Sea stacks are geological formations: solitary columns of rock rising dramatically from the sea. These natural monuments are remnants of former coastlines, separated from the mainland by the persistent power of oceanic forces. They illustrate the ongoing process of coastal erosion that relentlessly shapes the boundaries between land and water. Their existence is a testament to the varying resistance of rock to the sea’s energy.
The Geological Process of Sea Stack Formation
The creation of a sea stack follows a predictable sequence of coastal erosion, beginning on an exposed headland that juts into the ocean. The relentless hydraulic pressure of waves exploits weaknesses, such as faults and joints, within the rock face, gradually deepening them into sea caves. As wave action continues to pound both sides of the headland, these caves bore through the rock.
The continuous erosion eventually meets in the middle, forming a natural sea arch. This arch structure is inherently unstable, as the roof is constantly undercut and weakened by weathering and gravity. In time, the arch’s roof collapses into the sea, leaving the seaward pillar of rock as an isolated column—the sea stack. This sequence highlights the temporary nature of these formations, as the stack itself will eventually erode into a smaller feature called a stump.
Environmental Conditions Required for Sea Stacks
Sea stacks are not found on every coastline, as their formation requires a specific combination of geological and environmental factors. The rock type is paramount; the headland must be composed of moderately resistant material, such as hard sedimentary rock like limestone, or igneous rock like basalt. Softer rocks erode too quickly to allow the sequential formation of caves and arches, while harder rocks may resist erosion too effectively.
The coastline must also be classified as a high-energy environment, characterized by intense wave action and powerful currents. This energy drives hydraulic action and abrasion, concentrating erosion on the headlands due to wave refraction. This bending of waves around the headland focuses their destructive power, accelerating the cave-arch-stack sequence. The rate of sea level change must also be appropriate, allowing the formations to stand for a geologically significant period.
Iconic Sea Stacks Around the World
Sea stacks are distributed globally, marking coastlines wherever the necessary geological and oceanic conditions align. The most well-known collection is The Twelve Apostles, a group of limestone stacks lining the Great Ocean Road in Victoria, Australia. Although only eight remain standing today, their proximity to one another makes them an example of coastal erosion.
In the North Atlantic, the Old Man of Hoy in Scotland stands as a distinctive 137-meter column of red sandstone, isolated off the coast of the Orkney Islands. Iceland’s South Coast features the Reynisdrangar stacks, which are dark, basalt columns rising from the black sand beach of Reynisfjara. These structures were formed from ancient volcanic activity and are constantly battered by the powerful North Atlantic waves. On the west coast of the United States, Haystack Rock dominates the shoreline at Cannon Beach, Oregon, standing 72 meters tall and accessible by foot during low tide.