Plate tectonics in a nutshell

Our audience was this young Black-Crowned Night Heron (Nycticorax nycticorax), Oakland’s official city bird.

I sat down the other day for an interview with Liam O’Donoghue, producer of the exemplary East Bay Yesterday podcast. I had thought ahead about what he might ask, of course, but didn’t think he’d ask me to define plate tectonics, an all-encompassing Theory of the Earth, in a soundbite way.

I can discuss the details of plate tectonics all day without repeating myself, because I’ve soaked in the literature for fifty years. In my book I tried to minimize that while staying true to the science, avoiding lazy cliches and saying no more than what was absolutely necessary. I spent days boiling it down to a few essential pages. How could I deliver an “elevator pitch” version on the spot?

What I came up with may not make it into the final audio, but it went something like this.

The Earth is hot and gooey on the inside and cold and brittle on the outside. The inside part, despite being made mostly of solid rock, manages to move and stir, somewhat like the contents of a pot on a stove. And the outside part, cooled by the cold of outer space, responds somewhat like the surface of a cup of coffee as you blow on it. Between these two influences, cooling from above and heating from below, the hard outer crust breaks into about a dozen large segments, called plates although they’re really curved shells, and Oakland sits where two of those plates rub against each other.

Most discussions of plate tectonics don’t go this way; they start by listing the different working parts and then describe how they interact, then they tell how that all translates into other features on the Earth’s surface like mountains, oceans, volcano chains and so on. They also recount the history of how we arrived at this theory. The free textbook An Introduction to Geology, a sound text written by experienced teachers, does it that way. To my mind, this bottom-up approach has never been easy to digest, although serious geology students need to learn it all. I think it reflects the way, historically, that plate tectonics was first discovered.

I prefer to start with the gist. It starts on higher ground, and it leads directly to an intellectually exciting frontier in astronomy: Earth-like exoplanets that revolve around other stars. The latest generation of telescopes has shown us growing numbers of worlds similar to ours — small, with solid surfaces and relatively thin atmospheres. As we learn more about these planets, we should be able to distinguish the signatures of different tectonic styles in the light they reflect toward us.

Earth hasn’t always had plate tectonics. The evidence is subtle and the arguments are intricate, but Earth behaved differently for its first billion years (or two or three billion depending on different lines of evidence). And in the next billion years (or two or three), plate tectonics should come to a halt. Is this typical? Having only one specimen handy, we don’t know.

The same way we worked out the general life history of stars by looking at thousands of them, of random ages, so we ought to be able to do the same for planets like our own. Do they go through the same stages as Earth? If so, what’s the general timetable, and if not, what other possibilities has nature found? The answers are waiting for future scientists to discover them.

And then drilling down from this elevator-pitch version of plate tectonics, it’s a short reach to comprehend the situation in Oakland: We live where two plates are moving past each other, along the Hayward fault. The Hayward fault is one of several faults like it on either side, including the San Andreas fault on the west and the Calaveras and Concord faults on the east. Together, these make up the boundary between the North America plate and the Pacific plate.

What’s simple to describe in the elevator pitch is more complex close up. I picture the plate boundary as a set of several freight trains on parallel tracks very close — too close — to each other, rolling the same direction at different speeds. The tracks are so old and rickety that the cars keep swaying and colliding with the next train over. This analogy helps me understand the long-term effects on the landscape along the plate boundary over the last few tens of millions of years: the rise of the Coast Range. I think that gets the gist, too.

PS: Twelve years ago I wrote a slightly longer piece, “About Plate Tectonics,” for the site now known as, that still reads well. (They took it down in 2016.) I avoided the historical approach there, too.

For those with access to Naomi Oreskes’ 2001 essay collection Plate Tectonics: An Insider’s History of the Modern Theory of the Earth, David Sandwell’s wonderful essay “A Martian View of Plate Tectonics” imagines how the theory could have been assembled in an alternate history based purely on satellite observations. His publically available draft is pretty close to the printed version, and just as refreshing.

3 Responses to “Plate tectonics in a nutshell”

  1. Melodie Says:

    I kind of think of plate tectonics as a pan of softly boiling stew that has developed a top layer of “scum.” It helps me to visualize how the scum gets moved around and jumbled about from what is happening below it. It might not be very accurate, but it helps my mind to have something I can visualize in real time so that I don’t get overwhelmed by the size and time in which actual plate tectonics operate.

    I still struggle with translating this into the round earth.

    I also just recently read about subducted plates retaining aspects of their structures all the way down to the core. Which makes my stew analogy perhaps less accurate than it was to begin with. Maybe I need to think of it more as porridge with lots of vegetables etc in it … always a work in progress, and endlessly fascinating.

    I love that you ground all of this in a landscape that I can see outside my window.

  2. oaklanddots Says:

    When will the podcast be available to listen to?

  3. Christie Weiner Says:

    Andrew Alden, I’m awed by your clear verbal descriptions and visual similes for complex geology. I also love the photo of the night heron.

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