April 18, 2013 | 3
Here’s a word you don’t often apply to a forest: eroded. We don’t expect live trees to be eroded. The slope they’re standing on, sure: that can erode. Maybe the soft alluvial soil down by the river erodes in a flood, leaving roots exposed and trees more prone to fall in the wind. But would you say wind or water or slope failure is “eroding timber”? Probably not.
But it turns out that the force of a directed volcanic blast is very good at eroding timber. There’s really no other good way to describe what a hot, incredibly fast, powerful flow of gas and debris does to forests. It’s not just that it knocks trees down: it fragments and drags them, incorporates them into itself, doing to them what water does to earth and stones, and leaving behind patterns that can be read by geologists as they determine what the directed blast did.
Just imagine having to make sense of this:
It looks like chaos. People are so tiny compared to this devastation, but scientists have the tools to figure it out. After the eruption, once it was (relatively) safe to get back out there, they began listening to the trees, which had plenty to talk about.
One of the more obvious tales the trees told was about the direction of the currents.
There were two basics zones revealed when geologists mapped the directions the fallen trees pointed: a devastated area and a blowdown area.The fate of the trees was very different between the two, but one thing was the same for both: you wouldn’t have wanted to be a tree in either one.
Inside the inner zone, closer to the volcano, the force of the lateral blast was almost incomprehensible. It swept over and down ridges. Within 10 kilometers (6.2 miles) north, it didn’t matter whether the trees were on the near or far sides; the blast cloud “followed topography, eroding timber as it slowed down into each valley and up over each succeeding traverse ridge.” Where it passed, the trees were simply gone, leaving behind nothing but shattered stumps.
This collection of splinters used to be a substantial tree, reaching for the sun on Coldwater Ridge. It grew over generations; it was destroyed in seconds. When the lateral blast from Mount St. Helens hit it, the force of the flow knocked it sprawling and ripped it out, shredding it as it tore it away. The direction of the blast is clearly indicated by the direction its stump is lying in. There were thousands of those splintered stumps, all telling their stories. The remains of the trees, along with bushes and shrubs, ended up becoming part of the flow itself, joining juvenile dacite and older bits of the volcano.
Pause for a moment. Consider the force necessary to do that to a healthy Pacific Northwest forest. Ponder how powerful the blast would have to be in order to do the same thing to trees on the far side of the ridge. Try not to speculate on the fate of the woodland creatures caught in that current.
It’s about to get more wild.
When the flow reached 8 kilometers (5 miles), enough of its incredible energy had been spent for topography to start making a difference. Ridges and hills that presented a steep north face to the advancing blast cloud managed to nearly shelter some of the trees on their far sides. Instead of being reduced to a nub of splinters, some of them were merely snapped in half. As their tops toppled, they left a peel scar on the side of the tree facing away from the blast. Near the ground, then, the force of the flow wasn’t nearly so bad. This would prove important for people caught in the blast farther on: had they been subjected to its full force, they wouldn’t have survived.
Blast behavior started getting really odd around the 10 to 12 kilometer (6.2-7.5 mile) mark. Now trees were starting to be merely felled rather than pulverized and swept away. On the ridges facing St. Helens, the blast still sheared trees right off the near slopes. But as the flow descended particularly steep far slopes, it dropped the trees at oblique angles. At times, the trees even fell in the direction of the vent, seemingly facing into the blast. The flow was no longer clinging to the ground: it launched off cliffs and grabbed some air time before bits of it descended “as eddies 100s of meters in diameter.” As the main flow barreled on, those eddies reversed direction, circling around and flowing down the headwalls of cirques and valley walls, blowing back toward the mountain, and laying the trees flat in those directions. Those are incredibly powerful eddies.
This close to the volcano, the root balls of non-transported trees generally faced the direction the flow had come from. But at times, it would grab hold of root balls and slew the tree around 180°. Old growth trees were treated like pick up sticks.
Beyond 12 kilometers (7.5 miles), the lateral blast began to lose some of its momentum. Trees fell in more chaotic patterns, fanned out where bits of the flow separated, criss-crossed where those disparate flows came back together.
And then, the downed-timber zone simply ends. Many of the smaller, limber trees stand, stripped of branches and bark, but upright. The whippiest branches remain on the trees, bent away from the blast. A zone of scorched but intact trees follows, which, depending on the topography, might be as wide as 4 kilometers (2.5 miles) or as narrow as a tree length. And then, abruptly, healthy trees.
Four minutes after the lateral blast began, it reached its limits. It wasn’t energetic or dense enough to keep on keeping on. Warm and buoyant, its fury spent, it ramped up above the cooler, lower air the developing vertical eruption column sucked toward the volcano.
That’s the story the direction of the tree blowdown tells. There are other stories the trees tell of the blast cloud: its heat, and its abrasiveness. The trees tell us the order in which different aspects of the blast cloud arrived. Dead trees do tell tales, and we’ll continue listening as long as they have something to say.
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.