Habitats: Beaches - Coasts

There are many different types of coasts; beaches are just one type.

Coasts are divided into two categories: primary coasts, which were created by non-marine processes, and secondary coasts, which were formed by marine action. Primary coasts happen because of changes in the land, such as river deltas or lava flows. Secondary coasts are caused by changes in the ocean, such as the creation of barrier islands or coral reefs.

Primary coasts are created by erosion (the wearing away of soil or rock), deposition (the buildup of sediment or sand) or tectonic activity (changes in the structure of the rock and soil because of earthquakes). Many of these coastlines were formed as the sea level rose during the last 18,000 years, submerging river and glacial valleys to form bays and fjords (a type of estuary).

River deltas are an example of a primary coast. They form where a river deposits soil and other material as it enters the sea. River deltas are divided into three groups: the river-dominated delta, the tide-dominated delta and the wave-dominated delta.

River-dominated deltas, such as the Mississippi or Nile river deltas, are formed when there are large amounts of material in the water, and tidal action is relatively low. Tide-dominated deltas, which are found where the daily tidal range is more than a meter, have many branching channels and long narrow islands formed as the tide and river flow in different directions. Wave-dominated deltas are little more than a bulge on the shoreline since there is so much wave activity that all the sediment is spread evenly along the coast and does not accumulate at the river's end.

Primary coasts are divided into two categories: submergent coasts and emergent coasts. Submergent coastlines result from a general sea-level rise and crustal subsidence (a lot of heavy sediment on top of the bedrock is forcing the bedrock deeper into the earth). Most of the eastern United States has submergent coastlines. One example is the Chesapeake Bay. Emergent coastlines result from the land being lifted, either by tectonic activity or rebound from the weight of heavy glaciers, which exposes the former sea bottom bit by bit forming continuously new shoreline. A characteristic feature of emergent coasts are marine terraces, formed as tectonic uplift moves the land upward in short bursts, which are then worn by wave action into relatively flat surfaces, somewhat like a large staircase. Beach ridges can be formed by rebound, and are composed of cobblestones piled at the surfline by storm activity, which is slowly lifted higher over time.

Secondary coasts are caused by the action of the sea or by creatures that live in it. Sea cliffs, barrier islands, mud flats, coral reefs, mangrove coasts and salt marshes are all examples of secondary coastlines. While most of the eastern United States is considered submergent, a great deal of the coastline formed between submergent features is secondary, such as marshes, mangroves, sand beaches and islands. Large portions of the US Pacific coast are secondary as well, with eroded headlands and wave terraces.