Eyewitness accounts, photographs, and instrument 
records have been used to piece together the initial 
sequence of the May 18, 1980, eruption of Mount St. 
Helens, the youngest and most active Cascade volcano. 
A huge bulge formed rapidly, high on the north face of 
the mountain's volcanic cone, in the weeks prior to May 
18. Then, at 8:32 a.m. on that day, an earthquake struck 
the mountain, triggering an avalanche on the north face 
that is now thought to be the largest ever witnessed by 
humans. Superheated groundwater close to the magma 
flashed into steam, resulting in a lateral explosion that 
pulverized rock and trees and sent a hurricane-force, 
hot-gas-propelled bolt of ash off the north face and 
across the Toutle River Valley to the north and west. 
Temperatures in this inferno were estimated to exceed 
900° F. Comparable to a 400 megaton nuclear blast, the 
explosion blew down trees in a 160° arc up to fourteen 
miles north of the crater and totally devastated a 
somewhat smaller “blast zone.” Seconds later, overlying 
rocks were incorporated into a high-velocity debris flow, 
driven by rapidly melting glacial ice. On the western 
flank of the mountain, this chocolate-colored mass 
swept down the upper Toutle River Valley, eventually 
forming a mudflow that swept the entire drainage area. 
To the north, a lobe of this hot debris flow crashed into 
Spirit Lake while another swept over 500-foot-high 
Coldwater Ridge, removing all life in its path. As the 
summit of the mountain collapsed, two vertical columns 
of ash-laden gas and steam were injected more than 
65,000 feet into the air. This ash was eventually 
deposited, in layers up to five inches thick, over 49 
percent of Washington State and beyond. Close to the 
mountain, ash deposits were less thick, but they fell wet, 
forming a sticky goo on all surfaces. The plume of 
blasted ash also removed much of the remaining summit 
and lowered the peak from 9,677 feet to about 8,400 
feet. As a consequence, the feeding zone of all glaciers 
has disappeared. Shortly after these events and 
continuing throughout the next day, an indeterminate 
number of pyroclastic flows, or nuees ardentes (hot, gas- 
charged avalanches of fluidized rock fragments ), were 
ejected from the crater, so that much of the upper debris 
flow was covered with more than seventy feet of finely 
powdered ash and pumice. 
The volcano remains active, with frequent small, 
harmonic tremors and occasional bursts of steam and 
ash. Sizable eruptions, with their attendant small 
pyroclastic and pumice flows, occurred on May 25, June 
12, July 22, August 7 , October 17 and 18, and December 
27 of last year. Future eruptions are expected, but most 
geologists believe that another large eruption is not 
likely. 
THE ERUPTION 
■ ■ 
Blast zone 
TREE BLOWDOWN 
mam 
MUDFLOW 
■m 
LANDSLIDE AND DEBRIS FLOW 
PYROCLASTIC FLOW 
FLOOD 
40 
