Once Molehill, Now Mountain

For most people, mountains are part of the landscape. We don’t think about them except when we’re directly interacting with them, and even then, we don’t always ask ourselves where they come from. That’s a shame, because there’s some really interesting geology in mountains, and I’m not talking about the types of rocks, either. The processes that create mountains are fascinating. Molten rock rising through the crust? Continental plates hitting each other and forcing the crust into folds and waves? What’s not to love?

White Horse Bluff, which erupted under a lake

Volcanoes occur any time a rupture in the Earth’s crust allows magma to rise to the surface. This magma, or rather lava because it’s exposed to air (or water), can form all kinds of protrusions from the crust, depending on the composition and fluidity of the lava and the surrounding environment. A volcano that erupts to open air is different from one erupting underwater, which is different again from volcanoes that come up under a glacier. Submarine volcanoes often occur around ocean ridges, where there is tectonic activity. Land volcanoes often occur near tectonic plates as well, but they’re also formed by hotspots or weak points in the crust. (Hotspots are particularly neat because multiple volcanoes can form from a single one as the plates move across it.)

As for the shapes of land volcanoes … boy. Volcanoes are frequently complex and boast any number of features, but the main types are stratovolcanoes, cinder cones, and shields. Stratovolcanoes are what most people think of when they hear ‘volcano’. They’re conical, with lava, rock, and ash spewing out of the top in all directions, and build up over multiple eruptions. Mount Etna is a stratovolcano. So are Mount Fuji, Mount St. Helens, Mount Vesuvius, and Eyjafjallajökull. They occur at subduction zones, where one plate of the crust is moving underneath another, and tend to be rather explosive.

Parícutin, a cinder cone

Cinder cones are similar to stratovolcanoes, except that instead of lava they spew pyroclasts (also called tephra, also called rocks and ash). They can appear in isolation, as with Parícutin, or as part of another type of volcano when there are multiple vents.

Shield volcanoes, on the other hand, are made of flowing lava, and tend to be wider and flatter than other volcanoes as a result. They’ll also erupt for longer periods. The Hawaiian volcanoes are excellent examples of this type, though shield volcanoes pop up worldwide. They’re frequently the result of hotspots, but can occur at rifts and subduction zones as well.

Any and all off these can also be worn down by erosion, so that only the hardened lava from the vent or intrusions remains, or a shield volcano gets split into multiple mountains. It also bears mentioning that volcanoes are complex and that assuming a world only has the three classic types with no variation is erroneous. On Earth alone, you’ll find craters, calderas, and volcanoes inside calderas. Some volcanoes have fissures, linear vents that ooze more than they explode, and which yield a lot of the spectacular photos from Hawaii. And of course, a stratovolcano may sprout cinder cones or a small shield volcano, or a caldera may yield a lava dome, or, or, or.

The other major method of creating mountains, without any messy flying or liquid rocks, is by smashing tectonic plates into each other. This is how the bulk of Earth’s mountain ranges are formed, and why we find seashells and fossilized ocean creatures on top of mountains. When two plates meet and form a subduction zone, the top plate can be wrinkled by the force of the subduction, producing fold mountains such as the Rockies. Faults can also create fault-block mountains by raising a chuck of bedrock above another.

Of course, thanks to NASA and its probes, we now know a fair bit about the geology of the other planets and moons in our solar system. Starting close to the sun…

Pancake domes on Venus

Venus doesn’t have plate tectonics, but that doesn’t mean it has no mountains. Eighty percent of its surface is covered by lava plains and volcanoes. Venus boasts a large number of shield volcanoes, as well as coronae, scalloped margin domes, arachnoids, and pancake domes, features not found on Earth. (The only other spot we’ve found coronae is Uranus’ moon Miranda.) Venus is still volcanically active and its shield volcanoes tend to have far greater diameters than those on Earth.

The Moon’s maria are basalt planes formed by lava flowing into impact craters. Some of these maria also yield shield volcanoes and domes. The Moon also lacks tectonics, but this hasn’t stopped it from having mountain ranges. It’s also believed that the far side of the Moon was volcanically active longer than the near side.

Pretty much every volcano nut has heard about Mars’ Olympus Mons, the highest mountain in the solar system at 27 km. Pretty much every conspiracy buff has heard of the Face on Mars, which turned out to be a butte rather than an alien sculpture. It should surprise no one to learn that these aren’t the only mountainous features on the planet, or that Mars also seems to lack tectonic plates. Mars’s volcanoes likely come from hotspots, as the Moon’s do, and while they don’t erupt as often as the volcanoes on Earth, they’re of a much larger scale when they do finally happen, since lower gravity means magma bodies must be bigger if they’re going to reach the surface before cooling. Mars has several major volcanic regions, such as Tharsis where Olympus Mons is located, which boast giant volcanoes along with smaller ones called tholi. Tharsis also contains Alba Mons, another volcano with no Earth counterpart (it has an immense area but isn’t very high). Mars also has a number of volcanic plains, and may still be volcanically active—which may increase our chances of finding life there.

Due to tidal friction from Jupiter, the moon Io is the most volcanically active spot in the solar system. Its volcanoes eject a lot of sulphur and sulphurous compounds, along with the igneous rocks one expects. However, volcanic mountains are rare on Io, and most of Io’s mountains are formed by thrust faulting resulting from crust compression. What volcanic mountains there are, are shields.

Saturn’s Titan, the moon of methane lakes and NASA buzz, has both what may be cryovolcanoes spewing water, methane, and ammonia, and an icy mountain range in the southern hemisphere. This range may be caused by tectonic plates shifted after a meteor impact. Since we’re still studying Titan, though, and having a hard time seeing through its atmosphere at times, it’s uncertain whether the volcanoes are actually volcanoes. Further study is obviously needed.

The other major gas giant moons have many interesting and unexplained surface features as well as tectonic activity, but since none of them seem to actually have mountains, I’m skipping them here.

Any planet your characters are on, science fictional or fantastic, is likely to have mountains, volcanoes, plate tectonics, and the like. Earth provides a good model for just about any geographic feature you might want, but as we’ve seen with Venus and Mars, there are types of mountains and volcanoes that are only possible under different conditions. Higher or lower gravity, ratios of elements within the lava, the pull of a star or a planet, and plate tectonics will all come into play—and if there aren’t plates, there won’t be analogues to the Rockies, Alps, or Himalayas, which arise at plate boundaries.

  • We’ve seen the devastation and chaos a volcano like Vesuvius, Mount St. Helens, or Eyjafjallajökull causes. What happens when a supervolcano such as the one in Yellowstone blows? It’s going to take out a fair chunk of the continent.
  • Would it eventually be possible to sculpt mountains out of lava? To create islands on a water world from the planet’s own mantel?
  • If a planet’s mountains are made of different substances than Earth’s are, how will that effect what grows on them, or erosion?
  • Earth cultures near volcanoes tend to have volcano gods, and some cultures posit gods living on mountains. What might change if the mountains are different?
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