After my rather cataclysmic posts about geology last week, which looked at the volcano off the Greek island of Thera and the fault just waiting to rupture near the Pacific Northwest, one of my friends and a long-time loyal reader of Skeptophilia asked me if I'd ever done a post on the "Channeled Scablands." I told him I'd mentioned it once or twice, but always in passing.
So as befits a catastrophe so big it beggars belief, I thought a more thorough look was warranted.
There's a bizarre bit of terrain in what is now eastern Washington and Oregon that goes by the rather horrid-sounding name my friend referenced, and if you ever fly over it, you'll see why. It's formed of teardrop-shaped pockets of relatively intact topsoil surrounded by gullies floored with bare rock. The terrain has the look of what a shallow stream does to a sandy beach as it flows into sea, only on a gargantuan scale:
Then there's Dry Falls, in the upper Grand Coulee Basin, which even has a plunge pool basin at its foot... but almost no water:
Geologists figured out pretty quickly that the entire terrain was sculpted by a huge amount of running water. But the problem is, the entire area is a desert, and apparently has been for a long while.
So where'd all the water come from -- and where did it go?The answer turned out to be the Missoula Megaflood -- a tremendous flood (thus the name) that occurred eighteen thousand years ago, and which is right up there with Thera and the Cascadia Subduction Zone on our list of things that are big and scary and can kill you.
What apparently happened is that during the last ice age, a glacial dam formed across the northern Idaho Rockies, blocking the outflow of what are now the Columbia, Snake, Okanogan, Pend Oreille, and Spokane Rivers. As the climate warmed, the ice melted, but all that water had nowhere to go, so it backed up into an enormous lake -- called Lake Missoula -- that covered a good bit of what is now western Montana.
What made the flood even worse was a phenomenon called isostasy. We're used to thinking of the tectonic plates as moving back and forth, more or less parallel to the Earth's surface, but what is less obvious is that they can also move up and down -- perpendicular to the surface, like ice cubes bobbing in a glass of sweet tea. These chunks of the Earth's crust are actually floating in the semi-solid mantle beneath them, and the level they float is dependent upon how heavy they are, just as putting heavy weights in a boat makes it float lower in the water.
The whole western corner of the United States tilted toward the Pacific Ocean. It's like having a full bowl of water on a table, and lifting one end of the table. The bowl will dump over, spilling out the water, and it will flow downhill and run off the edge -- just as Lake Missoula did.
[Nota bene: This sort of isostatic tilt is still going on today, most notably underneath Great Britain. During the last ice age, Scotland was completely glaciated; southern England was not. The melting of those glaciers has resulted in isostatic rebound, lifting the northern edge of the island by ten centimeters per century. The problem is, the whole country is connected (however a lot of Scottish people might wish otherwise), so the entire island is tipping like a teeter-totter. The tilt is pushing southern England downward, and it's sinking, at about five centimeters per century. Fortunately, there's no giant lake waiting to spill across the country.]