Friday, January 20, 2012

Glacial Lake Missoula

We sometimes receive locality-specific questions, and often they are for localities far away from where the A-A-G responder lives.  This is an example of one that is close to where I live - and which I have personally climbed many times. By happenstance this question also opens the door to a phenomenal event: perhaps the largest flood in the history of the Earth.

I live in Portland, and there is an odd mountain east of here on the Columbia River called Beacon Rock. Someone told me that this was a volcano.
(no name, just an email address)

By a happy coincidence, I personally know what you are talking about - because I have climbed it many times. Beacon Rock is the core of a volcano that erupted about 55,000 years ago. You notice it doesn't look like a volcano? We will talk about volcanoes that don't look like volcanoes in a later chapter, but this one once did "look" like volcano.

A classical stratocone volcano like Mount Hood, Mount Fuji, etc., is formed not all at once, but over time. Lava works its way up to the surface and if it gets there quickly enough and with sufficient violence it will fragment into ash and tephra. Volcanic ash is self-explanatory, but tephra is basically lava foam: as the lava rises to the surface the pressure on it drops, gas dissolved in it comes out like a shaken bottle of Coke, and the result is small fragments from pea-sized to car-sized that are full of air-filled vesicles. These tend to drape down the sides and build up like cone around the core, which may just pour lava down its flanks in between tephra falls. This all means that the core of these volcanoes is generally solidified, hard lava, while the sides are looser stuff.

About 12,000 years ago a huge ice dam built up near what is now Missoula, Montana. It backed up a huge lake, a lake that grew until the ice dam couldn't hold it any further. One thing about an ice dam: when it fails, it fails very, very quickly. A lake variously estimated to be the size of one of our modern Great Lakes unloaded ALL of its water in a very short time. And it was a lot of water.

The wave that roared out into what is now eastern Washington State completely re-shaped the land. It left a region called the "Scablands" - strange hills that don't look right to anyone who doesn't live there. They don't have a normal dendritic (like veins on a maple leaf) drainage pattern, and this terrane confused geologists for a long time. Then someone saw them from an airplane and recognized Ripple Marks - the ripples you see on a stream-bed. Only these ripples were gigantic, thousands of times bigger.

People noticed things like Beacon Rock, which stands up like a thumb from the north edge of the Columbia River, before you get to Bonneville Dam. It has nearly vertical sides, and a visionary man named Henry Biddle bought it for $1 in 1915 before it could be dynamited for road gravel. With friends, he built a trail - complete with hand-rails, to the top. It stands almost 300 meters (850 feet) high, and affords a truly sublime view of the Columbia River Gorge and Bonneville Dam (you can't quite see the top of Mount Hood to the south because of the high scarp of the Columbia River basalts on that side). Beacon Rock stands up like it does because all the tephra was stripped from it by the first of the (now believed to be up to 72 separate) Missoula Floods.

Geologist mapping central Washington State found some phenomenal scours in the Columbia River basalt field there. Geologists mapping the Columbia River Gorge found even more startling things: boulders the size of a Volkswagon bus that had been lifted up over a 400 meter-high ridge on the south side of the river... and dropped on the other side. As they published their maps and talked among one another, they all started thinking: what kind of water force could do that? And amazement grew.

The early pioneers entered Clark County in southwestern Washington, and the Willamette Valley in northern Oregon and found wonderful flat valleys unlike the canyons they found farther north or south. They had no idea during their lifetimes that they were planting crops high above where there had been steep canyons just 12,000 years earlier.

This was all something too big to put numbers on... until side-scan sonar was used to map the seafloor off the coast of Oregon in the latter half of the 20th Century. Oceanographers found a debris fan stretching more than a hundred kilometers south and west of the port of Astoria, the gateway to the Columbia River. This debris fan has a volume of over 5,000 cubic kilometers. To put this into human perspective, this is a cube of debris 17 kilometers on an edge; imagine a block of rock, 100 square miles in area, standing up 50,000 feet into the Stratosphere. Specialists believe that almost all of this came from just the first and largest flood. By now, mapping had allowed geologists to find the source, and to thus put a name on what is believed to be the largest flood that ever happened in the Western Hemisphere.

The Missoula Floods.

Beacon Rock is just a tiny but very distinctive piece of evidence in a puzzle extending many hundreds of kilometers, from the Pacific Ocean to the Rocky Mountains, for one of the greatest landscape-changing events of prehistory. My house on Prune Hill, in Clark County, sits on something called the Troutdale Formation: I can walk outside and put my finger on fragments from one of the greatest catastrophic events of all time.


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