Sub-aerial processes are land based processes which alter the shape of a coastline. They’re a combination of both weathering and mass movement.

Weathering

Freeze Thaw

Freeze thaw weathering involves water entering cracks in rocks and freezing. When the water freezes it expands, fracturing the rock.

Exfoliation

The repeated action of heating and cooling rocks causing them to “shed” off layers.

Biological Weathering

Plant seeds get into cracks in rocks and begin growing. As they grow, they exert pressure on the rocks, causing them to fracture. Seaweed, under the sea, can attach itself to rocks so that, as the sea moves the seaweed, chunks of rock are pulled away.

Chemical Weathering

Corrosion is technically a form of weathering and not erosion. Processes such as hydrolysis and oxidation can weather away rocks. Hydrolysis involves the splitting of minerals due to their reactivity with water. Oxidation is, basically, rusting. Elements such as iron are susceptible to oxidation and can be found within minerals on coastlines.

Mass Movement

Mass movement can be defined as the large scale movement of weathered material in response to gravity. Essentially, it’s when a cliff or other structure that is not horizontally orientated has been weathered to the point at which it starts to collapse. There’s five types of mass movement: rockfall, soil creep, landslides, mudflow and slumping.

Rockfalls

Freeze thaw weathering on a cliff breaks the rocks up into smaller pieces which can then free fall. This occurs commonly on cliffs with lots of joints as the joints make it easier to break up the rock. If the cliff is undercut by the sea, it can loose some of its stability, increasing the likelihood that a rockfall will occur.

Soil Creep

Soil creep is an incredibly slow process. It occurs on very gentle slopes and produces an undulated (wavy) surface. Damp soil moves very slowly down hill due to the increase in its mass (since it’s wet).

Landslides

After being soaked by water, cliffs made from soft rock will begin to slip due to the rock being lubricated. Landslides are very similar to slumps.

(Rotational) Slumping

Slumping happens for similar reasons to landslides. Heavy rainfall makes the rock heavier due to it absorbing the water and the water also acts as a lubricant. The difference with slumping is that it happens on a concave surface, which causes the cliff to form a crescent shape.

Mudflow

Mudflow is a very dangerous form of mass movement which occurs on steep slopes with saturated soil and little vegetation. The lack of vegetation means that there is nothing to bind the soil together, promoting mass wasting. The saturated soil becomes heavier and is lubricated, leading to the rapid movement of a lot of mud downhill.