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.