Analysis and Anatomy of Several High Latitude, High Elevation Ice-Rock Avalanches

Wednesday, 17 December 2014
Bruce Franklin Molnia, US Geological Survey, National Civil Applications, Reston, VA, United States and Kim Angeli, US Geological Survey, Eastern Geographic Science Center, Reston, VA, United States
Since 2005, we have examined six high latitude, high elevation, ice-rock avalanches. All are located in southern Alaska or adjacent Yukon Territory. Each was investigated by combinations of oblique airborne photography and space-based electro-optical and multi-spectral imagery. Three were subject to field analysis. The location, failure surface elevation, and date of each are: 1) Mt Steller-S [60.5o N, ~3100 m, 09-14-2005]; 2) Mt Steller-N [60.5o N, ~2975 m, late 2005-early 2006]; 3) Waxell Ridge [61.6o N, ~2620 m, late 2005-early 2006]; 4) Mt. Steele [61.6o N; 4640 m, 07-24-2007]; 5) Mt. Lituya [58.8o N, 2260 m, 06-11-2012]; and 6) Mt. LaPerouse [58.5o N, ~2750 m, 02-16-2014]).

All display: a) evidence of failed near-summit hanging glaciers and/or perennial ice and snow; b) exposed hydrological features such as moulins, conduits, and collapsed englacial stream channels in failed ice and snow margins; and c) fresh concave bedrock failure surfaces exposed following hanging glacier/ice/snow failure. At the three visited sites, the rock fragment component of the slide debris was dominated by smaller cobble sized-particles, with a general absence of large angular blocks. At two sites, visual evidence of liquid water exiting conduits or collapsed englacial stream channels persisted for months. None of the failures is correlated with a seismic event. Although all six failures originated at latitude/elevation locations where mean annual temperatures are below freezing, the physical evidence strongly supports the presence and involvement of liquid water/meltwater in triggering each event.

We propose the following explanation for the origin of these events. Abnormally warm summer temperatures or extreme winter rainfall events produced unusual volumes of water which saturated summit snow and ice and/or filled summit glacier channels and conduits with liquid water. Liquid water reached the frozen water/bedrock interface, destabilizing the contact. The fresh concave bedrock failure surfaces suggest that glacier beds were adhering to steep bedrock surfaces composed of a mélange of freeze/thaw shattered rock fragments held together by interstitial ice. When the mass of saturated glacier ice failed, the bedrock mélange also failed, resulting in the typically fine grained debris and fresh bedrock scarp depressions.