Cartography: Plate Tectonics

Let me start by saying that no, you do NOT have to figure out functional plate tectonics for your world.

(For starters, plate tectonics assumes the existence of moving plates, which kind of presupposes a round planet with a crust and a mantle and a core, which may not be true depending on what you’re writing.)

But understanding plate tectonics can go a long way toward making you understand mountains and volcanoes and where earthquakes come from, which is handy in some cases. So I’ll give a nutshell version here. If you want more than a nutshell, the United States Geological Survey has an excellent primer online, which is where I checked much of my info on this and filled in the holes in my understanding.

Plates are, in highly non-technical terms, giant chunks of rock that make up the surface of the planet, and they move around. Since there is no part of the surface of the planet not already occupied by a plate, this means they smash into one another. There are four ways plates can interact:

  • In a divergent boundary or spreading zone, where they’re moving away from each other;
  • In a convergent boundary or subduction zone, where they’re moving toward each other;
  • In a transform boundary, where they’re sliding past each other;
  • Or in what they so elegantly call a plate boundary, where geologists aren’t quite sure what they’re doing.

Divergent boundaries are where new planetary crust is made. The plates keep pulling away from one another, and the magma beneath wells up to become rock. The middle of the Atlantic Ocean is a stellar example of this. When the boundary is on dry land — for example, where the mid-Atlantic boundary runs right through the center of Iceland — you can sometimes actually see cracks forming in the earth. There’s another case of this in East Africa. As you might expect, this can result in volcanoes, where the magma doesn’t just well up quietly and turn to rock before hitting the surface, but erupts upward violently.

Convergent boundaries are where old planetary crust is destroyed. They’re called subduction zones because when two plates fight, one of them loses, i.e. dives under the other and melts into magma once more. If an oceanic plate and a continental plate meet like this, the oceanic plate is the loser, because it’s mostly formed of a heavier stone than the continental plate is (basalt instead of granite). The result is a deep trench not far offshore and then often a mountain range not far inland, which may contain active volcanoes. See: the Andes. You also get earthquakes in a zone like this, as bits of the subducted plate stick and then slip loose. When two oceanic plates collide, the result is similar; again, one loses, there’s a good chance of volcanoes and earthquakes, and by this process you get things like the Aleutian Islands. (But not the Hawaiian Islands, which are weird and nowhere near a plate boundary.) They go in a curve along the edge of the trench formed by the subduction of one plate. Finally and perhaps most impressively, when two continental plates duke it out, neither one really sinks; they basically shove up and up and up, until you have the Himalayas, the highest region on earth.

Finally (because I’m not going to talk about muddled and confusing plate boundary zones that even trained geologists have trouble sorting out), you’ve got transform boundaries, where chunks of crust are sliding past each other. Apparently most of these are underwater, but they can be a huge source of earthquakes; the San Andreas Fault is of this kind, and is one of the only examples on dry land.

What we can get from this, therefore, is an idea of why volcanoes, earthquakes, and giant mountain ranges happen. Because of these processes, there are patterns in where stuff shows up; the “Ring of Fire,” the series of volcanoes at the edges of the Pacific Ocean, is a good example. By contrast, not so many volcanoes in the middle of the United States. They can happen, though. This is apparently caused by the same process that produced the Hawaiian Islands, namely, a buried “hotspot” powerful enough to melt enough crust to let stuff through. (Yeah, I’m all over the technical terms.) These are less common than the other kind, though, and — as a random detail — appear to produce volcanoes in a straight line, instead of along a curve.

So, the gist of all this discussion is, although you don’t need to figure out the complete plate tectonics of your world, if you’re attempting to make the place look physically natural, pay attention to where you’re putting your volcanoes.

Next: your guess is as good as mine . . . .