MORAINE AND VALLEY WALL COLLAPSE DUE TO RAPID DEGLACIATION IN MOUNT-COOK-NATIONAL-PARK, NEW-ZEALAND

The Tasman Glacier and its tributaries have been down-wasting since the 1880s. Until relatively recently reduction of ice mass had little effect on glacial valley topography but, beginning in the early 1960s, lateral moraines and even the bedrock valley walls began to undergo slope failure. The precursors to failure included tension cracks and the development of upslope facing fault scarps. The slope failure then rapidly progressed to widespread bedrock debris slides, topples, and frequent debris avalanches. Mountaineering huts and long established trails eithe have been moved or demolished as a result of this slope failure. The activation of all these slope failures is due directly to the rapid down-wasting of the valley glaciers. It is estimated that the Tasman ice surface is lowering 3.5 +/- 0.5 m annually. This rapid withdrawal of ice removes the lateral support of the valley walls which, as a consequence, are re-equilibrating, via slope failure, through periodic adjustments within lateral moraines and bedrock. Adjustments of moraines and valley walls is sporadic and appears to be associated with spring thaws and wet storms. This study indicates that dry moraine walls will become metastable and will re-equilibrate back to a lower relief via slope failure when the vertical relief between the moraine crest and glacial ice surface exceeds 129 +/- 10 m. Based on this information it is possible to predict when failure will likely occur on certain moraines.