Ice stream

The term ice stream - in addition to its use as a synonym for illustrative Glacier - a glaciological technical term that corresponds to the English term "ice stream". This refers to areas of ice sheets that differ from the surrounding ice through higher flow rates. About it a large part of the ice the ice sheets flows from - these are the Antarctic ice sheet 90 %, although ice streams account for only 13% of the Antarctic coastline. Metaphorically they are therefore referred to as " arteries of the ice sheets ." The flow rate of the ice often is significantly higher than that of mountain glaciers.

Ice streams in the narrow sense are not there laterally bounded by visible rock formations. If they are, there are outlet glaciers. However, this definition allows in practice there is no meaningful distinction. This is particularly evident when Rutford - ice flow in Antarctica, which is bounded on one side by mountains, on the other hand, by slower moving ice. When defining the wider fast flowing, fed by ice streams outlet glaciers are included.

As the largest ice stream applies the Lambert Glacier in Antarctica. The fastest-flowing ice stream known is the Jakobshavn Isbræ in West Greenland. This flow under normal circumstances at a rate of up to 7 km per year. A doubling of the speed to 14 kilometers per year was adopted in 1996 and in the following years at times. This shows the great dynamism that can develop ice streams. In the Antarctic, even ice streams were observed to change their direction of flow in a relatively short time.

Morphology

Due to the higher flow velocity pull ice streams surrounding ice down. Characterized the surface of a river of ice is lower than that of the flanking ice. Between this and the ice stream is huge longitudinal columns that are visible on satellite photographs well and have led to the discovery of the ice form. In the longitudinal direction, the surface profile of a river of ice from the "normal" flowing ice of the ice sheet is different. While the shape of an ice sheet is reminiscent of a parabolic shape and the inclination with distance from the ice divide increases, is a river of ice at its beginning point at the sharpest and becomes flatter. Thus its surface profile is convex along the flow line, in contrast to the predominant shape of the concave Eisschildoberfläche. Almost all the rivers of ice forming in the sea, they often feed an ice shelf.

Reasons for the high flow rates

The different behavior of different ice streams suggests that there are different causes for the high flow rates of ice streams:

  • Topographic causes: Most ice streams flow along subglacial Talstrukturen. The largest is the influence of the topography of the ground at the edge of the ice sheets, where they are thinnest. Basically, the flow of ice from glaciers concentrated in areas where the glacier base is sunk the deepest. That ice streams maintain the high speed in the lower regions, or even increase, but can not be explained solely by the topography.
  • Decrease in the viscosity of the ice: The concentration of the ice flow in an area leads to high voltages, and thus heat of friction. This leads to the increase in temperature inside and makes the ice cream softer.
  • "Lubrication" on the glacier reason: With most ice streams, it is assumed that liquid water and basal sliding plays an important role, and the glacier base is very slippery. Holes on the Whillans ice stream - have shown that the glacier bed is made of rubble that has a high proportion of clay minerals, and the water pressure at the interface of ice and debris nearly equal to the pressure exerted by the overlying ice. Either uncoupled this high pressure the ice from his bed or it weakens the dimensional stability of the debris on the ground, thus allowing its deformation, which favors the sliding - or both.

Fluctuations of the flow velocity

For the occurrence of high and often fluctuating flow rates in glaciers there are three categories: Surges, tidewater glaciers and ice streams. The fact that these categories overlap, is already alone seen, therefore, as virtually all fed by ice streams outlet glaciers are tidewater glaciers. At least for one such outlet glaciers, the Storstrømmen in the northeast of Greenland, is certain that he also surge behavior: From 1913 to 1978 he retired, and then rapidly in the following years - with speeds of more than 4 kilometers per year - push forward, with large masses of ice have been moved from the upper to the lower ablation zone.

However, there is no evidence that large areas of today's ice sheets and ice streams surge behavior show. In particular, measurements have been delivered not indicative of larger regions, where ice movement has almost come to a standstill and the ice thickness compared to lower lying areas continue to increase, which is characteristic of surge behavior in mountain glaciers. On the other hand Surges would be the most plausible explanation for the Laurentide ice sheet during Heinrich events during the Jungpleistozäns. However, it has so far not sufficiently aware of the former glacier dynamics.

In West Antarctica, the opening at the Siple Coast ice streams showed considerable fluctuations during the last centuries. While the flow rate of the Whillans - river of ice is between 300 and 800 meters per year, occurs the Kamb ice stream - are present in the same climatic conditions, since about 200 years virtually on the spot. One possible explanation for this behavior is that the subglacial water drains could have changed. This may keep you from freezing at the glacier bed, as the ice on the glacier base can only be produced by inflowing water latent heat remain at the pressure melting point. Perhaps this water now takes the path through another ice stream.

More interesting than the question of whether ice streams show surge behavior, seems at the moment the question whether the known tidal glaciers own dynamic evolving processes can lead to the disintegration of entire ice sheets. In the retreat of the grounding line, ie the line from which the ice begins to float on the sea, the frictional resistance, the flow rate is reduced which increases. Characterized further thins out the ice, which forms a feedback effect.

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