Coastlines are boundaries between land and water that surround Earth’s continents and islands. Scientists define the coast, or coastal zone, as a broad swath (belt) of land and sea where fresh water mixes with salt water. Land and sea processes work together to shape features along coastlines. Freshwater lakes do not technically have coastal zones, but many of the processes (waves, tides) and features found along ocean coastlines also exist in large lakes.
Coastal zone features
All coastlines include a thin strip of land that is submerged at high tide and exposed at low tide, called the shoreline. The coastal zone, however, extends far inland from the shore, across lowlands called coastal plains, and far seaward to the water depth where ocean waves do not reach the seafloor. The coastal zone includes lagoons, beaches, estuaries, tidal wetlands, tidal inlets, river deltas, barrier bars and islands, sand bars, and other shallow-water ocean features.
- Lagoons: Shallow, salt-water bays between barrier islands and the mainland.
- Beaches: Sand deposits along shorelines. Intense waves wash fine-grained mud from coastal sediments (particles of sand, gravel, and silt) leaving only sand-sized grains of resistant minerals like quartz and calcium carbonate. Beaches are common on the seaward side of barrier islands where wave energy is intense.
- Estuaries: The mouths of rivers and streams that receive a pulse of saltwater with the tides.
- Tidal wetlands (flats): The broad areas of marshy wetlands around lagoons and estuaries that flood with salt water during high tides.
- Tidal inlets: Openings through which water and sediment are washed in and out of lagoons by daily tides.
- Deltas: Deposits of sediments at the mouths (ends) of rivers that flow into the ocean.
- Barrier bars and islands: Long mounds, or bars, parallel to the shore into which near-shore ocean currents carry and deposit sand. Eventually, some barrier bars grow tall enough to stay exposed at high tide and become barrier islands. The outer banks of North Carolina as well as Galveston, Mustang Island, and South Padre Island in Texas are examples of barrier islands.
- Sand bar: A ridge of sand in rivers or along the coast built up by water currents.
Processes that shape coastlines
The coastal zone is constantly changing. Salt water rushes through tidal inlets into bays and estuaries twice daily. Waves, currents (steady flows of water in a prevailing direction), tides, and storms reshape coastal features over days, weeks, and months. Coastlines move landward and seaward as global sea-level rises and falls over hundreds and thousands of years.
All coastlines are at least somewhat affected by waves and tides. Waves straighten uneven shorelines by eroding (wearing away) points that extend into the ocean and depositing sediment in bays. They also generate strong, shallow currents that carry and deposit sediment parallel to the shore. Long shoreparallel features like barrier islands, spits (small strips of land that jut out into the sea), and sand bars border coasts where waves are the dominant force.
Tides move sediment and water in and out across the shoreline, and tide-dominated coasts have features like tidal inlets, natural jetties (protective rock barriers), and funnel-shaped estuaries that form a 90° right angle to the shore. Most coastlines are shaped by both waves and tides, and have some parallel and perpendicular features.
Types of coastlines
All coastlines are affected by waves, tides, storms, and currents, and every coast includes a shoreline. There are, however, many different types of coastlines. Some coastlines receive large amounts of sand and mud from rivers. Others accumulate the skeletal remains of animals like corals and shellfish. In some places, waves are eroding coastlines that are rising from the sea.
Coasts that receive a steady supply of sediment are called depositional coastlines. Rivers like the Mississippi in the United States and the Nile in Egypt erode sediment from continental interiors and deposit it in huge deltas at their mouths.
Waves, currents, and tides spread sediment into thin layers on the submerged continental shelf (the shallow seabed that stretches from the shore to the deeper ocean water). Over time, the weight of the sediment presses down on the edge of the continent, creating space for more sediment. New layers build on to the edge of the continent, and the coast moves seaward.
Depositional coastlines typically encompass a broad coastal plain, and a complex shoreline that includes long, wide beaches. These coastlines are almost flat, causing salt water to move far inland across coastal plains and up rivers during high tide. The Mid-Atlantic and Gulf of Mexico coasts of the United States are depositional coastlines.
Waves, tides, and currents sort and distribute incoming sediment into distinctive features along depositional coastlines. Some of these features include: barrier bars and islands, tidal inlets, lagoons, beaches, estuaries, and tidal wetlands.
Depositional coastlines also develop where plants and animals called carbonates live in clear, sunlit water away from river deltas. Carbonates like corals and shellfish have skeletons and shells made of the hard mineral calcium carbonate. The sediment supply on carbonate coastlines comes from the skeletal remains of the animals and plants that live there. Corals build giant ridges of rocks called reefs up from the seafloor. Florida and the Bahamas have carbonate coastlines.
Erosional coastlines occur where huge sections of the Earth’s crust called tectonic plates lift out of the sea. These coastlines are common along far northern coastlines that are bouncing back after being weighed down by thick ice sheets, and along coasts where tectonic plates meet. Erosional coastlines are the norm in Maine and eastern Canada, along the west coast of North America, and in Scandinavia.
Along these coasts, waves pound rocky shorelines and cut into the bottoms of cliffs. The shore retreats as blocks of rock and sediment fall into the sea. Isolated remnants of sea cliffs, called stacks, are left standing in the sea. All of these features are caused by movements of plates and wave action over time. The process of shoreline retreat claims much expensive real estate along erosional coastlines.
Life in the coastal zone
The plants and animals that live in the coastal zone have adapted to its cycles of change. Residents of tidal wetlands tolerate twice-daily drowning and drying. Fish in estuaries and lagoons adjust to large changes in water salinity (saltiness).
Plants grow with their roots in salt water and can survive burial by shifting beach sand and river mud. Many ocean and land animals spend their early lives in coastal wetlands where there is shelter and plentiful food before moving to dry land or the open ocean as adults.
Coastlines are also home to about two-thirds of Earth’s human population. People continue to work to understand the nature of coastal zones in order to protect coastal populations from their hazards (storms, waves, floods, erosion), but also to protect coastlines from the damaging effects of everyday human activities, such as eroding sand dunes by climbing them, or generating pollution.