The Cascade Mountains were formed as a chain of volcanoes, a manifestation of the Pacific Ring of Fire in the Pacific Northwest. The Ring of Fire is caused by the oceanic plates offshore subducting beneath the North American Plate. This destabilizes the hot mantle rocks below, liquefying them and forcing them upward, producing increases in elevation and even volcanic craters where magma pushed its way through.
The Cascade Mountains are only one sub-section of a ring of volcanoes surrounding the entire Northern Pacific Ocean. They are much less active than those elsewhere, although they are not entirely dormant, as was shown by Mount Saint Helens between 1980 and 1984. Indeed, the Cascade subduction zone is generally less active than many similar subduction zones. It lacks the large trench that tends to accompany a subduction zone and general seismic activity is lower than usual. While the Cascade Mountains do continue to grow, evidence suggests that their activity has been slowing for millions of years. This slowing of activity is likely due to the slowing of the relative movement between the oceanic plate and the continental plate beneath the Cascade Mountains, which has slowed by half in the past 7 million years.