A glacier is a large accumulation of many years of snow, transformed into ice. This solid crystalline material deforms (changes) and moves. Glaciers, also known as "rivers of ice," actually flow. Gravity is the cause of glacier motion; the ice slowly flows and deforms (changes) in response to gravity.

Many move at a rate between zero and about half a kilometre (0.3 miles) per year. The fastest moving glacier is in Greenland, rushing forward at 12.6 kilometres (7.8 miles) per year. The middle of a glacier moves much more quickly than its edges, which are held back by friction with the surrounding land. Apr 11, 2012