New Worlds: Plate Tectonics

(This post is part of my Patreon-supported New Worlds series.)

If you’re like me, you learned the basic ideas of plate tectonics in school: giant masses of stone, some of them with water on top, floating around on the earth’s molten core and making things like continents happen. But the details may have faded in your mind since then, and even if they haven’t . . . odds are good that nobody ever talked about how these things might work, or not work, in the altered circumstances of speculative fiction.

So let’s review, and then let’s speculate.

I’m going to keep it fairly simple, because I’m not really qualified to get complicated. (Dammit, Jim, I’m an archaeologist, not a geologist!)

As the plates of our planetary crust move around, there are three ways they can interact: they can move apart (a divergent boundary), slam together (a convergent boundary), or slide past one another (a transform boundary). The places where these things happen are called “faults,” as in the San Andreas Fault (to pick a famous example). You can roughly map the entire surface of our planet into a set of plates, though the maps you’re likely to find when you search for that are simplified — there are also cratons, i.e. smaller, more stable bits inside continental plates, which is why you have faults in places like Texas, far away from any major plate boundary.

Divergent boundaries are where our planet makes new land, through the action of lava rising up and cooling into stone, pushing plates apart. Most of these are found on seafloors, though a few are on land; there’s one running through East Africa and the Sinai Peninsula, whose sections are collectively known as the Great Rift Valley, which is in the (very. very. slow.) process of cracking Somalia and Ethiopia off the rest of the African Plate. The Mid-Atlantic Ridge system is, as the name suggests, largely underwater, but part of it runs through Iceland and is widening that island at a rate of a few centimeters per year.

Convergent boundaries, on the other hand, are where our planet destroys land and in the process lifts up mountains, as one plate slips/is subducted below another and shoves that one skyward. (Remember, I’m simplifying here. Divergent boundaries form ridges along their length, too, but since most of that is deep underwater, it won’t be that relevant to most stories, which tend to take place on dry land.) What kind of mountains you end up with depends on which plates are meeting: if it’s two oceanic plates, you get volcanic islands in the plate that’s on top. If it’s a continental plate and an oceanic plate, the latter (being heavier) gets subducted under the former, so that you end up with a deep offshore trench and a volcanically active coastal mountain range — this is why we have the “Ring of Fire” around the edge of the Pacific, with ranges like the Andes. And finally, if it’s two continental plates, you get the Himalayas.

Transform boundaries — like the aforementioned San Andreas fault — don’t either make or destroy land, but rather just slide one piece of it past another. They’re a type of strike-slip fault, but this is the point at which I’m going to stop falling down a Wikipedia hole of geological research, and talk about why this matters.

To begin with, it matters because it affects what your maps should look like. Assuming you’re writing about a world with plate tectonics like our own (which you might not be; more on that in a moment), it should have landforms a lot like our own. A steep, narrow range of coastal mountains like the Andes aren’t likely to have extensive shallow seas with coral reefs offshore, because those types of mountains come from an oceanic plate subducting under a continental one, and that means there should be an ocean trench not too far away. A broad band of mountains like the Himalayas and their neighboring cousins aren’t good candidates for extensive volcanic activity — you’re more likely to find that in the aforementioned coastal range. Ever notice how West Africa looks like you could fit it between North and South America? That’s because it used to be there (ish; remember that we’re simplifying here). Putting a similar echo of shapes into your continental coastlines will provide a bit of subconscious verisimilitude to your map.

Does that mean you have to put together a tectonic map of your entire world? No, of course not. I did for the Memoirs of Lady Trent (with the assistance of the guys who wrote a speculative geological history of George R.R. Martin’s Westeros), but that’s because I was writing about a globetrotting natural historian, and needed her natural environment to hang together sensibly. Having at least a general awareness of this means you’ll know when and where to include natural disasters like earthquakes, tsunamis, and volcanic eruptions, or natural features like steam vents (fumaroles), hot springs, and dramatic rift valleys. It will help you avoid things like the infamous Square Mountains of Mordor.

Unless you want square mountains. This whole Patreon series is geared primarily toward speculative fiction, where there are no guarantees that the world of the story is a round ball with plate tectonics and all their associated processes. Your novel might be set on a moon covered in water ice like Europa, or a flat land like Discworld. Earthquakes might be caused by the thrashings of a bound and tortured god, rather than the sudden movement of one plate against another.

But as with any aspect of worldbuilding, if you’re going to change some basic aspect of nature from what the reader is used to, then it helps to communicate that fact, and take into account how it will affect the the story. (Unless it isn’t relevant to your story at all, in which case it gets the same treatment as any other background aspect you’ve worked out: save it for a blog post or an easter eggs section on your website.) Characters living on a water-ice moon won’t be doing very much on dry land — but they might experience ice movement very similar to plate tectonics. Characters living on a flat world won’t see distant ships at sea appearing to “rise” and “sink” out of the water, because the curvature of the earth won’t obscure the hull and then the masts as they get farther away. When that bound and tortured god thrashes and causes an earthquake, people will respond in ways designed to alleviate his suffering or something else to prevent further destruction.

If that last sounds a lot like our own world prior to the advent of things like plate tectonic theory, well, you aren’t wrong. People act based on what they believe to be true, and in a fantasy world, what they believe might be physically and metaphysically correct. Or it might not be, and the reality might be something else that doesn’t match our world. But whether you’re talking about water-ice moons or bound gods, you want to make sure you’re internally consistent.

