Early last week, around 45 megatonnes of ice and rock plunged down the southeast flank of Mount Steele in Canada's Yukon Territory. The avalanche, which occurred in a remote and unpopulated area, was so large that it was initially detected by earthquake seismometers.
The Mount Steele avalanche happened on October 12 and was detected by Colin Stark and Goram Ekstrom of Columbia University using the global seismic network, a digital seismic network of over 150 stations that provides free, real-time data through the IRIS DMC. The avalanche was later confirmed by Landsat 8 satellite images, which shows a huge chunk of rock and ice displaced from the mountainside.
As reported at the AGU's Landslide Blog, the landslide was enormous. An estimated mass of about 60 million tonnes — the equivalent of 700 aircraft carriers — careened down the mountain over the course of 110 seconds. That's about 24 million cubic meters in terms of volume. The peak velocity of the avalanche is estimated at about 60 m/sec, which is about 220 km/h or 137 mph.
Back in 2007, geologists reported another avalanche on Mt. Steele. Here's an image from the paper:
Top image by NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey.