At the boundaries of two plates, one of the things that can happen is for one plate to slide under another.
That’s what’s happening on the southern coast of Alaska, where the Pacific Plate, moving northwesterly, is sliding under the North American Plate and, specifically, Alaska. The Pacific Plate, under the Gulf of Alaska, is sliding under Southcentral Alaska and the Being Sea. This is important. It gives us things like the Great Alaska Earthquake of 1964, the eruption of Mt. Katmai, the Aleutian Island arc and the Denali Fault. It has also given us a pretty nifty image of the Pacific Plate sliding under Alaska – the technical term is “subducting.” Here’s a plot of earthquake activity in Cook Inlet over that last 50 years:1The size of the round dots is proportional to the strength of the quake; the color of the dot is the depth of the epicenter of the quake. You can see that the blue dots – the deeper quakes – trend to the westerly region, and the red/purple/pink dots – the shallower quakes – to the east side. To better understand what this is showing, we’ll choose a cross-section, roughly from Homer on the east shore of Cook Inlet to the slopes of the Alaska Range, and view that cross-section from the side. If you prefer, think of it as a trench dug 250 kilometers deep, and looking at the wall of the trench. Here’s what you’d see:This is Pacific Ocean floor being pushed under the North American Plate. It could not be shown more clearly. Earthquake activity generally ceases at depths greater than 200-250 kilometers because rock heats enough from pressure and the earth’s internal temperature that is deforms plastically instead of fracturing and generating a quake. The approximately 60 degree angle of descent is what geology would predict and what is generally observed. Deeper than 200 kilometers, the subducted Pacific Plate melts and, for various reasons involving chemical composition, water content and density, rises as the volcanic arc of Augustine, Iliamna, Redoubt and Spurr, among many others.