Planet Geology – World-building Part 3

Note this is a series: Part One and Part Two.

Now that you have a sketch of your solar system, we can focus on the individual planets.

First, planets made of rock – terrestrial planets.  They are largely powered by geological processes such as hot spots, volcanism, and plate tectonics.  Rocky planets tend to have an iron core with a fairly hot mantle of nearly liquid rock.  Floating atop the mantle is the lithosphere, where the crust is this tiny sliver at the very top.  A geologically active planet often has the crust moving atop the mantle like floating islands, and this creates interesting features on the surface.

Hot spots are when a thin plume of magma rises from the very, very deep within the mantle and erupts through the crust.  The Hawaiian Islands on Earth are a good example of the results of a hot spot.  The reason the Hawaiian islands are spread out in a line is that the crust the hot spot punches through is moving slowly over time.   On Mars, Olympus Mons was formed due to a hot spot, but because the planet did not have plate tectonics – there was no movement really of any crust, Olympus Mons continued to just build up and up until the hot spot depletes itself or stops for whatever reason.  In essence, a hot spot can form on any rocky planet, but not all rocky planets may have a moving lithosphere.

To define: the lithosphere is the outer, rigid part of a rocky planet, where the crust is only the top most edge.  Most of the lithosphere is mantle.  This part of the planet can move, and when it does move, this process is known as plate tectonics.  Plate tectonics is where a lot of interesting geological features can start to arise.  Without plate tectonics, geological features formed by various processes are more limited and less diverse.

A planet without plate tectonics will not have any mountain ranges like those found on earth.  Instead, they may have sediments pile up in spots to create hilly terrain or sand dunes, and they may have one lone mountain here and there formed from a hot spot.  They may also have entire plains formed by basalt floods, where incredibly hot magma pours onto the surface from a break in the crust. When this plain cools, it will be a huge expanse of dark colored rock, and if the planet has any sort of soil or sand, over time the winds in the atmosphere will cover it.  If there is no atmosphere, then the plains will remain still and dark – like the dark “sea” areas of Earth’s moon.  If any moving liquid exists on the surface, canyons could be carved, though canyons could also form from fractures in the crust as well.  Other formations could come from meteor impacts.  Any pressure inside the planet could cause fractures in the crust, which can form other canyon like features.  Most of the processes for altering the geology of the surface would depend more on any atmospheric and liquid processes rather than active geologic processes.

For a planet with plate tectonics, the range of geological features expands considerably.  Take for example the Earth.  In Asia, the Himalayan Mountains are the result of two plates crashing into each other, pushing the roc upward to form mountains.  These mountain forming belts are known as orogonic belt, and in order for them to become taller then need a fairly deep root.  The Himalayans for instance are also know as foldbelt mountains because the collision folded the earth into the mountains, forming a deep root and pushing the excess earth upward.  Since the continent plates float atop the mantle much like an iceberg in water, the deformation of the colliding plates deform both upward and downward, but to reach the heights that the Himalayans do,  the foldbelt rock must be buoyed up by a less dense continental rock below in its root.  However, this root can only be so large before it becomes unstable – on earth this limit is around one hundred kilometers, and thus mountains on earth cannot truly exceed ten kilometers in height.  This is a principle in geology known as isostasy.  Depending on the geology of the planet and its materials, the limits on mountain height could change, but that would require calculations beyond what I will discuss in this preliminary material.

Another aspect of plate tectonics is the formation of new plates at spreading centers, where plates move away symmetrically from each other.  Magma flowing upward to fill in the space between those two plates forms new plates and a ridge of major volcanic activity.  An example of this on our Earth would be the mid-Atlantic ridge.

Plates collide in a variety of ways.  Convergent boundaries is the name of these types of collisions.  Head-on collisions as described earlier in my discussion of the Himalayan Mountains is a great way to build fairly tall mountains.  Collisions where plates slide next to each other can also cause mountain ranges as well, but not quite as dramatically.  An example of plates sliding next to each other on our Earth is along California in North American.

Another way for plates to interact is when one plate is consumed by diving back into the mantle, while the other moves above it. This is called a subduction zone.  The partial melting of the down-going plate generates a volcanic arc, which is where the crust is stretched like taffy and highly dangerous volcanic zones develop.  Eruptions from volcanic arcs can be some of the most explosive eruptions.  Island arcs can form in such regions, and a good example of this on our Earth would be Japan.  A deep oceanic trench also form in these regions as well.

By examining these processes, you can create a map of some of the major features of your planet.  This is just a preliminary explanation of these processes, so to explore it further it may be a good idea to examine some geology articles to discuss these points in detail. Especially if you may wish to really detail some of the geography of your world.

However, geological processes such as these are only half of what forms a planet.  The atmosphere a planet may have, any liquids on its surface, and the processes governing these also play a role in the formation of planetary features, the existence of life, the types of biospheres that can form, and formation of rivers, lakes, and oceans.


Now gas giants are a whole different class of planets.  They have no well-defined surface like a terrestrial planet.  They are teeming with helium and hydrogen, and have violent hot interiors formed from the enormous pressure of their atmospheres.  Heavy storms frequent the various levels of atmosphere, making it a not very hospital place for life.  Their moons are more likely to harbor the ability to hold life, and the moons are primarily rocky objects. Due to their mass, gas giants often form miniature accretion disks, which is why they often have a large amount of moons orbiting the planet itself.  Rocky, hotter moons form closer to the gas giant, while icier moons form further away.

Due to how massive gas giants often are, some scientists have speculated that a few may be aborted stars – for instance, Jupiter in our solar system could have ignited fusion at its core if it had been just a wee bit more massive.  Some very close binary stars with circular orbits may have come from a gas giant being massive enough to ignite fusion at its core, creating a small star instead of a planet.   Most gas giants never reach this point.   Due to how inhospitable gas giants are, their moons are a more likely choice for life, and thus we return to the beginning of this post, where I discussed how rocky planets can form.  Moons may form in a similar manner though this may depend on the composition of the moon, how large the moon is, and if it can hold an atmosphere or not.  Most moons have no atmosphere, but they may harbor seas or caverns under their surfaces that could be potentially used for life.

I think I’ll save a more thorough discussion of moons for a different post.

Categories: Physics, World Building, WritingTags: , , , , , , ,



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