How can a glacier flow while most of it remains solid ice?

A glacier can move even though most of its ice is solid. Its weight creates stresses within the thick body. Over time, ice crystals deform and rearrange, allowing the whole mass to change shape and flow.

This slow internal deformation is called creep. It is easier to see by comparing positions over time than by watching a single moment. Upper ice can also crack where the required deformation is too rapid for that brittle surface.

Motion at the glacier’s bed can add to the movement. Some glaciers slide over rock or ride above deforming wet sediment. Water beneath the ice can influence resistance, although not every glacier has the same basal conditions.

The direction of flow depends on the slope of the ice surface and the resulting pressure differences. Melting changes how much ice remains, while deformation and basal motion change where that ice goes. A glacier’s retreating edge can therefore coexist with ice continuing to flow forward.