Geomorphic Processes: Weathering, Mass Movement, Erosion, Transport and Deposition

How landscapes are reshaped

A cliff, river bank or valley side may look solid and permanent, but several geomorphic processes work together to change its shape. Weathering breaks down rock where it stands. Gravity can then move loosened material downhill. Rivers and waves erode surfaces, transport material and deposit it elsewhere.

These processes are connected, but they do different jobs. For example, weathering can loosen rock on a valley side, a slide can deliver it to a river, and flowing water can carry the fragments downstream. While travelling, the fragments may become smaller and rounder before being deposited.

Weathering: breaking down rock in place

Weathering is the breakdown of rock in situ, meaning in its original position. It does not itself carry the broken material away. There are three main types.

Mechanical weathering breaks rock physically without changing its chemical composition. An important example is freeze–thaw weathering. Water enters cracks in rock. When the temperature falls below freezing, the water freezes and expands, putting pressure on the sides of the cracks. When temperatures rise, the ice melts and water can enter farther into the cracks. Repeated freezing and thawing widens them until pieces of rock break off.

Freeze–thaw needs both water and temperatures that fluctuate around freezing point: permanently frozen water does not produce repeated freeze–thaw cycles. It is particularly relevant to wet upland areas of the UK, including mountains in Wales and Scotland.

Chemical weathering changes the minerals in rock through chemical reactions. In carbonation, slightly acidic rainwater reacts with limestone and gradually dissolves it. Some soluble minerals also dissolve directly in water. Another example is oxidation, in which iron minerals react with oxygen, producing rust and weakening the rock. Chemical weathering therefore changes the rock rather than simply breaking it into pieces of the same material.

Biological weathering is caused by living organisms. Plant roots grow into existing cracks and, as they become thicker, exert pressure that forces the cracks apart. Burrowing animals can also disturb and loosen material. Biological weathering is identified by its living cause, even when the actual breaking is physical.

Mass movement: gravity moves material downhill

Mass movement is the downslope movement of rock, soil or other material under gravity. Unlike river transport, it does not require flowing water to carry the material, although water often helps trigger it.

Heavy rainfall can saturate a slope, adding weight and reducing friction within the material. Weathering weakens rock, while erosion by a river or waves can remove support from the bottom of a slope. Together, these changes make slope failure more likely.

In sliding, a mass of material moves downhill along a relatively straight slip surface. The material may initially move together as a block before breaking apart farther downslope.

In slumping, the slip surface is curved. A block moves downwards and rotates as it slips, leaving a curved scar behind it. Slumping commonly affects weak material such as clay, especially when it becomes saturated and heavy. It can occur on coastal cliffs as well as valley sides.

Both processes reshape slopes and supply material that rivers or waves can later erode and transport.

Erosion: water and sediment wear surfaces away

Erosion involves the wearing away and removal of material. In river and coastal environments, moving water and its sediment load do this in several ways.

Hydraulic action is erosion by the force of water. Fast-flowing river water can loosen material from the bed and banks. At the coast, waves strike cliffs and force water and air into cracks. The compressed air puts pressure on the rock; repeated wave impacts weaken it and can break pieces away.

Abrasion, also called corrasion, happens when transported sediment scrapes or strikes a surface. Stones carried by a river grind against its bed and banks. Pebbles thrown against a cliff by waves wear away the cliff face. The effect is like sandpaper rubbing a surface.

Attrition happens when transported particles collide with one another. Edges chip off, so the particles become smaller, smoother and more rounded. Attrition changes the sediment load rather than directly scraping the river bank or cliff.

Solution, also called corrosion, happens when water dissolves soluble rock, such as limestone or chalk, and carries the dissolved material away. Its chemical action is similar to chemical weathering, but here the material is being removed by moving water.

These processes can operate together. Hydraulic action may loosen a fragment, abrasion may wear away the exposed surface, and attrition may round the fragment during transport.

Transportation: four ways water carries material

Transportation moves sediment and dissolved material from one place to another. The method depends on particle size and weight, as well as the strength of the water flow.

• Traction: large particles, such as boulders, roll or slide along the bed.
• Saltation: smaller particles, such as pebbles, bounce along the bed.
• Suspension: fine particles, such as silt and clay, are carried within the water rather than remaining on the bed.
• Solution: dissolved minerals are carried within the water. They are not visible solid particles.

The distinction between the two uses of solution is important: dissolving rock is an erosion process; carrying the dissolved material is transportation. Rivers and moving seawater can transport material by these methods.

Deposition: material is laid down

Deposition occurs when water can no longer transport part of its sediment load. This commonly happens when flow slows and loses energy, or when there is more sediment than the flow can carry.

A river may deposit sediment where its gradient becomes gentler, on the slower-flowing inside of a bend, or where it enters a lake or the sea. At the coast, sheltered areas have less energetic water, allowing sediment to settle. Constructive waves can also deposit material at the shoreline.

As flow weakens, larger and heavier particles generally settle before finer particles. Fine silt and clay can remain in suspension for longer and travel farther. Dissolved material is not simply dropped as solid sediment when the current slows.

Deposition builds up material in one place while erosion removes it from another. Alongside weathering and mass movement, this continual redistribution shapes the UK's river and coastal landscapes.

The process sequence

Weathering: breakdown in place → mass movement: gravity moves material downslope → erosion: wearing away and removal → transportation: material carried → deposition: sediment laid down.

These processes interact; they do not always follow one fixed sequence.

Weathering and mass movement

• Mechanical: physical breakdown; freeze–thaw requires water and repeated freezing and thawing.
• Chemical: minerals change or dissolve; carbonation affects limestone, oxidation affects iron minerals.
• Biological: organisms break down rock, for example growing roots widening cracks.
• Sliding: relatively straight slip surface.
• Slumping: curved slip surface with rotation; common in saturated weak material such as clay.
• Rainfall, prior weathering and undercutting can destabilise slopes.

Four erosion processes

• Hydraulic action: force of water; compressed air in cracks.
• Abrasion: sediment scrapes bed, banks or cliffs.
• Attrition: particles collide, becoming smaller and rounder.
• Solution: soluble rock dissolves and is removed.

Transport and deposition

• Traction: large particles roll or slide along the bed.
• Saltation: particles bounce along the bed.
• Suspension: fine sediment travels within the water.
• Solution: dissolved material travels within the water.

Deposition occurs when transport ability falls. Typical settings include slower river flow, river mouths and sheltered coastal water. Larger particles generally settle first; fine sediment remains suspended longer.