Interesting. I just watched a lecture on the Cascadia fault. Did you know that every 500 years, on average, the Juan DeFuca plate does its thing and the last time it popped was in 1700? That leaves us with another couple hundred years before the next big one is due right? BUT, here is the ticker. The southern part-from southern Ore to its end at N CA has slipped every 2 to 3 hundred years. BTW. It slips and the coast will move over 150 feet to the west while dropping some 30 feet. Portland, will move over 6 feet to the west and drop some 4 to 5 feet. EAST of the Cascades, the land will shift 1 to 3 feet and drop a few inches. That fault effects everything to Idaho. They know for a fact that The northwest area of the northwest rotates counter clockwise to the north east for 14 months, then stop and actually slides back to the southeast for a fraction of an inch. Then it continues its 3/4 inch or so travel to the Northeast-and the whole N American plate is moving to the southwesterly direction. They think that the fault is slipping under the land mass and not in the ocean and the discussion is..when does it trigger the big one?
What does this have to do with volcanoes? Well, those volcanoes are connected to that fault and have not erupted in a long while-other than Mount St Helens. They have measured Mnt Hood for example, and it is moving, as are the rest.
My question to that Professor would have been. When the vent system lines up correctly, do they erupt? No doubt those volcanoes vent system might be out of line because of the land mass movements? When I say vent systems I mean weaker down areas from the Peaks where the rock is easily fractured to allow magma to come up. He might answer that if that were the case then there would be a new appearance of a vent someplace else, but, do they know that the rock aside from the original vent systems might be harder and all.