{{short description|Geological landform consisting of a steep and often vertical column or columns of rock}}
[[File:The Twelve Apostles 2011.jpg|thumb|right|The Twelve Apostles stacks in Victoria, Australia]] A '''stack''' or '''sea stack''' is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion.<ref>{{cite book |last=Easterbrook |first=D. J. |title=Surface Processes and Landforms |page=442 |publisher=Prentice Hall |location=Upper Saddle River, New Jersey |year=1999 |isbn=0-13-860958-6 }}</ref> Stacks are formed over time by wind and water, processes of coastal geomorphology.<ref>[http://www.britannica.com/ebchecked/topic/123147/coastal-landform/49793/sea-stacks "Sea stacks"] britannica.com {{webarchive|url=https://web.archive.org/web/20090414225722/http://www.britannica.com/EBchecked/topic/123147/coastal-landform/49793/Sea-stacks |date=2009-04-14 }}</ref> 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<ref>{{Cite web |title=Tectonics, Sediment Supply, and Morphologies Along Rocky Coasts {{!}} EARTH 107: Coastal Processes, Hazards and Society |url=https://courses.ems.psu.edu/earth107/node/1007 |access-date=2025-11-07 |website=courses.ems.psu.edu}}</ref> 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. 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. Stacks can provide important nesting locations for seabirds,<ref>{{Cite web |date=2022-06-12 |title=Seabirds at Coquille Point |url=https://sea-edu.org/seabirds-at-coquille-point/ |access-date=2025-11-07 |website=Shoreline Education for Awareness, Inc. |language=en-US}}</ref> and many are popular for rock climbing, primarily in Ireland.<ref>{{Cite web |title=Sea Stack Climbing Ireland {{!}} Climb Donegal's Sea Stacks {{!}} Unique Ascent |url=https://uniqueascent.ie/seastacks |access-date=2025-11-07 |website=uniqueascent.ie}}</ref>
Isolated steep-sided, rocky oceanic islets typically of volcanic origin, are also loosely called "stacks" or "volcanic stacks".<ref>{{cite web|url=http://www.lordhoweisland.info/conservation/geo.htm|title=Geography and Geology|archiveurl=https://web.archive.org/web/20140912073751/http://www.lordhoweisland.info/conservation/geo.htm|archivedate=12 September 2014|work=Lord Howe Island Tourism Association|access-date=17 March 2026}}</ref>
==Formation== [[File:Downpatrick Head 2021.jpg|thumb|Downpatrick Head Dún Briste, County Mayo, Ireland. Clear horizontal bedding is visible, exposing 350 million years of geological history.<ref>{{cite web | title = Mayo - County Geological Site Report | work = Geoheritage | publisher = Geological Survey of Ireland | url = https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/MO050_Downpatrick_Head.pdf | access-date = 28 December 2023 }}</ref>]]
Stacks typically form in horizontally bedded sedimentary or volcanic rocks, particularly on limestone cliffs. The medium hardness of these rocks means medium resistance to abrasive and attritive erosion. A more resistant layer may form a capstone. (Cliffs with weaker rock, such as claystone or highly jointed rock, tend to slump and erode too quickly to form stacks, while harder rocks such as granite erode in different ways.)<ref name="Schwartz_2006">{{Cite book |url=https://books.google.com/books?id=VWnxpAxp6TMC&q=sea+stack+formation+high+compressional+strength&pg=PA238 |title=Encyclopedia of Coastal Science |publisher=Springer Science & Business Media |year=2006 |isbn=9781402038808 |editor-last=Schwartz |editor-first=Maurice |page=238}}</ref>
[[File:Clarkson Frederick Stanfield (1793-1867) - The Stack Rock, County Antrim - BELUM.U167 - Ulster Museum.jpg|thumb|''Stack Rock, County Antrim'' by Clarkson Stanfield, 1861]] The formation process usually begins when the sea attacks lines of weakness, such as steep joints or small fault zones in a cliff face. These cracks then gradually get larger and turn into caves. If a cave wears through a 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.<ref>{{cite web|url=https://wa100.dnr.wa.gov/willapa-hills/coastal-processes|title=WA100: A Washington Geotourism Website|publisher=Washington State Department of Natural Resources|access-date=17 March 2026}}</ref>
==See also== * Ball's Pyramid, the tallest sea stack in the world * List of sea stacks * Rauk
==References== {{reflist}}
==External links== {{Wiktionary|sea stack|stack}}
*{{Commons and category inline|Geological stacks}}
{{coastal geography}}
Category:Stacks (geology) Category:Coastal and oceanic landforms Category:Coastal geography