What causes glaciers to move?

By sliding

Explanation:

Gracious move by a combination of deformation of the ice itself and motion at the glacier base. At the bottom of the glacier, ice can slide over bedrock or shear subgiacial sediments.

Glaciers move by a combination of

(1) deformation of the ice itself and

(2) motion at the glacier base. At the bottom of the glacier, ice can slide over bedrock or shear subglacial sediments.