Anchor ice is defined by the

World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Internat ...

as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation". It may also be called bottom-fast ice. Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow sub or

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species of ...

during or after storms when the air temperature is below the

freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of the water, and the

subtidal The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

in the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

along

ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The ...

or near floating glacier tongues, and in shallow lakes.

Types and formation

In rivers

Anchor ice will generally form in fast-flowing rivers during periods of extreme cold. Due to the motion of the water, ice cover may not form consistently, and the water will quickly reach its

due to mixing and contact with the atmosphere. Ice platelets generally form very quickly in the water column and on submerged objects once conditions are optimal for anchor ice formation. Anchor ice in rivers tends to be composed of numerous small crystals adhering to each other in small flocculent masses. Anchor ice in rivers can seriously disrupt

hydro-electric power Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined and ...

plants by significantly reducing flow or stopping turbines completely. Another form of anchor ice may be observed at the mouths of

Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada ( Yukon, Northwest Territories, Nunavut), Danish Realm ( Greenland), Finland, Icel ...

rivers where fresh water seeps out of the river bed into the ocean up through the sediment. Anchor ice forms if the seawater is below the

of the river water.

In lakes

Shallow

tundra In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless m ...

lakes may feature anchor ice with a specific behavior. Lakes in the southwestern part of Nunavut, Canada typically freeze down to the bottom when the water level is low. On some cases spring meltwater flows into the lake under the ice cover, which becomes domed leaving a depressed "racetrack" ring around the shore where meltwater accumulates as well. The ice cover remains bottom-fast until the buoyancy force exceeds the freezing bond. At the latter moment the ice cover abruptly breaks off the bottom to form a flat sheet. In other cases the anchor ice becomes completely submerged into the meltwater and holes may be melted throughout the ice sheet. When the sheet finally lifts off the bottom, the meltwater accumulated at the surface is jetted through these holes with enough force to create small craters in the lake bottom where it is soft (sand or silt). This downward jet phenomenon was previously described for deltas into the

Beaufort Sea The Beaufort Sea (; french: Mer de Beaufort, Iñupiaq: ''Taġiuq'') is a marginal sea of the Arctic Ocean, located north of the Northwest Territories, the Yukon, and Alaska, and west of Canada's Arctic islands. The sea is named after Sir Fran ...

, where they were caused by periodic tidal buoyancy of holed ice.

Formed during storms

Anchor ice may be formed in the shallow

during storms in cold weather, when the uppermost layers of the water column are churned up by strong winds or waves. This type of anchor ice can be found primarily in the

, where submerged ice may be observed to completely cover the substrate to depths of up to 2m, with some anchor ice cover observed at more than 4.5m depth.

In the Antarctic

anchor ice is perhaps one of the most interesting phenomena of ice formation in the marine environment. The general mechanism of its formation is commonly assumed as the following: *Antarctic surface waters are forced to flow below a large, thick mass of floating ice (

ice shelf An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The ...

or glacier tongue) due to tidal motions or normal ocean circulation. *The surface water melts the underside of the mass of ice, causing a slight freshening of the water that brings the temperature of the water into equilibrium with the in-situ

at depth. *The water, at its

at depth (slightly lower than the freezing point at the surface due to the pressure effect on the freezing point), is advected from under the floating mass of ice and may rise towards the surface due to a variety of factors. *As the water rises the in-situ freezing point increases, leaving the water slightly

supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...

Supercooling Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...

is relieved by the formation of microscopic

ice crystal Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, ...

s in the water column. *Ice crystals may coalesce or adhere to submerged objects, including marine organisms, rocks, man-made structures, or other ice formations such as the

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's ...

, the ice foot, floating

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...

s or

iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...

s. Anchor ice crystals in the Antarctic are generally in the form of thin, circular platelets of 2–10 cm in diameter. Large masses of irregularly-oriented crystals form anchor ice formations, which may be as large as 4m in diameter when attached to large immovable objects on the sea floor. Anchor ice that forms on the underside of sea ice is often referred to as

platelet Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby in ...

or congelation ice, and can be hard to distinguish from that formed due to the cooling of the sea ice cover by cold atmospheric conditions. Anchor ice is thought to be relatively common in the Antarctic, due to large

that occupy many areas of the continental coast. Studies and observations of anchor ice formation in

McMurdo Sound McMurdo Sound is a sound in Antarctica. It is the southernmost navigable body of water in the world, and is about from the South Pole. Captain James Clark Ross discovered the sound in February 1841, and named it after Lt. Archibald McMurdo o ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

have shown that the phenomenon regularly causes the formation of ice on the seafloor to depths of approximately 15m, and rarely to depths of approximately 30m.

;Anchor ice in Antarctica

File:Antarctic Ice shelves drive the formation of supercooled seawater.png, Ice shelves can cause the formation of supercooled seawater and anchor ice. File:Anchor ice platelets on rope.JPG, Anchor ice growing on a rope.

Biological effects

Especially in the

, anchor ice has been implicated in drastic zonation of the

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Z ...

. Many animals are directly affected by the growth of anchor ice, and certain

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...

s have been shown to readily grow anchor ice and to be damaged by it. Anchor ice may also grow on animate or inanimate objects and lift them from the sea floor. In the

this will most likely result in the death of an organism, since during much of the year the ocean is covered by annual sea ice, and the organism is likely to become incorporated into this. Many organisms have actually been found on the surface of

in certain places in the

, likely due to the anchor icing phenomenon: *Organism accumulates anchor ice as it is bathed in supercooled water. *Organism becomes positively buoyant due to the accumulation of ice, and is lifted from the sea floor. *Organism floats to the underside of the

cover where it freezes in. *

Ablation Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for a ...

of the surface of the ice cover and additional growth of ice on the underside will result in the organism being transported "through" the ice and "deposited" on the surface, largely intact.

References

External links

Anchor ice formation and rafting in rivers: A video tour
* {{Ice , expanded Bodies of ice