blue-ice area
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A blue-ice area is an ice-covered area of
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 ...
where wind-driven snow transport and sublimation result in net mass loss from the ice surface in the absence of melting, forming a blue surface that contrasts with the more common white Antarctic surface. Such blue-ice areas typically form when the movement of both air and ice are obstructed by topographic obstacles such as mountains that emerge from the
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
, generating particular climatic conditions where the net snow accumulation is exceeded by wind-driven sublimation and snow transports. Only about 1% of Antarctic ice area can be considered to be blue-ice area, but they have attracted scientific interest due to the large numbers of meteorites that accumulate on them; these meteorites either fall directly on the blue-ice area and remain there or they fall elsewhere into the ice sheet and are transported to the blue-ice area by ice flow. Additionally, ice up to 2.7 million years old has been obtained from blue-ice areas. Blue-ice areas are sometimes used as runways for aircraft.


Appearance

Blue-ice areas have a generally smooth and often rippled appearance, a blue colour and a sparseness of bubbles in the ice. This light blue colour is a consequence of the absorption of light by ice and air bubbles encased within it and the source of the name "blue-ice area". It contrasts markedly with the white colour of Antarctic plains and can be seen from space and from aerial images while the density of the blue ice makes it appear on
radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
images as a dark ice form. Scalloped or rippled surfaces have almost regular surface patterns, although wholly smooth blue-ice areas exist as well and the terrain even of rippled surfaces features very low aerodynamic roughness, perhaps among the lowest of all permanent natural surfaces. This is because most aerodynamic drag is caused by surface anomalies less than a centimetre long, not larger uneven forms. Wave structures form through sublimation. The occurrence of supraglacial moraines at blue-ice areas has been reported, these form when debris contained within a glacier accumulates at the surface due to melting or sublimation. Small depressions in the ice known as cryoconite holes are common and are formed where rocks got embedded in the ice, but are absent on more mountainous blue-ice areas. Typical blue-ice areas often feature intense
katabatic wind A katabatic wind (named from the Greek word κατάβασις ''katabasis'', meaning "descending") is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are sometim ...
s, with average winds reaching and gusts of up to ; such winds can remove and take up large amounts of snow. They are usually warmer than comparable snow-covered areas, sometimes by up to , which makes them identifiable from
brightness temperature Brightness temperature or radiance temperature is the temperature at which a black body in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity of a grey body object at a frequency \nu. This con ...
imaging. This warming is due to the lower
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
of the blue ice compared to snow, which results in them absorbing more sunlight and warming more. Blue-ice areas also alter the climate above them. As commonly defined, blue-ice areas display little or no evidence of melting, thus excluding glaciers and frozen lakes in the Antarctic Dry Valleys where sublimation-dominated ice also occurs but which may be more comparable to the ablation areas of regular
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.


Occurrence

Blue-ice areas were first discovered in 1949-1952 by the
Norwegian–British–Swedish Antarctic Expedition The Norwegian–British–Swedish Antarctic Expedition (also known as NBSX or NBSAE) (1949–1952) was the first Antarctica expedition involving an international team of scientists. The team members came from Norway, Sweden and the British Commonw ...
. They have been identified only in
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 ...
although similar ice patches on
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
have been reported and blue ice is widespread at glaciers worldwide. Blue-ice areas make up only about 1% of the Antarctic surface ice; however, they are locally common and scattered across the continent, especially in coastal or mountainous areas, but not directly beside the coastline. They have been found in
Dronning Maud Land Queen Maud Land ( no, Dronning Maud Land) is a roughly region of Antarctica claimed by Norway as a dependent territory. It borders the claimed British Antarctic Territory 20° west and the Australian Antarctic Territory 45° east. In a ...
, the catchment of the
Lambert Glacier Lambert Glacier is a major glacier in East Antarctica. At about 50 miles (80 km) wide, over 250 miles (400 km) long, and about 2,500 m deep, it is the world's largest glacier. It drains 8% of the Antarctic ice sheet to the east and s ...
, the
Transantarctic Mountains The Transantarctic Mountains (abbreviated TAM) comprise a mountain range of uplifted (primarily sedimentary) rock in Antarctica which extend, with some interruptions, across the continent from Cape Adare in northern Victoria Land to Coats Land. ...
and
Victoria Land Victoria Land is a region in eastern Antarctica which fronts the western side of the Ross Sea and the Ross Ice Shelf, extending southward from about 70°30'S to 78°00'S, and westward from the Ross Sea to the edge of the Antarctic Plateau. I ...
. Individual locations in Antarctica include areas of the
Allan Hills The Allan Hills are a group of hills at the end of the Transantarctic Mountains System, located in Oates Land and Victoria Land regions of Antarctica. They are mainly ice free and about long, lying just north-west of the Coombs Hills near th ...
, the
Queen Fabiola Mountains Queen Fabiola Mountains is a group of mountains in Antarctica, long, consisting mainly of seven small massifs which trend north–south, forming a partial barrier to the flow of inland ice. The mountains stand in isolation about southwest of th ...
(the Yamato ice field there covers an area of and is the largest such structure), Scharffenberg-Botnen and the Sør Rondane Mountains. Their location has been correlated with specific
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
s, temperatures and a
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
of less than 100%.


