April 6, 2018

Tectonics: a Primer

One of the fundamental steps in worldbuilding is the creation and analysis of the plate tectonics, assuming your planet has them (Mars does not, for example).

Most importantly, the tectonic map can inform me where to place mountains, continents, and the like. Like any other topic I'll discuss here, the science of this can get extremely complicated. So I have to make some concessions and shortcuts. My apologies in advance to the geologists. I have tried to strike a balance between accuracy and simplicity.

I'll take a moment to mention GPlates. I've never used it, and I don't plan to for this project, but many people swear by it as a crucial step in accurate tectonics.

Tectonic plates themselves are not as much interest to me as what happens at the boundaries. There are four major kinds of boundaries:


I'll mostly ignore subductive and transform boundaries as individual categories - I've found it's easier for me to characterize them as either c or d depending on the relative directions of the relevant plates.

What these look like will depend on the type of crust covering the plates at each boundary, oceanic or continental. Since continental crust is less dense, it "floats" on top of the oceanic, and will generally be the one on top when one plate is forced on top of another in covergent or subductive processes. It's also important to note that oceanic crust is not necessarily covered by water, and vice versa for continental.

Hence, I can simplify my plate interactions to six total types.

  • Convergent oceanic/oceanic (cOO). Forms volcanic island chains as the submerging crust strikes magma plumes.
  • Convergent oceanic/continental (cOC). May form volcanic chains on the coasts of continents, as well as back-arc basins, which are very interesting.
  • Convergent continental/continental (cCC). This is where the largest mountain ranges will occur.
  • Divergent oceanic/oceanic (dOO). The source of new oceanic crust. Causes mid-oceanic ridges, but these are not usually visible above water. I'm not worried about these since I'm not mapping the surface of the ocean...yet. I must mention ├×ingvellir, a park in Iceland where the dOO is actually above water.
  • Divergent oceanic/continental (dOC). Doesn't really happen due to the underlying physics. Oceanic crust is itself formed by divergent boundaries (and eventually becomes continental over time).
  • Divergent continental/continental (dCC). When the divergence happens between continental crust, you get graben/horst features and sometimes volcanic activity. This opens an avenue for sea ingress, although I wouldn't build any port cities here that you wanted to last for more than a few million years.
Now what? Well, despite my raging against ad hoc, I've already committed that there will be certain features I want to see in my world. Part of this is to use my own brain as input to simulation, to prevent the homogenous worlds that many procedural generators produce. With this information, I can begin drawing my tectonics based on where I want mountains or continents to go, and the rest of the world will be "filled in" based on that.

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