History 200 DE

Week 2

Geology of the Northwest

Dr. Bruce Haulman

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To understand the modern Pacific Northwest, it is important to understand how the landforms go to be the way they are and how this geologic past affects us today.  The rainy side of the mountains, the deserts east of the mountains, Puget Sound, earthquakes, volcanoes all are part of the geologic past.

Plate Techtonics -

The Pacific Northwest was shaped in part by the collision of techtonic plates (large masses of the earth's crust (lithosphere) that move about floating on the magma that surrounds the earth's core.  As these plates collide a subduction zone is created as one plate rides up over the other and creates the potential for volcanoes and earthquakes.  The diagram above shows such a subduction zone. 

The next diagram shows the location of the Juan de Fuca Plate (in blue) which is moving eastward and slowly being subducted under the North American Plate that is moving northwestward.  The black dots on the map are the location of the chain of major volcanoes which run from Mt. Shasta in Northern California to Whistler Mountain in British Columbia.  See how many of them you can identify.

Because of this plate techtonic activity, the Pacific Northwest was shaped and continues to be shaped by volcanoes and earthquakes.

The Vashon Glacier -

The map below shows the maximum extent of the Vashon Glacier that carved out Puget Sound.  Seattle was under a mile of ice, and the glacier left marks on both the Cascade and Olympic mountain ranges. The glacier retreated some 14,000 years ago.

© Seattle Times

The Channeled Scablands -

The two maps below show how the channeled scablands were formed by huge floods of water, sometimes 200 feet high, that scoured the land in eastern Washington.  As many as 6 or more of these floods occurred as the ice sheet advanced and retreated creating Lake Spokane and Lake Missoula.   Each time as the ice dams collapsed, huge walls of water swept across the land, backing up at the Walulla Gap and flooding much of eastern Washington until the water slowly drained out the Columbia River.

The Osceola and Electron Mudflows -

As volcanoes heat up and release steam, or erupt; often times large mudflows result from the melting ice and superheated gasses.  About 4,800 years ago the Osceola Mudflow created the Enumclaw Plateau; about 600 years ago the Electron Mudflow filled much of the Puyallup River Valley.

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© Bruce E. Haulman 2008