Origin and processes

Blue-ice areas are regions where more snow is removed by sublimation or by wind than accumulates by precipitation or wind-driven transport, leading to the emergence of (blue) ice. In most of Antarctica, the net tendency is for snow to accumulate except in coastal Antarctica where melting occurs and blue-ice areas where sublimation dominates. This sublimation occurs at rates of
snow water equivalent Snow science addresses how snow forms, its distribution, and processes affecting how snowpacks change over time. Scientists improve storm forecasting, study global snow cover and its effect on climate, glaciers, and water supplies around the worl ...
and is balanced by ice flow, with the sublimation rate decreasing with elevation and increasing with temperature. Summer also increases the sublimation rate, although it still occurs during winter. Winds remove snow that rest on the surface and could even scour exposed ice away, although the occurrence of scouring is not established without doubt and the role of abrasion is also unclear. Such areas exist even in the coldest parts of Antarctica, and they are characterized by high mean wind speeds and low precipitation. Once they have formed, the smooth surface prevents snow from accumulating as it is quickly blown away by the wind, and the blue colour increases the absorption of sunlight and thus sublimation; both these phenomena act to maintain the blue-ice area, and wind-driven transport of warm air can cause the blue-ice area to expand downwind. Blue-ice areas are common in mountainous regions. Presumably, irregular surface topography obstructs ice flow and locally creates atmospheric conditions suitable for the development of blue-ice areas. Irregular topography does not need to be exposed to the surface to generate blue-ice areas, although they must have an effect on the ice surface topography to induce the formation of blue-ice areas. Consequently, many blue-ice areas form when ice thicknesses decrease, which has been postulated to happen during interglacials although in general the past history of blue-ice areas is poorly known. Such areas may not have existed at all during glacial times when the ice sheet was thicker. Changes in mean wind speeds cause short-term fluctuations in the land covered by blue-ice areas.
Global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
is predicted to decrease wind speeds across Antarctica causing a small decrease in the land surface covered by blue-ice areas.
Thermal contraction Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. The most well-known material with NTE is water at 0~4 °C. Water's NTE is the r ...
of blue-ice can cause
icequake A cryoseism, ice quake or frost quake, is a seismic event caused by a sudden cracking action in frozen soil or rock saturated with water or ice, or by stresses generated at frozen lakes. As water drains into the ground, it may eventually free ...
s.


Age

Ages of particular blue-ice areas have been inferred from the ages of the meteorites there discovered, although redistribution of meteorites between various areas through ice flow can cause this procedure to yield erroneous age estimates. The oldest blue-ice areas may be up to 2.5 million years old and the ice in them can be quite old as well, with ages of several hundred thousand years estimated on the basis of ice flow dynamics and
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
and the development of a horizontal stratigraphy. This occurs because ice blocked by obstacles stagnates and moves at a rate commensurate with the
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 ...
rate. Younger ages have been found as well however, such as 250,000 years old at the
Allan Hills The Allan Hills are a group of hills at the end of the Transantarctic Mountains System, located in Oates Land and Victoria Land regions of Antarctica. They are mainly ice free and about long, lying just north-west of the Coombs Hills near th ...
, 75,000 years old at the Yamato Mountains, and 25,000 years old at the Larsen blue-ice area.


Types

Several subtypes have been defined, which encompass most blue-ice areas. * Type I form in the lee of an obstacle and are the most common type of blue-ice area although they usually cover only a small surface area, compared to the other three types. They are often 50 – 100 times as long as the obstacle is high, which is often a mountain. * Type II form where
katabatic wind A katabatic wind (named from the Greek word κατάβασις ''katabasis'', meaning "descending") is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are sometim ...
s clear snow from the surface until ice appears. They form on valley glaciers. * Type III form where winds blowing on steep slopes – or even over flat terrain – remove snow from the surface. * Type IV form by wind removing snow from the lowest part of a glacier basin.


Meteorites

Blue-ice areas are known primarily for the meteorites that accumulate there. They originally fell on ice elsewhere and were transported by ice flows to the blue-ice area, where they accumulate when the ice they were encased in ablates away; this mechanism has been compared to a conveyor belt that transports meteorites to blue-ice areas. Additionally, meteorites that fell directly on the blue-ice areas are represented; because of the often great age of the surface a number of meteorites can accumulate even without ice-driven transport. Over 20,000 meteorites from blue-ice areas were known by 1999, a large share of all known meteorites on Earth. Meteorite findings occur only on a minority of all blue-ice areas and are mostly limited to inland blue-ice areas whereas coastal ones tend to be lacking in meteorites. This might reflect the fact that at low altitude the ice surrounding the meteorites can melt due to solar heating of the meteorite, thus removing it from view.


Research history

The earliest research in blue-ice areas occurred during the
Norwegian–British–Swedish Antarctic Expedition The Norwegian–British–Swedish Antarctic Expedition (also known as NBSX or NBSAE) (1949–1952) was the first Antarctica expedition involving an international team of scientists. The team members came from Norway, Sweden and the British Commonw ...
in 1949–1952, and was followed by two decades of mostly geological and geomorphological research. The discovery of meteorites in a blue-ice area of the Yamato Mountains led to an uptick in scientific interest; a number of programs to collect meteorites began. This also led to increased research in the glaciological and dynamical properties of blue-ice areas, and later to their meteorological and climatological implications.


Use

The hard, flat and smooth surfaces of blue-ice areas have been used as aircraft runways ( Blue ice runways) in parts of Antarctica. The very old ice in blue-ice areas has been used to reconstruct past climate, and the temporal resolution may be larger than in deep ice cores. Blue-ice areas are candidate sites for ice core drilling aimed at recovering 1.5 million year old ice, and 2.7 million year old ice has been recovered from such areas.


References


Citations


Sources

* * * * * * {{refend Glacial erosion landforms Environment of Antarctica Geomorphology Ice