A fast-moving ice or ice stream is a region of an ice sheet that moves significantly faster than the surrounding ice. Ice streams are a type of glacier. They are significant features of the Antarctic where they account for 10% of the volume of the ice. They are up to 50 kilometres (31 mi) wide, 2 km (1.2 mi) thick, can stretch for hundreds of kilometres, and account for most of the ice leaving the ice sheet.
The speed of an ice stream can be over 1,000 metres (3,000 ft) per year, an order of magnitude faster than the surrounding ice. The shear forces at the edge of the ice stream cause deformation and recrystallization of the ice, making it softer, and concentrating the deformation in narrow bands or shear margins. Crevasses form, particularly around the shear margins.
The Antarctic Ice Sheet is drained to the sea by several ice streams. The largest in East Antarctica is Lambert Glacier. In West Antarctica the large Pine Island and Thwaites Glaciers are currently the most out of balance, with a total net mass loss between them of 85 gigatonnes (84 billion long tons; 94 billion short tons) per year measured in 2006.
It has been suggested that the Antarctic ice sheet is losing mass. The past and ongoing acceleration of ice streams and outlet glaciers is considered to be a significant, if not the dominant cause of this recent imbalance.
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