests. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Slate - Slate is a compact, fine-grained metamorphic rock that possess slaty cleavage and hence can be split into slabs and thin plates. Most slate was formed from shale. Source: Bates and Jackson (1984)

Snout - see "Terminus"

Snowbridge - A snowbridge is a mass of snow that has accumulated in the top of an open crevasse, masking the existence of the crevasse. Frequently, a large void exists below the bridge. Source: Bruce Molnia

Soil - Soil is a dynamic resource that supports plants. It consists of mineral particles of different sizes (sand, silt, and clay), organic matter, and numerous species of living organisms. Soil has biological, chemical, and physical properties, some of which change in response to how the soil is managed. Source: Natural Resources Conservation Service (2001)

Soil Fertility - Soil fertility is the ability of soil to supply and sustain adequate amounts of nutrients for plant growth. Source: Saskatchewan Interactive

Solid - With respect to the classification scheme based on the three states of matter (solid, liquid, gas), in a solid, atoms and molecules are in close contact, sometimes in a highly organized arrangement called a crystal. A solid occupies a definite volume and has a definite shape. Source: Petrucci and Harwood (1993)

Solution - Solution is a process of chemical weathering by which mineral and rock material passes into solution (homogeneously mixed with a liquid). An example is the removal of calcium carbonate in limestone by carbonic acid derived from rainwater containing carbon dioxide, acquired during its passage through the atmosphere. Source: Katie KellerLynn

Spar - Spar is a general term used to describe any euhedral or subhedral crystal, regardless of mineral composition, where the crystal facets are easily discernable. Source: Katie KellerLynn

Species - Species is a classification of organisms all of which have a high degree of physical and genetic similarity, generally interbreed only among themselves, and show persistent differences from members of allied groups of organisms. Source: National Park Service, Executive Order 13112–Invasive Species

Speleothem - A speleothem (from the Greek for “cave deposit”) is a formal term for a cave formation. Speleothems are the result of the interactions among water, rock, and air within caves. Examples of speleothems are stalactites, stalagmites, colums, cave popcorn, aragonite crystals, and cave bacon. Source: Katie KellerLynn

Spires - Rock spires form as a result of weathering and erosion along joint and fault lines. Unlike needles or pillars, spires are typically solitary forms Source: Katie KellerLynn

Spodosols - Spodosols are soils with a subsoil accumulation of aluminum, organic matter, and usually iron. Source: Kohnke and Franzmeier (1995)

Spreading Center - Spreading centers, also called divergent margins, are the new, growing edge of a plate. They are coincident with a mid-ocean ridge. Source: Skinner and Porter (1995)

Stagnation - Stagnation is the in situ melting of glacier ice. Many glaciers have stagnant termini, covered by thick sediment debris. Some support vegetation, including mature forests. Source: Bruce Molnia

Steppe - A Russian term for grasslands, steppe landscapes occur in the semiarid midlatitudes where average precipitation is not great enough to support the growth of shrublands or forests. Source:

Stock - A stock is an igneous intrusion that is less than 40 square miles (100 km2) in surface exposure, is usually but not always discordant (not parallel to the foliation or bedding of the preexisting rock), and resembles a batholith except in size. Source: Bates and Jackson (1984)

Strata - Strata is plural for stratum—the layer of sedimentary rock, visually separable from other layers above and below; also referred to as a bed. Source: Bates and Jackson (1984)

Stratigraphy - Stratigraphy is the branch of geology that deals with the formation, composition, sequence, and correlation of layered rocks as parts of Earth's crust. Source: Linda Lutz-Ryan

Stratotype - The stratotype is the designated representative of a named stratigraphic unit, or of a stratigraphic boundary identified as a point in a specific sequence of rock strata. It constitutes the standard for the definition and recognition of that unit or boundary. Source: Bates and Jackson (1984)

Striations - Paralleling the direction of glacial flow, striations are lines of abrasion in the bedrock over which a glacier flowed. These are smaller scratches and streaks, compared to gouges and grooves. Source: Katie KellerLynn

Stromatolite - "Stroma" means mattress, bed in Greek; "litos" means rock. Stromatolites are commonly thought to have been formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, especially cyanobacteria. However, very few ancient stromatolites actually contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic precipitation. Source: Wikipedia

Subduction Zone - A subduction zone, also called a convergent margin, is the linear zone along which a plate of lithosphere sinks (subducts) down into the asthenosphere. Source: Skinner and Porter (1995)

