Click on the illustration above
to get up close and personal with this Cascadian view
The volcanic nature of many of these mountain peaks are all the fault of that boisterous Juan de Fuca plate. Or, if you prefer, you can blame it on Juan's kin, the troublesome North American plate. Skimming along towards each other atop the earth's mantle, at the rate of a couple of centimeters a year, both are headed on a disaster course.
As little Juan continues to converge into that big ole North American plate, watch out! Being more dense (of course since he's an oceanic plate) Juanito is just making trouble for himself. He's getting subducted under his bigger continental cousin. You'd think those oceanic plates would have figured out by now, that's the way the laws of plate tectonics work. But those kids never seem to learn!