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About Marie Brennan

Marie Brennan is a former anthropologist and folklorist who shamelessly pillages her academic fields for inspiration. She recently misapplied her professors' hard work to the short novel Driftwood and Turning Darkness Into Light, a sequel to the Hugo Award-nominated Victorian adventure series The Memoirs of Lady Trent. She is the author of several other series, over sixty short stories, and the New Worlds series of worldbuilding guides; as half of M.A. Carrick, she has written The Mask of Mirrors, first in the Rook and Rose trilogy. For more information, visit, Twitter @swan_tower, or her Patreon.


New Worlds: Plate Tectonics — 10 Comments

  1. As one who worked out tectonic plates for a world a few decades ago, figuring out how a square mountain range happened (suitable magic) and the results was kinda fun. Annihilating, but fun, with the changes ringing down millennia later in everything from climate to culture.

    • Yeah, you can justify all kinds of weird things with magic. It only annoys me when the actual sequence is that the writer didn’t realize things don’t work like that, and then tries to spackle over it later by claiming it’s because of magic. (Why did magic change the way this one thing works, and not everything else?) If you build that idea in from the start, I’m fine with it, and it can be a lot of fun.

  2. I remember reading an SF book about humans who colonised another world: Destiny’s Road by Larry Niven. The world was an old one, without plate tectonics.
    You’d think that would make it safer, without much vulcanism ( maybe an occasional magma plume could create a hotspot) or earthquakes.
    But that also means no new landmass-formation, while weathering of the existing landmasses does go on by way of rain and wind, lichen and algae.
    So not much in the way of high and steep mountains, either.
    So colonists landed, started farming and expanding their communities; everything appeared to be going well. And then gradually, they started turning into idiots.
    All the weathering without replenishment meant that a lot of the rare earth elements were shifting toward the ocean floor, and not being returned. That meant that the crops could take their needed trace elements out of the soil they were planted in for a couple of years, but then those trace elements were gone, pissed out and flushed downriver to the sea, even if a lot of waste was processed to manure the fields. And humans need traces of a lot of these rare trace elements to keep functioning well.

    If a new world has a relative scarcity of something like manganese or potassium, but otherwise fairly ideal circumstances, would we hesitate to colonise it? Would we realise that a certain source of that mineral could be essential to human life, as well as a way to distribute it fairly to all the inhabitants? Would any societal set-up which guarantees that be proof against power-hungry people wanting to exploit that?

    Even on earth, mining companies are starting to look at deep-ocean mining of manganese, because most of the stuff has migrated there over the eons (in the form of manganese balls IIRC); but that takes heavy technological and industrial investments perhaps not available to new-world colonists. And from what I understand, plate tectonics is far from a universal characteristic of planets and moons; nor is there any guarantee that all the trace elements earthlife is accustomed to will be present everywhere else in similar proportions.

    The book made me realise, viscerally, how we as humans are adapted to living on earth. How even some small differences could have very large unforseen consequences.

    Like rotational period: if a new planet’s day is 5 days long, or 5 hours, we’d probably more or less ignore it and keep a human sleep-wake rythm. Our circadian bodyclock appears to be set at 26-hour days (when living in total isolation from outside signals regarding day- and night-time during longer experiments), but to get reset by light each morning. So adapting to a slightly longer or shorter day would probably be possible. But what if the new days were 28 hours long, and we could live on the surface and farm and do ordinary outdoors jobs? Would we be able to adapt, or would we get further out of sync each day, and what would that do to us, hormonally, physically, and mentally?

    • Oh, wow. That is some grade-A attention to worldbuilding there, linking plate tectonics to nutrition! The circadian rhythm thing is an issue I’ve seen crop up in SF (though never in a portal fantasy, now that I think of it); the circulation of manganese, not so much.

      • It’s not just the plate tectonics that influences the availability of nutrients; weather patterns, wind and ocean circulation play a part as well. Look at how sandplumes taken high into the air by sandstorms in the Sahara Desert can fertilize the Atlantic ocean and even the east Amazon basin, and rain dust down all the way into northern Europe depending on the prevailing winds at the time – that’s one way to distribute newly weathered sand and rockdust with all its trace elements. Large tropical hurricanes not only distribute water vapour and heat from the tropics to the colder northern latitudes, but also whatever else they’ve picked up in the way of dust (that water vapour condenses around to form droplets). Dust is not something people ordinarily think about, unless it needs cleaning or there’s a sandstorm in the desert, but a book like Niven’s can suddenly open my mind to seeing how the whole world is interconnected in myriad ways.

        It’s probably more of an SF thing than anything you could expect in a fantasy, this much attention to physical geographic details, and with the shift away from the “isn’t science wonderful” exploration type of SF in recent decades towards (epic) fantasy or military / space opera SF and other subgenres, I don’t find that exact sense of wonder in the natural universe as much in most recent books I’ve read. Though maybe I’m not looking in the right places to find it, as your lady Trent does have it.

        • It’s probably more of an SF thing than anything you could expect in a fantasy

          Yeah, it’s hard to convey that kind of thing without having it be a central focus of the story, which is generally the domain not only of science fiction, but of a very specific kind of science fiction. (The kind that often gets called “hard” — a term people argue over, but my definition for it is “stories that are concerned with how stuff works and why.”)

    • Wow, that’s a lot of thought put into it. Very interesting. I might have to try and find this book.

  3. I never understood much of this until I discovered the Science Channel on satellite TV. They have wonderful uncluttered graphics that shows what happens with earthquakes and build up of magma inside a volcano. Suddenly all my geography and geology classes made sense and I redesigned my dragon world.

    • One of the things on the (endless) list of future topics for this series is the benefits writers gain by doing things like watching the Science Channel! Focused research into things you know you need to know about is good, but so is putting yourself in a position to constantly encounter a wide range of new topics.

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