Glaciers and other large ice masses

Abstract

Substantial progress has been made in recent years with the help of new techniques for the investigation of glaciers and other large ice masses in four major study areas: Flow properties of ice. These can be considered as arising from processes on three distinct scales: (a) lattice-scale processes responsible for the flow characteristics of monocrystals as functions of temperature and stress; (b) crystal-scale processes responsible for the flow characteristics of crystal aggregates and for crystal orientation `fabrics' associated with different stress states and histories; (c) large-scale processes of ice flow and deformation as revealed by vertical, longitudinal and transverse velocity gradients, in the contraction of ice boreholes and tunnels, and through deformations arising when the ice flows over bedrock irregularities. These large-scale processes can be described by relationships of the form ϵ dot above <sub>ij</sub> = λσ prime <sub>ij</sub> where ϵ dot above <sub>ij</sub> and σ prime <sub>ij</sub> are the components of the steady-state strain rate and stress deviator tensors and λ is a function of the stress invariants and the properties of the ice. The crystal scale processes have come to be regarded as especially relevant for the understanding of natural ice flow and feature in current laboratory experiments, reviewed in § 2. The large-scale processes form the objective of current field work, reviewed in § 3, where the emphasis is on deep drilling and on the use of radar for measuring both distance and strain along the surface and ice thickness and structure in the vertical. Micrometeorological and isotope studies used to establish current and past glaciological `régimes' are also considered. The dynamics of large ice masses, which is governed by their flow laws and the equations of motion without acceleration terms, which are invariably small compared with those involving the stress deviator σ prime _ij and the gravitational forces ρg _i . Mathematically the problem is to find velocities and displacements by integrating equations of the form partial differential σ _ij /partial differential x_i = ρg _i with prescribed values of the geometry, velocities and velocity gradients (strain rates) at the boundaries. The boundaries define the three major types of large ice masses, namely glaciers which have rigid boundaries on both sides and at their base, ice shelves which are bounded on two sides but mostly free-floating and hence not subject to shear stresses at the base, and ice caps which are bounded at their base only. Current work in this area, reviewed in § 4, is concerned especially with variations along the direction of flow which account for the detailed surface shapes of large ice masses and permit to deduce their effective flow law parameters. They also are believed to play a crucial role in the processes of glacier sliding and surging which occupy the centre of current discussions on the dynamics of glaciers and ice sheets. For the latter the ice temperature and its drastic effects on ice flow introduce a major complication and create a separate study area. Thermodynamics of polar ice masses. The temperature of large ice masses is controlled by heat conduction in response to boundary heat fluxes and strain heating inside the ice. Both depend on the ice flow in a feedback complicated near the bedrock by possible phase changes. On the other hand the heat fluxes arising at the glacier surface from varied atmospheric processes, while forming a wide study area in their own right, are mostly absorbed in a shallow surface layer penetrated by longer-term `climatic' variations only. Current thermodynamic studies, reviewed in § 5, have the aim of clarifying these effects to the point of filtering out the residue of climatic temperature variations. This is gradually being achieved by matching actual ice temperature profiles, measured in boreholes or estimated from the isotope-depth distribution in ice cores, with theoretical and computer solutions of the temperature conduction equation. Long-term changes in large ice masses. Flow law, dynamic and thermodynamic results can be combined with mass continuity and mass balance considerations to determine how large ice masses react to changes in climatic conditions. Current work, reviewed in § 6, is especially concerned with the present trends of the large polar ice sheets and with the possibilities of modelling glaciers and ice sheets by computer. Although still at an early stage and taxing the capacity of present-generation machines, this approach promises to clarify the complex temperature-velocity feedback operating in polar ice sheets and the melt-water processes in glacier sliding and surging. This makes it possible to outline in the final section (7) the dynamic studies which may be expected to bring further substantial progress in the understanding of glaciers and other large ice masses.