The Four Erosion Processes: How Waves Destroy Rock

H — Hydraulic Action: The Hammer and the Wedge
When a wave crashes against a cliff, it doesn't just push water against the rock — it traps compressed air in every crack and fissure in the cliff face. For a fraction of a second, that air is squeezed to enormous pressure. Think of it like inflating a balloon inside a sealed box: when the wave retreats, the pressure is suddenly released, and the crack walls are literally ripped apart. Over thousands of wave cycles, even solid rock is gradually fractured, widened, and eventually broken away. Hydraulic action is most effective in rocks that already have fractures or joints — it exploits weaknesses rather than attacking solid rock directly. A single breaking wave can exert up to 30 tonnes of pressure per square metre on a cliff face. That is roughly equivalent to a fully-loaded double-decker bus pressing against every square metre of rock.

A — Abrasion (Corrasion): The Sandpaper Effect
Waves do not only carry water — they carry whatever sediment the water has picked up: sand, pebbles, cobbles, sometimes boulders. When waves hurl this sediment against the cliff face and the base of the cliff, the rock fragments act as natural sandpaper, grinding away the surface and literally wearing it down. Abrasion is most effective at the base of a cliff, where repeated wave action creates a wave-cut notch — a horizontal groove at the waterline. It is also responsible for smoothing wave-cut platforms. The harder the sediment being thrown against the cliff, the more effective abrasion is: a wave carrying sharp angular flint fragments is far more destructive than one carrying fine sand.

S — Solution (Corrosion): The Chemical Dissolving Act
Seawater is slightly acidic — it contains dissolved carbon dioxide from the atmosphere, which forms weak carbonic acid. This acid reacts chemically with rocks containing calcium carbonate, particularly limestone and chalk. The rock literally dissolves into the water. This is not mechanical wearing — no physical force is needed. It is a chemical reaction that attacks the rock from within, weakening its structure over time. Solution is why chalk cliffs along the South Downs and limestone coasts in Ireland are particularly vulnerable to erosion. It is also why you sometimes see the sea looking slightly milky in these areas — dissolved calcium carbonate in suspension.

A — Attrition: Rocks Grinding Each Other Down
Attrition is not a cliff-erosion process — it is what happens to the sediment after it has been eroded. As rock fragments are transported by waves, currents, and longshore drift, they constantly collide with each other and with the seabed. Each collision knocks off corners and chips off edges. Over time, the sediment transforms: angular boulders become rounded cobbles, cobbles become pebbles, pebbles become sand. Pick up a beach pebble and hold it: its smoothness and roundness is the evidence of millions of collisions over thousands of years. Attrition explains why sediment gets finer as you move along a coast in the direction of longshore drift — the particles have simply been travelling longer and wearing down further.