Glaciers, ice sheets and ice caps represent tectonic systems driven by gravity. Their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements. All glacial ice has primary stratification, exhibited by variations in grain size, bubble content and debris content. As it deforms, with deformation dominated by plastic flow and recrystallization, accompanied locally by fracture under tension, a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation. Initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten. Foliation develops from inherited (mostly stratification) or introduced (mostly ice veins or fracture traces) fabric elements and from dynamic recrystallization. It is largely dependent on the accumulated strain, which is highest at the base and near the margins of glaciers, ice sheets and ice streams. Folds develop largely passively due to initial amplification of irregularities in the primary stratification, to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures. Buckle folds and boudinage, mostly on a small scale, occur where viscosity contrast is large, mostly in basal ice. Thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere. Many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures.
Bibliographical noteFunding Information:
Field work in the 1970s and 1980s was supported by grants from the National Science Foundation ( GA 19310, GA-42728, EAR 7721098, EAR 7712990, DPP 8308302, EAR 8609739 ), and this is gratefully acknowledged, as is support in various ways from the University of Minnesota . My interest in structures in ice was sparked by Roger Hooke, who has provided much advice, support and companionship in the field over the years. I thank Holger Stunitz for helpful comments and suggestions on the manuscript and Chris Wilson and Carl Stevenson for their thorough and thoughtful reviews.
- Ice caps