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The Cataclysm: “Stripped from the Proximal Forest”

The views expressed are those of the author and are not necessarily those of Scientific American.


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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).

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250.

Proximal downed tree, at Obscurity Lake 15 km north of Mount St. Helens, projecting to left beneath coarse layer A1, in turn overlain by layers A2 and A3 at right. Tree is darkened where tree was debarked and scorched where not protected by overlying layer A1. Photo by R.B. Waitt, Jr. Skamania County, Washington. 1980. Figure 266, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

 

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.

Warner Bros., I think, could have animated that sequence in the tradition of Wiley E. Coyote to fine effect.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250.

Stratigraphic section atop distal downed tree, 15-25 km from Mount St. Helens. Layers A2 and A3 overlie bark. Rule for scale. Skamania County, Washington. 1980. Figure 267, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

 

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.

 

Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250.

Scorched needles beneath layer A3 plastered on tree, about 20 km from Mount St. Helens. Needles beneath layer A3 are just as scorched as those not covered. Skamania County, Washington. 1980. Figure 268, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

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.

 

Previous: The Cataclysm: “Fully Down and Buried”

 


Dana Hunter About the Author: Dana Hunter is a science blogger, SF writer, and geology addict whose home away from SciAm is En Tequila Es Verdad. Follow her on Twitter: @dhunterauthor. Follow on Twitter @dhunterauthor.

The views expressed are those of the author and are not necessarily those of Scientific American.





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