A sea cave is a cavity formed at the base of a coastal cliff, primarily shaped by the relentless action of ocean waves. This geological feature is the product of mechanical erosion, where the physical force of the water exploits weaknesses in the rock structure to carve out a hollow. The main process driving the formation and enlargement of these caves is the sheer power of wave energy concentrated against the coastline. Sea caves are found globally where waves break directly against a rocky cliff face.
The Dominant Erosive Mechanism
The most powerful process in sea cave formation is hydraulic action, which is the force of the water itself impacting the rock face. As a wave crashes against the cliff, it forces water into small cracks and fissures under immense pressure. This action traps and compresses the air within these rock openings, which can exert a tremendous explosive force when the wave retreats and the pressure is suddenly released. The rapid compression and decompression of this trapped air acts like a pneumatic drill, gradually widening the existing structural weaknesses in the rock and dislodging fragments from the cliff face.
Working alongside the water’s force is abrasion, often considered the second most important process. Abrasion involves the grinding action of sediment, rocks, and sand that the waves carry and hurl against the cliff. These materials act as natural tools, wearing away the rock surface like sandpaper and accelerating the excavation of the developing cave. The combination of hydraulic action fracturing the rock and abrasion smoothing and deepening the cavity is far more significant than secondary processes like chemical weathering or biological activity.
Necessary Geological Conditions
The formation of a sea cave requires more than just powerful waves; it also depends on geological vulnerability. Sea caves begin where the coastal rock exhibits structural weaknesses. These weaknesses are pre-existing flaws such as faults, which are fractures where rock masses have shifted, or joints, which are smaller cracks caused by stress but without displacement.
The erosive power of the waves preferentially attacks and enlarges these vulnerable zones, which also include bedding planes and veins of softer rock. These planes and cracks provide the initial entry points for the water and compressed air to penetrate the cliff. This pre-conditioned geology ensures that the hydraulic action and abrasion are concentrated on specific, limited areas, allowing the cave to develop much faster than if the waves were attacking a homogeneous, unblemished rock face.
The Progression to Arches and Stacks
Once a sea cave forms, the continued erosive cycle often leads to a sequence of subsequent coastal landforms. As the cave widens and deepens on a headland, wave action can eventually erode through the entire mass of rock. When the cave breaks through to the other side of the headland, it creates a tunnel-like opening known as a sea arch. The arch is essentially a natural bridge, with the ocean flowing beneath it.
The arch, however, is a temporary structure. The roof is constantly weakened by continued erosion at its base and subaerial weathering from above. Eventually, due to a lack of support and the pull of gravity, the arch’s roof collapses into the sea.
This collapse leaves behind an isolated, vertical column of rock standing offshore, which is defined as a sea stack. The stack, now disconnected from the mainland, continues to be eroded at its base by waves. Over a long period, the stack is undercut and worn down until it is reduced to a low, flat platform or a small, submerged lump of rock known as a sea stump, completing the life cycle of the feature.