On the Mechanics of Surging Glaciers

Abstract

ABSTRACT. Observations of surging glaciers indicate that the flow regime is one dominated by extensional flow. The stress state has substantial longitudinal deviatoric stress. This regime is very different from the conventional model for glacier dynamics which is dominated by shearing flow. In addition, the initiation of surging often involves a compression front which travels down the glacier. The compression front seems to divide an up-stream region of high drag at the base of the glacier from one of low drag which allows the rapid sliding. We develop a framework for the mechanics of glaciers undergoing surging. Relevant issues are the extensional and compressional flows, high longitudinal deviatoric stress, and the stress state near the basal discontinuity. We find that some of the down-slope component of glacier weight is borne by longitudinal stress in the rapidly sliding region. This stress thrusts against the slowly moving parts of the glacier. We hypothesize that this effect causes the rapidly sliding part to spread and causes the compression front to travel down the glacier. A criterion for spreading of the rapidly sliding part is developed. The mechanics outlined above are used to develop a highly idealized model for glacier surging. We propose that regions of low drag are relatively common features of glaciers. The surge initiates when conditions are met which allow the surge nucleus to spread. The rapidly sliding region of low drag spreads to a large part of the glacier. Surging ends when the low-drag conditions terminate. Because of the changed state of the glacier, surge nuclei are now stable against spreading. Several years of rebuilding must occur before nuclei are once more unstable. Calculations are performed for the evolution of the shape of Medvezhy Glacier during the surge of 1963. We find a remarkable similarity between the data and our computations. RESUME. Sur le mecanisme des avances catastrophiques.

Surge dynamics in the Nathorstbreen glacier system, Svalbard

“…Iken, 1981;Bartholomaus et al, 2008). A surge nucleus is formed and an initial drawdown occurs and (Budd, 1975;McMeeking and Johnson, 1986). The effect of the frictional heat will be most important when sliding and basal drag are intermediate (Raymond, 2000).…”

Spatiotemporal dynamics of ice streams due to a triple-valued sliding law

“…Multivalued sliding laws in which the velocity is double or triple valued function of the bottom stress (Lliboutry 1968;Fowler 1986;Lliboutry 1987) were supported by different detailed physical mechanisms (Iken 1981;Fowler 1987a;Fowler & Johnson 1995;Schoof 2005;Gagliardini et al 2007). Quasi-periodic oscillatory motion of ice referred to as a surge (Raymond 1987) was explained by a triple-valued sliding law, in which bottom stress increases with velocity at high velocities (Lliboutry 1969;Hutter 1982a, b;McMeeking & Johnson 1986;Fowler 1986Fowler , 1987bFowler , 1989Greenberg & Shyong 1990;Fowler & Johnson 1995Fowler & Schiavi 1998). Fowler & Johnson (1996) also examined the effect of the possible instability in a laterally extensive flow due to a triple-valued sliding law.…”

Some Observations on a Recent Surge of Peters Glacier, Alaska, U.S.A.

“…The mixed longitudinal and transverse crevassing behind the propagating surge front indicates a changing stress field, with a strong compression as the front arrives and extension as it passes, as described in the model calculations by McMeeking and Johnson (1986) and T . Pfeffer (personal communication) .…”

“…Other theoretical ideas on surge-front propagation have been developed by McMeeking and Johnson (1986). In order to understand fully the surge phenomenon in its generality, the ideas developed in these studies must be tested using data derived from measurements on other glacier surges.…”