When you hear the word “cave” what do you think of? Darkness? Being trapped? Bats? Being lost? Or maybe you have been in a cave and you recall beautiful flowstone, cave bacon, delicate crystals, and other speleothems.
Do you know what karst is? When you hear the word “karst” what do you think of? Do you think of sinkholes? Disappearing streams? Catastrophic collapse?
Whatever you think of, this knowledge center is meant to address your fears and misconceptions about caves, as well as enhance your understanding of cave and karst systems.
Let’s get started. View the topics below to learn more.
What is a cave? This is a very good question. The Federal Cave Resource Protection Act of 1988 defines a cave as “any naturally occurring void, cavity, recess, or system of interconnected passageways beneath the surface of the earth.” The Natural Cave Conservation Association describes caves as naturally formed underground cavities large enough for a person to fit through. The term cavern is often used interchangeably with the word cave.
Caves exist worldwide and come in many shapes and sizes. A cave may or may not have an entrance at Earth’s surface. Sometimes caves are filled with air; sometimes they are filled with water.
Caves are unique, sensitive environments that house fragile mineral formations,
rare flora and fauna, irreplaceable artifacts, and distinctive fossils. They
also record Earth’s history and climate and are natural laboratories for
modern research.
Karst is a type of typography formed on limestone, gypsum, and other rocks that dissolve in natural acid. Karst describes landscapes characterized by caves, sinkholes, and underground drainage. In this kind of landscape, streams disappear into the ground and reappear elsewhere as large springs.
Valleys, plateaus, towers, pinnacles, and ponds are all surface features in
regions with karst. Below the surface are caves, fractures, and partings—all
components of a drainage network. The “type locality” of karst is
a limestone plateau in the Dinaric Alps of northwestern Yugoslavia and northeastern
Italy.
Karst landscapes are found just about everywhere on Earth: frigid tundra, dry deserts, and tropical jungles. Worldwide this is 25% of the land surface! If you live in an area underlain by carbonate rocks or evaporite rocks, there is a good chance you are familiar with karst, especially if it’s a warm, humid area where erosion via dissolution is rapid. Karst can also be found in arid terrains, however, where sulfuric acid can form large caves, such as Carlsbad Caverns in New Mexico.
A fifth of the landscapes in the United States are karstic. Major karst areas occur in 20 states, and smaller karst regions occur throughout the nation. Many major cities are underlain in part by karst, such as Saint Louis, Missouri; Nashville, Tennessee; Birmingham, Alabama; and Austin, Texas.
The majority of caves are found in karstic landscapes, but not all caves are.
There are caves that form in rocks other than carbonate and evaporate rocks.
Every state in the United States has caves. Tennessee has the most caves. Kentucky
has the world’s longest cave, and Hawaii has the world’s longest
and deepest lava tubes.
The longest cave is Mammoth Cave in Kentucky with 349 miles (563 km) of passageways. The deepest cave is Krubera (Voronja) Cave in the Republic of Abkhasia at over 5,600 feet (1,710 m) deep, recently explored to this depth in 2001 (Gee 1994). The largest cave chamber is in Sarawak Cavern in Sarawak, Malaysia. This room is approximately 2,300 feet (700 m) long, 1,400 feet (425 m) wide, and 390 feet (120 m) high. The vast majority of caves form in soluble rocks such as limestone, dolomite, gypsum, and marble, but caves also form in other types of rock.
There are at least 23 types of caves. We’ll discuss some of them here.
Select a type of cave to learn more about it.
The term ice cave requires some clarification because it has been applied to caves that form in ice and caves that form in rock. Ice caves that form in ice are also called glacier caves. Meltwater moving through glaciers forms this type of ice cave. Caves formed in rock that contain ice all year round are also referred to as ice caves; they are also called frozen caves. These caves may contain very large crystals of ice that form on the floors, walls, and ceilings of the cave.Ablation Zone
Lava caves or tubes are hollow spaces beneath the surface of a solidified lava flow.
Early people used sandstone caves for shelter. These shallow alcoves were more comfortable than deep, limestone caves because they were not as wet, cold, or dark.
Sea caves are clefts or cavities in the bases of cliffs at the edges of large bodies of water, typically the sea but also large lakes. Sea caves are usually at sea level and are affected by tides.
Naturally acidic waters form these caves by dissolving rock along and adjacent to joints and fractures. Physical erosion by groundwater is also involved.
The action of earthquakes form natural cracks in rock that can be considered caves. Typically the passages are narrow (less than five feet [1.5 m] wide) but may plummet hundreds of feet (meters) below the surface.
Typically we think of caves forming in carbonate rocks, such as limestone and marble, and other types of rocks that dissolve. Carbonate rocks are highly susceptible to the action of carbonic acid in rainwater and groundwater. Lowering the land surface by carbonation is a comparatively rapid process in areas underlain by limestone and marble, and where rainfall is abundant. Sulfuric acid is also known to form caves, particularly noteworthy in arid regions. Similar sulfuric acid reactions also operate in humid regions but tend to be masked by carbonation (Berger 1995). Other processes are at work forming caves in areas not underlain by carbonate or evaporite rocks.
To learn more about how caves and karst form, select a topic of interest.
Most formation and enlargement takes place at or just below the water table. The circulation of water and the dissolution of bedrock are greatest here because fractures are connected and most open. Cave passages created in this zone are generally horizontal and form along bedding planes.
In contrast, passages formed above the water table are vertical, as acidic water moves downward through fractures in bedrock. Caves above the water table serve as tributaries of water to caves below the water table.
