So you want to design tectonic plates, do you? Part 1
Well, after a long hiatus, I’m back with more worldbuilding tips. This one is about plate tectonics.
Many people begin their worldbuilding adventure with tectonic plates. It is a logical first step, and they set up the foundations of the world being built. Plate tectonics are largely responsible for the shape and arrangement of continents, the creation of oceans, and landform features like mountains, volcanoes and rift valleys. Once you know where your continents are, and how big they are, you can move on to meteorology, precipitation, establishing biomes, and generally deciding where life occurs on your world. And all that starts with plate tectonics.
My goal in this post is to look at what they are, and how they work, and give some tips on how to design them for your world. To make it a bit more user friendly, I’ll also post it in three separate goes.
A warning: this is a complicated subject. A lot of what follows is quite technical. I’ve also tried to simplify it, which means it is not 100% scientifically accurate, but it is hopefully accurate enough for your purposes. To put it simply: if you are having difficulty, ask a question. I can guarantee someone on the CG will try to help.
So, let’s get started.
What are they?
Tectonic plates are pieces of the Earth’s crust which…… wait, let’s go back a step.
A Kindergartener’s introduction to Earth’s internal structure (and other planets, too)
The core is the inner part of the planet; it is very hot, very dense, and has a large role to play in Earth’s gravitation pull. It is also not very relevant to plate tectonics;
The mantle is the middle part of the planet; it is perhaps easiest to think of it as a layer of molten rock (geologists would shake their heads at that, but we’re worldbuilders, not geologists). Due to the forces of thermodynamics…… hold on, what was that??
(Thermodynamics: the relationship between heat and energy/work. Put simply, when something is cold it condenses and sinks, and when it is hot it expands and rises; this is one of the simple scientific principles that governs many aspects of Earth’s systems. So, back to the Earth’s mantle…)
…as I was saying, due to the forces of thermodynamics, the stuff of the mantle (i.e magma) moves. It is hottest near the core, and so the mantle gets very warm there, and expands, causing it to rise through the mantle, leaving a vaccum that is filled with cooler stuff (magma); this process continues and forms a current of moving molten rock (magma);
The crust is the rigid outer layer of the planet; it is what we Earthlings know and love as the surface. Now, to understand plate tectonics you need to understand what the crust is actually like. Unfortunately, all this life and water on Earth’s surface makes it hard to see the true nature of the crust, but don’t worry, there is a solution.
Try looking for pictures of Jupiter’s moons, Europa and Ganymede. Y’see how they’re covered in lines? Those are fissures in the surface caused by stresses (heat but also gravitational pull). Even though it is made of stone, over geological time, it behaves like old leather or semi-dry clay – it bends and distorts, but the stress of doing so creates seams which crack and buckle, and may, after too much pressure, snap off the main crust entirely. The lines that appear are known a faults, and I’ll get back to them in a little while.
So, to put all that together: a planet with a hot core causes convection (movement) in the mantle; movement in the mantle causes the crust to flex and distort; distortion of the crust causes cracks to appear (faults) which can eventually grow large enough to break apart; when they break apart a new plate is formed.
Back to: What are they?
Now, this is where we started. Tectonic plates are pieces of the Earth’s crust which float on top of the mantle, and are constantly bumping against each other. Imagine a lake with two boats on it – the boats will bob around, and move slightly according to the currents. They probably won’t collide, though. Now add more boats – so many that the entire lake surface is covered in them. They can no longer move without hitting one another. It’s the same thing with the planetary crust and its tectonic plates.
When a part of the mantle is exposed, it cools and forms a new area of crust. That is why the entire surface of the planet is formed of tectonic plates/boats, and there is no exposed mantle/lake between them. They are constantly moving, but there is no space, so they jostle agaisnt each other, bumping here, and moving apart there.
So that is what tectonic plates are. That clear?
So you want to design tectonic plates, do you? Part 2
How do they work?
You need to know that there are two types of tectonic plate, and three types of tectonic boundary. Boundaries occur where two plates meet. The three types are convergent, divergent, and transform boundaries. The differences between them is the main point of this section.
The two types of tectonic plate are continental, and oceanic. The main difference between the two types is their mineral composition. Oceanic crust is denser, and therefore heavier, than continental crust. So, if we return to the boats analogy, imagine if some of the boats have 2 people inside, and the others have 10 – the heavier boat will sit lower in the water. Because oceanic crust sits lower, therefore, it is where all the water naturally drains to, and hence we get oceans. The continental crust sits higher, and so is not covered in water (actually, the edges of continents often are underwater, forming continental shelves; I’ll probably try to do a landforms post sometime in the future to cover these things). Just to be clear – the Pacific Ocean has its own tectonic plate; the Atlantic Ocean does not have its own plate.
