New ocean floor is being created offshore of Washington and Oregon. As more material wells up along the ocean ridge, the ocean floor moves toward and beneath the continent.
The North American Plate also moves, in a general southwest direction, overriding the oceanic plate. The Cascadia Subduction Zone is where the two plates meet.
Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mazama (Crater Lake) several thousand years ago and Mount St. Helens in 1980.