Maps and Geography

In this post we’ll discuss how to make a working map for your worldbuilding project, creating plate tectonics, wind zones, and ocean currents, turning your planet from a floating rock in space into a habitable world with continents and climates.

Hey everyone, my name is Matthew, at least according to my geography textbook, and this video is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time we created the solar system that our habitable planet called Locus has settled into, building some planetary neighbours and moons that will be visible from Locus.

For today’s discussion, we’re going to be establishing the geography of the planet, beginning the tectonic process beneath the surface, and giving rise to the continents that will form our first map of Locus, with which we can fill in our wind zones and ocean currents.

To start with, we have Locus as a blank planet. Well, not entirely blank. Locus, like earth, would become a water world once it has settled into its orbit within the habitable zone. It is expected that earth’s liquid water must have existed as far back as 4.4 billion years ago, within 150 million years after earth’s creation in the solar system. It’s widely accepted that for life to exist as we know it, or more specifically, for life to EVOLVE as we know it, then water would have to exist in this way on any planet that is going to support life.

While water is critical for life as we know it to develop, having a world map that is just completely coloured in blue isn’t particularly interesting. This is where plate tectonics come in. Plate tectonics is a well-accepted scientific theory wherein a planet’s lithosphere, that is it’s outermost layer, is divided into a number of plates that slowly move over the asthenosphere, the region of the planet’s mantle immediately below the lithosphere.

The lithosphere is cool and rigid, while the asthenosphere is hot and flowing. It is this difference in mechanical properties that cause the plates of the lithosphere to move.

So, in short, a planet requires a cool surface but hot interior for plate tectonics to occur. It is speculated that this is why a planet like Mars likely has plate tectonics, but a planet like Venus likely does not; it is simply too hot on Venus’ surface for tectonic activity to occur.   

Another important possible factor for Venus’ lack of tectonics is the absence of water, however, this is scientific speculation, and there are some researchers that maintain that plate tectonics can still occur even in the absence of oceans on the planetary surface.

Let’s say for Locus that some tectonic activity did occur prior to it forming oceans on its surface, but that the process was very slow, only accelerating to an earth-like pace once it had settled into the solar system’s habitable zone and developed an oceanic surface. While planets would start with just two or three plates across their surface, as tectonic activity continued those plates would break apart and we’d end up with a greater number of smaller plates, until you have something like… this. For reference, earth has 7 major plates, 10 minor plates, and a bunch of tiny sub-plates. I’ve given Locus 30 plates, which is slightly more than earth to represent the extra time it had for tectonic activity as a rogue planet.

The main important consideration here is that your fault lines shouldn’t ever make corners with each other; that is to say, having three lines meet is fine, but having four lines intersect is against the rules. You also want to consider that we’re dealing with a planet which is a sphere, not a 2d object, and so you’ll want the right and left sides of your tectonic map to match up. Also, everything at the top and bottom of the map is going to be elongated, so even though the plates I’ve got at the top and bottom of my map look massive, they’re really just visually bigger on this 2d map; in reality they’d likely be about the same size as the rest. If you’re having trouble with this kind of visualisation, there are applications out there that you can use that makes this process simpler. My personal recommendation is a free website called Experilous, which is linked here: http://experilous.com/1/project/planet-generator/2015-04-07/version-2.

Before we can put our plates into motion, we have to determine which ones are continental and which ones are oceanic, the main difference being their composition. A continental plate is felsic, meaning that it is thick but less dense, composed of lighter elements like silicon, oxygen, aluminium, sodium, and potassium. The density of a continental plate averages between 2600kg per cubic metre (162 pounds per cubic foot) to around 2750kg per cubic metre (172 pounds per cubic foot). Continental plates take a long time to form but are rarely destroyed, and some of the continental crust that forms in the early stages of the tectonic process will survive throughout the planet’s entire history.  

Oceanic plates in contrast are mafic, meaning that they are thin but denser, composed of heavier elements like magnesium and iron. The density of oceanic plates sits at around 3200kg per cubic metre (200 pounds per cubic foot). Due to their continued recycling as they sink back into the asthenosphere, oceanic plates are much younger than continental plates, rarely sticking around for more than 200 million years before sinking back into the mantle.