As water tables gradually fall, previously formed upper-level passages and rooms are drained, and new caves are formed at lower elevations. Older, higher caves are relatively dry except for dripping water and an occasional stream making its way from the surface to the water table. Water dripping or flowing into passages may deposit speleothems.
Collapse is part of cave formation and evolution. Ceilings of cave rooms and passages collapse when they become too wide to support the bedrock overlying them. In addition, the potential for collapse increases when water drains out, and its buoyant force is no longer present to help support a cave's ceiling.
Sandstone caves form through differential weathering along a shale-sandstone contact. Massive, cliff-forming sandstone can contain soft, erodible beds of shale. Water readily percolates down through sandstone but is trapped and cannot pass through shale beds because their pore spaces are so small. The groundwater is forced to move laterally along the contact between the two rock units until it seeps out on the face of the canyon wall or at the back of an alcove, creating a spring or seep. The prolonged flow of water along these spring and seep zones ultimately dissolves the calcium carbonate cement and loosens individual sand particles and blocks of sandstone, thus forming a void that enlarges with time.
When molten, fluid lava flows out of a volcano, it works its way downhill. In contact with air, the surface of this lava stream cools and hardens into a crust. The lava inside remains molten, however, and continues to flow downhill. When the molten lava eventually drains out of the interior of the hard-crusted passage, a lava tube or cave remains.
The action of waves pounding against rocks that line the shores of oceans and large lakes form sea caves. Sea caves form preferentially along natural lines of weakness in easily weathered rock. Wave-carried sand and gravel enhance the erosive power of waves and modify and enlarge sea caves.
A famous French speleologist, Norbert Casteret, described going into a cave as leaving the world he knew - of bird songs and blue skies - for a mysterious world of blackness filling him with mystical enthusiasm.
Will you see a mystical place when you enter a cave? Or a smelly dirty guano filled hole infested with bats? What you see in a cave is up to you.
Select a topic to find out more about cave environments.
Unlike a wilderness above ground, the cave wilderness cannot be explored without the aid of artificial light. You may have heard this anecdote but it is actually true: in the dark zone of a cave, you really cannot see your hand held up in front of your face when the lights go out. Cave animals called troglobites live their whole lives in total darkness. This may seem strange, or even threatening to us, but with proper precautions - such as carrying three sources of light while caving - dark wonderlands have been and remain to be discovered. These worlds without sunlight are important for challenging our assumptions about ecosystems and life, providing a sense of discovery, and opening up new avenues in research and science.
In general, caves are silent worlds. Very few critters live in caves, so noise is rarely produced by scampering animals. And, of course, in wild caves, clocks aren't ticking, machines aren't clanking, and refrigerators aren't humming. All you may hear in a cave is the sound of dripping water.
In addition to feeling air currents, in some caves you might hear wind blowing. As cavers say, "If it blows, it goes," meaning air currents indicate space beyond. For example, Lechuguilla Cave was discovered initially because of air blowing out of a pile of rubble.
During the heat of the summer tourist season, billboard and road signs for public caves entice travelers to come underground to cool off. If vacations were also common in the winter, these signs could be flipped to read come underground to warm up. Caves are pleasantly cool in the summer and far warmer than the surface in the winter. Daily and seasonal temperature fluctuations that occur at the surface tend to diminish as heat moves down through bedrock into caves. In short, cave temperatures are nearly constant throughout the year. Cave temperatures are approximately equal to the average annual temperature at the surface.
Surface temperatures and associated cave temperatures are determined by latitude and altitude. Cave temperatures in lower latitudes and higher altitudes are cooler. The amount of water that a cave receives also affects its temperature. Water is more efficient than air in transporting heat. Hence, the temperature effect of a stream extends much further into a cave than air currents. Also, if a cave receives large quantities of snowmelt, then its temperature will be much lower than other caves at the same altitude and latitude.
Seeping water moistens a cave's ceilings, walls, and floors. It is not surprising then that the air in most caves is nearly saturated with water vapor, that is, the relative humidity is close to 100%. Constant temperature at the inner part of a cave permits high humidity to be maintained indefinitely.
Near the entrances to caves, however, the humidity may be lower than the interior portions of caves. This is because cave entrances are the interface between the surface and underground. Outside humidity is usually lower than in caves, and cave temperatures differ from outside temperatures. A fall in temperature will increase the relative humidity; a rise in temperature will decrease it.
Have you ever followed the wind underground? Caves are places where you can do this. In fact, Wind Cave in South Dakota was discovered because of the air currents blowing out of it.
The exchange of air between a cave and the surface is a function of changing pressure of the outside atmosphere. Changes occur from day-to-night and result from changes in the weather, such as a storm front. Caves typically consist of many small, interconnected passages with relatively small entrances. Because of this, a considerable amount of time may be required to move air through a cave and equalize cave pressure with outside air pressure. The air in most caves, therefore, is constantly in motion, adjusting itself to surface changes.
Usually air currents in caves are so slow that they can be detected only with instruments. In constricted passages or at small entrances to large caves, however, air currents can be felt as breezes on your face.
Stronger cave winds, some of which are veritable gales, usually occur in caves with two entrances - one higher than the other. Cave winds normally have an annual cycle in which air blows out of the lower entrance all summer and out the upper entrance all winter. These caves are called blowing caves. In a few caves the cycle is shorter - the air moves inward for a few minutes and then outward for a few minutes. These are called breathing caves.
Caves and Karst in National Parks