How does plastic flow allow glaciers to move?

Glaciers move primarily through two mechanisms: basal sliding and internal deformation, the latter of which is driven by the plastic flow of ice.4

Because ice is a crystalline solid, it undergoes permanent deformation—often called creep—when subjected to shear stress over time. This plastic flow allows the ice to change shape and move without fracturing, functioning similarly to the behavior of metals. This process occurs through two primary internal mechanisms:123

• Intracrystalline gliding: Layers within an ice crystal shear parallel to one another, maintaining the continuity of the crystal lattice.1
• Recrystallization: The size and shape of crystal boundaries change in response to the orientation of adjacent crystals and the stresses applied to them.1

The speed of this deformation is controlled by the movement of dislocations, or defects within the crystal lattice. As gravity pulls the ice mass downslope, the weight of the ice induces shear stress, causing the glacier to deform and flow according to the constitutive law of ice.124