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.
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.
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.
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).
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.
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.