Geology Is Cool: Cook Inlet’s Plate Tectonics

WC freely admits that he doesn’t pay a lot of attention to popular and unpopular posts here at Wickersham’s Conscience. If all he cared about was high viewer counts, he’d write about The Quitter each day. But even by WC’s inattentive standards, geology is the least popular thing WC blogs about.

But that doesn’t stop WC – a geology minor in college – from returning to the subject.

Most people sort of/kind of understand the basics of plate tectonics. The Earth’s crust is made up of comparatively thin chunks that move around, sliding under each other, moving by each other and generally behaving like extremely slow motion bumper cars. 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:1

Earthquake Activity, Cook Inlet, 1950 - 1996

Earthquake Activity, Cook Inlet, 1950 – 1996

The 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:

Cook Inlet Quake Cross-Section

Cook Inlet Quake Cross-Section

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. The 1989 eruption of Mt. Spurr that closed Anchorage under a fog of volcanic ash was, in some part, millenia-old subducted Pacific Plate.

The same subduction process also created the Kenai Mountains, a bouillabaisse of scraped-off sea floor, continental plate and island fragments lifted up by the subducting plate. That rock bouillabaisse in turn is covered by the glaciers of the Harding Ice Field that grind it down as the subducting Pacific Plate lifts it up, pulling more snow out of the sky.

Like biological evolution, quantum mechanics, stellar evolution and relativity, plate tectonics is only a theory. It’s a theory corroborated by everything geologists can observe in the field. A few of WC’s friends strongly disagree with plate tectonics, ignoring the earthquake lots shown above, preferring instead to rely upon a single old book written by men to whom simple arithmetic was completely unknown. It’s a choice. But an impoverishing one. In Newton’s phrase, it’s playing with pebbles on a beach when the whole time the entire ocean is there beside you, waiting to be seen.





  1. WC apologizes in advance to any Real Geologist stumbling onto this site for the oversimplifications and generalities described here. 

3 thoughts on “Geology Is Cool: Cook Inlet’s Plate Tectonics

  1. I for one love your geology posts. I was not a geology major in college (girls didn’t do that in the 60s) so kind of sidled from ok-for-a-girl chemistry (couldn’t stand the smell) into physics. Which is, of course, the basis of everything (but don’t say that to a mathematician).


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