Tweaking Your World’s Geology

This post is 1 in the series: Shaping the Bones of Your World

Chances are your world is built on something.  If it’s anything like Earth, your characters are living in a thin veneer of air and water sandwiched between a dark, unforgiving void and a churning sea of white-hot iron. We’ve already looked at what’s above; now let’s take a look at what to put below. It’s not always obvious, but what happens beneath your feet can be pretty damn interesting.

Not to mention, geology defines your maps. Where are your coastlines, mountains, rifts, and volcanoes?  Well, that all depends on how your continents get along.

Geologic time is very long.* But depending on the geology you choose for your world, it can give rise to short-term effects that can rear up in a story or game. Earthquakes and volcanoes are the most noteworthy, but don’t rule out hot spring geysers, landslides, cataclysmic floods, and natural nuclear reactors.

Plate Tectonics

The first geology-related question a worldbuilder needs to answer is, “How active are your plate tectonics?” Almost everything follows this question – from the size of mountains, to the frequency of earthquakes, to what percentage of your world’s surface is covered in lava at any given time.

Let’s pause for a second to discuss plate tectonics. What are they?*

When the lighter bits of rock that floated to the top of Earth’s roiling magma soup finally dried out, they did so unevenly, like the parched landscape of a desert or a cracked film on a crème brûlée.* Plate tectonics are what happens when these thin* sheets of bedrock jockey for position, crumpling and rearranging the landscape as they scrunch into or slide past each other. This process is slow enough on Earth that humans rarely have to worry about redrawing global maps (besides political boundaries, of course). We don’t have to wonder if our favorite continent is still next door. But it’s fast enough that humans have occasional difficulties with dying under collapsed buildings or burn-drowning in volcano vomit.

The major plates of the Earth's crust Image by USGS (license)

The Earth’s active plate tectonics are responsible for:

  • The location of the continents
  • Earthquakes
  • Volcanoes
  • Most of the interesting rocks and minerals
  • The creative destruction of our planet’s surface – especially the bits that occasionally poke above the ocean and form hypervolcanic weirdscapes like Iceland.

To see how changes in geology would change a setting, let’s look at some alternate Earths.

The Geologically Crazed Earth

Hyperaggresive tectonic plates could force inhabitants to deal with frequent city-mulching earthquakes and non-stop lava flows around the world. Catastrophic? Absolutely. But it may not be anathema to human civilization because we can adapt to damn near anything. On our Earth, city-destroying earthquakes might strike a region (say, San Francisco Bay) at semi-random, 100-ish year intervals. For the hypertectonic world, these city-destroying earthquakes could strike about once a week. Except they wouldn’t be city-destroying anymore. The city wouldn’t have a hundred years to grow complacent. It would develop some remarkably quake-resistant architectural materials and styles, and any failures would be weeded out. Humans aren’t harmed by shaking; only our buildings are.

The same is not true of lava and ash, which does not play nice with human biochemistry. Constant lava flows and eruptions could mean the relocation of entire civilizations, as opposed to our Earth, where we rarely have to relocate small volcano-side villages. Here, there are only a handful of locations where an evil villain could hope to have a lava moat; on a world of crustal-plates-gone-wild, they could pick and choose between hundreds of convenient locations.* More subtle concerns include squabbles over the property rights for new lava-formed islands* and whether to replace the town’s waterwheel with one of them newfangled steam vent turbines.

You can expect civilizations to harness geothermal power sources in an early stage of technological development. England was full of waterwheels in the 10th century, and people who didn’t live near rivers or creeks decided that they wanted some power, too, so they invented windmills.* Steam vents dotting the landscape could be tempting sites for mill construction. Though not always as safe or reliable as streams, they’d offer more power (per “wheel size”) than the wind. Easy access to geothermal energy on a hypertectonic or thin-crusted world could make cheap and unlimited power available starting in the early Iron Age.

