Question

why do glacier move?

Answer 1

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 ofglacier motion; the ice slowly flows and eforms (changes) in response to gravity

Answer 2

There are two main ways that glacier move. The first one is called basal (base)sliding . Once a glacial ice reaches a thickness of about 10 m the weight of the ice is enough to cause basal melting due to the pressure of the ice above . This effect coupled with the angle of the underlying slope will affect the speed of the glacier ( steeper = faster ) .
There would also be movement from internal deformation.
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hope this helps !