Coast

The coast, also known as the coastline or seashore, is defined as the area where land meets the sea or ocean, or as a line that forms the boundary between the land and the ocean or a lake. Earth has around 620000 km of coastline. Because coasts are constantly changing, a coastline's exact perimeter cannot be determined; this measurement challenge is called the coastline paradox. The term coastal zone is used to refer to a region where interactions of sea and land processes occur. Both the terms coast and coastal are often used to describe a geographic location or region located on a coastline (e.g., New Zealand's West Coast, or the East, West, and Gulf Coast of the United States.)

Coasts are important zones in natural ecosystems, often home to a wide range of biodiversity, and important zones such as wetlands which are important for bird populations or mangroves or seagrass which provide nursery habitat for fish or other aquatic species. Some coasts fronts on the open ocean and are called pelagic coast while other coasts are more sheltered coast in a gulf or bay. A shore, on the other hand, may refer to parts of land adjoining any large body of water, including oceans (seashore) and lakes (lake shore).

While there is general agreement in the scientific community regarding the definition of coast, in the political sphere, the delineation of the extents of a coast differ according to jurisdiction. Government authorities in various countries may define coast differently for economic and social policy reasons. According to a United Nations atlas, 44% of all people live within 150 km of the sea. Because of their importance in society and high concentration of population, the coast is important for major parts of the global food and economic system. Important human activities happen in port cities, fisheries, and other spaces beaches and seaside resorts which are used for tourism.

However, the economic importance of coasts makes many of these communities vulnerable to climate change which causes changes in extreme weather and sea level rise and related issues such as coastal erosion, saltwater intrusion and coastal flooding. Other coastal issues, such as marine pollution and debris and marine ecosystem destruction, further complicate the human uses of the coast and threaten coastal ecosystems. International attention to these issues has been captured in Sustainable Development Goal 14 "Life Below Water" which sets goals for international policy focused on preserving coastal ecosystems and supporting more sustainable economic practices for coastal communities.

History
In its beginnings, the Earth's coasts have been an oxygen-producing oasis, but by around 4 billion years ago in the Hadean era, during the Great Oxidation Event, microbes living in the oceans were among the first organisms to appear on land, but then new forms of life, such as eukaryotes, a diverse type of cells, began to evolve into a variety of plants and animals. From prehistoric times onward, humans adapted their way of living through coastal environments. Today, coasts make up total of 7 percent of the Earth's oceans, with 95 percent of the world's marine productivity of aquaculture. As of 2016, more than 2 percent of the world's coasts have been found in marine protected areas.

Formation
Tides often determine the range over which sediment is deposited or eroded. Areas with high tidal ranges allow waves to reach farther up the shore, and areas with lower tidal ranges produce deposition at a smaller elevation interval. The tidal range is influenced by the size and shape of the coastline. Tides do not typically cause erosion by themselves; however, tidal bores can erode as the waves surge up the river estuaries from the ocean.

Waves erode coastline as they break on shore releasing their energy; the larger the wave the more energy it releases and the more sediment it moves. Coastlines with longer shores have more room for the waves to disperse their energy, while coasts with cliffs and short shore faces give little room for the wave energy to be dispersed. In these areas, the wave energy breaking against the cliffs is higher, and air and water are compressed into cracks in the rock, forcing the rock apart, breaking it down. Sediment deposited by waves comes from eroded cliff faces and is moved along the coastline by the waves. This forms an abrasion or cliffed coast.

Sediment deposited by rivers is the dominant influence on the amount of sediment located in the case of coastlines that have estuaries. Today riverine deposition at the coast is often blocked by dams and other human regulatory devices, which remove the sediment from the stream by causing it to be deposited inland. Coral reefs are a provider of sediment for coastlines of tropical islands.

Like the ocean which shapes them, coasts are a dynamic environment with constant change. The Earth's natural processes, particularly sea level rises, waves and various weather phenomena, have resulted in the erosion, accretion and reshaping of coasts as well as flooding and creation of continental shelves and drowned river valleys (rias).

Ecosystem services
Coasts make up a total of 7 percent of the Earth's oceans, with 95 percent of the world's marine productivity of aquaculture and the sustainability of marine habitats. Because of their importance to humanity, there are some coasts found in marine protected areas.

Coasts and their adjacent areas on and offshore are an important part of a local ecosystem. The mixture of fresh water and salt water (brackish water) in estuaries provides many nutrients for marine life. Salt marshes and beaches also support a diversity of plants, animals and insects crucial to the food chain.

The high level of biodiversity creates a high level of biological activity, which has attracted human activity for thousands of years.

Coasts also create essential material for organisms to live by, including estuaries, wetland, seagrass, coral reefs, and mangroves. They support 85 percent of U.S. migratory birds. Coasts also provide a habitat for sea turtles, marine mammals, and coral reefs.