Now, let’s get back to the different kinds of plate boundary.
Convergent boundaries are where two plates collide. The collision causes one plate to slide underneath the other one, getting pushed down towards the mantle, where it melts. This process is known as subduction, and creates a large trench at the subduction zone. Clearly, the heavier plate (eg. an oceanic plate) is more likely to get pushed down, which is why trenches usually appear in oceans and not on land.
As one plate gets pushed down, the other (usually the less dense) gets pushed up on top of it. I made a comparison to old leather and semi-dry clay before. Leather is flexible enough that it can be bent easily, but rigid enough that it doesn’t want to be distorted – the result is that it buckles. Another comparison would be when two cars collide, and the bonnet is compressed. Now imagine that happening at a much slower rate and on a much greater scale, and you’ll end up with something like the Himalayas or the Andes. Therefore, mountain ranges often form along convergent boundaries. If you take a look at a satellite image of a mountain range you’ll see that they often run in long parallel ridges – this is a result of this process, and may help you design geologically plausible mountain ranges.
When the subducted plate gets to a certain depth (70-80 miles, according to Wikipedia) it melts, and the magma rises. This results in volcanoes which either form part of the larger mountain range, or create whole new islands. These usually sit very close to the trench and are always on the uplifted side of the boundary.
Convergent boundaries can be:
• Oceanic-Continental: forms subduction zone (trench) and mountain range. Examples: oceanic plates being pushed under South America forming the Andes; Australasian plate getting pushed under the Eurasian plate forming Indonesia.
--> If you look at a map of plate boundaries, you’ll notice that ocean-continent boundaries often hug the edges of continents (as in South America, Indonesia, the Philippines and Japan).
This will become important when we look at designing tectonic plates for your world.
• Oceanic-Oceanic: generally the same thing as above. This is the type of convergence that causes volcanic island arcs which are not part of larger mountain ranges
• Continental-Continental: neither plate is subducted, but both buckle, forming a mountain range.
Divergent boundaries are where plates move apart (or more accurately, where the underlying currents of the mantle push them apart). Where two things move away from each other you get left with a gap between them. In this case, that gap gets filled with rising magma (forming volcanoes), which then cools and forms a new area of crust. On land, this becomes a rift valley; at sea it forms a mid-oceanic ridge. As the divergence continues, these features grow bigger and bigger. Rifting areas on land or underwater are always volcanic.
When a rift formed between Africa/Europe and the Americas, it drove the two continents apart and the rift kept getting bigger and bigger. The result is what we now call the Atlantic Ocean, and it’s still growing. The Mid-Atlantic Ridge is continuing to get wider, and in places (i.e. Iceland) it pokes above sea-level. If you lived on one side of Iceland and your friend lived on the other, each year you would be slightly further away from each other.
As the magma exists the fissure between the continental plates, it rises and then cools; successive layers of that gradually raises the height of the seabed on either side of the fissure, resulting in the ridge. As the magma rises and cools, it forms layers. Therefore the Atlantic Ocean is being created from the inside out, and the oldest bits are on the edge.
The Great Rift Valley in Africa is another (rather obvious) example of a rift valley that is continuing to grow. A chunk of south-eastern Africa is slowly tearing itself away from the main plate (which brings up a point – plates change. They continue to grow, shrink, divide, and, presumably, combine even as you read this). The Red Sea is where a rift valley has been flooded.
So, to recap: convergent boundaries are where plates collide, divergent are where they separate; when plates collide (in most cases) the edge of the continental crust gets destroyed (subducted and melted to become part of the mantle again) and when they separate new crustal material is created. Y’see – it’s all one big cycle.
Transform boundaries are where two plates slide alongside each other. They do not create or destroy crustal material. They are somewhat trickier than the other two, and probably less relevant, but I’ll try and give them a general overview.
When plates separate, the resulting fault (divergent boundary) is not uniform along its length. This results in rifting areas forming blocky segments, rather than continuous lines, and new seams emerging as they separate. It’s a bit hard to understand, and I’m not 100% myself, but I’ve attached an illustration that hopefully makes it clearer.
• The left diagram shows a single black line, which is the divergent boundary
• The middle diagram shows how that has separated as blocks rather than a continuous fault; the red lines are transform faults (they extend beyond the divergent boundary, which is shown by the area in the red outline)
• The right hand diagram shows what happens as they split apart, moving at different rates. The green spot (which could be a mountain range, a geological layer, or even an island) gets cut in half.
The San Andreas fault in the United States is on a transform fault, which means it is moving at a different pace to the rest of mainland America, and will, one day, be ripped away from the rest of the continent. New Zealand also has a large transform boundary across a part of it. As usual, doing some quick online research can give you a good visual idea of what a transform boundary can look like.
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]