Studying the Process of Dissipation of Energy of Motion in Highly Fractured Glaciers using Remote-Sensing Techniques (Abstract)

Abstract

New aspects of the physics of ice movement in glaciers are presented. An analysis of the equation for the energy of a moving ice mass shows that the energy which is dissipated into heat (internal friction in a glacier) occurs not only because of average ice-velocity profiles, as it is commonly considered when simulating glaciers by continuum laminar flow. It also turns out that a significant part of the internal friction is associated with the dissipation of motion energy on internal macroheterogeneities in a glacier: at places where fractures appear and, in the case of a block structure, at points of contact between individual blocks, on ice dams, ice bridges, etc. This mechanism seems to be dominant during rapid ice flow in highly fractured glaciers.

The process was previously examined on the surging Medvezhiy Glacier in the central Pamir Mountains. On Medvezhiy Glacier the above phenomenon was particularly distinctive because of anomalously high ice-movement velocities during a calm period; that is, between consecutive surges (ice velocity amounted to 3–5 m d⁻¹) and because of the block structure of the glacier against the background of its fracturing. Simple measurements of “instant” velocities of a large number of bench marks on the glacier surface, located across the middle flow course of the glacier, established that velocities at all points are of a fluctuating character, which outwardly resemble velocity fluctuations seen in turbulent liquid flow. It turns out that deviations of the “instant” velocity from the average velocity can amount to more than 50% of the latter. Using data obtained from periodic recordings of the “instant” velocity for fixed pairs of bench marks, which were located at different distances (so that it was possible to evaluate the difference in instant velocities), a structural function was constructed. This structural function meets the well-known, so-called Kholmogorov two-thirds law For local isotropic turbulence. This indicates that, as with turbulence, the energy of middle movement is transferred along a cascade of ice conglomerates of decreasing size down to the smallest, limited only by the dimensions of fundamental blocks. At the points of contact at dams and junctions, motion energy dissipates into heat. It is postulated that this phenomenon may have a much broader application; it may also be characteristic of the movement of any fractured mass of solid material.

These findings may be used to formulate a theory which describes the mechanism of auto-oscillations in a surging glacier. According to this theory, the pseudoturbulent character of internal friction during a quiescent period (between surges) is one of the factors which prevents the appearance of a surge as long as possible, thus ensuring the growth of a new active glacier after each ice catastrophe.

These studies also conclude research on the general concept of auto-oscillations of mountain glaciers — the so-called surging glaciers, originally formulated in the course of research on the cause of a surge on Medvezhiy Glacier in 1963. This concept was subsequently adopted and is now widely used in glaciology.