Introduction: The Power from Above

When we think about coastal erosion, we usually imagine powerful waves crashing against cliffs. However, waves aren't the only thing shaping our coastlines! There is another set of processes happening above the water line. These are called subaerial processes (from the Latin "sub" meaning under and "aer" meaning air).

In this chapter, we will explore how weathering and mass movement work together to weaken cliffs and change the shape of our landscapes. Don't worry if these terms sound technical—we’ll break them down step-by-step!

1. What is Weathering?

Weathering is the breakdown of rocks in situ (which is just a fancy Latin way of saying "in the place where they are"). Unlike erosion, which involves moving rock away, weathering just weakens it or breaks it into smaller pieces where it stands.

There are three main types of weathering you need to know:

A. Mechanical (Physical) Weathering

This happens when physical forces break the rock apart without changing its chemical makeup.

  • Freeze-thaw (Frost shattering): Water seeps into cracks in the cliff. When it freezes, it expands by about \( 9\% \), putting huge pressure on the rock. When it thaws, the pressure is released. Over time, this repeated "pushing" breaks the rock apart.
  • Salt Crystallisation: Waves spray salt water into cracks. When the water evaporates, salt crystals grow. These crystals put pressure on the rock, eventually causing it to crumble.

B. Chemical Weathering

This happens when a chemical reaction changes the minerals inside the rock, making it weaker.

  • Carbonation: Rainwater absorbs \( CO_{2} \) from the air to form a weak carbonic acid. This reacts with rocks like limestone or chalk, dissolving them slowly over time.
  • Oxidation: Some rocks contain iron. When they are exposed to oxygen in the air or water, they "rust," turning a reddish-brown colour and becoming much more brittle.

C. Biological Weathering

This is weathering caused by living things.

  • Plant Roots: Tiny seeds fall into cracks in a cliff. As they grow into trees or bushes, their roots act like wedges, forcing the cracks wider.
  • Burrowing Animals: Rabbits, birds, or even marine organisms can tunnel into soft cliff faces, weakening the structure from the inside.

Quick Review: Think of weathering as the "prep work." It weakens the cliff so that waves or gravity can finish the job!

2. Mass Movement: Gravity Takes Control

Once weathering has weakened the cliff, mass movement often follows. This is the movement of weathered material (rocks, soil, or mud) downslope under the force of gravity.

The type of mass movement depends on the lithology (the type of rock) and how much water is in the ground. Here are the main types found on coastlines:

A. Rockfalls

This happens on very steep or vertical cliffs made of hard rock. Weathering (like freeze-thaw) breaks off fragments, which then fall vertically to the bottom of the cliff. These form a pile of loose debris at the base called talus or scree.

B. Landslides

In a landslide, a block of rock moves rapidly down a planar (flat) surface. This often happens when the rock layers are tilted towards the sea. Rainwater acts as a lubricant, making it easier for the block to slide down the "slip plane."

C. Rotational Slumping

This is very common on UK coastlines where soft, unconsolidated material (like clay) sits on top of harder rock.
1. Heavy rain soaks into the clay, making it heavy and lubricated.
2. The material collapses along a curved slip plane.
3. The result is a "stepped" appearance on the cliff face.

D. Mudflows

When soil becomes completely saturated with water, it loses its structure and begins to flow like a liquid. Mudflows are often very fast and can be extremely dangerous.

Common Mistake to Avoid: Don't confuse a slide with a slump! A slide happens on a flat surface, while a slump happens on a curved surface.

3. How These Processes Influence Coastal Landforms

Subaerial processes don't work in isolation; they interact with marine erosion (which you can learn more about in the chapter on Marine erosion and erosional landforms).

  • Cliff Recession: While waves attack the "toe" (the bottom) of the cliff, weathering and mass movement attack the top. This causes the cliff to retreat inland.
  • Changing Cliff Profiles: Hard rocks that experience rockfalls keep a steep, vertical profile. Softer rocks that experience slumping create a shallower, stepped profile.
  • Sediment Supply: Mass movement provides "food" for the sea! When a cliff slumps, it dumps a huge amount of sediment onto the beach, which waves then transport along the coast.

Summary: Key Takeaways

1. Weathering is the breakdown of rock in situ. It can be mechanical (physical), chemical, or biological.

2. Mass Movement is the downslope movement of material caused by gravity. Key types include rockfalls, slides, slumps, and flows.

3. Water is the "enemy" of cliff stability—it increases the weight of the cliff and reduces friction, making mass movement much more likely.

4. Lithology (rock type) determines which process will happen. Hard rocks tend to fall; soft, wet rocks tend to slump or flow.

Top Tip for Exams: If you get a question about why a coastline is receding quickly, don't just talk about waves! Mention that subaerial processes like heavy rain causing slumping can be just as important as the sea itself.