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The Cataclysm: “A Horrible Crashing, Crunching, Grinding Sound”

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


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At the risk of sounding flippant, I wish to share a bedrock truth of the Pacific Northwest. Only, it’s more like a harsh woody truth. The fact is that, on the seaward side of the great mountain chains, you can’t see much geology. It’s covered in trees. Whenever I go out looking for some nice geology, say an outcrop or a vista or a wee bit o’ bedrock, all I seem to encounter is trees. I find an apt quote from Rocko’s Modern Life scrolling through my brain: “Stupid trees everywhere!”

In the vicinity of Mount St. Helens, trees have not been a problem for the last 33 years.

Mount St. Helens from Johnston Ridge, September 2011.

Mount St. Helens from Johnston Ridge, September 2011. Note the shattered stump in the foreground.

When the directed blast roared from the abruptly-unroofed cryptodome, it annihilated trees within 25 kilometers (15.5 miles) of the volcano. The the thick gray volcanic deposits, the roaring mudflows, the soaring ash clouds were awe-inspiring enough, but one of the most lasting impressions is acres upon acres of trees, lying stripped and bare. Vast old-growth forests and thriving younger ones vanished in minutes. If you’ve ever stood under those trees, experienced their enormous height and girth and their ability to block out the sky, what the force of a lateral blast did to them should leave you speechless.

Aerial view of timber blowdown - destroyed by May 18 eruption of Mount St. Helens. Skamania County, Washington. June 8, 1980. Image and caption courtesy USGS.

Aerial view of timber blowdown - destroyed by May 18 eruption of Mount St. Helens. Skamania County, Washington. June 8, 1980. Image and caption courtesy USGS.

Witnesses fortunate enough to survive described trees bending and breaking in the hot wind, disappearing under the blast cloud. The cloud ripped trees from the ground and hurled them ahead of it. Some witnesses saw a high green wall sailing through the air ahead of the thick, roiling cloud filled with ash and stone, before the cloud overtook it. Those caught within the devastated zone experienced a very peculiar event: at one instant, there were trees standing all around them. The next instant, all of the trees were down. Strangely, only James Scymanky heard anything: “a horrible crashing, crunching, grinding sound” as the blast cloud approached. For everyone else, the forest fell in near-total silence. If a tree falls in a forest during an eruption, it seems it may not make a sound even when there are people there to hear it.

Where the blast cloud passed, it left devastation in its wake. Three distinct zones were marked out by what it did to the forest.

Schematic map of devastated area and generalized directions of streamlines of the blast flow , as indicated by directions of alinement of fallen trees. Streamline arrows are dashed where information is lacking. Devastated area is divided into two zones (dashed line), the inner direct blast zone and the outer channelized blast zone. Note that near new vent (approximate location indicated by X) , margin of blast zone lies south of vent in spite of the fact that flow emerged to north; however, at a distance of 10 to 20 km, margins have been deflected to northeast and northwest. Note also that singed zone is narrow compared to devastated area, and is wider where blast approached rising terrain (as in the northwest) and narrower where the blast approached falling terrain (as in the east) at its outer limits. Fig. 219 from USGS Professional Paper 1250. Image and caption courtesy USGS.

Schematic map of devastated area and generalized directions of streamlines of the blast flow , as indicated by directions of alinement of fallen trees. Streamline arrows are dashed where information is lacking. Devastated area is divided into two zones (dashed line), the inner direct blast zone and the outer channelized blast zone. Note that near new vent (approximate location indicated by X) , margin of blast zone lies south of vent in spite of the fact that flow emerged to north; however, at a distance of 10 to 20 km, margins have been deflected to northeast and northwest. Note also that singed zone is narrow compared to devastated area, and is wider where blast approached rising terrain (as in the northwest) and narrower where the blast approached falling terrain (as in the east) at its outer limits. Fig. 219 from USGS Professional Paper 1250. Image and caption courtesy USGS.

Close to the mountain, from the vent up to 12 kilometers (7.5 miles) away, not a single tree remained in many places. There were some stumps: short, splintered remains of once-majestic trees. Many trees had simply been ripped up by the roots. The force of the blast shattered them, shredded them, mixed them with the contents of the cloud and swept them away. Old Pacific Northwest trees are not demure little things, but this blast cloud tore giants apart without trouble.

When the blast zone had reached distances of 12-25 kilometers (7.5 – 15.5 miles), the blast cloud began merely stripping trees of their branches and laying them flat, most pointing away from the volcano. Depending on how strong they were, or how sheltered they were, they were either uprooted completely, snapped off near their bases, or only had their tops ripped off.

And then there is the very odd zone where the trees remained standing, but were seared and singed badly enough they later died. That zone could be as narrow as a tree length, or as wide as 4 kilometers (2 miles).

Margin of the devastated area in the Green River drainage near the intersection with Shultz Creek. View north. Note transition from devastated area in foreground through a narrow singed zone into undamaged forest in background. A from Figure 220 in USGS Professional Paper 1250. Image and caption courtesy USGS.

Margin of the devastated area in the Green River drainage near the intersection with Shultz Creek. View north. Note transition from devastated area in foreground through a narrow singed zone into undamaged forest in background. A from Figure 220 in USGS Professional Paper 1250. Image and caption courtesy USGS.

After the eruption, USGS geologists studied the former forests intensively. There were stories in the wood fragments and other tree debris mixed in with the blast deposits. Stumps and stripped logs gave testimony to the power, direction, duration, and order of deposition in the blast cloud. The direction they lay in showed how the currents of the pyroclastic density flow moved. Temperatures of the blast cloud could be measured. There was a lot to learn from those perished forests, as you’ll soon see.

Norman Banks in timber destroyed by May 18 eruption of Mount St. Helens near headwaters of the Green River. Skamania County, Washington. June 8, 1980. Image and caption courtesy USGS.

Previous: Interlude: What Vehicles Say About Temperatures Within a Volcanic Blast.

Next: The Cataclysm: “All of the Trees Seemed to Come Down at Once”

 

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.

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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