The Geologically Deceased Earth

A world lacking plate tectonics could look different depending on how long ago Earth’s plates decided to stop fighting and go to sleep. If Earth’s plate tectonics ended yesterday, almost no one would notice. In fact, the pent-up energy from plates smashing into each other would keep being released as volcanoes and earthquakes for some time.*

It’s a much different story if plate tectonics ended a billion years ago. The ancient mountains would have been weathered short and smooth by the vagaries of wind and wave. Much of the sedimentary rock pushed up from seabeds to the surface by ancient plate tectonics would have worn away, but what remained would be eroded in incredible ways. For example, the world could boast some enormous cave systems, since there’d be no earthquakes to collapse them. But there could still be volcanoes.

Volcanic Activity

Volcanism isn’t purely a result of plate tectonics; some volcanoes form above hotspots in the mantle.* A world without shifting plates could still have these hotspots, but they would result in different sorts of volcanoes. On Earth, plates slowly shift above the hotspots, and so a chain of volcanoes forms in sequence with only the “lead” volcano active. The best Earth examples are the Hawaiian islands, with the larger active volcanic island (Hawaii’s Big Island) over the current hotspot and smaller, ancient, mostly-sunken islands and atolls (like Midway Atoll) half an ocean away.

On a world where the plates above the hotspot didn’t shift, the volcano would stay in the same place, growing in size indefinitely. This is pretty much what happened on Mars, which has enormous volcanic mountains like Olympus Mons* because magma and/or lava* kept building up at the same location indefinitely. So a world with no plate tectonics might not have many mountains, but it could have a dozen or so really big ones.

Giant Impactors

Another way to get mountains on a tectonics-free world – the way that most of the Solar System gets its mountains  – is huge impact craters. Why wait for giant slabs of rock to slowly grind into each other when we can summon really FAST giant slabs of rock FROM SPACE!  This is a quick way to spread ocean sediments onto land without plate tectonics and to get lots of very big, jagged mountains more or less instantaneously. With no plate tectonics to gradually erase them,* they can stick around awhile. That is, until they eventually wear away from wind and water erosion.

Pent-Up Apocalypse

Think about how and why your world’s plate tectonics have shut down; is your world geologically dead or just asleep? Tectonic plates are driven around the world by convection in the plastic-y mantle, and their motions (with all the accompanying earthquakes and volcanic eruptions) help transfer heat from the interior to the surface. If the plates fuse together, stopping volcanism and earthquakes, an unsettling amount of energy could build up in the red-hot depths of the world. Venus provides a horrific example. As if Earth’s evil twin wasn’t terrifying enough,* it is thought to have “resurfacing events” about every 400 million years. Instead of occasional earthquakes and volcanoes, it gets them all at once: enough to turn nearly the entire surface molten. If you’re looking for a scientifically plausible apocalypse in your world, just freeze plate tectonics on a planet that needs plate tectonics. Complex life would have enough time to evolve between apocalypses, and tiny pockets of survivors* could seed the next era.

Water and Geology

Any world with some water and ice is going to have interesting geology,* whether or not the plates are shifting. The advance and retreat of ice can redraw maps much faster than lumbering continental landmasses. Glaciers can plow through solid rock and reshape the land. Melting and refreezing ice sheets move coastlines back and forth. Ice sheets compress the rock below them. When they recede, the land springs back up in a process called isostatic rebound, which can form new islands in shallow seas.* Glaciers can also fill enormous lakes behind ice dams, then empty them all at once in apocalyptic floods. Not only do these make great end-of-the-region* scenarios for roleplaying fiction,* they also scrape up lots of topsoil and debris and put them in one place. For example, Washington State’s Walla Walla valley is sitting on a hundred feet of topsoil, scoured by the inconceivable force of the Missoula floods.


