A sea stack is an isolated column of rock that stands vertically in the sea, separated from the mainland coastline. These formations are the result of coastal erosion, sculpted over millennia by the relentless forces of the ocean. They represent the final stage in a progressive sequence of landform development that begins on the rocky shore. The majestic appearance of a sea stack is a testament to the immense power of wave energy acting upon a geologically specific location.
The Necessary Coastal Precursors
Sea stacks require a specific geological foundation, typically a rocky headland or cliff line. Headlands are sections of the coastline made of resistant rock that jut out into the sea because the surrounding softer rock has eroded away. This difference in rock strength creates the initial platform for stack development. These formations are exposed to the full force of wave action, concentrating the sea’s energy upon them. The headland must also contain lines of weakness, such as joints, faults, or bedding planes, which erosional processes can exploit.
Initial Erosion and Sea Cave Formation
The formation process begins when high-energy waves attack the base of the headland. Two primary erosional mechanisms exploit the pre-existing weaknesses: hydraulic action and abrasion. Hydraulic action occurs when waves force air into cracks, and the compressed air rapidly expands as the wave retreats, widening the cracks. Abrasion (corrasion) acts as a grinding tool, where loose rocks and sediment carried by the waves are hurled against the cliff face. This combined attack gradually undercuts the headland, carving out sea caves on either side.
The Transition to Arches and Final Collapse
As waves continue to erode the base of the headland, the sea caves deepen and expand. Erosional forces target weaknesses from both sides until the two caves meet, creating a natural bridge known as a sea arch. The arch is not a permanent structure, remaining under constant attack from wave erosion and sub-aerial weathering. Continued wave action and the weight of the rock above eventually cause the arch to become structurally unstable and collapse. The isolated column of resistant rock that remains standing is the sea stack, which continued erosion will eventually reduce to a low-lying feature called a sea stump.
Factors Influencing Formation Speed and Shape
The rate at which a sea stack forms and its final appearance are governed by several variables. The most significant factor is the resistance of the rock type; harder rocks like granite erode much slower than softer rocks like chalk. The energy level of the wave environment also dictates the speed of change, with coastlines exposed to strong winds and large swells experiencing faster erosion. The height of the tidal range determines the zone of wave impact on the cliff face, concentrating erosional power in a specific vertical band. The geological structure, including the angle of the bedding planes and the spacing of the joints, influences the stack’s shape, as erosion preferentially removes rock along these lines of weakness.