Figure 16.20(a) In a rock avalanche, large masses of broken rocky material flow, rather than slide, downslope at high velocity. (b) The two rock avalanches shown in the photo were triggered by the November 3, 2002, earthquake along the Denali fault in Alaska. The rock avalanches traveled down south-facing mountains, moved across the 1.5-mile-wide Black Rapids Glacier, and flowed partway up the opposite slope.