Human settlements
More and more of the world’s people live in coastal regions. Many major cities are on or near good harbors and have port facilities. Some landlocked places have achieved port status by building canals.

Nations defend their coasts against military invaders, smugglers and illegal migrants. Fixed coastal defenses have long been erected in many nations, and coastal countries typically have a navy and some form of coast guard.

Emergent coastline
According to one principle of classification, an emergent coastline is a coastline that has experienced a fall in sea level, because of either a global sea-level change, or local uplift. Emergent coastlines are identifiable by the coastal landforms, which are above the high tide mark, such as raised beaches. In contrast, a submergent coastline is one where the sea level has risen, due to a global sea-level change, local subsidence, or isostatic rebound. Submergent coastlines are identifiable by their submerged, or "drowned" landforms, such as rias (drowned valleys) and fjords

Concordant coastline
According to the second principle of classification, a concordant coastline is a coastline where bands of different rock types run parallel to the shore. These rock types are usually of varying resistance, so the coastline forms distinctive landforms, such as coves. Discordant coastlines feature distinctive landforms because the rocks are eroded by the ocean waves. The less resistant rocks erode faster, creating inlets or bay; the more resistant rocks erode more slowly, remaining as headlands or outcroppings.

Other coastal categories

 * A cliffed coast or abrasion coast is one where marine action has produced steep declivities known as cliffs.
 * A flat coast is one where the land gradually descends into the sea.
 * A graded shoreline is one where wind and water action has produced a flat and straight coastline.

Landforms
The following articles describe some coastal landforms:


 * Bay
 * Headland
 * Cove
 * Peninsula

Cliff erosion

 * Much of the sediment deposited along a coast is the result of erosion of a surrounding cliff, or bluff. Sea cliffs retreat landward because of the constant undercutting of slopes by waves. If the slope/cliff being undercut is made of unconsolidated sediment it will erode at a much faster rate than a cliff made of bedrock.
 * A natural arch is formed when a headland is eroded through by waves.
 * Sea caves are made when certain rock beds are more susceptible to erosion than the surrounding rock beds because of different areas of weakness. These areas are eroded at a faster pace creating a hole or crevice that, through time, by means of wave action and erosion, becomes a cave.
 * A stack is formed when a headland is eroded away by wave and wind action.
 * A stump is a shortened sea stack that has been eroded away or fallen because of instability.
 * Wave-cut notches are caused by the undercutting of overhanging slopes which leads to increased stress on cliff material and a greater probability that the slope material will fall. The fallen debris accumulates at the bottom of the cliff and is eventually removed by waves.
 * A wave-cut platform forms after erosion and retreat of a sea cliff has been occurring for a long time. Gently sloping wave-cut platforms develop early on in the first stages of cliff retreat. Later, the length of the platform decreases because the waves lose their energy as they break further offshore.

Coastal features formed by sediment

 * Beach
 * Beach cusps
 * Cuspate foreland
 * Dune system
 * Mudflat
 * Raised beach
 * Ria
 * Shoal
 * Spit
 * Strand plain
 * Surge channel
 * Tombolo

Coastal features formed by another feature

 * Lagoon
 * Salt marsh

Other features on the coast

 * Concordant coastline
 * Discordant coastline
 * Fjord
 * Island
 * Island arc
 * Machair

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


 * Attrition
 * Currents
 * Denudation
 * Deposition
 * Erosion
 * Flooding
 * Longshore drift
 * Saltation
 * Sea level change
 * eustatic
 * isostatic
 * Sedimentation
 * Coastal sediment supply
 * sediment transport
 * solution
 * subaerial processes
 * suspension
 * Tides
 * Water waves
 * diffraction
 * refraction
 * wave breaking
 * wave shoaling
 * Weathering

Animals
Animals that live in coastal areas include puffins, sea turtles and rockhopper penguins, among many others. Sea snails and various kinds of barnacles live on the coast and scavenge on food deposited by the sea. Most coastal animals are used to humans in developed areas, such as dolphins and seagulls who eat food thrown for them by tourists. Since the coastal areas are all part of the littoral zone, there is a profusion of marine life found just off-coast, including sessile animals such as corals and sea anemones.

There are many kinds of seabirds on various coasts. These include pelicans and cormorants, who join up with terns and oystercatchers to forage for fish and shellfish. There are sea lions on the coast of Wales and other countries.

Plants
Many coastal areas are famous for their kelp beds. Kelp is a fast-growing seaweed that grows up to a metre a day. Mangroves, seagrasses and salt marsh are important coastal vegetation types in tropical and temperate environments respectively. Restinga is another type of coastal vegetation.

Length of coastline
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