Subhedral - The term subhedral is used to describe a mineral grain that is bounded partly by its own rational (natural) faces and partly by surfaces formed against preexisting grains as a result of either crystallization or recrystallization. Source: Bates and Jackson (1984)

Subsistence - Subsistence refers to the required items an organism needs in order to survive. Source: National Park Service

Suncups - Suncups are a series of bowl-like depression melted into a snow or ice surface, separated by a network of connected ridges. Individual suncups may be more than 3 feet (0.9 m) deep and 10 feet (3 m) in diameter. Suncups form during warm, sunny conditions. Source: Bruce Molnia

Sunspots - Sunspots are relatively dark areas on the Sun's surface. They represent lower temperatures. Source: Katie KellerLynn

Surf Zone - The surf zone is the section of the coastal zone between the shoreline and the breaker zone. It is known as the zone of active breaking waves. Source: Katie KellerLynn

Surge - A surge is a short-lived, frequently large-scale, increase in the rate of movement of the ice within a glacier. Ice velocities may increase 10 to 100 times that of normal flow rates. In some surges, the terminus of a glacier rapidly steepens and advances. Although not all glaciers surge, those that do, often surge with some sort of a periodicity. Source: Bruce Molnia

System - In geology, a system is a major chronostratigraphic unit of worldwide significance. It is the fundamental unit of chronostratigraphic classification, extended from a type area or region and correlated mainly by its fossil content. Source: Bates and Jackson (1984)

Systematics - Systematics, also known as taxonomy, is the science of classification, in particular the classification of organisms. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)


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Talus - Talus are rock fragments of any size (usually coarse and angular) derived from and lying at the base of a cliff or very steep, rocky slope. Source: Neuendorf et al. (2005)

Tapirs - The tapirs are large, browsing animals, roughly pig-like in shape but with a short trunk that can grasp or hold things. Source: Katie KellerLynn

Tarn - A tarn is a lake that develops in the basin of a cirque, generally after the melting of a glacier. Source: Bruce Molnia

Taxa - "Taxa" is the plural form of "taxon." Taxa are the groups or entities of taxonomy--the scientific classification of both living and fossil organisms. Source: Katie KellerLynn

Taxonomy - Taxonomy is the classification of life (plants and animals) into groups of related features. Source: National Park Service

Tectonic - "Tectonic" pertains to the forces involved in, or the resulting structure of, tectonics (see Tectonics). Source: Bates and Jackson (1984)

Tectonics - Tectonics is a branch of geology dealing with the broad architecturae of the ourter part of Earth, that is, the major structural or deformational features and their relations, origin, and historical evolution. It is closely related to structural geology, but tectonics generally deals with larger features. Source: Bates and Jackson (1984)

Tectonism - Tectonism is a general term used for all movement of Earth's crust produced by tectonic processes, including the formation of ocean basins, continents, plateaus, and mountain ranges. Source: Neuendorf et al. (2005)

Temperature - Temperature is the degree of hotness or coldness. To establish a temperature scale, scientists arbitrarily set fixed points and temperature increments called degrees. Two commonly used fixed points are the temperature at which ice melts and the temperature at which water boils, both at standard atmospheric pressure. On the Fahrenheit temperature scale the melting point of ice is 32°, the boiling point of water is 212°, and the interval between is divided into 180 equal parts, called Fahrenheit degrees. On the Celsius (centigrade) scale the melting point of ice is 0°, the boiling point of water is 100°, and the interval between is divided into 100 equal parts, called Celsius degrees. Another temperature scale, the Kelvin scale, assigns a value of zero to the lowest conceivable temperature. This zero—0 Kelvin—comes at -273.15°C. Source: Petrucci and Harwood (1993); Katie KellerLynn

Tephra - Tephra is a collective term for all clastic materials ejected from a volcano and transported through the air. It includes volcanic dust, ash, cinders, lapilli, scoria, pumice, bombs, and blocks. Source: Bates and Jackson (1984)

Terminus - The terminus is the lower margin or extremity of a glacier, also called the snout, toe, or end. Source: Katie KellerLynn

Terrace - In marine geology, a terrace is a bench-like structure on the ocean floor. Source: Katie KellerLynn

Terrestrial - Terrestrial is a term used to describe species that live on land. This includes all species that live in trees and other plants that grow on land. Source: National Park Service

