Coast

A coast (also called the coastline, shoreline, or seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are shaped by the land around them and by the constant action of water, such as waves. The type of rocks and soil along a coast determines what the shore will look like. The Earth has about 620,000 kilometers of coastline.

Sunrise on the Jersey Shore coastline at Spring Lake, New Jersey, U.S.

Coasts are very important for nature. They are home to many different plants and animals. You can find wetlands, which are great for birds and land animals, as well as special areas like salt marshes, mangroves, and seagrasses that help tiny fish, shellfish, and other sea creatures grow. Rocky shores, often found where the ocean crashes against the land, provide homes for animals like mussels, starfish, and barnacles, as well as seaweeds.

Many people live near coasts—about 44% of the world’s population lives within 150 kilometers of the sea. Coasts are important for jobs and food. Port cities, fishing, and farming of sea animals all depend on the coast. Beaches and seaside towns also bring in money through tourism. Coasts can also help protect us. For example, mangroves and seagrasses can store carbon and help reduce the effects of climate change. However, coasts face many challenges. Climate change brings higher seas and stronger storms, which can wash away land. Pollution and overfishing also harm coastal animals and plants. Because coasts are always changing, it can be hard to measure exactly how long a coastline is.

Size

The Earth has about 620,000 kilometres of coastline. Coastal areas make up around 7 percent of the Earth's oceans, and many fish depend on these areas at some point in their lives. As of October 2010, about 2.86% of special ocean zones were protected areas.

Somalia has the longest coastline in Africa.

The way we think about coasts can vary. Some scientists consider wet areas near the water, like seagrass and salt marshes, to be part of the coast. Others might only think of land plants close to the shore as coastal. While scientists generally agree on what a coast is, different governments may define it differently for various reasons.

Formation

Atlantic rocky coastline, showing a surf area. Porto Covo, west coast of Portugal

Tides help decide how far sediment moves along the shore. In places where the tides rise and fall a lot, waves can reach higher up the coast. In places where the tides don’t change much, sediment builds up in a smaller area. Tides themselves don’t usually wear away the land, but strong waves called tidal bores can push up rivers and erode the land.

Seaside in Budelli, Italy. Budelli beach is famous for the color of its sand, which is pink due to the presence of fragments of a microorganism called Miniacina miniacea.

Geologists group coasts by how much the tides rise and fall. Macrotidal coasts have tides that move more than 4 meters (13 feet). Mesotidal coasts have tides between 2 and 4 meters (6.6 to 13 feet). And microtidal coasts have tides less than 2 meters (7 feet). The difference between macrotidal and mesotidal coasts is important. Macrotidal coasts don’t usually have barrier islands or lagoons. They often have estuaries shaped like funnels with sand ridges. Waves play a bigger role in shaping the land along mesotidal and microtidal coasts.

Waves wear away the coast as they crash onto the shore. Bigger waves have more power and move more sediment. Long, open shores let waves spread out their energy, but steep, short shores don’t, so the waves hit harder. This can break rocks apart and shape the land. Sediment from eroded cliffs is moved along the coast by waves, creating rough or cliff-like shores.

Rivers bring sediment to the coast, but dams and other human-made structures often stop this sediment from reaching the sea. Coral reefs can also add sediment to tropical island coasts.

Coasts are always changing. Natural processes like rising sea levels, waves, and weather shape and reshape them. These processes can cause erosion, add new land, and even create flooded valleys and wide underwater areas called continental shelves.

Importance for humans and ecosystems

Human settlements

Many people around the world live near the coast. About 44% of all people live within 150 km (93 mi) of the sea, according to a United Nations atlas. Big cities often sit near good harbors and have port facilities. Some places without access to the sea have built canals to become ports too.

Countries protect their coasts from threats by having coastal defenses, navy forces, and coast guard services.

Tourism

Coasts with beaches and warm water attract many tourists. This leads to the growth of seaside resort towns. In island nations like those in the Mediterranean, South Pacific Ocean, and Caribbean, tourism is very important for the economy. People enjoy activities like swimming, fishing, surfing, boating, and sunbathing at the coast.

Managing growth and taking care of the coast can be difficult for local leaders. They need to build infrastructure for new residents, and sometimes poor building practices can make these areas more vulnerable to coastal erosion and sea level rise. Solutions can include beach nourishment or managed retreat when moving communities away from the coast becomes necessary.

Ecosystem services

Types

Emergent coastline

Further information: Emergent coastline and Submergent coastline

An emergent coastline is where the land has risen above the sea because the sea level dropped or the land lifted up. You can see this in places where old beaches are left high above the water. The opposite is a submergent coastline, where the sea level rose or the land sank, leaving old valleys and inlets underwater.

Concordant coastline

Further information: Concordant coastline and Discordant coastline

A concordant coastline happens when different types of rocks run parallel to the shore. These rocks wear down at different speeds, creating special shapes like small bays called coves. In discordant coastlines, the rocks are worn away by ocean waves, leaving behind points of land called headlands and areas where the land dips in called inlets or bays.

