World-building Part 5 – Geomorphology (planetary features)


Note this is a series: Part One, Part Two, Part Three, and Part Four.

A note for my readers, especially those interested in my paper mache planet project: This post in particular describes how I created my own tectonic plate maps, determined the geological features of my own planet including major rivers and lakes; I’ve also created climate zone maps, but that will be discussed in part six of this series. It’s these maps that will be essential when it comes time for me to start painting my model of Elivera.

At this point in the world building series, we’ve created a basic solar system and a star. We’ve dealt with the geology of a planet – particularly its plate tectonics and what powers the volcanism and plate tectonics of a planet. We’ve also covered its atmosphere and natural cycles. Now, I want to discuss the features on the planet itself, for this will play a fairly crucial role in determining the flora, fauna, and most importantly where and how your people will live on this planet. To do this, I’ll be using a lot of the science we’ve already discussed in parts one through four.

A great podcast that discusses geomorphology in depth is available to you at Astronomy Cast – Ep. 221 Geomorphology. For a simplistic definition, geomorphology is the study of planetary features. However, Dr. Pamela Gay has a much more precise definition that will also sum up what I hope to cover in this fifth part of my world building series:

It’s basically a really long, fun-to-say word that means the surface of a planet isn’t flat due to a variety of processes, ranging from tectonic processes (this is the plates that make up the surface of the planet moving around), to aeolian processes (things getting blown about by the wind), and fluvial processes (which basically means stuff that’s been affected by liquids, like water). You also get Imbrian processes, which is volcanism. So all of these different things, basically earth, wind and fire (if you consider volcanism fire) — they have an effect on the shape of the surface of the planet. We don’t have perfect spheres, and where we deviate from that perfect sphere — that’s geomorphology.

An important fact for a planet is whether or not the planet is still active. An active planet means tectonic processes are still in play, and they are one of the larger roles in planetary features. As discussed in part three of this series, how the plates move against each other can often create mountains, rifts, trenches, hot spot volcanoes which in turn can create mountains and/or islands, and other rock based features. If a planet is not active, like Mars, then hot spots do not move due to plate tectonics, and so you end up with ridiculously high mountains like Olympus Mons where the hot spot kept pouring out eruptions in one area; the smaller gravity of Mars in turn allowed Olympus Mons to grow much larger than any mountain could ever grow on Earth. The stronger gravity on Earth would pull down bits of the mountain, preventing it from every growing much larger than Mount Everest, which is currently the tallest mountain on Earth. Due to the plate tectonics on Earth, the plates move slowly across the mantle, and so a hot spot does not penetrate the crust in one spot at all times – it will penetrate it in several areas. The Hawaiian Islands are a good example of this. Due to the movement of the Pacific Plate moving toward north-east toward the Eurasian plate, the hot spot under the Hawaiian Islands ended up creating the island chain. This image shows the process fairly well:

image credit USGS Geology Survey
Image Credit: http://geology.com/usgs/hawaiian-hot-spot/

Here you can see how the hot spot has “moved” so to speak, forming each of the individual islands. To be more precise, it is mostly the Pacific plate that moves, forcing the hot spot to burrow upward through the crust yet again to create a new island. There is some evidence that the hot spots plume may shift slightly over the millions of years due to the convection currents in the mantle, but for the most part, they seem to stay in relatively the same location.

Many islands are created in this manner, so identifying hot spots like the Hawaiian Islands’ hot spot, would be a good idea for mapping out island chains. To do this properly, you also have to map out the tectonic plates on your active planet, and determine the general directions they are moving. I don’t really see it necessary to map out the entire history of your planet’s tectonic plates, but you can if you really want to, and it could help you to visualize how the land looked if you wanted to write a story set in the distant past. I’d say it’s easier to pick a general time block for your planet with your “present time” being near the end of this time block. When you map out how the continents move, it’s always good to examines the boundaries of plate movement to make sure it makes sense. If it helps, take a look at the tectonic plate maps of Earth and see how the plates move on Earth over the course of a couple million years. You can use that to help you craft the movement of your planet’s own plates.

Once you know the directions of each of your planet’s plates, you can then map out the island chains formed by hot spots. If your plate is moving north-east, then the oldest islands in the chain will be more to the north-east, and the newer ones – including the one that’s currently active – will be to the south-west.

Another fun point about mapping the direction of your plates movements, is that the ones that are crunching into each other are essentially forming mountains. As I mentioned in part three, orogenic belts is from two plates crashing into each other, and the crust is being folded upward to form a chain of mountains. These will be some of the higher mountain chains on your planet.

Another area of mountain forming is where one plate is being subducted under the other plate, and these mountains will be more volcanic. A good example of this on our own planet is the Aleutian Islands off the coast of Alaska as well as Japan and the smaller islands surrounding it; here the Pacific plate is being subducted under the neighboring plate. This area will have not only a deep oceanic trench, but as the Pacific plate is subducted under the other plate, the North American plate rises upward to form a chain of mountains, many of them volcanic.

