Benutzer:Saehrimnir/Blue-ice area

Blue-ice areas have a generally smooth and oftenVorlage:Sfn rippled appearance, a blue colourVorlage:Sfn and a sparseness of bubbles in the ice.Vorlage:Sfn 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 plainsVorlage:Sfn and can be seen from space and from aerial imagesVorlage:Sfn while the density of the blue ice makes it appear on radar images as a dark ice form.Vorlage:Sfn Scalloped or rippled surfaces have almost regular surface patterns, although wholly smooth blue-ice areas exist as wellVorlage:Sfn and the terrain even of rippled surfaces features very low aerodynamic roughness, perhaps among the lowest of all permanent natural surfaces.Vorlage:Sfn This is because most aerodynamic drag is caused by surface anomalies less than a centimetre long, not larger uneven forms.Vorlage:Sfn

The occurrence of supraglacial moraines at blue-ice areas has been reported,Vorlage:Sfn these form when debris contained within a glacier accumulates at the surface due to melting or sublimation.Vorlage:Sfn Small depressions in the ice known as cryoconite holes are common and are formed where rocks got embedded in the ice,Vorlage:Sfn but are absent on more mountainous blue-ice areas.Vorlage:Sfn

Typical blue-ice areas often feature intense katabatic winds, with average winds reaching Vorlage:Convert and gusts of up to Vorlage:Convert; such winds can remove and take up large amounts of snow.Vorlage:Sfn They are usually warmer than comparable snow-covered areas, sometimes by up to Vorlage:Convert, which makes them identifiable from brightness temperature imaging. This warming is due to the lower albedo of the blue ice compared to snow, which results in them absorbing more sunlight and warming more.Vorlage:Sfn Blue-ice areas also alter the climate above them.Vorlage:Sfn

As commonly defined, blue-ice areas display little or no evidence of melting,Vorlage:Sfn 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 glaciers.Vorlage:Sfn

Blue-ice areas have been identified only in AntarcticaVorlage:Sfn although similar ice patches on Greenland have been reportedVorlage:Sfn and blue ice is widespread at glaciers worldwide.Vorlage:Sfn Blue-ice areas make up only about 1% of the Antarctic surface ice;Vorlage:Sfn however, they are locally commonVorlage:Sfn and scattered across the continent, especially in coastal or mountainous areas,Vorlage:Sfn but not directly beside the coastline.Vorlage:Sfn

They have been found in Dronning Maud Land, the catchment of the Lambert Glacier, the Transantarctic Mountains and Victoria Land.Vorlage:Sfn Individual locations in Antarctica include areas of the Allan Hills,Vorlage:Sfn the Queen Fabiola Mountains (the Yamato ice field there covers an area of Vorlage:Convert and is the largest such structure),Vorlage:Sfn Scharffenberg-BotnenVorlage:Sfn and the Sør Rondane Mountains.Vorlage:Sfn

Blue-ice areas are regions where more snow is removed by sublimation or by wind than accumulates by precipitation or wind-driven transport,Vorlage:Sfn 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.Vorlage:Sfn This sublimation occurs at rates of Vorlage:Convert snow water equivalent and is balanced by ice flow, with the sublimation rate decreasing with elevationVorlage:Sfn and increasing with temperature. Summer also increases the sublimation rate, although it still occurs during winter.Vorlage:Sfn Winds remove snow that rest on the surface and could even scour exposed ice away, although the occurrence of scouring is not established without doubtVorlage:Sfn and the role of abrasion is also unclear.Vorlage:Sfn

Such areas exist even in the coldest parts of Antarctica,Vorlage:Sfn and they are characterized by high mean wind speeds and low precipitation.Vorlage:Sfn 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.Vorlage:Sfn

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 in order to generate blue-ice areas,Vorlage:Sfn 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 interglacialsVorlage:Sfn 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.Vorlage:Sfn Changes in mean wind speeds cause short-term fluctuations in the land covered by blue-ice areas. Global warming is predicted to decrease wind speeds across Antarctica causing a small decrease in the land surface covered by blue-ice areas.Vorlage:Sfn

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 oldVorlage:Sfn 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 and the development of a horizontal stratigraphy. This occurs because ice blocked by obstacles stagnates and moves at a rate commensurate with the ablation rate.Vorlage:Sfn Younger ages have been found as well however, such as 250,000 years old at the Allan Hills and 75,000 years old at the Yamato Mountains.Vorlage:Sfn

Types

Several subtypes have been defined,Vorlage:Sfn which encompass most blue-ice areas.Vorlage:Sfn

• Type I form in the lee of an obstacle and are the most common type of blue-ice areaVorlage:Sfn although they usually cover only a small surface area, compared to the other three types.Vorlage:Sfn They are often 50 - 100 times as long as the obstacle is high, which is often a mountain.Vorlage:Sfn
• Type II form where katabatic winds clear snow from the surfaceVorlage:Sfn until ice appears.Vorlage:Sfn They form on valley glaciers.Vorlage:Sfn
• Type III form where winds blowing on steep slopes - or even over flat terrain - remove snow from the surface.Vorlage:Sfn
• Type IV form by wind removing snow from the lowest part of a glacier basin.Vorlage:Sfn

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 accumulateVorlage:Sfn 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.Vorlage:Sfn 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.Vorlage:Sfn Over 20,000 meteorites from blue-ice areas were known by 1999, a large share of all known meteorites on Earth.Vorlage:Sfn

Meteorite findings occur only on a minority of all blue-ice areasVorlage:Sfn and are mostly limited to inland blue-ice areas whereas coastal ones tend to be lacking in meteorites.Vorlage:Sfn 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.Vorlage:Sfn

The earliest research in blue-ice areas occurred during the Norwegian–British–Swedish Antarctic Expedition 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 glaciologicalVorlage:Sfn and dynamical properties of blue-ice areas, and later to their meteorological and climatological implications.Vorlage:Sfn

The hard, flat and smooth surfaces of blue-ice areas have been used as aircraft runways (blue ice runways) in parts of Antarctica.Vorlage:Sfn 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.Vorlage:Sfn Blue-ice areas are candidate sites for ice core drilling aimed at recovering 1.5 million year old ice,^[1] and 2.7 million year old ice has been recovered from such areas.^[2]

Citations

Vorlage:Reflist

Sources

Vorlage:Refbegin

• 3, 1999, S. 337–359, doi:10.1029/1999RG900007, bibcode:1999RvGeo..37..337B (englisch). 
• 6, Juni 2001, S. 807–816, doi:10.1111/j.1945-5100.2001.tb01918.x, bibcode:2001M&PS...36..807H. 
• 2, 1. Januar 2003, ISSN 0009-2819, S. 93–147, doi:10.1078/0009-2819-00031, bibcode:2003ChEG...63...93H. 
• 2, 2005, ISSN 1365-2079, S. 225–236, doi:10.1017/S0954102005002634, bibcode:2005AntSc..17..225H (englisch). 
• 66, 2014, ISSN 0260-3055, S. 129–137, doi:10.3189/2014AoG66A069, bibcode:2014AnGla..55..129H (englisch). 

Vorlage:Refend

Category:Glacial erosion landformsCategory:AntarcticaCategory:GeomorphologyCategory:Ice