This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
A rather extensive forest became part of a directed blast deposit: that’s the summary. One moment, you’re a green and pleasant home for much of the local wildlife; the next, you’ve been rudely ripped apart and incorporated within a bunch of rock and ash by a volcano having a bad turn. So it goes.
When Rick Waitt traced the fate of Mount St. Helens’s magnificent forests, he found they’d had quite the adventure (aside from being knocked flat, bruised, battered, buried, and burnt).
Within the down-timber zone, it was clear some rather spectacular force had been applied. It wasn’t piddly little wood fragments and needles that became deposits, but entire tree trunks. Whole limbs had been ripped off, splintered, and subsequently dumped. The heavier bits, as heavy bits tend to do, remained close to the ground as the blast carried them along. As the flow lost energy, the heavy bits of layer A1, including its compliment of ex-trees, settled out first, fining upward as the deposit accumulated. Mind you, when I say “fined upward,” I don’t mean they got all demure and small, even close to the volcano. No, the ex-tree bits in subsequent layers within layer A2 and the pieces that landed atop layer A3 were as mind-blowingly large as 75 centimeters (29.5 inches). Not only that, but the way they landed show they were first torn loose by that erosive front of the blast, then heaved high in the air by the following phase, held airborne by convection, then unceremoniously dumped moments later.
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Warner Bros., I think, could have animated that sequence in the tradition of Wiley E. Coyote to fine effect.
Other branches, pine cones, and bits ripped from the unfortunate forest were light enough to continue traveling. They sailed the volcanic winds even beyond the boundary of layer A2, past the devastated area, and came to rest in a bed of silty layer A3, then were covered with a blanket of the following air-fall deposits left by the central eruption column. Some of those fragments were as long as 15 centimeters (6 inches). Imagine how much force it requires to take pieces of wood half the length of a school ruler and keep them in the air for twenty minutes or more.
Yeah.
Mixed up in all that were smaller remains, a mulch of fir needles, splinters, and twigs. In most areas, they can be found in all three layers, but to the north the energy of the blast was so ferocious it wouldn’t let them settle out until layer A3 did. Almost everything was burnt black, no matter where it landed, showing it all got seared before coming to rest. Only the needles and branches flying through the southern edge of the east side of the blast managed to come out without a thorough scorching, showing the blast cloud wasn’t so hot there. Still fast and furious enough to rip trees apart and turn them from biology into geology, though.
Thus ends the story of The Forest that Was. From here on, our relationship with the blast deposits will get decidedly rocky.
References:
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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