seastack

A seastack, also called a sea stack, is a standalone vertical or near-vertical column of rock that rises from the sea near a coast. They form where continuous wave action erodes headlands.

Formation process: Cracks and faults in coastal rock are widened by hydraulic action and abrasion, creating sea caves. When a sea arch collapses, a free-standing stack remains. Over time, the base is undercut and the stack may topple, leaving a rock stump that erodes away.

Geology and ecology: Stacks occur in various rock types—sandstone, limestone, basalt—and reflect local geology and wave exposure. They are dynamic features, shaped by long-term erosion and sea-level changes. They often support seabird colonies, lichens, and marine invertebrates on their surfaces, while waves at the base deter vegetation.

Examples: Notable sea stacks include the Old Man of Hoy in Scotland and the Twelve Apostles along the Great Ocean Road in Victoria, Australia.

Conservation and hazards: Seastacks are inherently unstable and subject to collapse; some are protected as scenic or geological resources and monitored to manage coastal access. They can pose hazards to ships and shorelines during storms.