The Juan De Fuca tectonic plate was formed around 30 million years ago when the ancient Farallon plate pushed against the North American plate. The Farallon plate would split as it subducted underneath the American Continent, splitting into several smaller plates.  The Juan De Fuca plate formed as a result of this collison, and is currently subducting underneath the Americans at inches per year. 

As the Juan De Fuca plate is subducted beneath the Americas, the material of the plate began to melt as it went deeper inside the earth's mantle. This heated material, now magma, began to rise underneath the American plate, exerting enormous pressure on the area. This pressure and heated material began to erupt and spill out onto the surface, cooling and forming mountains. Some of these mountains are still prone to eruption, such as Crater lake and the more infamous Mt. St. Helens.