Mount Saint Helens Erupts!

By Robert I. Tilling, Lyn Topinka, and Donald A. Swanson (USGS)

May 18, 1980, a Sunday, dawned bright and clear. At 7 A.M. Pacific Daylight Time (PDT), USGS volcanologist David A. Johnston, who had Saturday-night duty at an observation post about six miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. These measurements showed no change from the pattern of activity of the preceding month. Seismic data, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature readings revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later. About 20 seconds after 8:32 A.M. PDT, apparently in response to a magnitude 5.1 earthquake approximately one mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston.

Debris Avalanche

Although the triggering earthquake was of greater magnitude than any of the shocks recorded earlier at the volcano, it was not unusual in any other way. What happened within the next few seconds was described by geologists Keith and Dorothy Stoffel, who at the time were in a small plane over the volcano's summit. Among the events they witnessed, they

"noticed landsliding of rock and ice debris  in-ward into the crater... the south-facing wall of the north side of the main crater was especially  active. Within a matter of seconds, perhaps 15 seconds, the whole north side of the summit crater began to  move instantaneously. ... The nature of movement was eerie.... The entire mass began to ripple and  churn up, without moving laterally. Then the entire  north side of the summit began sliding to the north  along a deep-seated slide plane. I (Keith Stoffel) was amazed and excited with the realization that we were watching this landslide of unbelievable proportions. ... We took pictures of this slide sequence occurring, but before we could snap off more than a few pictures, a huge explosion blasted out of the detachment plane. We neither felt nor heard a thing, even though we were just east of the summit at this time."

Realizing their dangerous situation, the pilot put the plane into a steep dive to gain speed and thus was able to outrun the rapidly mushrooming eruption cloud that threatened to engulf them. The Stoffels were fortunate to escape, and other scientists were fortunate to have their eyewitness account to help unscramble the sequence and timing of the quick succession of events that initiated the May 18 eruption.

Warning Signs? It was hard not to notice the bulge on St. Helens in the months before the eruption. The increase of seismic activity was also a clue. Image courtesy of USGS.

The collapse of the north flank produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) 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 155 to 180 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about four miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. 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. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet.

Check here for comparisons to other 20th Century volcanoes.

Source: USGS "Eruptions of Mount St. Helens: Past, Present, and Future."

Share Your Memories!

What do you remember about Mount Saint Helens? Have you any compelling stories to share? Share your stories with the world! (We print the best stories right here!)

		Your Memories Shared!

"I remember very well where I was on May 18, 1980 - I was sitting on the river bank of the Clark Fork River in Missoula, Montana (not far from the seismic lab at UofM) thinking about Dorothy Stoffel because she and Keith were friends of mine - and it was Dorothy's birthday! I saw the huge black clouds coming in from the west and thought it was a thunderstorm when someone came from the college lab and said Mt. St. Helens had blown up. I had been trying to reach Dorothy all week by phone to wish her a happy birthday and was back at home trying once more after the ash had fallen on Missoula, when I saw Keith on the Nightly News! Was it any wonder she didn't answer her phone. I have lost touch with the Stoffels, been married and divorced and married again, moved, went to college and moved again but I was happy to see news of the symposium and that they are still alive and well. None of us will forget that day." --Jan

"I lived in the small town of Centralia, Wa. After going to the drive in & not getting home till 1 a.m. My baby boy woke up wanting a bottle. As it was pitch black outside, I turned on the kitchen light. the clock said 9:09 a.m. I just assumed the clock was broken, but as i fed my son I saw that the bedroom clock also said the same time. I knew immediately what had happened. I rushed to wake my husband as our dog was outside. We had to cover our mouths with bandanas as you could not breath. I consider myself very lucky that all of my family were in a safety zone." --Nicki

"My boyfriend and I decided it was time to leave L.A. and move to Washington in March 1980. The day we drove past Mt. St. Helen's was March 20, 1980 - the same day the mountain woke up... I have always felt a kinship with this mountain. On the day of the big blast we lived in Everett. We were awakened by the explosion, and should have saved the ash that collected on our car!" --Anonymous

"When this magnificent eruption occurred, I was in the process of coming into this world! Born one hour to the minute after MSH erupted, my name means "one who will rise again." As we all know, this is basically what Mt. St. Helens did, after being dormant for over 120 years! My entire life has been a series of metaphorical deaths and rebirths, so being born when the volcano erupted just cements this eruption as being a very strong personal symbol for me." --Stacie

"I was on the top of Silverstar Mountain when she blew. At first I felt a rumbling then I woke up, got out of the tent and saw a huge cloud of ash and rock rising into the sky." --Anonymous

"I had the television on as usual and i heard them talking about an eruption at MSH, and sure enough, i ran to my window, facing south, and i saw it with my own two eyes. a volcanic plume, 60,000ft in the air. Watching the volcano make history from so far away, yet, seeming so close at the same time. Seeing it drop from the 9th highest peak in WA to the 30th. Ash was falling, and i tell you, i'll never forget the day that Mt St Helens erupted, May 18th 1980. Around 8:30 when the first earthquake rumbled beneath the mountain to trigger the explosion, i was in WA, snug in my home." --Anonymous

" I remember that I was 10 years old at the time, playing in the front yard of our home in Spokane, WA. I remember looking up and seeing the dark clouds looming overhead, thinking to myself that it was going to rain, and my day of playing outside was ruined! After an hour or two, it was as dark as night, the ash that fell from the sky strongly resembled snow. We had to wear breathing masks so the ash would not disrupt our lungs. It was so eerie seeing the world so dark in the middle of the day. It must have built up an inch or two, or so it seemed like, and for years afterwards people collected the ash and sold it as memorabilia. " --Leo