Tertiary - Traditionally the Tertiary Period consisted of five epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene and covered the time from 65.5 million years ago to 1.8 million years ago. Recently instead of breaking the Cenozoic Era into the Tertiary and Quaternary, however, the International Commission on Stratigraphy breaks the Cenozoic into the Paleogene and the Neogene Periods. Hence, although still used, the terms Quaternary and Tertiary are no longer formally accepted terms of geologic time. Source: Naomi Lubick (2003); International Commission on Stratigraphy (2003)

Texture - The general appearance or character of a rock is referred to as its texture. Source: Bates and Jackson (1984)

Theory - A theory is a model or way of looking at nature that can be used to explain and to make further predictions about natural phenomena. It is a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses. Source: Petrucci and Harwood (1993); Kennedy et al. (1998)

Therapsids - Therapsids, previously known as the "mammal-like reptiles," are an order of synapsids. Traditionally, synapsids were referred to as reptiles; however, the term now includes mammals. Therapsids became the dominant land animals by the Middle Permian, replacing the pelycosaurs. Source: Wikipedia

Thermocline - Thermocline is the region in a thermally stratified body of water that separates warmer surface water from cold deep water and in which temperature decreases rapidly with depth. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Tidal Flat - A tidal flat is an extensive, nearly horizontal, marshy or barren tract of land that is alternatively covered and uncovered by the tide, and consisting of unconsolidated sediment (mostly mud and sand). It may form the top surface of a deltaic deposit. Source: Katie KellerLynn

Till - Till is an unsorted, unstratified accumulation of sediment deposited directly by a glacier. Till is a heterogeneous mixture of different sized materials deposited by moving ice or by the melting in-place of stagnant ice. After deposition, some tills are reworked by water. Source: Katie KellerLynn

Tillite - Tillite is a sedimentary rock formed by lithification of glacial till, especially pre-Pleistocene till. Source: Katie KellerLynn

Time-Transgressive - Another term for "diachronous," a time-transgressive rock is of varying age in different areas or cuts across time planes or biozones. For example, a time-transgressive marine sand may have formed during an advance or recession of a shoreline, becoming younger in the direction in which the sea was moving. Source: Neuendorf et al. (2005)

Tissue - Tissue is an aggregate of cells usually of a particular kind together with their intercellular substance that form one of the structural materials of a plant or animal. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Toe - see "Terminus"

Trace Fossil - A sedimentary structure consisting of a fossilized track, trail, burrow, or tube resulting from the life activities of an animal, such as a mark made by an invertebrate creeping, feeding, hiding, or resting on or in soft sediment. Many trace fossils were formerly assumed to be bodily preserved plants or animals. Source: Bates and Jackson (1984)

Transform Boundary - A transform boundary is a boundary in which two plates (portions of Earth’s lithosphere) slide past each other without creating or destroying lithosphere. Source: Katie KellerLynn

Transgression - Transgression is the spread of the sea over land areas. Source: Bates and Jackson (1984)

Translocation - Translocation is the movement of materials in soil, primarily by water but also through the action of soil organisms. Source: Bradshaw and Weaver (1993)

Triassic - The Triassic is the first period of the Mesozoic Era (after the Permian of the Paleozoic Era, and before the Jurassic) thought to have covered the span of time between about 251 and 199.6 million years ago; also, the corresponding system of rocks. The Triassic is so named because of its threefold division in the rocks of Germany. Source: Bates and Jackson (1984)

Trilobite - Trilobites are extinct arthropods in the class Trilobita. They appeared in the Cambrian period and flourished throughout the lower Paleozoic before slowly declining to extinction. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago. Trilobites are well-known, possibly the second most famous fossil group after the dinosaurs, and are the most diverse group of animal species preserved in the fossil record, consisting of eight, possibly nine, orders, and more than 15,000 species. Most were simple, small marine animals that filtered mud to obtain food. Source: Wikipedia

Trimline - A trimline is a sharp boundary that marks the maximum upper level of the margins of a glacier that has receded from an area. It usually coincides with a break in slope or change in color of bedrock indicating the separation of weathered from unweathered bedrock. The trimline of a long-extinct glacier may be marked by a sharp change in the age, constitution, or density of vegetation. Source: Bates and Jackson (1987); Bruce Molnia

