So you want to design tectonic plates, do you? Part 3
Tips on Designing Tectonic Plates
I think that’s probably enough on how tectonic plates work. That’s 2000 words of text in the paragraphs above. The last thing I wanted to do was give some advice on how to apply all that info to worldbuilding.
If you are designing tectonic plates for your world, you need to be aware that, although it is a creative process, it is based on scientific principles which can only be ignored through magic (and weird magic, at that).
The first thing to remember is that when one continent is moving away from another, it is almost certainly moving towards a different one. Therefore if Continent A has a divergent western edge, then it will probably have a convergent eastern one. This is not always the case, but it is a helpful rule to keep things simple.
Second: oceanic plates are heavier than continental ones, and therefore get subducted under the latter.
Third: planets are round. If you have a continent going off the eastern edge of the map, you’ll find the rest of it on the western edge. This is particularly important when doing the north/south poles. Tectonic boundaries do not go to the north pole and just stop.
Fourth: look at maps of the Earth. There are many online maps of Earth showing the tectonic boundaries and plates. Look at how many there are, and how they vary in size and shape. Some are huge, others are tiny. Mostly they are large and blocky shapes, but sometimes they are a bit more unusual (eg the Austral-Indian plate). Notice also that plate boundaries do not exactly imitate coastal boundaries – many continents are surrounded by seas and oceans which are still included on their plate.
Fifth: while looking at Earth’s tectonic plates, don’t forget that the entire Earth is covered in fissures, and the maps tend to only show those which are clearly defined and large-scale. Other faults exist, and may have some similar attributes to the ones described above, but are less visible at a global scale. This is also worth bearing in mind because, in a few million years, those minor faults might suddenly start to grow.
Sixth: still looking at the Earth maps, search for patterns – they are rarely coincidences. Here are two that I’ve noticed:
• As I mentioned previously, convergent ocean-continent boundaries often seem to hug the shape of the land (as in western South America, southern Indonesia, Japan, parts of the Mediterranean, and New Zealand)
• If you follow the line of divergent boundaries around the world, you’ll find they all seem to be part of the same system. It runs from north of Iceland, down the Atlantic, underneath Africa and Australia (with a branch rising to the Red Sea), underneath the Pacific and up the eastern edge of the Americas. That is an almost continuous line of divergent boundaries, which is never interrupted. A similar thing can be seen with convergent boundaries (from Europe/Mediterranean across southern Asia to Indonesia, rising past Japan to Siberia and jumping over to Alaska, and then sort of running the length of the American coast to the southern tip of South America). Why does this pattern exist? It goes back to my first tip – if it is splitting along one seam, it is being joined along another.
Seventh: landforms. I did say that I would write about landforms in a future post, but since I don’t know when that will be, I’ll give a few brief comments here.
• Mountains: on the non-subducted side of convergent boundaries. Volcanoes are often included, near the continental margin. Also appears where two continents collide.
• Island arcs: often volcanic, these form along convergent boundaries out at sea, on the non-subducted side.
• Trenches: these occur where one plate slides under another (is subducted)
• Rift valleys: obviously, where two continental plates diverge. Often volcanic, and the rising magma tends to form mountainous terrain. These mountains usually have a different character to convergent-zone mountains.
• Mid-Oceanic Ridges: where oceans have grown between two continents. If these rise above the sea level they form islands (like Iceland) but that appears to be rare (at least, it is rare on Earth today).
• Volcanoes and earthquakes: these go hand in hand with tectonic activity. Whether it is converging or diverging volcanoes seem to appear; they are less common far away from plate boundaries. Earthquakes are also common near tectonic boundaries, including transform boundaries. Just ask San Franciscans.
Lastly, when drawing tectonic plates on your map, I would recommend using different colours (or at least different line styles) to designate convergent, divergent, and transform boundaries. I also highly recommend that, at convergent boundaries, you show which side of the fault is being subducted and which isn’t (again, look at Earth maps for some ideas). This will help a lot later when adding landforms and ocean-forms, like mountains, volcanoes, and trenches.
I think that’s it for plate tectonics. A huge topic, and this post has probably left things out (or gotten them wrong). As usual, any corrections, questions or comments are welcome.
[EDIT: the university semester resumes in Australia soon, so I don't know how long it will be before I can provide any more tips. I'll still read people's comments, and try to respond to quick questions, when I am able]
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