This morning I saw a short notice in the newspaper and followed it up. A huge chunk of mountain and overlying ice, starting near the summit of 11,200 foot Lituya Mountain in Alaska, came loose and spread itself 5.5 miles down a valley glacier with a gradient of only 13.7 degrees. Though no one witnessed it, there’s evidence of a huge air blast with a force estimated at 200 miles per hour. The landslide registered on seismographs as a 3.4 magnitude earthquake. http://www.nationalparkstraveler.com/2012/07/massive-landslide-coats-glacier-bay-national-parks-johns-hopkins-glacier-chocolate-frosting10200 and http://www.huffingtonpost.com/2012/07/13/glacier-bay-national-park-landslide_n_1669832.html?utm_hp_ref=green
The shoulder of the mountain where the slide started is at c. 11,000 feet and the head of the Johns Hopkins Glacier is at c. 4,400 feet–about a 6,600 foot drop for the top of the slide. A deep, 150-meter (450 feet) wide chunk of that long, steep mountainside/cliff gave way from top to bottom–a huge amount of rock and ice falling a long way, trapping air as it disintegrated and pushing more air ahead of it. When it hit the glacier below, its momentum and the air trapped within it sent it hurtling for five and a half miles along the glacier’s gently sloping floor.
There have been other big events like this, and there will be many more. It’s one of the ways that mountain ranges get worn down as they’re being pushed up.
In 1958, triggered by an earthquake, a 3,000 foot cliff peeled into Lituya Bay, creating a wave up to 1,700 feet high. http://geology.com/records/biggest-tsunami.shtml
The article includes maps, pictures, witness descriptions and two videos. Be sure to watch the videos. The one on the right describes the discovery of the trim line in the forest far above the water caused by an earlier wave (“What happened here?”. The video on the left is the account of a survivor of the 1958 wave, whose fishing boat was carried by the wave above the forest before being deposited back in the bay. It includes simulations of the wave and the experience of the survivors. Awesome.
This reminds me of the big rockfall in Yosemite Valley In 1996, from the cliff above Happy Isles. A huge slab peeled off near the top. The shape of the cliff launched it outward into thin air. When it hit the talus below after free-falling for 1,800 feet, imagine the whomp! The air blast devastated the forest in the area, knocking down big trees. http://en.wikipedia.org/wiki/1996_Yosemite_Valley_Landslide and http://gsabulletin.gsapubs.org/content/112/1/75.abstract
In 1991 the side of Mt. Cook in New Zealand gave way in a massive rock avalanche, with the top falling nine thousand feet to the Tasman Glacier http://www.teara.govt.nz/en/landslides/2/1 . In this video, climbers who were camped at the Plateau Hut 300 metres to the side describe the experience. http://www.youtube.com/watch?v=oSAmOhqvVZQ
This planet is an awesome, dynamic thing!