The Cataclysm: “The Path of Maximum Abrasion”

Back at my old high school in Arizona, some genius or set thereof had made two independent but intersecting decisions regarding physical education:

  1. They made shorts mandatory, and
  2. 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.

Image shows a splintered stump, with the splinters angled off to the left. In the distance, bare ridges are visible.
A nearer view of tree torn from base on Harry’s Ridge, five miles north of Mount St. Helens crater. Skamania County, Washington. 1980. Image and caption courtesy USGS.

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.

Detail of the half-debarked tree at Johnston Ridge Observatory. The left side shows the side of the tree facing the volcano: it's stripped of bark and a bit polished. The right shows the side facing away, which still retains its thick bark.
Detail of the half-debarked tree at Johnston Ridge Observatory. The left side shows the side of the tree facing the volcano: it’s stripped of bark and a bit polished. The right shows the side facing away, which still retains its thick bark.

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.

Image is a close up black and white photo of a splintered stump. What must have been a substantial conifer is shredded like wicker. Rocks have been driven into cracks.
The area devastated by Mount St. Helens and damaged tree stumps. Lithic fragments and scoriaceous gray dacite driven into upstream side of a splintered stump on ridge east of Studebaker Creek. Hammer for scale. Photo by S.W. Kieffer. Skamania County, Washington. 1980. Figure 220-D, U.S. Geological Survey Professional paper 1250. Image and caption courtesy USGS.

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.

 

Previously published at Rosetta Stones.

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The Cataclysm: “The Path of Maximum Abrasion”
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3 thoughts on “The Cataclysm: “The Path of Maximum Abrasion”

  1. 2

    Hiking we used to carry panty hose for just such an occasion. Thin enough to breath on exposed skin but remarkably protective.

    They are also good in cool or rainy weather under jeans they keep the dew, or swamp soaked fabric far enough away from the skin to avoid chaffing and clamminess. Several people have claimed ticks won’t feed through them. With a bit of synthetic-safe repellent added the mosquitoes don’t attack my nearly naked, and still well ventilated, legs. Is is far less oily and messy than repellent smeared on directly.

    The synthetic material doesn’t hold water, breaths well, and is remarkably tough. I think of them as skin protection and as insulation under layers as ultra-ultra-light long-john bottoms. Select the thicker hose for more insulation and the thinner hose for warm weather uses.

  2. 3

    That is an old logger’s trick from decades ago in the pre-sports-wear times! In cold weather, my stepdad’s co-workers would wear support hose instead of long john’s underneath your work clothing. Breathable, thin, flexible, doesn’t hold sweat, and being snug against the skin creates a heat pocket between garments. For upper body, cut out the crotch for your head and snip the toes to put your hands through. Voila, warm.

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