The third way to form a mountain is if at a hole in the crust magma rises upward, breaking through to the surface and flowing onto the surface of the crust; over time, this will slowly build the mountain higher and higher. There’s a set limit, due to the gravity of the planet, as to how high a mountain can form, and this type of mountain building is limited to how long the hot spot stays under the mountain. The plate it is on will move slowly over time, thus the hot spot will no longer feed that mountain, causing it to become an extinct volcano. Islands are often formed this way, but this can happen on land masses as well.

Notice also that the areas where plates are moving away from each other. These spreading ridges is where not only new crust is being formed, but will also be a ridge on the crust. On Earth, these ridges are on the bottom of the ocean; the Atlantic ocean has a spreading ridge, and thus the Atlantic is the ocean that’s growing larger over time, while the Pacific Ocean is shrinking slowly due to it being subducted under the plates to the north-east. This type of dynamic will be present on your planet as well, and if you plan on writing novels that span several million (or billion) years, then this may be a factor in how your planet will look over that time period.

You can take a look at some of these features on earth in depth through this Geologic website: Plate Boundary Map.

Once you have a good handle on how the plates move and what features are often formed at the boundaries of plates, you can start marking features on your world. This is the start of determining how your planet’s features were formed and what they will look like for the characters in your novel.

So far, I’ve only discussed the tectonics process and the imbrian process influences on planetary features. Since in part four we discussed the atmosphere of your planet and the various cycles the planet will have, let’s discuss a bit the aeolian and fluvial processes that influence the features of a planet.

Moving water on the surface of the planet will cut into the crust and form riverbeds and canyons. Take a look at the Grand Canyon in North America; the Colorado River over the course of millions of years cut deeper and deeper into the crust, forming the canyon over time. Moving water erodes rock and soil over time, cutting deeper deeper as long as the river exists. Rivers can shift over time due to various processes that may block a section of the river, causing it to take a easier path, and thus altering the direction of the river.

Even lakes cause erosion to the surface over time, and a lot of the time lakes are generally fed by some sort of water source – whether an underground spring, a river flowing into it, rainwater, ice melting, and so forth. There are times when lakes will no longer have any river flowing into it because the river itself has shifted due to geological processes, and depending on the region, the lake may slowly evaporate over time. Other lakes are formed due to glacier processes on the earth, where giant sheets of ice move slowly across the planet, often carving valleys and/or depressions into the surface. Many of these depressions often fill with water due to the melting of the glaciers, as they retreat due to a slight raise in the planet’s temperature, or an underground spring will fill it, or a stream might fill it and so on. Another way for a lake to worm is when a volcano violently erupts and forms a giant crater; over time this crater may fill with water – Crater Lake in Western North America is a good example of this type of lake. Tectonic processes can also form lakes; for example, when two plates adjacent plates move past each other, forming fault lines – California in North America has a long fault-line that is a good example of this – and these fault lines can form gaps called grabens, which in turn can fill with water. Some of the deepest lakes on Earth are grabens filled with water. Here’s a good article that explains lakes in depth: Lake Origins.

Wind on the other hand wears away at the surface of the earth at a slower rate than liquids that cut into the surface. Wind and water both can eventually erode even mountains. Take for example the Appalachian Mountains of Eastern North America, where wind, water, and in this case even vegetation has all played a role in eroding the mountains, causing them to shrink a bit. They’re not as large as they were millions of years in the past. This type of process will eventually happen to other Mountain chains as well, especially if the process that is folding them upward or is building them with magma shifts over time. Wind has the potential to carry bits of dirt and sand all over the world. For example, there has been studies that show the desert sands of the Sahara in Northern Africa on Earth are often carried on the wind to South America and much of the time are dumped there. Deserts in particular are highly susceptible to wind erosion due to how dry the ground is, and how easy it is for the wind to pick up the top layer of particles and blow them away; this is how sandstorms form, when large gusts are wind pick up a large amount of soil and then blow them in a specific direction. Depending on the strength of the wind and the amount of soil picked up will determine how thick and fierce a sandstorm can be; this type of storm tends to really only exist in dry areas.

Which brings me to another point. Now that you have a pretty good idea of the geologic features of your planet, your next step is to figure out which areas will be relatively dry, fairly wet, very cold, fairly temperate, and very hot, which will help you determine where deserts are, forests thrive, glaciers may form, and so on and so forth. This will involve a discussion on climate, which I will save for part six of my world-building series.

If you have any questions, feel free to ask them in the comment section! Also, if you have any thoughts or things you’d like to see discussed or expanded, also feel free to comment.

Categories: Physics, Science, World BuildingTags: , , , , ,

4 comments

  1. I always enjoy reading this blog due to all the information in it. Your Dad is getting really interested in Wisconsin geology due to all the great rocks he has been finding. We have a rock with rough gemstones in it; rocks shaped like monks, animals, with round holes worn through. So he has checked out geology books on it. Very interesting.

    Like

    • That’s really cool, and thank you for reading.

      Wisconsin definitely has some really fascinating geology there. I think I still have two of the rocks I picked up from explorations around the countryside there.

      Like

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