How Does the Sea Cut a Cave Into a Cliff? Water Finding the Weakness
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Waves exploit any weakness in a cliff and enlarge it, and the result is a cave that grows in a predictable sequence towards a collapsed arch and a standing stack. The processes involved are more varied than pounding.
How the sea does the cutting
Several distinct processes attack a cliff and they act together. Hydraulic action is the direct force of water striking rock, and its effectiveness comes largely from air trapped and compressed in cracks by an arriving wave and released explosively as it withdraws, which levers the rock apart. Abrasion uses the sand and stones the waves carry, which grind the surface and do most of the shaping. Attrition wears those tools down into rounded beach material. Solution dissolves soluble rock chemically. And ordinary weathering works on the rock above the waterline, weakening it by salt crystallisation, by wetting and drying and by frost, so the cliff face is being attacked from several directions at once.
Why caves form where they do
Erosion concentrates on weaknesses rather than working evenly:
- •Joints and fractures in the rock, which are the commonest starting point
- •Faults, where rock has been broken and crushed along a plane
- •Bedding planes between layers of different resistance
- •Contacts between rock types, where one erodes faster than the other
- •Softer bands within otherwise resistant rock
- •Headlands, where wave energy is concentrated by refraction while bays are sheltered
The sequence that follows
The development from crack to isolated rock is consistent enough to be a textbook sequence and is genuinely observable along many coasts. A crack in a headland is enlarged into a notch and then a cave. The cave deepens, and where two caves form on opposite sides of a narrow headland they may meet, producing an arch. The arch widens until its roof cannot support itself and collapses, leaving a pillar separated from the land, which is a stack. The stack is undercut in turn and eventually falls, leaving a stump visible at low water. Each stage is visible at different sites and occasionally several stages appear along a single stretch of coast, which is why such coastlines are used for teaching.
What lives in them
Sea caves support communities structured by how long each part is submerged and how much light reaches it. The entrance carries the same zonation as any rocky shore, with barnacles, limpets and seaweeds arranged by tidal height. Further in, light falls away and photosynthetic organisms disappear, leaving filter feeders including sponges, sea squirts and anemones that depend entirely on what the water brings, and those communities can be unusually rich because wave action delivers food continuously. The darkest interiors resemble deep water communities at depths of a few metres, which makes them accessible study sites for organisms that otherwise require diving far deeper. Several seal species pup in sea caves, and bats, birds and invertebrates use the parts above the waterline.
What else the roof does
Two further features are worth distinguishing. A blowhole forms where a cave extends inland and a weakness in its roof is enlarged from below by compressed air and water, eventually breaking through to the surface, after which each arriving wave forces water and spray out of the hole, sometimes to considerable height. A geo is a narrow steep inlet formed where a cave collapses along its whole length rather than only at one point, leaving an open cleft cut into the cliff. Both are common on exposed rocky coasts. Sea caves also form by other means entirely in a few places, including lava tubes opening at a coast and caves dissolved in limestone that the sea has since reached, which are not cut by the sea at all.
The takeaway
Compressed air in cracks, sand and stones carried by waves, chemical solution and ordinary weathering all attack a cliff together. Erosion concentrates at joints, faults and softer bands rather than working evenly. A cave deepens into an arch, the arch collapses into a stack and the stack falls to a stump, and several stages frequently appear along one coast.