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


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:

 Tree blowdown, Smith Creek. Note two geologists at lower right for scale. Skamania County, Washington. September 24, 1980. Photograph by Lyn Topinka. Image and caption courtesy USGS.

Tree blowdown, Smith Creek. Note two geologists at lower right for scale. Skamania County, Washington. September 24, 1980. Photograph by Lyn Topinka. Image and caption courtesy USGS.

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.

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.

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.

 Splintered tree on Coldwater Ridge after May 18 eruption of Mount St. Helens. Geologist for scale. Skamania County, Washington. May 19, 1980. Image and caption courtesy USGS.

Splintered tree on Coldwater Ridge after May 18 eruption of Mount St. Helens. Geologist for scale. Skamania County, Washington. May 19, 1980. Image and caption courtesy USGS.

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.

 Trees torn from their base, on Harry's Ridge five miles north of Mount St. Helens crater. Skamania County, Washington. 1980. Image and caption courtesy USGS.

Trees torn from their base, on Harry’s Ridge five miles north of Mount St. Helens crater. Skamania County, Washington. 1980. Image and caption courtesy USGS.

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.

 Tree blow-down on east side of Smith Creek, northeast of Mount St. Helens. Aerial view .5 mile east and.4 mile north of SW corner sec. 32, T. 9 N., R. 6 S. Skamania County, Washington. June 4, 1980. Image and caption courtesy USGS.

Tree blow-down on east side of Smith Creek, northeast of Mount St. Helens. Aerial view .5 mile east and.4 mile north of SW corner sec. 32, T. 9 N., R. 6 S. Skamania County, Washington. June 4, 1980. Image and caption courtesy USGS.

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.

Fawn Lake, a small cirque lake about 0.5 km in diameter. Trees in northwest half of the basin, above line X-X' , show direct blowdown. Trees in southeast half of the basin below line X-X', show reverse blowdown; this is particularly easy to see on small island in southeast part of lake. Note that, in detail, blowdown pattern is complex; some partial trees are still upright on cirque wall, as indicated by two enclosed areas with dots, and some zones intermediate between direct and reverse blowdown occur (near line X-X'). Portion of Figure 220-B from USGS Professional Paper 1250. Image and caption courtesy USGS.

Fawn Lake, a small cirque lake about 0.5 km in diameter. Trees in northwest half of the basin, above line X-X’ , show direct blowdown. Trees in southeast half of the basin below line X-X’, show reverse blowdown; this is particularly easy to see on small island in southeast part of lake. Note that, in detail, blowdown pattern is complex; some partial trees are still upright on cirque wall, as indicated by two enclosed areas with dots, and some zones intermediate between direct and reverse blowdown occur (near line X-X’). Portion of Figure 220-B from USGS Professional Paper 1250. Image and caption courtesy USGS.

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.

 A different view of edge of blast zone which left an almost definite line between standing and downed trees. Standing trees were killed by the blast. Skamania and/or Cowlitz County, Washington. 1980. Image and caption courtesy USGS.

A different view of edge of blast zone which left an almost definite line between standing and downed trees. Standing trees were killed by the blast. Skamania and/or Cowlitz County, Washington. 1980. Image and caption courtesy USGS.

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.

 Tree blowdown near Elk Rock, 11 miles from Mount St. Helens. (Photo by Lyn Topinka. Cowlitz County, Washington. August 22, 1980. Image and caption courtesy USGS.

Tree blowdown near Elk Rock, 11 miles from Mount St. Helens. (Photo by Lyn Topinka. Cowlitz County, Washington. August 22, 1980. Image and caption courtesy USGS.

 

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

 

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 Scientific American/Rosetta Stones.

Comments

  1. rq says

    Ah, life, so fleeting… Spend hundreds of years growing straight and tall and green, and *poof* all gone in a few seconds! Although *poof* might be an understatement…

    Near the ground, then, the force of the flow wasn’t nearly so bad.

    Ah yes, this was a chapter in fluid dynamics, no? Or at least, I remember it from biomechanics as the reason why bacteria are able to stay attached to surfaces even in what amount to a hard current to them – because as you approach a surface, the rate of flow approaches zero (because the surface isn’t moving), with the gradient being a factor of how fast and strong the current is – the faster and stronger, the sharper and lower the gradient (in the sense that it lies closer to the surface and is steeper).
    Or something.
    This walk down vague memory lane brought to you by: rq!!!

  2. says

    I keep having to pick my jaw up off the floor whenever I see these pictures. When you realize there are teeny tiny humans standing in the first one, and that those trees are not puny little suburban front yard trees…well my mind is still having trouble processing it.

    P.S. Dana, there’s a terrible-looking action movie coming out that takes place in Pompeii circa mountain-go-splodey times. Can I count on you to see it and critique the geology so I don’t have to?

    • rq says

      Please enlighten? I am not aware of such movies – but I am ready to be! Perhaps we can pay Dana to go, or at least reimburse her, if it ends up being that terrible? (Which, let’s face it, it probably will…)

    • Lithified Detritus says

      Wow. That looks like it is really bad. And it will be coming out at about the time I will be covering volcanoes, which means I will get to deal with some “teachable moments.” Fortunately, I already have a Pompeii lesson ready to go.

      • Trebuchet says

        Holy Crap! Er, make that “Wholly Crap!” It doesn’t even look like Vesuvius. And I never knew Pompeii was destroyed by a tsunami.

        Meanwhile, Dana’s MSH series continues to be awesome.

        • Lithified Detritus says

          Trebuchet – I thought of you when I saw these pictures. We’ve had a bit o’ weather here in flyover country.

          On topic, I was blown away (so to speak) by the diagram of Fawn Lake. I’m trying to visualize what would have resulted in those patterns. Amazing!

        • Lithified Detritus says

          Oh, and Pliny the Younger did describe something that sounds suspiciously like a tsunami, but there is no evidence that it caused serious damage. From Wikipedia:

          These events and a request by messenger for an evacuation by sea prompted the elder Pliny to order rescue operations in which he sailed away to participate. His nephew attempted to resume a normal life, continuing to study, and bathing, but that night a tremor awoke him and his mother, prompting them to abandon the house for the courtyard. At another tremor near dawn the population abandoned the village. After still a third “the sea seemed to roll back upon itself, and to be driven from its banks …,” which is evidence for a tsunami. There is, however, no evidence of extensive damage from wave action.

          • Trebuchet says

            LD: Awesome lighthouse pictures, but shame on them for not identifying the actual locations!

      • rq says

        But it’s got to be good! People riding horses! Overlay dramatized music! Real Hollywood Science!
        I wouldn’t pay for this, but it looks like fun for the laughs. Kinda like Krull.
        Yup, we have to sponsor Dana to go to this movie.