Spec Tech: Making Mountains

One part of the massive fun that is world building in fantasy or science fiction is sketching out maps for your protagonists to explore and conquer. Mountain ranges that form the bones and teeth of the land are an important part in the creation of an environment. Mountains are often used entirely as obstacles, as ways to isolate countries and prevent free movement.

While mountain ranges often act as the borders of countries, it’s important to note that borders obey the location of mountain ranges, not the other way around! Mountains form through unimaginably long and powerful geological processes that are constantly active and always changing – even if that change happens at a scale humans have a difficult time comprehending. It’s important to understand why mountains form as the key to where they’re going to end up.

Generally, mountains are formed by tectonic processes. The Earth is a giant heat engine fueled by residual heat and radioactive decay in the core, which causes convection in the mantle, where rocks are already so hot they tend to deform like silly putty. This constant convention and the processes on the surface that are driven by it are the reason Earth is called a “living” planet. Tectonic processes constantly change the surface landscape and recycle rocks. As strange as it may seem, this is important to life on Earth.

Convection in the mantle pushes around the tectonic plates that form the crust of the Earth, and that is ultimately what drives most mountain formation. Plates move around and collide, which causes the crust to rumple and deform in interesting ways. These tectonic collisions and mantle convection also can cause magma to build up under the surface, where it can cool into giant bodies of rock called plutons – or a series of plutons can amalgamate together to make a batholith. Uplift these plutons or batholiths through more tectonic forces, and they become mountains on the surface. To give you a sense of scale, the small part of the Pikes Peak Batholith in Colorado that shows on the surface covers hundreds of square miles, and the 14,000 foot tall mountain called Pikes Peak is just one part of the batholith.

Most mountain ranges are formed under one of three generalized processes:

Continental Collision: When tectonic plates collide, there’s a lot of compressive force brought to bear. Most generally, imagine having a sheet of paper flat between your hands. Push on both sides of the paper at the same time, and it rumples. When crust rumples, you get relief in the landscape, and if that relief is high enough, the high points are mountains. This pure compression of two continental plates colliding is what formed the Himalayas – and it’s also the reason that many of the mountains in that range are still getting taller, because tectonic forces are still driving the Indian and Asian plates together. These collisions tend to build up mountains like a cartoonish 90-car-pile-up, where more cars are continually smashed into the growing pile of wreckage.

What this looks like on a map: Generally, think about how the Himalayas and the Alps look – both are fairly thick rather than long bands of mountains, which pinch off on either side. This band of mountains could conceivably be straight, but will likely have at least a gently arced shape that generally mirrors the shape of the two coastlines that have crashed together.

When an oceanic tectonic plate collides with a continental plate, the effect is a little different. Oceanic plates tend to be more dense than continental plates, which means the continental plate will ride up over the oceanic plate. The oceanic plate is subducted down into the mantle, where it is eventually consumed. This scenario will still cause some rumpling of the continental plate – because even if the continental plate is “winning,” there is still a lot of compressive force. But the oceanic plate being melted beneath the continental plate also provides a lot of magma, which can either crystallize underground to make batholiths, or escape to the surface and cause volcanoes to form. Oceanic plates also sometimes have island arcs associated with them, and these can be “stitched on” to the continent.

What this looks like on a map: These sort of mountain ranges will stay near the coastline that’s along the actively subducting plate, and will have a similar trend. For example, if your coast runs north to south, you’ll end up with a long range of mountains that will also trend north to south.

There are other interesting cases that can come from these basic collisions, however. For example, if you look at a topographic map of North American, there are mountain ranges ranging roughly north to south over the entire western half of the continent. This is because the North American plate “ate” an oceanic plate. Instead of that plate quickly sinking into the mantle to be recycled, it stayed just under the continent as subduction continued, grinding along and uplifting mountain range after mountain range. The oceanic “slab” finally broke off and subducted after uplift of the Rocky Mountains. This is one reason why the mountain ranges get progressively younger from west to east in the western US and Canada. This also means that clustering mountain ranges together to make large, mountainous regions is very feasible.

What this looks like on a map: This starts out similar to the normal oceanic/continental collision, but then successive mountain ranges pop up further and further inland. This will have the effect of a series of long mountain ranges that also follow the same trend. So if the coast is north-south, all of the successive ranges moving inland will also be generally north-south.  As an added dimension, the further inland you go, the younger the mountains will be, which means steeper slopes and a more forbidding profile.  Also keep in mind that while such mountain ranges may follow the same trend, that doesn’t mean they will precisely mirror each other; there is some wiggle room.

Tension:  Instead of being compressed by a collision, tectonic forces pull two sides of a plate in opposite directions. As happens when you try to do the same thing with the piece of paper, something eventually gives. However, since – unlike paper – the continental crust is quite thick, this normally means that large blocks of crust, bounded by faults (the proverbial tears in the paper) will just drop down. This may seem counter intuitive, but this too can cause some significant relief in the landscape.

What this looks like on a map: Tensional features have a very distinctive look of large, flat valleys bounded by bizarrely linear high regions, which also follow the same trend. For an example of this, Google “Basin and Range Province.”

Hot Spots: It is possible to get isolated mountains, or long chains of spaced out, single mountains. The most common cause of this is a hot spot, where a mantle plume heats a geographically limited area of the underside of a tectonic plate. This is the situation that has formed the Hawaiian island arc. The Pacific Plate has basically drifted over a stationary plume; the current active volcanoes are the ones at the plume, while the older, now inactive volcanoes form a trailing string of mountains which are slowly being eroded away. This process works on continental plates as well. In the United States there is a string of calderas (ancient volcanoes that have collapsed in on themselves to create massive, deep basins) with the Yellowstone Caldera at one end.

What this looks like on a map: Solitary islands or mountains in a long chain. If the hot spot is still active, the chain ends at an active volcano. Since the further a mountain is from the volcano, the older it is, the mountains will become successively smaller and have gentler slopes due to erosion.

You’ll notice that none of the above scenarios produce mountain ranges, for example, at right angles to each other. While I can imagine a scenario that could produce ranges at wildly different angles – plate A subducts under Fictional Square Continent’s east coast and causes mountain range A, then plate B subducts under the south coast and causes mountain range B – one thing to remember is that geology is very slow.  So even if this odd scenario happened, mountain range A would be unroofed and being eroded down while mountain range B would be in the process of being created.

Nature often curves in interesting ways, but rarely does it work at right angles.

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