The story of the Scablands

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:

[Image licensed under the Creative Commons DKRKaynor, Channeled Scablands, CC BY-SA 4.0]

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:

[Image licensed under the Creative Commons © Steven Pavlov / http://commons.wikimedia.org/wiki/User:Senapa / CC BY-SA 3.0, Dry Falls (Washington), CC BY-SA 3.0]

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. 

As the ice age ended, the warming continued.  Eventually this caused the ice dam to collapse, and all that water drained out, sweeping across what is now eastern Washington, and literally scouring the place down to bedrock. 

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. 

Well, as the Cordilleran Ice Sheet melted, that weight was removed, and this caused the flat piece of crust underneath it to lift upward on its eastern edge.

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.

The result was a colossal flood that at its peak was traveling at an estimated one hundred kilometers an hour.  Dry Falls was, at that point, an enormous waterfall five times the width of Niagara Falls, with a flow rate ten times higher than all of the rivers in the world combined.

[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.]

These kinds of megafloods aren't uncommon, usually during the transition between a glacial and an interglacial period.  Another place this happened is not far from where I live -- around thirteen thousand years ago, there was an ice dam across what is now the St. Lawrence River, blocking the main outflow from the Great Lakes.  This backed all that water up into a huge lake called Lake Agassiz, which encompassed all of the Great Lakes and then some.  It was the same story as Lake Missoula; as the climate warmed, the ice dam collapsed, and a large percentage of the water drained out into the North Atlantic.  Some climatologists think that this may have been responsible for the Younger Dryas event, when the warming trend in North America suddenly reversed, the temperatures dropping in only a few decades by an estimated 7 C -- because all that fresh water temporarily shut down the North Atlantic Meridional Overturning Circulation, stopping the Gulf Stream in its tracks and sending eastern North America, Greenland, and western Europe back into the deep freeze for over a thousand years.  (The link between the draining of Lake Agassiz, the Younger Dryas, and the shutdown of the NAMOC is far from settled, however.)

This sort of thing, besides being fascinating in its own right, always makes me think of the people who talk about Earth being "fine tuned" for human habitation.  When you start looking into geology, climatology, and astronomy, you realize how tenuous our existence actually is.  That we've done as well as we have, despite all the natural calamities, is impressive, but our continued survival is hardly guaranteed.  We'd better start understanding the past history of our home, including the violent and uncomfortable bits -- lest our own reckless and heedless actions trigger a catastrophe we're ill prepared to deal with.

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