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Dana’s Super-Awesome Mount St. Helens Field Trip Guide IV: Hummocks Trail

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


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We’ve left the lovely breezes and rippling blue of Coldwater Lake; a road crossed, a tiny distance traversed, and we are in a rather grimmer place.

If you had been standing here in the North Fork Toutle River Valley on the morning of May 18th, 1980, you would have died. Never mind if you had your car carefully pointed towards a speedy escape. By the time you realized it was time to flee, it would have been far too late. There are people still entombed in the debris avalanche not far from here. This is the place to pause and reflect a moment on the power of geologic processes. Earth demands respect.

We’re about to hike over the results of a sector collapse. When a volcanic edifice becomes over-steepened and weakened, it can come down catastrophically. A major part of the mountain roars and tumbles down at incredible speeds. Clouds of dust and debris boil up as the mass churns and slides down-mountain. Some unfortunate valley is filled with hummocky debris. A volcano is left with an enormous gouge in its face. And this is without a lateral blast: what happened here would have been impressive enough alone. “Maclargehuge” is a word you might use to describe it, but when you get a good look at the thing from ground level, you’ll want something a bit stronger. I think “gigantinormous” will just about cover it, but should any other terms occur to you, please share them here.

Before you start down this trail, get prepared. It’s sunblock time. Slather that stuff on – I’ll not have my readers die of melanoma. Make sure you’ve got more water than you know what to do with. Yes, I know, we’re only going half a mile in, then turning back, but trust me on this. It will suck you dry if the sun’s shining. You are going to get baked without mercy. Shade is nearly non-existent. And what if you get all intrigued by the outstanding geology and decide to keep on till the river, eh? I’ll not have my readers suffer heatstroke and dehydration, either, so take as much water as you can carry.

Right? All right. Let’s go see some geology.

We’re going to take the trailhead on the left, following the loop clockwise. You’ll know you’re on the correct bit if you see an interpretive sign: the first quarter-mile has lots, and they repay a perusal. This is a scientific research area, so please do stay on the trail. There are scientific studies of the area’s recovery going on; this is a fantastic chance for us to see how the landscape evolves and ecosystems recover after a catastrophic eruption, so don’t muck it up. Besides that, this is an area you really don’t want to get lost in.

The early part of the trail moseys through some beautiful, lush meadows and baby forest. The tall grasses are tangled with abundant wildflowers, and skinny young alders partially shade everything. You can see hummocks under thick green mantles: notice their steep, almost conical shapes. Some are more rounded than others, but a lot of them look like debris piles dumped any-old-how – and that’s basically what they are.

Lovely meadow scenes along the Hummocks Trail.

Lovely meadow scenes along the Hummocks Trail.

You’ll soon see bare or sparsely-vegetated hummocks peeking from behind thin screens of trees. Some are too steep and too well-drained to support plant life. This is the kind of thing that makes geologists scream for joy, because we can actually see what’s going on.

Let’s pause a moment and get a handle on what we’re seeing. If you’ll recall from reading up on the subject, the gigantinormous landslide came down in three fairly distinct blocks. Blocks II and III are the ones that made it this far. They actually turned 90° to the west when they hit Johnston and Harry’s Ridges. The landslide decapitated the North Fork Toutle River and left a jumbled, lifeless surface behind. This particular lumpy terrain is a dead giveaway for a huge debris avalanche.

Debris piles too steep and well-drained for even the ubiquitous Pacific Northwest flora to conquer.

Debris piles too steep and well-drained for even the ubiquitous Pacific Northwest flora to conquer.

Those lumps contain quite a bit of Mount St. Helens’ history: watch for it as you walk. There’s some lovely pastel-hued rock, which is hydrothermally-altered dacite from domes erupted during the Pine Creek eruptive period. Black basalt and basaltic andesite were erupted during the Castle Creek period. The bluish-gray and reddish-brown andesites hail from the Kalama eruptive period. Young light-gray dacite comes from the domes formed during the Goat Rocks eruptive period. And you might see some brand-new breadcrust bombs: they surfed in on the debris avalanche. All of these varied rocks keep the hummocks from being a uniform shade of blah. There’s a nice sign along the trail that will show you where on the mountain all those bits came from. You can look from it to the crater and then the hummocks, and marvel that all that stuff from up there ended up way down here. And despite the chaos, geologists can actually figure out which is what and where. (I’d say hats off to ‘em, but don’t doff your cap unless it’s overcast. That sun is fierce. A respectful tap on the brim should do.)

