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Stack (geology)

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Stack (geology)

Stacks at Torre Sant'Andrea, southern Italy

A stack or seastack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by erosion.[1] Stacks are formed over time by wind and water, processes of coastal geomorphology.[2] They are formed when part of a headland is eroded by hydraulic action, which is the force of the sea or water crashing against the rock. The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island. Without the constant presence of water, stacks also form when a natural arch collapses under gravity, due to sub-aerial processes like wind erosion. Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing.

Stacks at the island of Fårö, southern Sweden


Stacks typically form in horizontally-bedded sedimentary or volcanic rocks, particularly on limestone cliffs. These rock types' medium hardness means medium resistance to abrasive and attritive erosion. A more resistant layer may form a capstone. (Cliffs with weaker rock - such as clay - tend to slump and erode too quickly to form stacks, while harder rocks such as granite erode in different ways.)

The formation process usually begins when the sea attacks small cracks in a headland and opens them. The cracks then gradually get larger and turn into a small cave. When the cave wears through the headland, an arch forms. Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast - the stack. Eventually, erosion will cause the stack to collapse, leaving a stump. This stump usually forms a small rock island, low enough for a high tide to submerge. There are other stages after this.

Some stacks last for a very long time - possibly because they do not get submerged easily (for example the Ball's Pyramid volcanic stack in Lord Howe Island, New South Wales, Australia.)

Other languages

The term kekur (Russian: кекур) is used for the stacks in the Arctic and Pacific oceans, in the Russian Far East, and for stone columns in the watersheds of the Lena, Yana and Indigirka Rivers.[3]

The Swedish rauk refers to a stone column created by natural erosion. Examples are found on the islands of Gotland, Fårö and Öland.

Ball's Pyramid, Australia, is the tallest volcanic stack in the world.[4]
The "Three Sisters" sea stacks at Eatonville, Nova Scotia





(coastal countries only)


Faroe Islands





  • Branaunmore, Cliffs of Moher, Ireland
  • Dun Briste Sea Stack, Ballycastle, Co. Mayo


United Kingdom

North America



See also


  1. ^ Easterbrook, D.J. "Surface Processes and Landforms", p. 442, Prentice Hall, Upper Saddle River, New Jersey, 1999.
  2. ^ "Sea stacks"
  3. ^ Cловарь по естественным наукам. Кекур
  4. ^ Geography and Geology, Lord Howe Island Tourism Association. Retrieved on 2009-04-20.
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