- Ice wedge
An ice wedge is a crack in the ground formed by a narrow or thin piece of
icethat measures anywhere from 3 to 4 meters wide and extends downwards into the ground up to 10 inches. During the winter months, the water in the ground freezes and expands. Once temperatures reach -17 degrees Celsiusor colder, the ice that has already formed acts like a solid and contracts to form cracks in the surface known as ice wedges. As this process continues over many years, ice wedges can grow up to the size of a swimming pool.fact|date=May 2008
The origin of ice wedges has many theories but only one has consistently been backed by the most prominent scientists: the thermal contraction theory.
Thermal contraction theory
The Thermal Contraction Theory states that during the winter months, thermal contraction cracks form only a few centimeters wide and a couple meters deep because of the extreme cold [Cite web |url=http://arctic.fws.gov/permcycl.htm |title=Ice wedges, polygons and pingos |date=2006-02-14 |accessdate=2008-05-26 |publisher=U.S. Fish & Wildlife Service - Alaska |work=Arctic National Wildlife Refuge] . Over the next few, the snow melts and the remaining water fills the cracks and the
permafrostbelow the surface freezes it. These tiny cracks turn into permafrost. Once the summer months arrive, the permafrost expands; the fact of horizontal compressionproduces upturning of the frozen sediment by plastic deformation. The next winter the cold refreezes and cracks the already forming ice wedge and opens way for the eventual melting snow to fill the empty crack. The mean annual air temperature thought needed to form ice wedges is -6° to -8° C or colder.Cite web |url=http://www.britannica.com/eb/article-65736/permafrost |title=permafrost: Origins |publisher=Britannica Online Encyclopedia |accessdate=2008-05-26]
Three forms of ice wedges
There are three different forms of ice wedges: Active, Inactive and Ice Wedge Casts. All three forms are prevalent today and can be found in different parts of the world.
Active ice wedges
Active ice wedges are those that are still evolving and growing. During each year, a layer of ice will be added if cracking occurs, but cracking need not occur every year to be considered active. The zone in which most ice wedges remain active is along the permafrost zone. The amount of active ice wedges that are cracking yearly are consistently declining and becoming inactive.Cite web |url=http://www.britannica.com/eb/article-65737/ |title=permafrost: Active wedges, inactive wedges, and ice-wedge casts |publisher=Britannica Online Encyclopedia |accessdate=2008-05-26]
Inactive ice wedges
Ice wedge casts
In areas of past permafrost, ice wedges have melted and are no longer filled with ice. The wedge, which is now empty, is filled with sediment and dirt from the surrounding walls. These are called ice wedge casts and are used to calculate the climate of hundreds of thousands of years ago.
What ice wedges tell us about history
Ice wedges can tell a very great deal about history. After a while, when the ice wedge gets large enough and is no longer active,
sedimentswill fill the crack left by the ice wedges. These, in turn, are called pseudomorphsand could contain important hints of the past, including animal remains.fact|date=May 2008
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