ROCK AVALANCHES ONTO GLACIERS IN NORTHWEST NORTH AMERICA

Rock avalanches onto glaciers are a common occurrence in the mountains of northwest North America (Washington, British Columbia, Yukon Territory, and Alaska). Our inventory shows that 40 events with volumes greater than ca. 1 Mm³ have occurred in the region in the 62 yr period 1947-2009, the largest being the 2007 Mount Steele rock avalanche, Yukon Territory (ca. 80 Mm³). They are of interest since 1) they commonly afford an excellent opportunity to examine rock avalanche emplacement mechanisms without either substantial topographic interference in runout or additional earth materials being entrained in the debris and 2) they are an important geomorphic process in glacierised volcanic centres in this part of the Pacific Rim. The path geometry of rock avalanches onto glaciers is characterised by longer runout distances and lower travel angles compared with those of rock avalanches on non-glacier surfaces of a similar volume reflecting the lower friction of glacier surfaces. We have used remote sensing to characterise the fragmentation of initial rock masses in a number of rock avalanches. In the 1997 Mount Munday (est. volume 3.2 Mm³) case for example 75 % of the source rock mass was fragmented during travel to a size smaller than 4.6 m³. Simulation of velocities indicate that velocity decay is a simple function of debris spreading on open glacier surfaces. Rock avalanches make a major contribution to the denudation of glacierised watersheds. 20 of the 40 events in our inventory were triggered by three earthquakes in 1964, 1979, and 2002. Of interest is the relative contribution to denudation of earthquake triggered events compared to background occurrence frequencies. Finally, we explore the magnitude and frequency relations of rock avalanches onto glaciers and find they are characterised by higher temporal frequencies in the low magnitude range (<100 M m³) than events on non-glacier surfaces.