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 9,677 ft ( 2,950 m ) to 8,365 ft ( 2,550 m ) , replacing it with a 1 mile ( 1.6 km ) wide horseshoe-shaped crater .
This caused the eruption , which reduced the height of the mountain 's summit from 9,677 ft ( 2,950 m ) to 8,365 ft ( 2,550 m ) and replacing it with a 1 mile ( 1.6 km ) wide horseshoe-shaped crater .