- Waves are the most powerful agents of marine erosion.
- Waves are created by winds sweeping over the water surface, setting up a series of undulating swells surging forward.
- Waves are curved and refracted against the alignment of the coast as they approach shallow water near the shores.
- When the depth of the water is less than the height of the waves, the wave crest curls over and breaks into shore.
- The water rushing up the beach and hurling rock debris against the land is called swash.
- The water that retreats or is sucked back is called backwash.
- An undertow is another element in offshore drift, which flows near the bottom away from the shore.
- The undertow exerts a pulling effect that can be dangerous for sea bathers.
Types of Erosion
- Sea waves, aided by winds, currents, tides, and storms carry on the erosional and depositional processes.
- There are 4 types of erosion:
- Corrasion or Abrasion:
- Waves armed with rock debris charge against the base of the cliffs and wear them back by corrosion.
- On coming tides and waves complete the work by sweeping the eroded material into the sea.
- Attrition:
- The constantly moving waves that transport beach materials also hurl these materials against each other until they are broken into small pieces.
- The grinding and polishing of these materials against the cliff faces and against each other is largely responsible for the fine sand which forms the beaches.
- Hydraulic Action:
- The erosion that occurs when the motion of water against a rock on the surface produces mechanical weathering.
- It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.
- Solvent Action:
- The solvent or chemical action of waves is another mode of erosion, but it is pronounced only in case of soluble rocks like limestone and chalk.
- On limestone coasts, the solvent action of the sea water on calcium carbonate sets up chemical changes in the rocks and disintegration takes place.
- Corrasion or Abrasion:
Marine transportation
- Solution – minerals are dissolved in the water and carried along in solution.
- Suspension – fine light material is carried along in the water.
- Saltation – small pebbles and stones are bounced along the sea bed.
- Traction – large boulders and rocks are rolled along the sea bed.
Marine Landforms – Erosional
- Headlands and Bays:
- Cliffs erode at different paces along the coastline
- Different types of rock can form headlands and bays
- Soft rock such as clay and sand erodes quickly forming bays
- Bays are inlets of the sea where land curves inwards, usually with a beach
- Hard rock such as chalk is more resistant to erosion
- Soft rock erodes inwards, while hard rock sticks out into the sea forming a headland
- Headlands are more open to waves and have erosion features such as wave-cut platforms and cliffs
- Bays are more sheltered with constructive waves which deposit sediment and form a beach.
- Cliffs and Wave-Cut platforms:
- Cliffs are shaped through erosion and weathering.
- Soft rock erodes quickly and forms gentle sloping cliffs, while hard rock is more resistant and forms steep cliffs.
- A wave-cut platform is a wide, gently sloping surface found at the foot of a cliff.
- A wave-cut platform is formed when the sea attacks the base of the cliff between the high and low water mark.
- A wave-cut notch is formed by erosional processes such as abrasion and hydraulic action – this is a dent in the cliff usually at the level of high tide.
- As the notch increases in size, the cliff becomes unstable and collapses, leading to the retreat of the cliff face.
- The backwash carries away the eroded material, leaving a wave-cut platform.
- The process repeats, and the cliff continues to retreat.
- Caves, Arches, Stacks, and Stumps:
- Caves, arches, stacks and stumps are found on a headland due to erosion.
- Cracks are widened by hydraulic action and abrasion.
- The widening of the crack forms a cave.
- The cave grows larger and eventually breaks through the headland, forming an arch.
- The base of the arch widens through further erosion until it collapses, leaving a stack.
- A stack is an isolated column of rock.
- When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry, or chimney rock.
- The stack is undercut at the base until it collapses to form a stump.
- Ria, and Cove:
- A ria is a coastal inlet formed by the partial submergence of an unglaciated river valley.
- It is a drowned river valley that remains open to the sea.
- A cove is a small bay or inlet with a sheltered or restricted entrance.
- Coves are usually formed by the erosion of soft rock formation.
- This leaves hard rock that can form a circular or oval bay with a small entrance.
- A cove is generally less than 1000 feet across and can be much smaller, sometimes less than 100 feet in diameter.
- Ria:
- Cove:
- Geos and Gloups (blowholes or marine geyser):
- Waves splashing against the roof of a cave may enlarge the joints when air is compressed and released repeatedly inside them.
- This can create a natural shaft that may eventually pierce through the surface.
- When waves break into the cave, they may force water or air out of this hole, which is known as a Gloup or blow hole.
- The enlargement of blow holes and continual action of waves weakens the cave roof.
- When the cave roof collapses, a long, narrow creek may develop which is known as a Geos.
- Chasms:
- Chasms are narrow, deep indentations formed due to headward erosion by wave action through vertical planes of weakness in rocks.
- Lateral erosion of the chasm mouth widens it till a bay is formed.
- Creek, and Inlet:
- Creeks are natural streams of water tributary to a river, or a narrow sheltered waterway or an inlet in a shoreline or channel in a marsh.
- Inlets are long and narrow indentations of a shoreline that often lead to an enclosed body of salt water such as a bay, lagoon, or marsh.
- Creek:
- Inlet:
Marine Landforms – Depositional
- When water loses its energy, any sediment it is carrying is deposited. The build-up of deposited sediment can form different features along the coast.
- Beaches:
- Beaches are formed by eroded material that is transported from elsewhere and then deposited by the sea.
- Waves must have limited energy for beaches to form in sheltered areas like bays.
- Constructive waves build up beaches with a strong swash and a weak backwash.
- Sandy beaches are usually found in bays where the water is shallow and the waves have less energy.
- Pebble beaches often form where there are higher energy waves and cliffs are being eroded.
- The cross-profile of a beach is called the beach profile and has ridges called berms which show the lines of high tide and storm tides.
- A sandy beach typically has a gently sloping profile, whereas a shingle beach can be much steeper.
- The size of the material on the beach is larger at the top due to high-energy storm waves carrying large sediment.
- The smallest material is found nearest the water as the waves break down the rock through attrition.
- Spits and Hook:
- A spit is a stretch of sand or shingle jutting into the sea from the land.
- Spits are formed due to changes in the landscape or river mouth.
- Longshore drift carries sediment.
- Deposition occurs when the coastline changes shape.
- A long thin ridge of material is deposited, forming the spit.
- Hooked ends can form due to changes in wind direction.
- Waves cannot get past a spit, creating sheltered water behind it.
- Silts are deposited here to form salt marshes or mudflats.
- Bars, Lagoons, and Barrier:
- Sometimes a spit can grow across a bay and join two headlands together. This landform is known as a bar. They can trap shallow lakes behind the bar, these are known as lagoons. Lagoons do not last forever and may be filled up with sediment.
- Barrier: It is the overwater counterpart of a bar.
- Tombolos & Dumb Ball:
- A tombolo is a sandy isthmus. Sometimes, islands are connected with mainland by a bar called tombolo.
- If two islands are connected to each other by a bar is called Dumb Ball.
- Marine Dunes & Dune Belts:
- With the force of on-shore winds, a large amount of coastal sand is driven landwards forming extensive marine dunes that stretch into dune belts
- Their advance inland may engulf farms, roads & even the entire villages;
- Hence to arrest the migration of dunes, sand binding species of grass & shrubs, such as marram grass & pines are planted.
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