How do internal deformation and basal sliding move a glacier downhill?

Glaciers move downhill primarily through two mechanisms: internal deformation and basal sliding, both driven by the force of gravity.12

Internal Deformation

Internal deformation, or strain, occurs as a response to shear stresses generated by the weight of the ice and the slope of the glacier surface. Because ice is a crystalline solid, it behaves like a metal near its melting point, undergoing plastic deformation known as creep. Under constant shear stress, ice crystals shift and flow, a process described by the Glen flow law, where the rate of shear strain is approximately proportional to the cube of the shear stress.321

Basal Sliding

Basal sliding occurs when a glacier slides over its bed, a process most common in temperate glaciers where the base is at the pressure-melting point. This movement is facilitated by two main processes:32
* Plastic flow: Stress concentrations cause the ice to deform and flow around bedrock obstacles.3
* Regelation: Pressure on the upstream side of obstacles causes ice to melt; the resulting water refreezes on the downstream side.3

Sliding is further influenced by subglacial water pressure, which can reduce friction between the ice and the bed. Some glaciers may also move by riding on a deforming layer of water-charged sediment.43