Ice flow

from Wikipedia, the free encyclopedia
Velocity of the ice of the Antarctic Ice Sheet . The blue, yellow or white colored areas mark the faster flowing ice streams.

The term ice stream is - in addition to its use as a clear synonym for 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 in terms of higher flow velocities. A large part of the ice of the ice sheets flows over them - in the case of the Antarctic Ice Sheet this is 90%, although ice flows only make up 13% of the Antarctic coastline. This is why they are pictorially referred to as the “ arteries of the ice sheets”. The flow speed of the ice streams is often significantly higher than that of mountain glaciers.

Ice flows in the narrower sense are not laterally limited by visible rock formations. If they are, they are outlet glaciers . However, this definition does not allow any meaningful delimitation in practice. This is particularly evident in the Rutford ice flow in Antarctica, which is only bounded on one side by mountains, but on the other side by slower moving ice masses. Fast-flowing outlet glaciers fed by ice streams are included in the definition in a broader sense.

The Lambert Glacier in Antarctica is the world's largest ice flow . The fastest flowing known ice current is Jakobshavn Isbræ in West Greenland. Under normal circumstances, this flows at a speed of up to 7 kilometers per year. In 1996 and in the following years this speed doubled to 14 kilometers per year. This shows the great dynamics that ice flows can develop. In the Antarctic, ice flows have even been observed that changed their direction of flow in a relatively short time.

morphology

Due to the higher flow velocity, ice streams pull the surrounding ice downwards. As a result, the surface of an ice stream is deeper than that of the flanking ice. Huge longitudinal gaps form between this and the ice flow , which can be clearly seen on satellite images and which led to the discovery of the ice flows. In the longitudinal direction, the surface profile of an ice stream differs from the "normally" flowing ice of an ice sheet. While the shape of an ice sheet is reminiscent of a parabolic shape and the slope increases with the distance from the ice divide, an ice stream is steepest at its starting point and becomes shallower. Thus, its surface profile is convex along the flow line, in contrast to the predominant concave shape of the ice sheet surface. Almost all ice streams end in the sea, often they 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 velocities of ice streams:

  • Topographic causes: Most of the ice flows flow along subglacial valley structures. The influence of the subsurface topography is greatest at the edge of the ice sheets, where they are thinnest. Basically, the flow of ice from glaciers is concentrated in areas where the glacier floor is deepest. The fact that ice flows maintain or even increase the high speed in the lower areas cannot be explained by the topography alone.
  • Decrease in the viscosity of the ice: The concentration of the ice flow in one area leads to high stresses and thus to frictional heat. This increases the temperature inside and makes the ice softer.
  • “Lubrication” on the glacier bottom: With most ice flows, it is assumed that liquid water and basal sliding play a major role, and that the glacier bottom is very slippery. Drilling in the Whillans Ice Stream has shown that the glacier bed consists of rubble, which has a high proportion of clay minerals, and that the water pressure at the interface between the ice and rubble is almost the same as the pressure exerted by the ice on top. This high pressure either decouples the ice from its bed or it weakens the dimensional stability of the rubble on the ground and thus enables its deformation, which promotes sliding - or both.

Fluctuations in the flow rate

There are three categories of the occurrence of high and often fluctuating flow velocities in glaciers: surges , tidal glaciers and ice flows. The fact that these categories overlap is evident simply because practically all outlet glaciers fed by ice streams are also tidal glaciers. At least for one such outlet glacier, Storstrømmen in northeast Greenland, it is certain that it also shows surge behavior: It retreated from 1913 to 1978, and then quickly in the following years - at a speed of more than 4 kilometers per year - to advance, with large ice masses being shifted from the upper Zehr area to the lower.

However, there is no evidence that larger areas of today's ice sheets and ice currents show surge behavior. In particular, measurements have so far not provided any indication of larger regions where the ice movement has almost come to a standstill and the ice thickness is continuously increasing compared to deeper lying areas, which is characteristic of surge behavior in mountain glaciers. Surges, on the other hand, would be the most plausible explanation for the Heinrich events at the Laurentide Ice Sheet during the Young Pleistocene . However, there is currently insufficient knowledge of the glacier dynamics of that time .

In West Antarctica, the ice flows flowing into the Siple Coast have shown noticeable fluctuations over the past few centuries. While the flow speed of the Whillans Ice Stream is between 300 and 800 meters per year, the Kamb Ice Stream , which has the same climatic conditions, has been treading on the spot for about 200 years. One possible explanation for this behavior is that the subglacial water drainage may have switched. This may cause the glacier bed to freeze solid, as the ice on the glacier floor can only stay at the pressure melting point through the latent heat generated by inflowing water . It is possible that this water is now taking its way across another ice stream.

More interesting than the question of whether ice flows show surge behavior seems to be the question of whether the processes that develop their own dynamics from tidal glaciers can lead to the disintegration of entire ice sheets. When the grounding line retreats, i.e. the line from which the ice begins to float on the sea, the frictional resistance decreases, which increases the flow velocity. This further thins the ice, creating a feedback effect .

See also

literature

  • Kurt M. Cuffey, WSB Paterson: The Physics of Glaciers. Fourth Edition. Butterworth-Heinemnn, Burlington 2010, ISBN 978-0-12-369461-4 .
  • Roger LeB. Hooke: Principles of Glacier Mechanics. Second edition. Cambridge University Press, Cambridge 2005, ISBN 0-521-83609-3 .
  • Terry Hughes: Glacier Motion / Ice Velocity. In: Vijay P. Singh, Pratap Singh, Umesh K. Haritashya (Eds.): Encyclopedia of Snow, Ice and Glaciers. Springer, Dordrecht 2011, ISBN 978-90-481-2641-5 , pp. 408-414.

Individual evidence

  1. ↑ Günther Drosdowski: Duden style dictionary of the German language. Bibliographisches Institut, Mannheim 1971, ISBN 3-411-00902-0 , p. 315. ( online )
  2. ↑ a b c d Cuffey, Paterson: The Physics of Glaciers. Fourth Edition. 2010, pp. 360–372.
  3. ^ Matthew R. Bennett: Ice streams as the arteries of an ice sheet: their mechanics, stability and significance. In: Earth Science Reviews. Volume 61, 2003, pp. 309-339. ( online ; PDF, 1.6 MB)
  4. ↑ When glaciers flow rapidly. In: Neue Zürcher Zeitung . October 2, 2002.
  5. ↑ Glacier in the bathtub. In: Deutschlandfunk , Research News. September 29, 2008.
  6. ↑ Antarctic ice flow in reverse gear. On: Wissenschaft.de from October 4, 2002.
  7. ^ A b Terry Hughes: Glacier Motion / Ice Velocity. 2011, pp. 408-414.
  8. ↑ a b Roger LeB. Hooke: Principles of Glacier Mechanics. 2005, pp. 105-110.
  9. ↑ Central Institute for Meteorology and Geodynamics (ZAMG): Antarctica: More dynamic than assumed , accessed on April 24, 2013
  10. ^ Garry KC Clarke: Fast glacier flow: Ice streams, surging, and tidewater glaciers. In: Journal of Geophysical Research. Volume 92, 1987, pp. 8835-8841. ( Summary )
  11. ↑ a b c d Cuffey, Paterson: The Physics of Glaciers. Fourth Edition. 2010, p. 537f.
  12. ^ Cuffey, Paterson: The Physics of Glaciers. Fourth Edition. 2010, pp. 365-370.