PORT TOWNSEND — In “Assembling the Pacific Northwest,” geology professor Ralph Dawes will take listeners back 200 million years, when the West Coast was located at Spokane, at the edge of the “craton” — the old part of the continent.
How did the rest of the Pacific Northwest land mass form, and where did it come from?
The answers will be presented in an illustrated lecture that starts at 4 p.m. Saturday at Quimper Unitarian Universalist Fellowship, 2333 San Juan Ave., Port Townsend.
The presentation is free and open to the public, although an optional donation of $5 would be appreciated to defray expenses, according to the Jefferson Land Trust Geology Group, which is hosting the lecture.
Dawes says the geological history of the Pacific Northwest gives insight into the landslides, floods, volcanic eruptions and earthquakes that are experienced today.
The West Coast became an active plate boundary when an oceanic plate on the sea floor started to plunge beneath the North American (craton) plate.
As the oceanic plate subducted, blocks of Earth’s crust that it carried, such as island arcs and oceanic plateaus, were too big to be overridden.
They were scraped off and added to the edge of the continent in a process known as accretion.
That was the way the land mass of the Pacific Northwest west of Spokane was formed.
Accreted terranes
Accreted terranes have very different geologic histories than bodies of rock immediately adjacent to them, and they are separated from their surroundings by major faults, according to Dawes.
An example on the Quimper and Olympic peninsulas is the rock layer known as the Crescent Basalt, which is part of a large accreted terrane.
This huge region-sized ribbon of basalt underlies much of the Pacific Northwest coast.
The Crescent Terrane originated as a massive layer of basalt that formed from lava erupting from the sea floor. The lava built broad oceanic islands in some places.
All this basalt, along with other oceanic sediments, were then pushed under the North American Plate, with some of it accreted to the edge of the continent.
Deep thrust faults separate the Crescent Terrane from terranes that had accreted earlier to the coastline.
Jigsaw puzzle
The accreted terranes of the Pacific Northwest are a four-dimensional jigsaw puzzle — in time as well as in space — that is still being assembled, according to Dawes.
Dawes will delve into the discovery that some Pacific Northwest terranes might have moved north more than 1,000 miles along the edge of the continent.
Western North America has been the main testing ground for the development of the accreted terrane theory.
Dawes also will discuss the current state of knowledge of accreted terranes in the Pacific Northwest, with the Olympic Peninsula representing the leading edge of the North American continent, where terrane accretion continues today.
Originally from Edmonds, Dawes earned a degree in literature from Antioch College in Yellow Springs, Ohio, followed by degrees in geology from Western Washington University, where he earned a bachelor’s, and University of Washington, where he earned a master’s and doctorate.
He has taught geology for the past 23 years, the past 16 at Wenatchee Valley College.
For more information, visit the Jefferson Land Trust Geology Group’s website at www.quimpergeology.org.
