But it turns out that the force of a directed volcanic blast is very good at eroding timber. There's really no other good way to describe what a hot, incredibly fast, powerful flow of gas and debris does to forests. It's not just that it knocks trees down: it fragments and drags them, incorporates them into itself, doing to them what water does to earth and stones, and leaving behind patterns that can be read by geologists as they determine what the directed blast did.
Until the summer dry season comes, things in the Pacific Northwest are perpetually wet. Edward Smith and his companions, camped 18 kilometers (11 miles) north of Mount St. Helens, had set their tent on its side to dry out. At 8:32 am, an unusually strong wind gusted, again, and again: the tent tumbled, and a sound like a trio of rifle shots sounded. The surrounding pressure seemingly changed; they found themselves forced to the ground. And then the leading, black edge of the blast cloud soared overhead. Chunks of juvenile gray dacite fell like hail, some as large as golf balls. In the rock rain, Edward and his companions watched the cloud rush to the north before abruptly pulling back to the south. Blue sky appeared, briefly, though the black cloud never completely left his sight. Then the interlude ended. The blackness roared back. A cedar fell; seconds later, he told geologists, "there were no trees left." They tumbled in eerie silence, in eerie stillness, no sensation of the blast that ripped them down. "Whatever happened," Edward said, "it happened over our head."
"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."
Brain tweaking -
And just to get you through the day -
Few talked to the trees as extensively as Richard Waitt. He was investigating the blast deposits, and found the trees to be quite helpful. His work in the field led him to identify three primary layers: A1, the base, was pretty full of gravel. A2, the next level up, was a coarse sand, and the final, A3, a fine air-fall sand.** Throughout those layers were trees and bits thereof, and he queried them closely to figure out the progress of the lateral blast and how it had left its deposits.
Geologists: "Hey, boss person, we need to order vehicle parts and then destroy them. For science!"
Boss person: "Ummm... okay."
The thing is, things happen to vehicles when they're caught up in a directed blast. What the volcano did to them can tell us a lot about what was taking place inside that blast cloud. Vehicles in the blast zone at Mount St. Helens sustained all sorts of damage.
It did not look so calm as it sounded.
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