Weathering happens when natural events, like wind or rain, break up pieces of rock. In coast al areas, strong winds and powerful waves break off soft or grainy rocks from hardier rocks.
The harder rocks are left as cliffs. The tiny pieces of rocks broken off by weathering are called sediment or alluvium. Erosion is the process of transportation of this sediment. On sea cliffs, sediment becomes part of the seafloor and is washed away with the waves. On inland cliffs, sediment is often carried away by rivers or winds.
Larger rocks broken off by sediment are called scree or talus. Scree builds up at the bottom of many inland cliffs as rocks tumble down. These piles are called scree slopes or talus piles.
Some scree slopes can be so large that soil and sediment can build up between the rocks, allowing trees and other vegetation to grow on the slope. Most scientists and mountaineer s think the Rupal Flank of Nanga Parbat, a mountain in the Himalayas, is the highest cliff in the world.
The Rupal Flank rises 4, meters 15, feet above its base. Others say the highest cliff in the world is the east face of Great Trango, in the Karakoram mountain range, which is 1, meters 4, feet tall and one of the most difficult rock-climbs in the world. Steep coastal cliffs can keep out invaders. Photograph by James P. Verona Rupes. The repeated freeze-thaw action puts pressure on the rocks until they eventually crack and break the rock.
Freeze-thaw usually occurs when the temperature oscillates around freezing point. The shattered angular fragments of rock accumulate around the base of the cliff as scree talus. As coastal climates tend to be milder this process is only really effective during very cold winters. A common type of mechanical weathering found at coasts is salt crystallisation. This is when salt crystals are deposited in cracks and over time the salt accumulates and applies pressure to the crack similar to freeze-thaw weathering.
Wetting and drying is common along coastlines. Clay-rich rocks are prone to expand when they are wet and contract when they dry.
This results in cracks which are vulnerable to both freeze-thaw and salt crystallisation. Biological weathering is mechanical when there is a physical effect. An example of this is when growing plant roots widen cracks as does the leverage created by bushes and trees swaying in strong winds. Burrowing animals and nesting birds excavate material in partially weathered and eroded cliffs. Chemical weathering involves decomposition of rocks literally, changing the minerals.
Chemical weathering occurs as a result of a weak chemical reaction between water and rock. Rainwater mixed with carbon dioxide in the atmosphere forms Carbonic acid. This then reacts with calcium carbonate in limestone to form calcium bicarbonate. Bicarbonate is soluble in water and the limestone gets weathered when carbonation occurs. Mass movement is the movement of material downslope as the result of gravity.
This can be a slow process in the case of soil creep or fast in the case of rockfalls. During the day, the sun heats up the surface of the rock and at night the outer layers cool down. This repeated expansion and contraction of the surface layers results in the gradual disintegration of the top layers.
On cliff faces, both animal and plant action can loosen large amounts of rock material. Trees and plant roots can prise apart rocks by growing in between the bedding planes and joints enlarging them as they increase in size.
Equally biological damaging is when small animals burrow underground and loosen rocks such as sand martins. Microorganisms in the soil are also able to break down the rocks; particularly carbonate rocks. Below high tide level, seaweed will often attach itself to rocks and when waves crash on the shore, seaweed will be subject to powerful currents and forces that will pull pieces of the rock away with it. The chemistry that makes up coastal rocks can be affected by rain and sea water leading to the gradual disintegration of solid rocks.
Hydrolysis occurs in certain rocks with minerals that are reactive to water or chemicals in the water. Here, large and small fragments of rock are continually weathered and eroded until they separate and fall from the cliff as whole parts.
In Britain this is often due to freeze thaw weathering. Where rocks are laid down in beds that slope dip down towards the sea whole layers can slide down slope along a slide plane.
This is where saturated soil and weak rock flows down a slope and produces a lobe - sometimes called a solifluction lobe.
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