Subaerial processes in coastal systems

Coastal landscapes are shaped by a combination of marine processes, such as erosion, transport, and deposition caused directly by the sea, and subaerial processes, which occur on land and are not directly linked to marine action. Subaerial processes include weathering and mass movement. These processes break down rock material, producing sediment that enters the coastal system. Weathering weakens coastal cliffs, making them more susceptible to collapse, while mass movement shifts material downhill.

Types of weathering on coasts

Weathering is the gradual breakdown of rock through physical, chemical, or biological means, without involving movement. It occurs on coastal cliffs and weakens the rock structure, increasing vulnerability to erosion and mass movement. Different types of weathering dominate depending on the climate, rock composition, and location.

Mechanical weathering

Mechanical weathering breaks down rock without altering its chemical makeup, often through physical forces that exploit existing cracks or pores.

Key processes in mechanical weathering:

• Freeze-thaw weathering - Water seeps into fractures in the rock. When temperatures drop below 0 °C, the water freezes and expands, putting pressure on the rock. Repeated cycles of freezing and thawing cause fragments to break off. This is common in coastal areas with variable temperatures.
• Salt weathering - Seawater or spray enters rock pores and cracks. As the water evaporates, salt crystals form and expand, exerting pressure that dislodges rock pieces. This occurs frequently on exposed cliffs at high tide.
• Wetting and drying cycles - Rocks containing clay absorb water and expand when wet, then contract when dry. These repeated cycles weaken the rock, leading to fragmentation. This process affects clay-rich coastal formations.

Chemical weathering

Chemical weathering changes the rock's chemical composition, often through reactions with water or air, leading to dissolution or weakening.

Key processes in chemical weathering:

• Carbonation - Rainwater absorbs carbon dioxide from the atmosphere, forming weak carbonic acid. This acid reacts with rocks containing calcium carbonate, such as limestone, gradually dissolving them. Certain seaweeds also release mild acids that contribute to this process when they break against rocks.
• Oxidation - Rocks with iron content react with oxygen in air or water, forming iron oxide. This weakens the rock, causing it to crumble and often giving it a rusty red colour. It is prevalent on iron-rich coastal outcrops exposed to the elements.

Biological weathering

Biological weathering involves the actions of living organisms that physically or chemically break down rock.

Key processes in biological weathering:

• Animal activity - Marine animals burrow into rock fractures for habitat, breaking apart the structure. Some secrete chemicals that dissolve the rock, further undermining cliffs.
• Plant growth - Seeds lodge in rock cracks and germinate. As roots grow, they exert pressure, widening fractures and eventually splitting the rock. This is common on vegetated coastal slopes.

Mass movement on coastlines

Mass movement is the downhill shifting of materials like rock, sand, soil, and clay under gravity, often triggered by coastal conditions. It typically happens when waves undercut cliffs, creating unstable overhangs that collapse. Unconsolidated rocks, which lack strong binding between particles, and weak geological structures are particularly prone. Heavy rainfall saturates materials, reducing friction and increasing the likelihood of movement. Surface runoff can also erode fine particles, transporting them downslope.

Types of mass movement

Different types of mass movement vary in speed and mechanism.

Landslides:

• Involve the rapid sliding of material down a straight slope, often after heavy rain or storms.
• Rainfall lubricates the slope, creating a slip plane where friction is overcome.
• This leaves a landslide scar, an unvegetated area on the cliff face above the moved material.

Rotational slumping:

• Material slides as a single mass along a curved slip plane, usually over time.
• Occurs where permeable rocks overlay impermeable ones, with water creating the slip plane.
• The top vegetation often remains intact.

Blockfalls (rockfalls):

• Broken blocks of rock fall downslope, common on steep cliffs with joints and bedding planes.
• Waves undercut the base, destabilising blocks that then topple and accumulate as talus or regolith at the cliff foot.

Coastal landforms formed by mass movement

Rotational scars

A curved, vegetation-free mark on the cliff face left after rotational slumping, resembling a scar where material has detached.

Terraced cliff profiles

Formed by multiple rotational slumps over time, creating a series of small, stepped terraces down the cliff face, giving the appearance of layered clifftops.

Talus scree slopes

Steep piles of rockfall debris at the cliff base, fanning out from the fall point.

Characteristics of talus scree slopes:

• Material is naturally sorted, with larger rocks at the bottom and smaller ones on top.
• These slopes temporarily protect the cliff from wave undercutting until waves transport the debris away.

Influence of weathering and mass movement on coastal recession

Subaerial processes like weathering and mass movement influence the rate of coastal recession, sometimes accelerating it and sometimes slowing it.

How weathering affects coastal recession

Weathering gradually weakens cliff faces by breaking down rock, making them more prone to erosion by waves. This increases the rate of recession. Additionally, weathered fragments provide scree that waves use as tools for abrasion, further eroding the coast.

How mass movement affects coastal recession

Mass movement deposits rock material onto beaches, increasing their size and acting as a buffer against wave energy. This can strengthen beaches and reduce erosion, thereby decreasing the rate of coastal recession.