Trophic Level - Trophic level is one of the hierarchical strata of a food web charcaterized by organisms that are the same number of steps removed from the primary producers. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Tsunami - A Japanese word meaning "harbor wave," a tsunami is a wave, or series of waves, that are generated in a body of water by a sudden disturbance that displaces water. They are typically caused by earthquakes and landslides in coastal regions. Volcanic eruptions, nuclear explosions, and even impacts from meteorites, asteroids, and comets from outer space can generate a tsunami. Source: National Park Service

Tufa - Tufa is a chemical sedimentary rock composed of calcium carbonate, formed by evaporation as an incrustation around the mouth of a spring, along a stream, or exceptionally as a thick, concertionary deposit in a lake or along its shore. It may also be precipitated by algae or bacteria. Source: Bates and Jackson (1984)

Tuff - Tuff is a general term for all consolidated pyroclastic rocks. Source: Bates and Jackson (1984)

Type Area - The type area is the geographic area or region that encompasses the stratotype or type locality of a stratigraphic unit or stratigraphic boundary. Source: Neuendorf et al. (2005)

Type Locality - The type locality is a reference point; it is the place where a geologic feature (i.e., ore body, a particular kind of igneous rock, or the index specimen of a fossil species or subspecies) was first recognized and described. Source: Bates and Jackson (1984)

Type Section - A type section is a sequence of strata that constitutes a stratigraphic unit and its stratigraphic boundary. It serves as an objective standard with which spatially separated parts of the unit may be compared. There is only one type section. Source: Neuendorf et al. (2005)


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U.S. Constitution - The U.S. Constitution is the basic principles and laws of the United States. It determines the powers and duties of the government and guarantees certain rights to the people in the nation. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Unconformity - An unconformity is a break or gap in the geologic record, such as an interruption in the normal sequence of deposition of sedimentary rocks, or a break between eroded metamorphic rocks and younger sedimentary strata. Source: Bates and Jackson (1984)

Understory - Understory is the underlying layer of vegetation, especially the trees and shrubs between the forest canopy and the ground cover. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Undertow - Undertow is the seaward return flow, near the bottom of a sloping beach, of water that was carried onto the shore by waves. Source: Katie KellerLynn

Ungulate - Ungulates are the group of mammals that have hooves (e.g., bighorn sheep, elk, deer, and bison). Source: National Park Service

Union - The federal union of states during the period of the American Civil War is referred to as the Union. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Uplift - An uplift is a structurally high area in Earth's crust produced by positive movements that raise or upthrust the rocks, as in a dome or arch. Source: Bates and Jackson (1987)

Upwelling - Upwelling is the rise of cold, heavy seawater toward the surface from depth. Upwelling also refers to the relatively quiet eruption of lava and volcanic gases, with little force. Source: Bates and Jackson (1984)

U-shaped Valley - A valley with a parabolic or “U” shaped cross-section, steep walls, and generally a broad, flat floor. Formed by glacier erosion, a U-shaped valley often results when a glacier widens and over-steepens a V-shaped stream valley. Source: Bruce Molnia


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Vascular Plant - Plants that have conducting systems to transport water and nutrients to cells are referred to as vascular. The xylem and phloem are parts of the conducting system. Source: National Park Service

Vector - The mode of transporting a species from its native habitat to the new habitat that it invades is referred to as a vector. Source: National Park Service

Veins - Veins appear in rocks, animals, insects, and plants. In rocks, magma forms veins that upon solidification often contain useful minerals. In animals, blood flows through veins. In plants, veins are vascular bundles that form the framework of a leaf. In insects, stiff ribs form veins on wings. Source: Katie KellerLynn

Vent - A vent is an opening in the Earth's crust from which molten rock and volcanic gases escape onto the ground or into the atmosphere. Source: U.S. Geological Survey

Vertebrate - Any animal with a spinal column is a vertebrate, for example, mammals, reptiles, fish, and birds. Source: National Park Service

Virus - Viruses are classified as any of a large group of submicroscopic infective agents that are regarded either as extremely simple microorganisms or as extremely complex molecules, that typically contain a protein coat surrounding RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth and multiplication only in living cells, and that cause various diseases in animals and plants. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Viscosity - Viscosity is a liquid's resistance to flow. The stronger the intermolecular forces of attraction are, the greater the viscosity. Source: Petrucci and Harwood (1993)

Vitrification - Vitrification is the process of converting clay, and other substances, into glass or a glassy substance by heat and fusion. Source: Merriam-Webster’s Collegiate Dictionary (11th ed.)

