Right, we'll be back to Mount St. Helens soon, but we're going to take a few side trips first. Don't worry - it's all related. And there's a place in Oregon, not too far south of St. Helens, where you walk across a parking lot and see all the components of the subduction zone that fuel her.
First, we'll have a bit of a diagram.
This is the idealized version of a subduction zone, showing you all the relevant bits. There's an even better diagram here, showing you the structural high that can be created as the oceanic plate dives beneath the continental one. I encourage you to study both images for a moment. As with all diagrams, these are simplified versions of real-life structures. You usually don't find anything so simple in the field. But I can show you a place where you can actually see those diagrams in action, without squinting.
Take a short drive out of Corvallis, Oregon to Marys Peak, the highest peak in the Oregon Coast Range. The road goes to the top. Once you've parked, walk west, toward that lovely meadow with the butterflies and lilies and large interesting boulders. If you look over the trees on a clear day, you'll see a scene much like this:
Look between the trees, and you can see the gently-rounded tops of other Coast Range mountains, and beyond them, you can just make out the sea. It's a lot easier to see in person - unfortunately, all the hydrocarbon haze from the conifers round here plays havoc with the camera. Never mind that. Just stand here a moment, and look at the sea. Beneath the waves, about 50 miles (80km) out, the Juan de Fuca plate is subducting beneath the North American plate.
Wow, right? Now, turn east and amble across the parking lot.
You're walking across the forearc ridge right now. This is the structural high caused by those two colliding plates. I'll be babbling about Marys Peak and the Coast Range in detail one o' these days, but it can basically be described as a bunch of stuff plastered to North America, wrinkled up into a lovely little mountain range.
Continue east to the other side of the mountain. I know, "other side of the mountain" sounds like a substantial hike, but it's only a few hundred yards at worst. This is the easiest geological excursion ever. Just head over to where you can get a view through the trees toward the east.
Now, this is rather exciting. The Willamette Valley, broad and flat, is a forearc basin on dry land. Many forearcs are drowned by seawater, but in this case, the Coast Range has risen high enough to cut off the ocean, so here we are: a nice, dry forearc basin you can see all the bits of. And from here, you can see all the way to the volcanic arc, the Cascades. See? That line of low mountains in the distance is the Western Cascades, and beyond it, rising like a snowcone, is Mount Jefferson. No, really, it's there. It's the triangular white bit between the two short trees in the center of the photograph.
It's a lot more impressive in real life. Again, haze. Here's a photo of it from a photographer lucky enough to be up there on a not-as-hazy day. That's one of the famed Cascade volcanoes, that is. Like St. Helens, it's a stratovolcano. Its last major eruption happened somewhere around the time Mount St. Helens got started, about 35,000 years ago. But don't count it as defunct - as long as the subduction zone is active, it has a chance of being refueled and rekindled.
Oregon has more geological goodness than a person can see in a single lifetime, some of it truly spectacular, but this is one of the least-known gems. How many other places can you take a leisurely walk of a few hundred feet and say you've seen the best part of a whole subduction complex?
If you want to see how all this affects the weather, as well as get a more detailed description of what's going on, see Lockwood's post. For an excellent diagram that will help you visualize the geologic structure of this part of Oregon, visit this Oregon Historical Society page - they certainly didn't neglect Oregon's geologic history!
Lockwood DeWitt, who knows this area better than just about anyone, and guided me across the forearc ridge last time I visited him. Look him up if you're ever in Oregon. He knows all the best places.