Rock avalanches that travel onto glaciers and related developments, Karakoram Himalaya, Inner Asia

Catastrophic rock slope failures in rugged terrain commonly generate rock avalanches. When these occur in glacier basins the extent of landslide run out and its emplacement geometry are affected by movement over ice. Substantial modifications of glacier activity and sedimentation can follow. Ice activity leads to rapid modification, transport and dispersal of landslide debris. The nature of these interactions is described from observations over 20 years at Bualtar Glacier in the Karakoram. In August 1986 rock avalanches descended onto the ablation zone of the glacier. Their moisture content and patterns of deposition were affected by travel over ice. Glacier movement increased sharply at and below the rock avalanche deposits and, within a few months, the glacier surged. A second surge occurred 2 years later. Major slope failures and debris flows were triggered beyond the ice margins, and ponded melt water led to small outburst floods. By 2005 the landslide material had been transported some 9 km, about one third of this distance in the surges. Eventually it was fully reworked to become less readily distinguishable from other heavy supraglacial debris. A large area of thickened ice persisted where the debris reduced ablation; a positive impact on mass balance equivalent to a 20% increase in annual accumulation. However, it occurred as a moving segment of the ablation zone. In 10 years it had, in effect, replaced the mass transferred in the surges. Data for the mid-1980s indicate the rock avalanches exceeded pre-existing supraglacial debris by roughly five times and, over a 30 year period, will equal almost 500 years of normal supraglacial transport to the glacier margins. Other impacts of the episode suggest a doubling of this contribution to denudation. Similar developments were observed at three other glaciers where recent rock avalanches occurred and at twenty-one prehistoric rock avalanches newly identified as having spread over glaciers. It seems inadequate to treat such events only as singular extremes, or the landslide and glacial processes separately. Here they are interpreted as contributing to'disturbance regime' landscapes, whose history reflects episodic and regionally specific responses to strong interference among different earth surface processes. They generate polygenetic deposits and landforms that are morphologically complex. Individual episodes involve decades to millennia; their numbers and repetition can over-ride or redirect geotectonic and climatic conditions. It is suggested they are major, but hitherto misread or neglected, elements of landform evolution in the Karakoram and, perhaps, in other high mountains. (C) 2008 Elsevier B.V. All rights reserved.