Bits of Mount St. Helens' history. Can you identify their origins?

Bits of Mount St. Helens' history. Can you identify their origins?

All of this stuff came roaring down the valley at incredible speeds. You don’t usually think of land breaking speed limits, but out in the center of the valley, it could’ve given a sports car a challenge, and possibly outrun the police. Consider: the landslide came roaring along at 150 miles per hour (around 70 meters per second). Porche’s lovely Cayman only beats it by 15 miles per hour – and that’s on a lovely smooth track, not a lumpy-bumpy river valley floor filled with enormous old trees. Also, landslides haven’t got sleek aerodynamic design. Wowza, right?

Speaking of enormous old trees, you’ll see a few buried in the debris here and there, some barely visible and some sticking up any-old-how. Like this giant one, here, which really makes you give your best Keanu-Reeves “woah!” That poor thing was probably treated like a pickup stick that’s got in the way of a bulldozer, which combined with the lateral blast, completely ruined its century.

Moi with enormous trees entombed by the debris avalanche.

Moi with enormous trees entombed by the debris avalanche.

Just beyond it, after you’ve wandered along between more hummocks and been treated to some truly spectacular views of St. Helens, you’ll come to the sign marking the junction with the Boundary Trail. Look left, and you’ll see a fabulous dike exposed in Johnston Ridge. This is a beautiful remnant of Tertiary-age volcanic activity, and an excellent reminder that our own belligerent beauty hasn’t been the only fire mountain on the scene here. The dike is big and sold, which means it formed a bit of a barrier to the debris avalanche here. You know what groins do to coastal sediment. Sort of the same thing happened here, with 16 feet (5 meters) worth of landslide piling on that (“upstream”) side of the dike relative to the other (“downstream”) side. And the way that debris piled on tells us it was moving at a leisurely 22 miles per hour (10 meters per second) out here on the margins of the flow. Which, come to think of it, still isn’t the kind of speed you want earth and rock achieving on its own.

View of Mount St. Helens from intersection of Boundary and Hummocks Trails. The Tertiary dike is on the left.

View of Mount St. Helens from intersection of Boundary and Hummocks Trails. The Tertiary dike is on the left.

This is a nice place to look out over the lumpy terrain and consider relief. No, not relief from the sun, although you’re probably considering that pretty closely by now. We’re talking terrain. So between the hummocked-up bits and the low bits, we’re talking up to 246 feet (75 meters) of topographic relief. And it’s got a distinctive appearance that will allow geologists to tell this was teh result of a mega-huge debris avalanche for centuries to come.

As the flow came down-valley, it was able to spread out laterally. Some of the resulting deposit compacted more than other portions, causing much more lumpiness. Then you’ve got your phreatic explosions leaving monster holes all over the place as water from streams and ice from ex-glaciers eventfully encountered hot rocks. New stream banks failed after the lahars roared through. And as time passed, chunks of ice that had survived everything else eventually melted, leaving kettles behind.

And changes continue, as change does. Erosion began having its say about 10 seconds after everything came to rest and hasn’t stopped since. The hummocks slump and ravel as gravity asserts itself. Rain carves gullies and causes debris flows, which change the face of the deposit. And Mount St. Helens occasionally contributes, although it’s been quite quiet lately. The land gives the sense that it’s a huge hunk of clay plunked down by a potter: the potter’s hands still knead it, prodding it toward the shape it will assume as time ticks on.

Time to head back, now, to the stands of slender trees filled with cavorting birds, and the meadows with their bobbing grasses and flowers. If you’re lucky, like we were, you’ll see a wee froggy scrambling out of the path, back to its peaceful pond. Life is assertive, and returns, enjoying the boundless opportunity to be embraced between catastrophes.

We’re about to come face-to-crater with the instigator of the most recent catastrophe. I know we’ve seen a lot today, but, my darlings: it is the merest prelude to what comes next.

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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