Introduction to coastal recession and lithology

Coastal recession occurs as erosion gradually moves the coastline inland, reshaping landscapes over time. This process varies greatly depending on the underlying geology, leading to uneven rates of change along different coastal sections. A key factor is lithology, which describes the physical characteristics of rocks, such as their hardness and resistance.

As a result, some areas experience differential erosion, where more resistant sections wear away slowly while weaker parts recede faster.

How different rock types influence erosion rates

Rocks along coastlines fall into three main categories based on their formation and properties: igneous, metamorphic, and sedimentary. Each type has distinct levels of resistance to erosion, affecting how quickly coastal recession happens. These differences arise from their internal structures, which determine how they withstand wave action and weathering.

Formation and characteristics of rock types

• Igneous rocks - These form from the cooling and solidification of magma, resulting in a hard, crystalline structure with few joints or weaknesses. This makes them highly resistant to erosion, typically receding at rates of 1 mm to 5 mm per year. An example is basalt, which is the most common igneous rock.
• Metamorphic rocks - Created when existing rocks undergo intense heat and pressure, these develop a crystalline structure. However, the crystals align in one direction, making the rocks prone to folding and faulting, which can introduce weaknesses. They offer moderate resistance, eroding at 1 mm to 10 cm per year. Slate serves as a typical example.
• Sedimentary rocks - These develop from layers of fragments (clastic) or the remains of dead plants and animals, often resulting in porous and fractured structures. Their limited resistance leads to erosion rates of 2 cm to 6 cm per year, as waves exploit these weaknesses. Limestone is a common example.

Erosion rates of unconsolidated materials

Coastlines sometimes consist of unconsolidated materials, such as loose layers of clay that are not cemented together into solid rock. Without this bonding, they are extremely vulnerable, eroding at rates up to 10 m per year.

The role of lithological structure in coastal recession

Beyond basic rock types, the overall structure of coastal cliffs plays a crucial role in recession rates. Most cliffs have complex profiles, combining multiple rock layers (strata) along with deposits from past geological events, such as glacial till or fluvial deposits.

Specific factors influencing lithological structure

Several structural elements within cliffs determine their stability and erosion speed. These include the arrangement of rock layers, how water interacts with the rocks, and chemical reactions with the environment. Each factor builds on the basic lithology to influence overall coastal recession.

Order of rock types in strata

The sequence of strata affects cliff stability. When less resistant rocks form the base and more resistant ones sit on top, waves undercut the weaker lower layers, leading to overhangs that eventually collapse. This process speeds up recession through mass movement events like rockfalls.

Additionally, strata with multiple folds introduce cracks and fractures throughout the cliff. These weaknesses reduce resistance to wave action.

Rock permeability and its effects

Permeability refers to a rock's ability to allow water to pass through it. This property influences erosion by controlling how water interacts with the cliff structure.

How rock permeability affects erosion:

• Impermeable rocks (e.g. clays) - These prevent water infiltration, leading to high surface runoff. The flowing water erodes the cliff face directly, acting as an additional force alongside waves.
• Permeable rocks (e.g. chalk) - Water seeps in, creating pressure known as pore water pressure that weakens the internal structure and widens existing cracks or joints. Rainwater absorption also adds mass, which can cause slumping — a type of landslide where saturated layers slide downward. This often occurs along slip planes, smooth surfaces created by water that facilitate movement.

Rock reactivity and chemical weathering

Some rocks undergo chemical weathering, a process where compounds in the rock react with seawater or atmospheric elements, breaking them down over time. For example, carbon dioxide in the air combines with water to form weak carbonic acid, which dissolves calcium carbonate in limestone, gradually eroding the material.

This reactivity is more common in unconsolidated materials.