A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale caused an eruption that reduced the elevation of the mountain 's summit from to , replacing it with a wide horseshoe-shaped crater .
A massive debris avalanche was triggered by an earthquake measuring 5.1 on the Richter scale . This caused the eruption , which reduced the height of the mountain 's summit from 9677 ft to 8365 ft and replacing it with a 1 mi wide horseshoe-shaped crater .