Volcanism, The Defining Feature of the Pacific Northwest

More than any other single feature, lava has come to define the physical character of the Pacific Northwest.  Igneous rock, lava in its solidified form, has helped to shape our weather, define our vegetation, set the paths of our rivers, and determined the locations of our towns.  The Pacific Northwest’s volcanic activity is caused in large part by the subduction of the Juan de Fuca Plate that migrates under the westward moving North American Plate.

The Juan de Fuca Plate is one of the Earth’s smaller plates, and it is actually a remnant of the once far larger Farallon Plate.*  As the Juan de Fuca Plate continues to peal under the North American Plate, the melting Asthenospheric mantle pushes magma upward.  Over the last 37 million years, this activity has created the area known as the Cascade Volcanic Arc.  In this zone, lava and pressurized gases are released through over 4,000 individual volcanic vents including stratovolcanoes, shield volcanoes, dikes, fissures, cones, and domes.  Each has played a role in creating the Cascade Mountains.  Yet even the Cascades are part of a much larger system of volcanic activity that encircles the Pacific Ocean known as the Pacific Ring of Fire.

Tectonic plate subduction isn’t the only source of the region’s volcanic activity, however, and basalt is certainly found outside of the Cascade Volcanic Arc.  Areas simply called Hot Spots also contribute to the Northwest’s volcanic character.  Hot Spots are simply thin spots in the Earth’s crust, either the lithosphere or the continental crust, where pressure breaks through to cause volcanic activity on the surface.  Some 10-15 million years ago, hundreds of lava flows were released through dikes above Hot Spots in eastern Oregon and the very southeastern tip of present day Washington, creating one of the largest basalt floods on Earth.

At more than 63,000 square miles and over 1 mile thick, the Columbia River Basalt Group covered much of the Pacific Northwest.  As the magma chamber discharged and eventually emptied, the continental crust sunk in its place and created a gigantic depression known as the Columbia River Plateau.**  This simultaneous discharge and sinking of the mantle allowed the lava to ooze all the way out the Pacific Ocean, and traces of the basalt can be seen on the coast as far south as Yaquina Head.  Time and erosion have exposed hundreds of layers of basalt throughout the region.  In particular, much of the Columbia River Basalt has been revealed by the Missoula Floods, themselves products of the melting ice sheets at the end of the last ice age.

In 1872, another Hot Spot was designated the country’s first national park…Yellowstone.  Referred to many geologists as a supervolcano, it might appear that the Yellowstone Hot Spot is slowly migrating eastward from its early emergence near the Steens Mountains in southeastern Oregon.  In fact, the Hot Spot has actually remained relatively stationary over the last 18 million years, and it is rather the western migration of the North American Plate that has marked the path of this volcano.  Like the Columbia River Plateau, the volcanic activity of the Hot Spot created the depression of the Snake River Plain, evidence of the North American Plate’s migration.

*The Farallon Plate is now completely submerged beneath the North American Plate.  It is what created the Sierra Nevada Mountains beginning roughly 100 million years ago, and it also created the Coast Mountains of British Columbia.
**Magma release and the subsequent collapse of the emptied magma chamber is essentially the same phenomenon that created Crater Lake and all other such calderas.