This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Right. On with our Mount St. Helens field trip extravaganza. We've brunched at Hoffstadt and got a nice overview at Castle Lake Viewpoint. Now we're on to get intimate with a bouncing baby lake.
Stop 3. Coldwater Lake
This is an ideal place to be on a hot summer day. Easy little trail, lovely cool water, and unlike many places within the blast zone, there's even some shade! In just over half a mile, you'll see a wide variety of geology, some brand new and some a little more mature.
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As you start down the trail, you'll notice some rather large lumps. You're walking on and alongside the portion of the debris avalanche that dammed Coldwater Lake. The tall lumps are hummocks, knobby little hills made up of bits that until recently formed the beautiful symmetrical come of Mount St. Helens. Chunks of volcanic rock, some quite large, peek out from a matrix of ashy, stony rubble. It's rather like glacial till, all sorts of unsorted bits all jumbled together. You can tell it wasn't transported by water not only because it's not arranged neatly with larger bits sorted from the itty bitty bits, but also because water transport would have rounded off the angular edges. These are unapologetically pointy.
Get yourself out on one of the little docks that give you a good view of the lake. Here, the water's shallow, and quite clear. Have a close look at the lake bottom. It's telling us more about this lake than you may suspect, and I'll bet you can puzzle it out. Take a moment to ponder.
You've noticed that the bottom's mostly rocks of all shapes and sizes, but virtually no mud. You'll also have noted that this jumble of rocks isn't your typical beach rock bed - their edges are pretty jagged, and there's a big range of sizes. We're on top of the debris avalanche here, and the lake covers the part of it that thins out toward the middle of the valley. We know there was a lot of fine-grained material in that deposit. So what we're seeing here is water energetic enough to wash out the fine stuff, but too sedate to tumble the rocks. They've been left pretty much in place. This isn't a body of water prone to roaring floods, strong currents, or storm waves.
But here's a strange thing: logs. Many large logs, washed up on shore, much like the gigantic ex-trees you find piled on the berm of any storm-wrecked Pacific Northwest seashore. But why, if these waters aren't able to toss rocks around, would they be able to give entire mature trees the old heave-ho? And without crumbly bluffs for those stately old forest citizens to fall off of, why so many?
Well, of course, wood floats, so it wouldn't take a lot of mad wave action to nudge logs ashore. Notice the strand line isn't very far back. As for where those logs came from, you've probably already realized they're the remains of trees mowed down by the debris avalanche and lateral blast. Once it was reasonably safe to return to the area, loggers came in to salvage as many downed trees as they could, using tugboats to transport rafts of logs across the baby lake. The trees you see are some of the logs left behind.
Speaking of trees, you'll notice a fine young forest growing practically in the lake between you and Mount St. Helens. This is the South Coldwater Creek delta, a fine bit of fluvial geology that started growing almost the instant enough water backed up to create Coldwater Lake. South Coldwater Creek flows behind that ridge St. Helens is peeking over, and heads into the lake. It's carrying a goodly amount of stuff eroded out of the volcanic deposits it flows through. When it arrives in the lake, that sediment-laden water slows beyond the point where it can transport its load. Suspended particles drop out, forming the delta those trees have found to be such a happy home.
The delta would be smaller, but in 1985, engineers built a tunnel to provide a safe outlet for Spirit Lake. The water carried by that tunnel is deposited into the headwaters of South Coldwater Creek, increasing its power to erode. Coldwater Lake, only 8 kilometers long and 55 meters deep, will eventually fill in with sediment from the delta and other sources, becoming a marsh, and eventually an ordinary meadow. So enjoy this gem while it lasts.
Head on out to the end of the board walk, which provides you an unobstructed view out over the lake. We've lots of delicious geology to see, some of it older than the May 1980 eruption.
