Movement of material, especially rocks and ice, under intense pressure, when it flows like a very viscous substance and does not revert to its original shape when pressure is removed. As it moves, it shears. In ice, plastic flow is due to pressure at depth; a thickness of at least 22 m is needed for plastic flow temperate glaciers. Plastically flowing ice will flow around and over an obstacle, which may cause deposition in the lee of the obstruction: this is plastic moulding; see R. LeB. Hooke (2005). Plastic deformation is an irreversible change in the shape of material, without fracturing, resulting from compression or expansion. See Moeyersons et al. (2006) Geomorph. 76, 3–4 on plastic deformation in a vertisol.
Subjects: Earth Sciences and Geography.