Sub-aerial processes

weathering and mass movement

weathering definition

the physical disintegration and chemical decomposition of rock in situ at or near the Earth’s surface

positive feedback example involving weathering

if rate of debris removal exceeds rate of weathering and mass movement then rate of weathering and mass movement could increase

negative feedback example involving weathering

if debris removal is slow and ineffective this leads to a build-up of scree reducing cliff face exposure and weathering and mass movement rates decrease

3 types of mechanical weathering

Frost shattering (freeze-thaw), salt crystallisation, wetting and drying

process of frost shattering

water enters a crack or joint in the rock when it rains, it then freezes in cold weather and expands by 10%, exerting pressure on the rock forcing it to widen. Repeated freezing and thawing causes fragments of rock to break away and collect at the base of the cliff as scree, these angular rock fragments are then used by the sea as tools in marine erosion

process of salt crystallisation

salt water evaporates leaving salt crystals behind, which grow over time and exert stresses in the rock causing rock to break up. Salt can also corrode rock particularly if it contains traces of iron

wetting and drying

rocks rich in clay e.g. shale expand when they get wet and contract as they dry, causing the rock to crack and break up

what is mechanical weathering

the break up of rock without any chemical changes taking place

what is chemical weathering

chemical reaction where salts may be dissolved or clay-like deposit may result which is then easily eroded

3 types of chemical weathering

carbonation, oxidation, solution

carbonation process

rainwater absorbs carbon dioxide from air forming a weak carbonic acid which reacts with calcium carbonate in rocks such as limestone and chalk forming calcium bicarbonate which is easily dissolved, the cooler the temperature of rainwater the more carbon dioxide is absorbed so carbonation is more effective in winter

oxidation process

reaction of rock minerals with oxygen e.g. iron to form a rusty red powder leaving rocks vulnerable to weathering

solution process

the dissolving of rock minerals such as halite

what is biological weathering

the breakdown of rocks by organic activity

how plants contribute to biological weathering

thin plant roots grow into small cracks in a cliff face, the crack widen as roots grow breaking up the rock

how water contributes to biological weathering

water running through decaying vegetation becomes acidic leading to increased chemical weathering

how animals increase biological weathering

birds e.g. puffins and animals e.g. rabbits dig burrows into cliffs

what is mass movement

the downhill movement of material under the influence of gravity

types of mass movement

rockfall, mudflow, landslide, soil creep

rockfall

involves the sudden collapse or breaking away of individual rock fragments at a cliff face, when the angle is significant e.g. over 45 degrees, in heavily jointed and often quite resistant rock

landslide

can be rotational or planar, rotational occurs when there is a mix of geologies, planar occurs on a bedding plane roughly parallel to the ground surface, the moving block of material remain largely intact

mudflow

occur over weak or unconsolidated material such as clay, when water gets trapped in the rock it increases pore water pressure which forces rock particles apart and leads to slope failure

soil creep

result of freezing and thawing, very slow- 1cm a year, cooler temperatures needed, gravity pulls the water that is contained in the soil down a slope, the soil moves downhill with the water, forms terracettes

solifluction

similar to soil creep but specific to cold periglacial environments, in summer the surface layer of soil thaws out and becomes extremely saturated because it lies on top of impermeable permafrost, the sodden soil with vegetation slowly moves downhill by a combination of heave and flow

runoff

link between the water cycle and the coastal system, when overland flow occurs small particles are moved downslope to enter the littoral zone, potentially forming an input into the sediment cell, transfers both water and sediment from the rock face to the beach