Countdown to the Mt. St. Helens Eruption: the Suspense!


For anyone who loves a good fireworks show, there's something about the build, something about knowing that for all of the spectacle you've witnessed so far, the finale is going to be an incredible, heart-thumping display of explosive power.

That must have been the feeling going through scientists' minds in the two-month run up to Mt. St. Helens' historic eruption on May 18th, 1980. To celebrate the 30th anniversary of the event, the USGS has captured every moment of suspense and anticipation in an excellent feature on their website (the image above is of the "bulge" — that giant thing on the right that looks like a mountain unto itself — filled with magma and fit to burst on April 27,1980).

You can read daily logs starting with March 15th, 1980 when the snow-capped, picturesque St. Helens first awoke with a series of small earthquakes. At the time, seismologists had no sense of what they were seeing — seismic monitoring in the area was new, and these were only a few little tremors.

Then, on March 20th the mountain rumbled with a magnitude 4.2 quake. Avalanches slid down its flanks, and the U.S. Forest Service closed St. Helens above the treeline to protect visitors. No one had ever seen anything like this before, but seismologists were still puzzled, and the big quake appeared isolated.

A normal sequence of diminishing aftershocks were to be expected, but over the weekend of Mar 22-24 quake activity exploded, with dozens of tremors registering over magnitude 3.0, and several over 4.0. It became clear the volcano was threatening to erupt.

On the 27th, a small explosion flung ash into the air (left), and quakes continued. Scientists scrambled to deploy seismometers to record the ongoing earthquakes, and the FAA restricted flights over the volcano. The local power company partially drained nearby Spirit Lake to make way for flooding and mudflows. Roads were closed, hundreds of people evacuated.

The wait was on.

Over the next few weeks, a crater developed on the summit, sporadic small eruptions were common, and all the while a large, ominous bulge grew on the north side of the mountain like a giant volcanic pimple.

The bulge grew 5 feet a day, the mountain's summit sank and its flanks cracked as a cauldron of

pressurized magma heaved northward. By May 12 parts of the mountain had pushed up 450 feet higher than before the first eruption.

On may 15th, a final bit of ash erupted, and then all went quiet. Only the bulge kept growing, and the earthquakes kept coming.

On May 17th, there was this entry in the daily log:

May 17th – The mountain remained quiet. Seismic activity reached the lowest level for May, with only 18 earthquakes larger than 3.0 recorded (including 6 larger than magnitude 4.0). In response to pressure from property owners and with the Governor's consent, law enforcement officials escorted about 50 carloads of property owners into the Red Zone to retrieve possessions. Those who entered were required to sign liability waivers at the roadblocks and to leave by nightfall. Authorities agreed to allow another caravan of property owners in at 10:00 a.m. the following morning.

That afternoon, a photo was taken of geologist Dave Johnston, as he took up a monitoring post 5 miles away from the mountain, seemingly a safe distance:

On the morning of May 18, Mt. St. Helens finally exploded, more powerfully than any expected. Fifty-seven people were killed in the blast, which registered a magnitude 5 on the Volcanic Explosivity Index. Johnston was never seen or heard from again.

It was one of the most significant eruptions of the 20th century, both for what we learned scientifically, and because it brought the awesome power of a volcanic eruption into the living rooms of everyone in America for the first time.

Thirty years later, it's worth going back for a second look.

Images: USGS, Peter Lipman; Dan Miller; Harry Glicken

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