From right to left, you'll see some peaks peeking up over the unnamed ridge between us and South Coldwater Creek. One of the tallest is Coldwater Peak, where geologists have set up an observation post to keep a close eye on its feisty young neighbor. The peak right in front of you is Minnie Peak. It's not a volcano: it's formed from the hard grandiorite of the Spirit Lake pluton, which intruded and cooled between 20-23 million years ago. You remember all the babbling I've done about batholiths, right? This is the same thing, only smaller, and it's been uplifted a good distance - Minnie's 1,711 meters (5,610 feet) high. She's all carved and sharpened by glaciers.
You can actually see some cold glacial action close by. Have another look at our unnamed ridge. It's got glacial drift plastered all over it - stuff left by the ice of the Hayden Creek glaciation, which was around 140,000 years ago. It's kind of exciting to find glacial deposits that old - newer glaciations often wipe out traces of the old. And we know glaciers were here more recently. Take a look toward the narrows there in front of Minnie Peak and see if you can spot the lateral moraine.* Its from the Evans Creek glaciation, which happened only about 11,000 to 22,000 years before our times. You can see a big landslide scar in it - glacial deposits aren't very well consolidated, alas, and gravity works.
All right. Look to your center-left. You're seeing the long slope of Coldwater Ridge, and in front of it, the brown lump of an island. That's a hummock, tall enough to avoid being buried by the rising lake waters. Its face is so steep that even our determined PNW plants can't get a root-hold. It shows that the debris avalanche made it a fair way up the valley. I've taken to calling it Hummock Island, but if you come up with a suitably awesome name, we can switch.
Now for some rather more mature geology. Look at Coldwater Ridge, a little ways to the right of Hummock Island. You see outcrops of lava there, jutting from the flanks.
See how they're genly tilted eastward? Those belong to the Pole Patch Syncline, a broad downwarp in the crust. To give you an idea of its size, its axis is around 25 kilometers (15 miles) away. The lava flows are basalt and andesite. Geologists think they were part of an Oligocene shield volcano, erupted on its flank sometime between 34-23 million years ago. Mount St. Helens is just the most recent volcano in a long, varied, and exciting area eruptive history.
Coldwater Ridge is covered in stumps, making it look like a ridge that's decided not to shave for several days. You'll also see logs lying about here and there. The ridge was being logged before St. Helens exploded, so you'll notice some stumps are relatively smooth-topped while others are jagged and splintery. A pre-May 18 photo shows its top neatly shaved. The volcano finished what the loggers started, and then some.
You can spot the Coldwater Ridge Visitor's Center, which will be a fabulous place to stop if you've got time on your way back. The vista of Coldwater Lake and Mount St. Helens is breathtaking. Take a moment to remember Gerry Martin, a ham radio operator working for the Washington Department of Emergency Services on the morning of the cataclysmic eruption: he died in the blast.
Take the other limb of the loop trail back to the parking lot. If you've come in summer, you'll be treated to a riot of flowers growing happily on the hummocks. This part of the trail is a botanist's delight. Don't worry - geology gets its own back further down, where one of the hummocks is too steep and crumbly for much vegetation. This is a good place to pause and get a feel for the size of these things. No wonder they formed an effective debris dam. It's pretty stable now: the channel that engineers cut to control lake levels in July of 1981, forming Coldwater Creek in the process, worked a treat. They monitored it for some time, concerned about the possibility of an outburst flood, but discontinued that monitoring in 1998 - the blockage is now so stable that it's considered safe.
We've gotten a feel for the hummocks, see a quite young lake with a spiffy delta, and enjoyed some icy geology along with the hot. Now we're going to leave the cool lake breezes and shady trail behind. It's time to go walkies on the largest landslide ever witnessed.
*It's at the center right, at the end of the ridge; it looks almost like someone dumped it there to build a road bed, dunnit?
References:
Burns, Scott (2011): Field Guide to Mt. St. Helens north. Portland State University.
Decker, Barbara and Robert (2002): Road Guide to Mount St. Helens (Updated Edition). Double Decker Press.
Doukas, Michael P. (1990): Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington. USGS Bulletin 1859.
Pringle, Patrick T. (2002): Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity. Washington DNR Information Circular 88.