High and low energy coasts

The Cinque Terre, along the Italian Riviera

Coasts can be high energy or low energy. High energy coasts face strong waves and storms, which shape the land by wearing it away and creating cliffs. Low energy coasts are protected from big waves and change very slowly, often building up sand and dirt over time.

Destructive and constructive waves

When waves reach the shore, the water moving onto the beach is called swash, and the water running back down is called backwash. If the swash is weak and the backwash is strong, the beach can wear away. This happens with steep, close waves that crash down hard. These are called destructive waves. If the swash is strong and the backwash is weak, the beach can build up. This happens with gentle, spaced-out waves that roll onto the shore. These are called constructive waves.

Rivieras

The word "Riviera" comes from an Italian word meaning "shoreline." It originally described the coast along the Ligurian Sea near Genoa, and later spread to include parts of France and other places like Australia and Turkey. Today, "Riviera" is used to describe any beautiful, sunny coast that attracts many visitors.

Other coastal categories

A cliffed coast has steep cliffs formed by the action of the sea.
A flat coast gently slopes down into the water.
A graded shoreline is flat and straight, shaped by wind and water.
An erosional coast is where the land is being worn away, while a depositional coast is where material is being added.
An active coast is near the edge of a moving tectonic plate, while a passive coast is farther away from such edges.

Landforms

The following articles describe some coastal landforms:

Cliff erosion

Many sediments along a coast come from the erosion of nearby cliffs or bluffs. Sea cliffs move inland because waves constantly wear away the slopes. If the slope or cliff is made of loose soil, it erodes faster than a cliff made of solid rock.
A natural arch forms when waves wear away a headland over time.
Sea caves happen when certain rocks wear away faster than surrounding rocks, creating holes that become caves.
A stack forms when a headland is worn away by waves and wind, or when an arch collapses, leaving a piece of land standing in the water.
A stump is a smaller sea stack that has worn down or fallen because it was unstable.
Wave-cut notches happen when waves wear away overhanging parts of cliffs, making the cliff more likely to break apart. The pieces fall down and are later moved away by waves.
A wave-cut platform forms after a sea cliff has been worn back for a long time. These platforms start gently sloping and later become shorter as waves break further out.

Coastal features formed by sediment

Coastal features formed by another feature

Other features on the coast

Coastal waters

Overview of different zones of coastal waters: Input, production, transport and storage pathway of carbon in marine waters, including movement across maritime zones of national jurisdiction: territorial sea, Exclusive Economic Zone (EEZ), continental shelf, high seas (international waters), and deep seabed.

Coastal waters, also called coastal seas, refer to the area of water near the land where the ocean meets the shore. The meaning of this term can change depending on how we use it. Sometimes it means just a few kilometers from the coast, and other times it can refer to a much larger area called the continental shelf, which may stretch over a hundred kilometers from land.

This term is used in different ways for different purposes. For example, in environmental management in the European Union, it covers a smaller area close to the coast, while in the United States, it can include a larger region farther out to sea. Coastal waters are important for many reasons, including shipping, military operations, and supporting many types of sea life, from enclosed areas like estuaries to open waters on the continental shelf.

The littoral zone is the part of the sea, lake, or river that is close to the shore. It stretches from the high water mark, which is rarely covered by water, to areas that are always underwater. Coastal waters can sometimes face problems like harmful growths of algae.

In geology

Geologists study rocks formed from sediments left by shorelines and nearshore areas. These rocks help us understand how ancient continents looked and how big the oceans were long ago.

Sediments from the shore become layers of sandstone. These layers show how the ocean moved away from the land over very long periods. Some of the best examples of these shoreline rocks are found in Utah and Colorado.

The following articles describe the various geologic processes that affect a coastal zone:

Attrition
Currents
Denudation
Deposition
Erosion
Flooding
Longshore drift
Marine sediments
Saltation
Sea level change
- eustatic
- isostatic
Sedimentation
Coastal sediment supply
- sediment transport
- solution
- subaerial processes
- suspension
Tides
Water waves
Weathering

Wildlife

Animals

Coastal fish

Plants

Coastal areas often have special plants. Kelp is a type of seaweed that can grow very quickly—up to half a meter in just one day when conditions are right. Other important plants in coastal areas include mangroves, seagrasses, and salt marsh plants. In some places, you can find Restinga vegetation.

Threats

A settled coastline in Marblehead, Massachusetts. Once a fishing port, the harbor is now dedicated to tourism and pleasure boating. Observe that the sand and rocks have been darkened by oil slick up to the high-water line.

Coasts can be harmed by many things people do and by natural changes. Important problems include pollution and rising sea levels.

Pollution can come in many forms, like dirty water, too many nutrients causing harmful algae, oil spills, and trash that washes ashore. Rising sea levels can cause erosion and push salty water into places where it doesn’t belong.

This stretch of coast in Tanzania's capital Dar es Salaam serves as a public waste dump.

Global goals

The world is working together to protect coasts through Sustainable Development Goal 14, called "Life Below Water." This goal helps keep marine coastal ecosystems safe and supports better jobs for people living near coasts. The United Nations also started the UN Decade on Ecosystem Restoration from 2021 to 2030, but more work is needed to restore coastal areas.