Twelve Apostles, Victoria, Australia-2June2010 (1)

A stack or sea stack is a

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...

landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by

wave erosion Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward ...

. Stacks are formed over time by wind and water, processes of coastal geomorphology."Sea stacks"
britannica.com They are formed when part of a headland is

eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...

hydraulic action Hydraulic action, most generally, is the ability of moving water (flowing or waves) to dislodge and transport rock particles. This includes a number of specific erosional processes, including abrasion, at facilitated erosion, such as ''static eros ...

, 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 A natural arch, natural bridge, or (less commonly) rock arch is a natural landform where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion ...

collapses under

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

, due to sub-aerial processes like

wind erosion Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...

. 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

seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same enviro ...

s, and many are popular for rock climbing. Raukar

Isolated steep-sided, rocky oceanic

islets An islet is a very small, often unnamed island. Most definitions are not precise, but some suggest that an islet has little or no vegetation and cannot support human habitation. It may be made of rock, sand and/or hard coral; may be permanent ...

typically of volcanic origin, are also loosely called "stacks" or "volcanic stacks".

Formation

Stacks typically form in horizontally-bedded

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

s, particularly on

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

cliffs. The medium hardness of these rocks means medium resistance to abrasive and attritive

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...

. A more resistant layer may form a

capstone CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) is a lunar orbiter that will test and verify the calculated orbital stability planned for the Lunar Gateway space station. The spacecraft is a 12- ...

. (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 Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

erode in different ways.) The formation process usually begins when the sea attacks lines of weakness, such as steep

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

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.

References

External links

* {{coastal geography Coastal and oceanic landforms Coastal geography

Formation

See also

References

External links