Just as their names imply, continental plates will include a continent, and oceanic plates will be predominantly oceanic. I know right, mind blown. We’ve already determined when we created Locus that there’s more water on Locus than on earth, so let’s limit our continental plates to 5 out of the 30 plates we’ve got, making the planet around 85% covered in water compared to earth’s 70%. Let’s also put those 5 plates in groups so that they can form larger continents, like… so.

Now that we’ve got the plates, we have to determine their movement. All plates move at least in some way, though some are more active than others. A good rule of thumb is that across the planet there’s probably at least one major section among oceanic plates where they’re moving apart, and one major section among continental plates where they’re coming together. Note that plates don’t just have to move in one direction, they can also rotate. Let’s give our plates… these movements.

Where plates meet on their boundaries, tectonic activity occurs. What type of activity depends on the movement of the plates that are bordering each other. There are three main types of plate boundaries; divergent, convergent, and transform. Simply put, divergent is where plates are moving apart, convergent is where plates are coming together, and transform is where they are moving alongside each other. What occurs at these boundaries then depends on whether the plates are continental or oceanic, but at pretty much every boundary type, expect earthquakes.

For divergent boundaries, whenever two oceanic plates diverge, magma will come up from the mantle and harden to form a ridge. This will either form an underwater mountain range, or if the ridge becomes large enough to emerge from the water, a chain of volcanic islands will form. The best real-world example we have of this is the Mid-Atlantic Ridge.

Whenever two continental plates diverge, the exact same thing happens but the rift will progressively drop below sea level. The Red Sea is an excellent real-world example of a well-developed continental divergent boundary, where the plates have fully separated and the resulting rift has dropped below sea level.

If a continental plate and an oceanic plate diverge, the space created will just be filled by water, turning the boundary into just an oceanic divergent boundary, so don’t worry about this boundary type.

For convergent boundaries, whenever two oceanic plates converge, one will slide under the other causing an ocean trench and pushing   the other plate upwards causing island arcs with volcanoes. Real-world examples of this include Japan and the Caribbean islands.

Whenever two continental plates converge, the two thick plates will often buckle and fold, usually upwards, causing huge mountains. Easily the best real-world example of this is the Himalayan Mountain range.

Whenever an oceanic and a continental plate converge, a combination of the above two examples occur. The oceanic plate is thinner and denser, and so is forced underneath the thicker less dense continental plate. An oceanic trench is created on the ocean side of things, and a mountain range is created on the continental side, often with volcanoes. The Washington-Oregon coastline of the United States is a good real-world example of an oceanic-continental convergent plate boundary.

It's important to note that convergent boundaries are likely to cause the strongest earthquakes out of any type of plate boundaries, which is unsurprising given the real-world examples of these boundaries we’ve just looked at.

Finally, we have transform boundaries, which are pretty straightforward. Neither plate moves underneath the other, they simply move parallel to each other. There won’t be too much environmental change to the area like mountains or trenches, but instead you can just expect earthquakes.  

Given the very strange shape that tectonic plates can take, it’s likely that each plate will have multiple different types of boundaries. As long as you have a rough idea of what’s going on with the plates, then you can make an educated assumption of what’s going to be occurring on the surface.

This leads us to something like… this. Whoa, that’s a BIG jump from where we were. And yes, you’re right, but you can see if we overlap our tectonic plates that all I’ve done is made the edges a little prettier, and add in some mountain ranges where the plate boundaries are occurring on land. There are island chains of different kinds across the oceanic plate boundaries, and considering plates move, the land boundaries don’t need to be exact to the plate boundaries. Think of the plate boundaries more like… guidelines. You can also tell that I’ve added some pretty extensive polar caps, which I’ve done due to Locus having a much lower average temperature than modern earth, which is consistent with maps of what earth would have looked like during our last glacial maximum.  

Now that we’ve got our map, let’s establish its wind and ocean currents. First, we need to separate the planet into 6 parts at 30 degrees separation each. Of course, at 90 degrees north and south, you’ll be standing on the north and south poles respectively.

