Answer:
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.
