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

Abstract

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 m 3 ) ice-contact rockslides can deform ice at several mm/year, increasing to several m/year for very large (>10 8 m 3 ) 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.

Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

“…The relationship between bedrock slope instability and glacier retreat is well known (Jarman, 2006;McColl, 2012;McColl and Davies, 2012) and recent research in the Alps has highlighted the effects of permafrost degradation associated with bedrock slopes (Gruber et al, 2004;Gruber and Haeberli, 2007;Deline et al, 2015). Development of Smørbotn 7 is likely to have proceeded rapidly following deglaciation and to have been triggered by debuttressing of the bedrock slopes below the ice-sheet limit at~1500 m a.s.l.…”

Rapid decline of snow and ice in the tropical Andes – Impacts, uncertainties and challenges ahead

“…Physical process understanding and modeling, and field evidence from the past and present suggest that periods of warming and associated glacier shrinkage and permafrost degradation result in decreasing slope stability (Gruber and Haeberli, 2007;Huggel et al, 2012;McColl and Davies, 2013). In the tropical Andes no systematic analysis concerning observed or projected slope stability exists and little is known on the related impacts of climate change on populations and infrastructure in the region.…”

Two early Holocene rock avalanches in the Bernese Alps (Rinderhorn, Switzerland)

“…Moreover, the efficacy of glacial buttressing to provide significant lateral support to adjacent rock slopes has been called into question because of the ductile behavior of ice over long time scales (McColl et al, 2010;McColl and Davies, 2013). Therefore, debuttressing of valley slopes during glacial retreat is increasingly regarded as an unlikely predominant trigger for large paraglacial rock slope failures.…”