Erosion
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Coastlines. (2022). In Q-files Encyclopedia, Earth, Erosion. Retrieved from
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"Coastlines." Earth, Erosion, Q-files Encyclopedia, 10 Feb. 2022.
https://www.q-files.com/earth/erosion/coastlines.
Accessed 19 Mar. 2024.
Coastlines 2022. Earth, Erosion. Retrieved 19 March 2024, from
https://www.q-files.com/earth/erosion/coastlines
Earth, Erosion, s.v. "Coastlines," accessed March 19, 2024.
https://www.q-files.com/earth/erosion/coastlines
Coastlines
Coastlines are where ocean or sea meets land. Some coastlines are shaped by the action of waves crashing against them, wearing away the rocks. Along others, sand and shingle are piled up in beaches or mudflats. The coast is a continuing battle between sea and land. Sometimes the sea “loses” as shingle, sand or mud piles up and the land grows. In other places, the sea “wins” as waves, currents and tides batter and break up the coast. Even hard rocks like granite are gradually worn away, especially during storms when high winds whip up huge waves powerful enough to smash pebbles and boulders against the shore.
Coastal erosion
The shape and features of a coastline depend on its rocks, winds and currents. Very hard rocks erode slowly and stand out as high headlands. Waves build up higher in the direction of the prevailing (or main) winds in the region. This means waves have their biggest effect on coasts exposed to these winds. Sometimes waves may carve a hole, or arch, right through a headland. If the roof of the arch later collapses, part of it is left standing alone in the sea. This is called a stack.
Bays
Bays, wide, curving areas of sea between two headlands, form where soft rock has been worn away by the sea. Pounding waves wear away the land one “bite” at a time. Once a weak spot in a wall of hard rock is broken through, the softer rocks behind are more easily worn away. A small cove is carved out. Then the waves get to work on the sides of the cove, gradually enlarging it into a bay.
Cliffs
Cliffs are sculpted both by the waves and by rockfalls and landslides. Air trapped inside joints and faults in the rocks is compressed by the massive force of the waves when they strike the cliffs. This causes explosions that break up the rock, sending whole blocks crashing into the sea. When waves undercut soft rocks, creating a notch at the base of the cliff, the rocks collapse on to the shore and break up into tiny fragments. Wide beaches may eventually form, protecting the cliffs behind them from the full erosive force of the sea.
Wave-cut platform
A wave-cut platform forms when a rocky coastline is eroded over a long period of time. The cliff gradually “retreats” after more and more rock is worn away by the waves. A gently sloping area forms at the base of the cliffs, widening over time. In some places a wave-cut platform appears as an area of flat rock. In others, it is covered by sand or shingle, forming a beach.
Beaches
Rocks worn away from a coastline are eventually ground down by the pounding waves into tiny fragments: shingle or sand. These fragments, known as sediments, are suspended in the water before being laid down in sheltered parts of a coast, such as bays. As the waves pile up more and more shingle or sand, a beach is created. Beaches also form on the flat area between cliffs and the sea, the wave-cut platform. During storms, large waves crash on to the beach, carrying away the sand or shingle into deep water offshore. In this way, a beach may disappear in a matter of hours.
Sand dunes
Dunes form on wide beaches where onshore winds blow sand inland. Vegetation or large pebbles on the edge of the beach slow the wind down, causing it to drop the sand grains. Over time, these may build up into large mounds. As a dune forms, plants start to grow on it in a process known as ecological succession. Rotting seaweed, brought in by storm waves, adds nutrients to the sand. These allow sea grasses well adapted to harsh conditions, such as marram grass, to take root. The grasses add more nutrients to the soil, enabling other, less hardy plants to grow on the dunes. Eventually heathland or coniferous forests take over.
Longshore drift
The sea scours away rock fragments from one part of a coast, then, where the current slows down, deposits them at another, creating a beach. The fragments, shingle or sand, are carried along the beach in a zig-zag pattern, called longshore drift. When waves approach the coastline at an angle, the shingle or sand fragments are swashed up the beach at this same angle. As the wave retreats, they move back down at right angles to the beach's edge: the backwash.
Spits and bars
When the path of the current depositing sand and shingle is interrupted—where it meets a headland or enters an estuary, for example—the current weakens. The sediment is then dropped on to the sea bed. As longshore drift continues, a mound of sand or pebbles gradually extends out from the land into the sea, forming a finger-like feature called a spit. This may form a curved shape because of the action of the waves.
When the drift is strong, the spit may extend right across the bay, forming a bay-mouth bar. Sometimes a spit links the coast to an island, forming what is called a tombolo.
Barrier islands
Where coasts have long, gentle slopes stretching far out to sea, waves tend to break in the shallow waters well offshore. The breakers pile up sand on the sea bed to form what are called offshore bars (also known as sandbars or shoals). Running parallel to the coastline, the bars eventually build up above the water’s surface. They may then be breached in places by storm waves to form barrier islands. These are often covered by sand dunes. Barrier coasts, consisting of chains of barrier islands, make up 12–15% of the world’s coastlines. Examples are to be found along the east coast and Gulf coast of the USA, and Europe’s North Sea coast from the northern Netherlands to Denmark.
Lagoons and salt marshes
The shallow water between a bar or barrier island and the coast may eventually become cut off from the open sea, forming a shallow pool of water known as a lagoon. Over time, the lagoon fills up with sediment—mud and silt—from inflowing streams, forming salt marshes. Salt marshes provide rich feeding grounds for birds and other wildlife. The marshes may be drained to become dry land, useful for farming or grazing.
Tidal flats
Tidal flats are coastal wetlands that form in intertidal areas, the area that lies above water at low tide but which is submerged under water at high tide. Where sediments—silts, clays and the ground-up remains of marine animals—are deposited by the incoming tides or rivers flowing out to sea through estuaries, mudflats form. They provide habitats for many species, including migratory birds and mud-dwelling animals. Some estuaries have tidal flats several kilometres wide; the tide can come in faster than a person can run.
Submerged coasts
At the height of the last glacial period, around 26,500 years ago, the sea level across the globe fell 120 metres (about 400 feet) lower than it is today. This was because so much water was frozen, or "locked up", in the massive ice sheets. The climate has warmed since then, melting the ice and causing sea levels to rise, flooding low-lying areas. As a result, new islands, such as Great Britain, and new inlets, such as the Gulf of Bothnia between Sweden and Finland, were created. New coastlines have been attacked and shaped by the erosive force of the rising sea.
Fjords and rias
Along coasts of mountainous lands, where rising seas flooded into the deep U-shaped valleys gouged out by glaciers, fjords—long, narrow inlets with very steep sides—have been created. There is often a sill, a shallow area at the mouth of a fjord, the result of the glacier's diminished erosive power near its snout. Most fjords are actually deeper than the sea they link to. Fjords are found in Norway and the South Island, New Zealand.
In some areas of the world, rising sea levels have created submerged coasts. Here, the sea has gradually flooded across lowland areas and valleys, leaving hills as islands. The result is known a ria coastline. River valleys have become a branching network of inlets, while the hillsides that once sloped down to the winding river are now pointed headlands. Examples are found in the Galician coast of Spain (from where the term ria originates), southwest Ireland, the Adriatic coast of Croatia and the northern tip of the South Island, New Zealand.
Consultant: Ian Fairchild