A few seconds after the beginning of the directed blast, life within roughly ten kilometers (6.2 miles) of Mount St. Helens within the blast zone was about to be extinguished.
Oblique aerial view of the laterally blasted material deposited in the North Fork Toutle River valley approximately seven miles away from Mount St. Helens. Note the dense ash in the air, the ash-covered blast deposits, and the brown mudflow deposits. Cowlitz County, Washington. May 19, 1980. Image courtesy USGS.
?Directed blasts,? Rick Hoblitt, Dan Miller, and James Vallance wrote in their 1981 paper on the blast deposits, ?typically devastate large areas? and kill essentially all above-ground life within these areas.? Human and animal, arthropod and avian, tree, bush and flower, all perished. Between its kinetic and heat energy, the directed blast released the equivalent of 24 megatons of energy in a few short moments. This is one megaton short of the theoretical yield of the largest hydrogen bombs the United States ever created; the first nuclear bombs to destroy a city were only 15 and 21 kilotons, respectively ? orders of magnitude smaller. Instruments measuring the blast saw ?amplitude variations comparable to those caused by detonation of nuclear devices in the 1-10 mt [megaton] range.?
There are words for eruptions like this. They belong to the Volcanic Explosivity Index. The qualitative description for Mount St. Helens?s climactic eruption is ?paroxysmal,? but it achieved a mere ?very large? as a descriptor. Some versions of the VEI become speechless at that point; others scale up to ?huge,? ?humongous? and ?indescribable.? Mount St. Helens was just above average.
In the figure, the volumes of several past explosive eruptions and the corresponding VEI are shown. Numbers in parentheses represent total volume of erupted pyroclastic material (tephra, volcanic ash, and pyroclastic flows) for selected eruptions; the volumes are for uncompacted deposits. Each step increase represents a ten fold increase in the volume of erupted pyroclastic material. Image and caption courtesy USGS.
What made her so wildly destructive was the direction that energy was released in. She didn?t just explode up into empty air; when her north face collapsed, it allowed the confined energy of the cryptodome to escape sideways, channeling that release along the ground, where the people and plants and animals were.
The initial blast accelerated from around 50-100 meters per second (112-224mph) at Sugar Bowl dome to at least 156 m/s (349mph) by the time it destroyed seismic station SOS four kilometers (2.5 miles) away. Leaving the high pressure of the vent and encountering the relatively low pressure of the open air, it spread out, covering more than 180?, bearing down on people to the west and east in the South and North Fork Toutle River drainages.
Map showing the area devastated in the directed blast, and locations of witnesses interviewed by USGS geologists. Modified after Figure 35, Rosenbaum and Waitt 1981, in USGS Professional Paper 1250. Image courtesy USGS.
Canadian geology student Catherine Hickson and her husband Paul watched the eruption unfold from fifteen kilometers (9.3 miles) east of the summit as their terrified dogs cowered beside them. At first, the cloud raced smoothly down the crumbled north face of the mountain; then it began billowing eastward, toward them, overtaking the avalanche. Watching it, realizing it would plow right through the river valley between them and the mountain, Catherine thought of two words: ?nu?e ardente,? glowing cloud. An eruption cloud like this had raced down from Mount Pel?e and reduced the thriving seaport of Martinique to ruins, killing nearly thirty thousand; one had descended upon Pompeii from Mount Vesuvius and left no survivors. She knew a car stood no chance. ?So we fled,? she said simply in a letter she wrote to a friend after the eruption.
As her husband sped down a narrow logging road, dodging frantic elk and falling rocks, she snapped a few pictures of the cloud bearing down on them.
Nine kilometers (5.6 miles) west of the summit, Joe Sullivan, his wife, brother-in-law, and friend Mark Dahl fled west down the South Fork Toutle River as the enormous black eruption cloud plowed over the hills. They were speeding at 121-129 kilometers per hour (75-80 mph,) but after less than four minutes, the ash-laden cloud billowed over a ridge ahead of them to the northwest ? it had outraced them down the North Fork Toutle River valley.
Behind them, only eight kilometers (5 miles) west of the exploding volcano, Dave Crockett found himself trapped.
All of them were in line of the blast.
In this schematic of the directed blast, the blast is just beginning to overtake the debris avalanche. It will rapidly accelerate to at least 562 kilometers per hour (349 mph) before slowing to a more sedate 100 kph (62 mph) 25km (15.5 miles) away. Some estimates place its top speed at 650 kph (404 mph). Image courtesy USGS.
The pyroclastic density current of the directed blast would later be described by Hoblitt, Miller and Vallance as ?a subhorizontal fountaining mass of debris and expanding gasses.? Those gasses ripped and plucked existing rock from the debris avalanche and the ground, mixed that rock with the hot young dacite from the cryptodome, and hurled the whole mass along at incredible speeds. Where the cloud passed over the debris avalanche, it entrained more cool material, which reduced its temperature, but to the east and west, it had less loose rock to mix in and remained ferociously hot ? up to 300?C (572?F). Witnesses on Mounts Rainier and Adams watched it flow through valleys and over ridges, hugging the ground, ignoring topography. Where it passed, nothing was left standing. Trees were ripped down, blown away, many of them within 12 kilometers (7.5 miles) of the volcano reduced to splinters and mixed with the debris. Cars, trucks, and logging equipment were sandblasted, battered, scorched: the velocity of the blast cloud combined with its pummeling rocks and logs sometimes punched vehicles hard enough to move or flip them within all but a third of the downed timber zone.
People, animals, and plants caught within this part of the devastated area did not survive. In less than four minutes, a brilliantly sunny morning had turned into a searing, dark, fatal blizzard of stone, and they were gone, most of them asphyxiated by incandescent ash and gasses.
Near view Columbian photographer Reid Blackburn's car, buried four feet deep by the deposits left by the directed blast and subsequent ashfall. He was at Coldwater I, 10.5 kilometers (6.5 miles) northwest of Mount St. Helens's summit, and died in the blast. Image credit D. Dzurisin via USGS.
The atmosphere saved the Hicksons and others who had been in the path of the blast cloud to the east and west. As rarefaction (expansion) waves reflected off the atmosphere and became compression waves, the blast was deflected toward the north. It stopped just 3 kilometers (1.9 miles) short of the Hicksons; it missed Dave Crockett by perhaps half that distance. The vagaries of fluid dynamic had steered destruction away from those fortunate few.
But people to the northwest, north, and northeast were still in its path. Nineteen kilometers (11.8 miles) north, in the green river drainage, Al Brooks and his friends Dale and Leslie Davis snapped a photo as the blast cloud rose over a ridge north of Coldwater Creek. Seconds later, they would tell USGS geologists, ?it looked like that whole mountain range had just exploded.? Other witnesses to the north saw the wall of ash and rock, now a few thousand feet high, suddenly rise in front of them. And that blast cloud, ?a boiling mass of rock? just as high as you could see? was bearing directly on them.
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References:
Hickson, Catherine J. (2005): Mt. St. Helens: Surviving the Stone Wind. Vancouver, BC, Canada: Tricouni Press.
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.
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