Ocean basins serve as collecting bins for eroded debris that is washed from continents. Sources of sediment that end up in coastal environments, and ultimately the deep sea, are: (1) sedimentary debris produced by weathering and erosion of rocks on land; (2) grains derived from the hard parts of organisms, such as skeletal debris, shells, and teeth; (3) tiny grains—typically small, magnetic spheres and silicate chondrules (bit of asteroids originating from outer space, so-called extraterrestrial debris); (4) material, such as manganese nodules, that is precipitated by chemical or biochemical reactions in seawater; and (5) material, such as ash, that is ejected from volcanoes.
Coastal sediments come in a wide range of colors: from the brilliant white sands of Gulf Islands National Seashore (Mississippi) to the black volcanic beaches of Hawaii Volcanoes National Park (Hawaii). Sand coloration depends on the parent rock from which the beach sediments have eroded. The parent material may be from a local source (eroded from a nearby bluff or cliff) or may be transported long distances; for example, Mississippi River delta sediments may be transported from Montana. Additional coloration of coastal sediments may occur as a result of the influence of color-producing minerals such as hematite (red), limonite (yellow), magnetite (black), and olivine (green), or the introduction of chemicals and pollutants. Still other beaches are composed mainly of calcium-carbonate shells, which are a variety of colors depending on mineral staining.
This black sand beach in Hawaii Volcanoes National Park is the site of some of the newest land on Earth. When molten lava cools rapidly in seawater, it shatters, forming the black sand that is washed onto the shore.
White sand beaches are typically composed of quartz-rich sediments. Due to its hardness and chemical structure, quartz is a very durable mineral that is difficult to weather and erode. Therefore, quartz is often the most prevalent mineral found in beach sediments. White sand beaches often include accessory minerals such as garnet, magnetite, and ilmenite. These minerals are often found in dark streaks along shorelines, which demonstrate the winnowing by waves and wind action of heavier minerals. Sand ripples created by wind action are clearly visible in this photo.
Some beaches, like the one shown here, are composed primarily of the carbonate remains of marine organisms. The calcium-carbonate shells of mollusks and gastropods may be the dominant component of these beaches and make great locations for shell collecting. If you look closely, these beaches may also include fossils such as shark teeth, whale vertebrae, or bony fish remains. In general, fossils are typically heavier than their modern counterparts, and may be different in color because of mineral staining that occurs with prolonged burial. Bone fossils may maintain textural features such as striations and porous matter. This beach at Cape Lookout (North Carolina) is composed of shelly fragments and quartz sands.
With respect to size, the most common sedimentary deposits in the ocean are mud and sand, with gravel a distant third; boulder and tiny particles (colloids) are extremely rare in the sea (Pinet 1992). However, beaches may be composed of sediments of various sizes: (from finest to coarsest) mud (silt and clay), sand, and gravel (cobbles and boulders). Scientists generally use Wentworth’s grain-size distribution chart for sediment size analysis. Typically, people associate beaches with quartz sand, but remember, sand is a grain size (1/16 mm to 2 mm), not a rock type, so beach sediments are found in all shapes and sizes and may be composed of any rock type (i.e., igneous, metamorphic, or sedimentary).
Finding muddy beaches in the United States is rare, although this particle size (silt and clay) is commonly found in back-barrier regions of barrier islands. For muddy sediments to dominate, low-energy conditions must exist that will allow very fine-grained particles to settle out from being suspended in water. Except during periodic storm events, tidal creeks, marshes, and mud flats are low-energy coastal environments where clays and silts are commonly found. This image shows a tidal flat exposed during an early morning low tide in Florida.
Sandy beaches are found throughout the United States, ranging from the fine, white sands of Padre Island National Seashore (Texas) to the coarser sands of Cape Lookout National Seashore (North Carolina). Sandy beaches are typically associated with passive (trailing edge) margins, a wide continental shelf, and a sedimentary geologic framework. These regions often contain barrier structures such as barrier islands, barrier spits, and barrier beaches. This image shows the primary dune complex on Canaveral National Seashore (Florida). Dunes protect inland areas from wind, waves, and storm events. Vegetation aids dune stability, which decreases beach erosion.
Gravel (cobbles and boulders) beaches are typically found in high-energy coastal environments such as the northeastern United States and along the Pacific coast. Rocky coasts are commonly associated with active (leading edge) margins and narrow continental shelf widths. These areas typically have an igneous or metamorphic framework, which provide a source of more resistant rocks that form rocky shores. Acadia National Park in Maine is a good place to see a gravel shoreline.
Wind, waves, and currents constantly move and redistribute coastal sediments along shorelines. Beach sediments that have been transported great distances will be sorted according to grain size. For example, Gulf Coast sediments may have traveled the length of the Mississippi River. Winnowing of more uniform sediment shapes and sizes results in a beach with well-sorted sediments. In contrast, poorly sorted sediments are composed of a variety of sediment shapes and sizes. Low-energy conditions that allow fine sediments to settle from suspension must exist for the formation of muddy or sandy beaches. High-energy conditions are typical of rocky beaches that allow for the transportation of coarser sediments. Extreme storm events are necessary to move the largest sediments (i.e., boulders).
Well-sorted beach sands are visible in this photograph of Padre Island National Seashore in Texas. Note that the sand grains are all of similar size and shape. The sand dunes found at Padre Island support a wide variety of coastal vegetation, such as this sea purslane (Sesuvium portulacastrum), which serves to stabilize dunes and decrease overwash.
Poorly sorted sediments display a variety of sizes including cobbles, pebbles, and sand. The sediments come in many shapes, although rounded cobbles dominant (on account of abrasion during sediment transport). Kelp, a marine plant that provides critical habitat for many marine species, has washed ashore on this beach. The keys in the photo provide a sense of scale. Zonation is visible in this photo: coarser-grained sediments are located in the upper corner (inshore), and finer-grained sediments are found in the lower corner (offshore).