Stack (geology)

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Big Flowerpot, Flowerpot Island Ontario, Canada

Ball's Pyramid, Australia, is the tallest volcanic stack in the world.^[1]

Old Man of Hoy, Scotland

Example of stack in the making, Yesnaby, Mainland Orkney

A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast. Stacks 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. Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing.

Without the constant presence of water, stacks also form when a natural arch collapses under gravity, due to sub-aerial processes like wind erosion. A stack may collapse or be eroded, leaving a stump.

Stacks typically form in horizontally bedded sedimentary or volcanic rocks particularly on limestone cliffs, because of the medium resistance to 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 sea attacks small cracks and opens them. The cracks then gradually get larger and turn into a small cave. When the cave wears right through the headland, an arch forms. Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast called a stack.

Examples

• The Big Flowerpot, Flowerpot Island, Lake Huron, Canada.
• Three Sisters, Eatonville, Nova Scotia.
• Goat Rock, California, USA.
• Sugar Loaf, Mackinac Island, Michigan, USA.
• Haystack Rock, Cannon Beach, Oregon, USA.
• Old Harry Rocks, Dorset, England.
• The Needles, Isle of Wight, England.
• Am Buachaille, Sutherland, Scotland.
• Ball's Pyramid, Lord Howe Island Group, Australia
• Old Man of Hoy, Orkney, Scotland.
• The Twelve Apostles, Victoria on the Great Ocean Road, Victoria, Australia
• Percé Rock, Percé Quebec, Canada
• Hopewell Rocks, Hopewell Cape New Brunswick, Canada
• Po Pin Chau, High Island, Hong Kong
• Étretat, Seine-Maritime, France
• Lange Anna on Heligoland
• Stac an Armin, St Kilda, Scotland
• Stac Dhomnuill Chaim, Lewis, Scotland
• Tri Brata and Sail Rock, Russia
• Les Jumeaux, Hendaye, France.

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References

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