Vog - Vog (volcanic smog) is a mixture of gases and aerosols (tiny particles and droplets) formed when volcanic gas reacts with moisture, oxygen, and sunlight. Source: U.S. Geological Survey - Hawaiian Volcano Observatory

Volcanic Arc - A volcanic arc is a generally curved linear belt of volcanoes above a subduction zone, including the volcanic and plutonic rocks formed there. Source: Bates and Jackson (1987)

Volcanic Gas - Magma contains dissolved gases that are released into the atmosphere during eruptions. The most common gas released by magma is steam (vaporized water), followed by carbon dioxide, sulfur dioxide, hydrogen chloride, and other compounds. Source: U.S. Geological Survey

Volcano - A vent at Earth's surface through which magma (molten rock) and associated gases erupt; also, the cone built by effusive and explosive eruptions. Source: U.S. Geological Survey

Volcano, Active - A volcano that is erupting is considered active; also, one that is not currently erupting but is expected to do so. Source: Bates and Jackson (1987)

Volcano, Dormant - A dormant volcano is a volcano that is not now erupting but that has done so within historic time and is considered likely to do so in the future. There is no precise distinction between a dormant and an active volcano. Source: Bates and Jackson (1987)

Volcano, Extinct - An extinct volcano is one that is not now erupting and is not considered likely to erupt in the future. Source: Bates and Jackson (1987)

Volume - Volume is the amount of space occupied by a three-dimensional object. It has the unit (length)3, and the international standard is cubic meters (m3). Source: Merriam-Webster's Collegiate Dictionary (11th ed.); Petrucci and Harwood (1993)

Volunteer - A person who volunteers their service to help others without benefit of pay. Volunteers are vital to the success on NPS research expeditions. Source: National Park Service

Vug - A vug is a small cavity in a rock or rock vein that is usually lined with crystals of a different mineral composition from the enclosing rock. Source: Bates and Jackson (1984)

Vulcan - Vulcan is the Roman god of fire and metalworking. The term relates to the working of metals or volcanic eruptions. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)


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Water - Water is the liquid that descends from clouds as rain; forms streams, lakes, and seas; and is a major constituent of all living matter. When pure, water is odorless and tasteless. It is the oxide of hydrogen (H2O). It appears bluish in thick layers, freezes at 0°C, boils at 100°C, and has a maximum density at 4°C. It is a poor conductor of electricity, a good solvent, and is very slightly compressible. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)

Water Table - The boundary between the zone of aeration (above) and zone of saturation (below). The water table is the upper boundary of groundwater reservoirs. Source: Katie KellerLynn

Watershed - A watershed is the area drained by a river and all its tributaries; it is a catchment or drainage basin, which is bounded peripherally by a divide (e.g., topography). Source: Bradshaw and Weaver (1993)

Wave - Defined with respect to water, a wave is a disturbance that represents energy propagating or moving across the water surface (a surface wave) or along a density discontinuity within a water column (an internal wave). Source: Pinet (1992)

Weathering - Weathering is the breakdown and decomposition of earthy and rocky materials in response to atmospheric processes. Source: Bradshaw and Weaver (1993)

Wetlands - Wetlands is a general term for a group of wet habitats. It includes areas that are permanently wet or intermittently water-covered. Source: Katie KellerLynn

Whaleback - Whalebacks are smooth, glacially sculpted bedrock knobs of modest size that resemble the back of a sounding whale. Source: Sharp (1988)


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Xeric - The term xeric refers to a habitat or plant, for example, that requires only a small amount of moisture. Source: Merriam-Webster's Collegiate Dictionary (11th ed.)


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Zircon - A mineral (ZrSiO4) that is a common accessory mineral in siliceous igneous rocks, crystalline limestone, schists, and gneisses, in sedimentary rocks derived therefrom, and in beach and river placer deposits. When cut and polished, the colorless varieties provide exceptionally brilliant gemstones. Source: Bates and Jackson (1984)

Zone of Aeration - The zone of aeration lies above the water table. Although some of the pore spaces in the soil and rock are filled with water, others are filled with air; the ground is not saturated in this zone. Source: Katie KellerLynn

Zone of Saturation - The zone of saturation lies below the water table. All of the pore spaces in the soil or rock of this zone are completely filled with water. The ground is completely saturated. This is where groundwater is found. Source: Katie KellerLynn

Zooplankton - see "Plankton"

Views of the National Parks
Views of the National Parks was created February 21, 2001 by the National Park Service
as an education and interpretation program for those
interested in experiencing the wonders of America's national parks.