Large ice-contact slope movements: glacial buttressing, deformation and erosion


Correspondence to: S. T. McColl, Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. E-mail:


Glaciers and slope movements may act simultaneously to erode and modify glaciated slopes. Undercutting by glaciers can destabilize slopes but the extent to which slope failure may progress prior to subsequent glacier withdrawal has not hitherto been considered. The traditional view has been that the buttressing effect of ice prevents slope movement. The problem with this view is that ice is one-third the density of rock and flows under low applied stress. Consequently, failed slopes may move into the glacier if they exert a stress in excess of the resistance provided by the glacier. Slope movement rate depends on ice rheology and other factors influencing driving and resisting stresses. Simple viscous equations are used to investigate these variables. The equations predict that small (<125 000 m3) ice-contact rockslides can deform ice at several mm/year, increasing to several m/year for very large (>108 m3) rockslides. To test these estimates, field evidence is presented of slope movements in glaciated valleys of New Zealand; narrowing or squeezing of glaciers adjacent to unstable rock slopes is demonstrated and considered to be the result of slope movement. For one site, geomorphic mapping and slope movement monitoring data show that movement rates are of similar order of magnitude to those predicted by the viscous equations; closer agreement could be achieved with the application of modelling techniques that can more realistically model the complex slope geometries and stability factors encountered, or by obtaining additional empirical data to calibrate the models. This research implies that, while the concept of glacial debuttressing – the reduction of slope support from withdrawal of glaciers – is valid, complete debuttressing is not a prerequisite for the movement of ice-contact rock slopes. These slope movements may contribute to the erosional processes of glaciers and the evolution of glaciated slopes in a previously unrecognized way. Copyright © 2012 John Wiley & Sons, Ltd.