The Cascadia subduction zone is a long convergent plate boundary that separates the Juan de Fuca , Explorer , Gorda and North American Plates . Here , the oceanic crust of the Pacific Ocean sinks beneath North America at a rate of per year . Hot magma upwelling above the descending oceanic plate creates volcanoes , each of which erupts for a few million years . It is estimated that the subduction zone has existed for at least 37 million years ; in that time it has created a line of volcanoes , called the Cascade Volcanic Arc , that stretches over along the subduction zone from Northern California to Vancouver Island . Several volcanoes in the arc are potentially active . All of the known historic eruptions in the arc have been in the United States . Two of the most recent were Lassen Peak in 1914 to 1921 and the major eruption of Mount St. Helens in 1980 . It is also the site of Canada 's most recent major eruption , about 2,350 years ago at the Mount Meager massif
major eruption of Mount St. Helens in 1980
1980 eruption of Mount St. Helens
Silverthrone Caldera