This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
At long last, we return to our long-running series on the May 18th, 1980 eruption of Mount St. Helens. When last we visited our raging volcano, it was busy roasting trees. And it's not through with them yet...
Back at my old high school in Arizona, some genius or set thereof had made two independent but intersecting decisions regarding physical education:
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They made shorts mandatory, and
They decided we'd do tennis in March.
Now, neither decision is necessarily bad, at least not on its own, but there were critical components of meteorology, geography, geology, and architecture no one had factored in. Namely: March is the absolute windiest month in Northern Arizona, Page is covered in thick sand eroded from the magnificent Page Sandstone, and the tennis courts were outdoors. I often had occasion to reflect upon the fact my youngest uncle had stripped the old paint from our Datsun 240z by sandblasting it. I now knew how that car felt. My legs had never been so completely free of dead skin, which had been stripped off along with several layers of epidermis. If I'd known much about religion in those days, I may have converted to a variety with an extreme emphasis on modesty, just so I could've claimed a religious exemption and put on trousers. That wind-blown sand hurt like blazes.
And that was just the ordinary spring wind, carrying plain old sand, not a volcanic blast cloud carrying all sorts of super-sharp, sometimes large, and definitely hot fragments of rock and glassy ash. I can't imagine how much worse that would be.
Fortunately, we've got science to tell us enough to fill in the blanks. Geologists James Moore and Thomas Sisson were all over the trees after Mount. St. Helens hit them with all she had. You might think it's weird, how two science guys can crawl around trees and stumps measuring what's not there (on account of it having been abraded away), but they managed - and thus wrote another chapter in the volcanic saga.
They were able to figure out how much of these poor trees was removed by a kind of forensic reconstruction of their original thickness, using the bits of their outer surfaces St. Helens had missed, plus growth rings. They relied mostly on stumps and small trees that hadn't toppled. And those measurements tell a story that make me realize just how good I had it on those windy tennis courts.
If you were the side of the tree facing the volcano, you had it very bad indeed. Nearly equally, from bottom to top, you would have been completely sandblasted. Not even falling would protect you: it just meant that your roots got the brunt of it on the parts exposed to the volcano. The abrasive phase of the lateral blast may not have lasted long, but it was long enough to hit you both when you were up and when you were down. Yeow.
Moore and Sisson's measurements show that all the bark was stripped on the side facing the blast. If you were a large tree, that was 1-2 centimeters (.4-.8 inches) of you ripped right away. If you were small, you'd lost an average of a millimeter - which may not sound like a lot, but it's all relative. If you were close to the volcano, more than 180° of your trunk had its bark blasted off. If you were a stump, you'd end up more abraded than if you were a small tree thin enough to bend over in that searing, grit-filled wind. Stumps within 8 kilometers (5 miles) were actually abraded several centimeters deep. My near-microscopic layers of missing epidermis ain't nothing compared to that.
Farther away wasn't that much better. Small trees 14-20 kilometers (8.7-12.4 miles) distand (disregarding the southern margin) had 180° of bark removed. One of the eeriest things to see at the Johnston Ridge Observatory is a display of one of the trees where that's happened.
Further away, the near sides are roughed up but not completely debarked by the sandblasting they endured - my legs can sympathize.
USGS geologist Richard Waitt had a look at downed trees that had retained their branches, and found their former undersides, which faced the volcano after the trees toppled, were "debarked and abraded," showing that even after the phase of the blast powerful enough to fell old giants where they stood, the blast was still hurling projectiles horizontally - a mind-boggling amount of force.
Abrasion decreased with distance; the patterns show there was a "simultaneous decrease in size, velocity, and number of fragments carried by the surge" the further away it got. It was losing energy and dropping its load; the debris it buried the trees in tells us even more about the complex phases of the blast. The trees had been sandblasted, felled, and scorched, but the volcano wasn't quite through with them yet.
Previous: The Cataclysm: "From Unbaked Fragments to Vitreous Charcoal"
Next: The Cataclysm: "Fully Down and Buried"
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.