Lake Missoula’s ice dam broke every 40 years or so for millennia, draining an entire inland sea – on par with one of the Great Lakes – through eastern Washington State. It bottlenecked at the entrance to the Columbia Gorge, where it was stalled for long enough to drop its sediment along with amazing house-sized boulders from two states away. This topsoil isn’t just an interesting geological quirk and farming resource. It’s also weirdly easy to reshape; a flash flood combined with a burst irrigation dike created Burlingame Canyon in a matter of days.

Point is: the cycle between ice and liquid water can reshape the landscape on timescales that are lightning fast, geology-wise. Most of these effects are fairly local, so a world wouldn’t even need to deviate from Earth, per se, for bizarre consequences. But think about changing ice in your setting. Is the hero’s home city threatened by an advancing glacier? Imagine an island where the docks keep drying out and being rebuilt due to the land rebounding as the pressure of the melted ice is released. Local lords may squabble over who has rights to the newly formed land. Or perhaps the low-lying city is threatened by a nation-destroying flood as a faraway ice damn is about to release a frigid sea the size of Lake Superior. None of these require any supernatural influence (or even deviation from Earth) to make a setting feel noteworthy.

The Living Rock

Earth is a living world in more ways than one. We don’t just have active plate tectonics but also an active biosphere. A non-trivial percentage of the bones of the Earth is literally bones – or at least their processed remains. Entire layers of rock are comprised of fossilized creatures. Let’s look at how life can actively shape the landscape – and not just by dying off in large enough numbers to form another sedimentary strata or oil deposit.

Coral: Winner of “Most Productive Organism” Award

Animated gif showing how a coral atoll forms as a volcanic island sinks away Image by NOAA (public domain)

Humans are not the planet’s most accomplished builders. That record goes to coral, which have built* enormous islands and reef structures. The caveat: coral can only build these massive structures in shallow water. If the water level drops, they dry out and die. If the water level rises too quickly, they’ll get insufficient light and die. When the water rises just fast enough, they can keep up and go on a construction spree.

This is why the Pacific Ocean is dotted with so many coral atolls: they are the ghosts of once-mighty volcanic mountains that slowly eroded and sunk away. Millions of years after an island-volcano becomes extinct, it can finally wear down into nothingness, but its ring of coral saves it from total oblivion. That is, as long as the mountain doesn’t sink too fast.

Biology is amazingly diverse, and there’s no reason coral has to be the only mega-project engineers.* Does that mountain look strange to you? That’s because it’s a stonewasp hive; do not go near that thing. Or perhaps the world’s best engineers are still coral, but they’ve been given superpowers. One of my favorite personal homebrewed settings is a world with coral so industrious that every square mile of continental shelf is a maze of coral reef. Sharp boundaries delineate the bluewater (safe, sheltered, and shallow) from the blackwater (deep, scary, and teeming with large monsters that snack on ships). Nothing is different from Earth except a bit of coral biochemistry.

A Fun Aside: Geological Necromancy

The next time you ask yourself, “What’s the worst that necromancer can do?” consider that you live on a planet where entire layers of rock are the remains of once-living creatures. Why would a powerful necromancer bother walking all the way to a graveyard to animate corpses, when billions of cubic feet of dead plankton and coral are all around them?  Why can’t a necromancer summon a fossil fuel phantasm from the oil well? Arbitrary rules for game balance, that’s why. Feel free to flaunt these in your story, and watch the geo-necromantic chaos unfold when the heroes, hunting a wandering monster, instead encounter a wandering mountain.


Once you know what’s going on beneath, you can tweak Earth-normal geology to make your setting memorable. Next time we’ll move past destroying cities and focus on all the good things geology has to offer.

P.S. Our bills are paid by our wonderful patrons. Could you chip in?



  1. Kingsville Geology

    I’m a bit late to the party, but I wanted to comment and thank you for bringing attention to geology in fantasy worlds. I’m a geologist and one of my favorite parts of worldbuilding is literally that – building the physical processes that shaped the landscape. Thanks for the article!

Leave a Comment

By submitting a comment, you confirm that you have read and agree to our comments policy.