Mount St. Helens	"Godzilla's Attacking Babylon" September 22, 1999

On May 18, 1980. Mount St. Helens--called Louwala-Clough, or smoking mountain by some Northwest Coast Indians--exploded violently. It was the worst volcanic disaster in U.S. history. The following chronological outline of the eruption (adapted from Tilling, Topinka, and Swanson 1990), gives a good idea of the range of destructive events that we tend to lumped together in the word "eruption." What is really impressive--and frightening--is not the scale but the speed with which the action takes place: debris from a lateral blast traveling at 670 mph, an ash column rising 12 miles in ten minutes. Comparison of the Mount St. Helens eruption with Pliny the Younger's description of Vesuvius in A.D. 79 suggests the Pompeiians could have had it a lot worse.

March 20. A magnitude 4.2 earthquake

March 27. Begins spewing ash and steam; ash column rises to about 6,000 feet above the volcano; initial explosions form a 250-foot-wide crater within the larger, preexisting snow- and ice-filled summit crater, and new fractures break across the summit area.

Through April 21. Intermittently ejects ash and steam; craters enlarge and ultimately merge.

May 7. After lull, small steam-blast eruptions resume. Through mid-May about 10,000 earthquakes are recorded (concentrated in a small zone less than 1.6 miles directly beneath the bulge on the mountain's north flank).

May 12. Parts of the bulge near the summit are more than 450 feet higher than before the magma intrusion began.

May 18, 8:32 A.M. A magnitude 5.1 earthquake about 1 mile beneath the volcano collapses the bulged, unstable north flank.

About 10 seconds after the earthquake large blocks slide away, merging downslope into a gigantic debris avalanche, which moves northward at speeds of 155 to 180 miles an hour. The avalanche deposit--intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake--covers an area of about 24 square miles, advancing more than 13 miles down the North Fork of the Toutle River and filling the valley to an average depth of about 150 feet.

Sudden removal of the upper part of the volcano by the landslides triggers the almost instantaneous expansion (explosion) of high temperature-high pressure steam present in cracks and voids in the volcano and of gases dissolved in the magma, blowing out the side of the volcano in a blast heard hundreds of miles away in parts of British Columbia, Montana, Idaho, and northern California.

The blast's initial velocity of about 220 miles an hour quickly increases to about 670 miles an hour, approaching but not surpassing the speed of sound (735 mph).

The near-supersonic lateral blast, loaded with volcanic debris, causes widespread devastation as far as 19 miles from the volcano.

A strong, vertically directed explosion of ash and steam begins very shortly after the lateral blast. In less than ten minutes, the ash column reaches an altitude of more than 12 miles and begins expanding into a characteristic mushroom-shaped ash cloud. It reaches an altitude of 16 miles in 15 minutes. Near the volcano, the swirling ash particles in the atmosphere generate lightning, which starts many forest fires. Moving at an average speed of about 60 miles an hour, the cloud reaches Yakima by 9:45 A.M. and Spokane by 11:45 A.M., screening out nearly all sunlight. The eruption continues vigorously for more than nine hours, pumping ash into the atmosphere and feeding the drifting ash cloud. About 540 million tons of ash fall over an area of more than 22,000 square miles.

Destructive mudflows and debris flows begin within minutes, as hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melt snow and glacial ice on the upper slopes of the mountain (such flows are also called lahars). The largest and most destructive mudflows develop several hours later in the North Fork of the Toutle River, when the water-saturated parts of the massive debris avalanche deposits begin to slump and flow, ultimately dumping more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia Rivers. On the upper steep slopes of the volcano, the mudflows travel as fast as 90 mph, slowing to about three mph in the flatter and wider parts of the Toutle River drainage.

May 19. The eruption nearly stops; the ash cloud reaches the central United States (it is detected in the northeast two days later and some of the ash drifts around the globe within about two weeks).

Autopsies indicated that most of Mount St. Helens' 57 victims died by asphyxiation from inhaling hot volcanic ash.

© 1999 by the Archaeological Institute of America archive.archaeology.org/online/features/godzilla/helen.html