To begin with, let’s look at wind zones. Assuming your planet is similar to earth, rotating in the same direction with roughly the same speed, then there are six major wind zones. From the equator moving poleward, there is the intertropical convergence zone, the tropical easterlies, the subtropical ridges, the westerlies, the polar front, and finally the polar easterlies.

The intertropical convergence zone, which on earth we call ‘the doldrums’, is the wind zone across the equator where the two hemispheres meet, hence its name. The winds here are usually very weak, and weather is unusually calm, with low pressure warmth across the entire band. The tropical easterlies blow east-to-west due to earth’s rotational direction, which is a phenomenon called the ‘Coriolis effect’. They exist between the equator and the 30 degrees north and south mark, which on earth we call ‘the trade winds’. Most tropical storms, including cyclones, exist as tropical easterly winds. Next there are the subtropical ridges at around 30 degrees north and south, which are a narrow band of very light winds that don’t blow in a particular direction, making rainfall in these areas practically non-existent. The air here cools and sinks, creating a high-pressure zone. On earth we call these the ‘horse latitudes’, derived from the expression ‘beating a dead horse’. The westerlies then follow the subtropical ridges, between 30 and 60 degrees north and south, blowing west-to-east, giving them their name. Westerlies have an enormous impact on ocean currents, which we’ll discuss in just a little bit. Then, the polar front is a low-pressure zone that sits along the 60 degrees north and south line, categorised by cold polar air meeting warm tropical air, with drastic temperature differences between the two sides of the front. Finally, we have the polar easterlies, running from the poles to the polar front, which are dry and, unsurprisingly, very cold.

If your world rotates in the opposite direction, then everything is going to be flipped, with tropical westerlies, then easterlies, then polar westerlies. If your planet rotates at a different rate, then the zones start to divide into strange variations. Specifically, if a planet rotates at less than half the speed of earth, it will lose everything more poleward than the tropical easterlies, and if it rotates more than twice as fast as earth, it will effectively double everything in each hemisphere. For Locus though, we already know that it rotates at a similar speed and in the same direction, so we’ll be keeping things the same as earth.

Now let’s complete our geographical foundations by establishing some ocean currents, which are pretty straightforward. Keeping the same 30 degree splits we were using before; we can start to fill in circular current systems called gyres that rotate in opposite directions every time we move into a different band. Starting at the equator, we can make two westward flowing currents. If your planet spins the opposite direction, your current will be eastward flowing. This current is broken only by meeting land, and any current that moves east or west is usually a neutral current that is neither warm nor cold. Where the current meets land, it will then start to move away from the equator as a warm current until it hits the westerly winds, which blow from the west towards the east. However, the current doesn’t immediately shift east as soon as it hits the 30 degrees mark. Instead, it takes some time for the winds to be strong enough to shift the current, starting to move west at around the 40-45 degrees mark, and to keep things simple I’m going to go with 45 degrees. So, at this 45-degrees mark, the current then flows east as a neutral current. When it hits land, it’s now going to travel back towards the equator as a cold current, completing our gyres. You’ll notice that currents flowing towards the equator are always cold, and currents flowing away from the equator are always warm.

Now in the 45-to-75-degree cell the rotation of our currents will switch. Repeat this process until you’ve effectively closed all loops across the entire planet. If there are any sections where an island exists within a loop, use your best judgement for what would be occurring. Islands are generally small enough for variations in current temperature to not drastically effect their climate, and they’re more homogenous with their biomes. And that’s it! With wind zones and ocean currents done, we can establish more specialised geographical climates, which we’ll work on in the future.

So, to recap, as our primordial planet developed oceans, tectonic activity increased, giving rise to the tectonic plates that have shaped the planet into a workable geographical map. We’ve given Locus 30 tectonic plates, 5 of which are continental and the remaining 25 are oceanic, giving us an oceanic coverage of around 85%, which is slightly higher than earth.  

Join me next time where we will look at the origins of life, establishing how life is going to be created on Locus. And until next time… stay awesome!