Volcano Hazards - 1 of 8
Volcano Hazards FAQs - 8 Found
On May 18, 1980, at Mount St. Helens, data shows that an estimated 7-20 seconds elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles per hour. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.