"The Largest Historic Gravitational Slide Known"

That’s what this is: one helluva huge debris avalanche. Thing goes nearly fourteen miles down the Toutle River Valley. It’s bloody ginormous is what it is. That’s it, there in the brown, on this lovely little map.

Generalized map of flowage deposits from the May 18, 1980, eruption, around Mount St. Helens. Skamania and Cowlitz Counties, Washington. Image courtesy USGS.

We all focus on the bits of St. Helens that went boom: do we really give the bits of it that went thump proper credit? We certainly will when I’m finished here. You see, the Rosetta Stones post I linked above is just the beginning: I’ve got Harry Glicken’s posthumously-published, nearly-as-large-as-the-debris-avalanche write up of his studies of the thing, and I’ve got a bazillion photos of the results of the avalanche as it was then and 32 years later, and I will, over time, make you intimately familiar with the thing. There are so many stories to tell emerging from just this one aspect of the eruption. Pretty scenery, now, too!

But here’s one image I want you to take away with you right now:

Close-up view of Mel A. Kuntz in front of elastic material from May 18 debris flow. Skamania County, Washington. August 11, 1980. Image courtesy USGS.

Keep in mind: that’s not one of the bigger boulders.

Go marvel at the largest landslide ever witnessed by lil ol’ us. Take to heart this lesson: what falls down the mountain can be as catastrophic as what explodes up and out of it – sometimes moreso. Because, my darlings, this shit can happen even in the absence of an eruption. Gulp.

"The Largest Historic Gravitational Slide Known"
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6 thoughts on “"The Largest Historic Gravitational Slide Known"

  1. 1

    Wonderful. I likes volcanoes I does, and you bring Mt St Helens so beautifully to life.

    A minor quibble: “2.5 cubic kilometers (1.5 cubic miles) of Mount St. Helens thundered in to the valleys below” – one of these numbers is wrong

  2. 2

    Your photo caption describes the rocks as “elastic.” While they may have had that quality when they were still in the Earth’s mantle, I think you meant to type “clastic.”

  3. F

    Actually, I recall the debris flows being the bigger deal at the time. They could reach farther and affect more people than ashfall or the potential of getting whacked by a hot rock. Plus, there was video footage of this, unlike the the main eruptive events. (For which I’ve seen some truly awful attempts at animation via image morphing.) To me, lahars aren’t things separate from “eruptions”, they are just one consequence of magma movement. Like the side of a mountain falling off.

  4. rq

    Wow. Those are some incredible speeds; that’s what caught my attention, at any rate. For heavy earth (rocks, soil, trees, etc.), things that aren’t ordinarily in motion , to move that fast… Just wow.

  5. 5

    I remember a field trip through the Pacific Geoscience Centre many years ago, not long after the eruption in question. (My family lived in Central Saanich, just north of Victoria. We got some of the ash.)

    I will always remember how they described the eruption there. The entire top of the mountain lifting off on top of a cloud of hot volcanic gas… and then sliding down the side. Still with the gas.

    Have you ever played air hockey, where the little plastic puck can go amazingly fast because it doesn’t have to deal with much in the way of friction? Imagine millions of tons of rock, acting much like that little plastic puck with no appreciable friction as it ‘falls’ down the side of the mountain at an angle, accelerating all the way.

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