In memory of Dr. Harry Glicken, 1958-1991.
Eruptions seem like simple matters: pressure builds, something goes boom, lots of stuff comes out. But that's not the story of every volcanic eruption, and it doesn't capture the complexity by half. Pressure was building within Mount St. Helens. It had been booming, and promised a bigger boom, and delivered on that promise - but not in the way anyone expected.
Dr. David Johnston and his colleagues had expected the bulge to come down eventually - things that steep aren't stable - but no one knew just how big that fall would be, or that it would only be the beginning of a complicated, cataclysmic chain of events. It would take painstaking geologic detective work on and around a still-smoldering volcano and some fortunate photos to piece the sequence together.
It began with an earthquake, an insistent 5.1 magnitude shake at 8:32:11 am. Jolting an already overstressed mountain causes issues: on St. Helens that morning, the heavily fractured summit split dramatically. In less than ten seconds, cracks raced one and a half kilometers (nearly a mile) across the apex of the bulge. This was Act I in a three-act landslide that would become the largest in volume in recorded history.
Slide I, as Harry Glicken, Barry Voight and their colleagues named it, took the face off the mountain. It included broken blocks of old basalt and andesite flows, along with loose volcanic breccia, that had comprised the pre-eruption edifice. It took down Goat Rocks dome, and incorporated a chunk of the dacite summit dome. It hurtled down the mountainside at a speed of 50 meters per second (over 160 feet/second), accelerating as it went. Twenty-six seconds after it started, it had slid 700 meters (about 2,300 feet) down the mountain. As it fell, it ripped the confining pressure off the cryptodome that had caused all that dramatic bulging and the phreatic fireworks.
As slide I plunged, the summit area continued failing behind the brand-new, 600 meter (nearly 2,000 foot) landslide scarp. Slide II took down the rest of the summit north of the graben; it sliced deep, excising most of the rest of the summit dome and appreciable portions of modern St. Helens, and ripped out part of her ancestral heart. In the next eleven seconds, as the two slides moved downslope almost as one unit and stripped the cover off the magma of the cryptodome beneath, pressurized gasses found themselves free to expand. Water that had been heated to high temperatures by hot, fresh magma flashed to steam. Gasses in the magma came out of solution. And with the face of the mountain gone, steam and gas emerged sideways: the lateral blast had begun.
Those gasses propelled the slides: they were now screaming downward at speeds of 70-80 meters per second (230-260 feet/s), and covered the next 700 meters (2,297 feet) in 11 seconds. Portions of Forsyth Glacier broke free, cascading over Sugar Bowl dome. The remains of the mountaintop behind the slide II scarp came apart. We'll never know the precise details of how it failed - blast clouds now shielded it from view - but we know this slide III contained the remains of St. Helens's summit, including the hydrothermally-altered base of her summit dome. Brand-new dacite from the cryptodome mixed in with slides II and III.
2.5 cubic kilometers (
.6 cubic miles) of Mount St. Helens thundered in to the valleys below.
As it hit the stream-carved topography below, the debris avalanche divided. One lobe leapt a 380 meter (1,247 foot) ridge, filling South Coldwater Creek's valley with up to 195 meters (639 feet) of hummocky debris. It carried blocks of basalt and andesite up to 100 meters (328 feet) across. Pause for a moment to think of the kind of force it takes to transport massive rock that far from its source.
Another lobe slammed into Spirit Lake. Its speed and bulk sent a catastrophic water wave up the ridges bordering the lake, mowing down any trees not felled in the blast for 260 meters (853 feet) up the slopes. Erosion happened in an instant as the lake water sloshed back and poured over the avalanche deposits. At the end, the lake stood 60 meters (197 feet) above its former level, choked with debris, covered in a raft of logs, a shadow of its former self.
The bulk of the avalanche headed for the North Fork Toutle River valley. It carried chunks of rock up to 170 meters (558 feet) across, and it didn't stop until it had gone over 20 kilometers (12.4 miles) down the valley. It plowed up trees and hauled them along, leaving a distal deposit of chaotic logs, splinters, plowed-up soil, and other detritus at its terminus. Fragments of ripped-apart glaciers up to 12 meters (39 feet) across, were buried within. Debris up to 150 meters (492 feet) deep buried the river.
Other units of the debris avalanche choked tributary valleys and clung to the remnants of the mountain. In the end, it buried an area of almost 39 square kilometers (15 square miles), leaving a lunar landscape of bumpy, jumbled rock, ice and dirt.
When we think of avalanches and rock slides, we think of ice and snow, chilly stones, cool earth. We don't expect intense heat. But this wasn't an ordinary debris avalanche: it wasn't just glaciers and old, cold rock coming down, but parts of the cryptodome. And that dome was hot - parts of its dacite ended up embedded in the avalanche deposit at temperatures above 600C (1,100F). The avalanche itself was heated by magmatic gasses and steam to temperatures approaching 100C (212F). Slides II and III, especially, were toasty - they incorporated appreciable bits of the cryptodome and the hydrothermal system that had developed around it, and they'd gotten mixed up with the blast cloud. They became large portions of the North Fork and Spirit Lake lobes. Measurements of the temperatures of the North Fork lobe ranged from 98C (208F) close to the mountain to 68C (154F) at its end. This smeared-out temperature profile wasn't an effect of cooling in transit - the avalanche moved too fast for that, and there wasn't enough mixing with surface water to cool it so drastically, so it's likely hotter material from deeper within the mountain didn't travel quite so far.
But some hot material was about to travel very far indeed...
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.
*Edited to correct unit conversion, with thanks to Lockwood DeWitt.