The Contribution of Discontinuous Rock-Mass Failure to Glacier Erosion

Abstract

Geomechanical rock-mass properties control the response of bedrock to applied stresses and can be summarized in a linear Mohr-Coulomb equation, which defines the principal parameters determining failure. Nevertheless, in studying the erosion of bedrock by glacier ice, little attention has been paid to failure criteria though a coincidence of erosion all and forms with fracture systems at regional and local scales has been demonstrated. Few studies have analysed the precise nature of the fracture geometry, or proposed its mechanical impact in association with glacier ice.This investigation proposes that, since almost all bedrock possesses identifiable fracture systems, the properties of discontinuous rock mass (DRM) be regarded as defining primary conditions of stress and stability which are subsequently modified directly and indirectly by glacier ice. Consequent rock-mass failure modes are prescribed by discontinuity geometry and applied stresses, and evidence from North Wales confirms the validity of the theoretical treatment of rock-mass properties, and explains the accordance of landforms with structure.

Control of glacial quarrying by bedrock joints

“…Others have pointed to a predominance of bedrock joint sets with orientations systematically related to sliding direction (e.g., Trainer, 1973). On the other hand, many studies have emphasized the role of preglacial joints in controlling orientations of bounding surfaces of roches mountonnées and other erosional landforms (Addison, 1981;Gordon, 1981;Rastas and Seppälä, 1981;Laitakari and Aro, 1985;Rea and Whalley, 1996;Olvmo and Johansson, 2002;Krabbendam and Bradwell, 2011). Thus, existing field observations do not provide a clear picture of whether quarried surfaces generally result from subglacial crack growth.…”

Allometric development of glacial cirque form: Geological, relief and regional effects on the cirques of Wales

“…The importance of joints, faults and other planes of parting in cirque development was emphasized for example by Haynes (1968) and Addison (1981). In Snowdonia, the fracture network disregards lithological boundaries, but lithology controls fracture spacing and rock mass strength at smaller scales (Addison, 1981).…”

“…In Snowdonia, the fracture network disregards lithological boundaries, but lithology controls fracture spacing and rock mass strength at smaller scales (Addison, 1981). However, data are not available on a broad basis and for the present survey the units mapped by the British Geological Survey and others are used.…”

Medial Moraines of the Haut Glacier D’arolla, Valais, Switzerland: Debris Supply and Implications For Moraine Formation

“…The precise mechanics of such a shearing process are far from clear and, in any case, the extreme angularity of the contained fragments is not consistent with the observed morphology and characteristics of subglacially derived and transported c1asts (Boulton, 1978). The most likely mechanism to account for the debris bands is rock falls in the firn zone, resulting either from freeze-thaw weathering of supraglacial faces or discontinuous rock-mass failure (DRMF) of oversteepened sections of the valley head-wall (Addison, 1981). The resultant surface debris layers are then incorporated within the glacier by burial beneath annual layers of winter snow, and -as they are subsequently transported down-glacier -are transformed into a series of comparatively narrow bands parallel to the direction of glacier flow and dipping steeply inwards towards the centre line of the glacier.…”