Ice Wedges

The most conspicuous and controversial type of ground ice in permafrost is that formed in large ice wedges or masses with parallel or subparallel foliation structures. Most foliated ice masses occur as wedge-shaped, vertical, or inclined sheets or dikes 0.025 to 3 metres (0.1 to 10 feet) wide and 0.3 to 9 metres (1 to 30 feet) high when viewed in transverse cross section. Some masses seen on the face of frozen cliffs may appear as horizontal bodies a few centimetres to 3 metres (10 feet) in thickness and 0.3 to 14 metres (1 to 46 feet) long, but the true shape of these ice wedges can be seen only in three dimensions. Ice wedges are parts of polygonal networks of ice enclosing cells of frozen ground 3 to 30 metres (10 to 100 feet) or more in diameter.

The origin of ground ice was first studied in Siberia, and discussions in print of the origin of large ground-ice masses in perennially frozen ground of North America have gone on since Otto von Kotzebue recorded ground ice in 1816 at a spot now called Elephant's Point in Eschscholtz Bay of Seward Peninsula. The theory for the origin of ice wedges now generally accepted is the thermal contraction theory that, during the cold winter, polygonal thermal contraction cracks, a centimetre or two wide and a few metres deep, form in the frozen ground. Then, when water from the melting snow runs down these tension cracks and freezes in early spring, a vertical vein of ice is produced that penetrates into permafrost. When the permafrost warms and re-expands during the following summer, horizontal compression produces upturning of the frozen sediment by plastic deformation. During the next winter, renewed thermal tension reopens the vertical ice-cemented crack, which may be a zone of weakness. Another increment of ice is added in the spring when meltwater again enters and freezes. Over the years, the vertical wedge-shaped mass of ice is produced.

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