A key part of the plot in a story I'm working on revolves around the main region being on top of volcano that is set to erupt. The trouble is that the region in question in my setting is roughly the size of Wyoming. It's on an Earth-like planet.

Clearly, the volcano needs to be a supervolcano. However, I'm having my doubts about it being so big. I also think that perhaps a volcano this big isn't necessary, because sizable volcanic activity can be had from much smaller volcanos (for an extreme dramatization, see Supervolcano.

How big can a volcano be?

Note: The setting doesn't take place on the sides of a volcano, but at the very top of it. I originally set it in a caldera, but it turns out that the volcano doesn't have to have erupted yet for the idea to be plausible. I'd also like answers rooted in .


  • The planet is Earth-like, but plate tectonics don't have to be Earth-like.
  • I care about width, especially at the top, but overall height isn't important.
  • It seems like things are as I thought they might be - the idea is unrealistic. I might consider using smaller volcanic features, but they would all have to be tied together. The answers don't have to be edited to cover this; it's just my musings.
  • No hand-waving, please! I can change the gravity of the world if necessary (in my story), but the gravity is still as I originally said - like that of Earth.
  • The volcano doesn't have to exist on the Earth we live in, though if the biggest volcanic feature(s) on Earth is/are the limit, I'll be happy to hear about them.

All the answers so far are good - don't change them! I'd like some more sources (a la the tag) - not that I doubt them, but because I'd like to do a bit of reading afterwards about this topic. A volcanologist I am not.

Related question on Skeptics, mentioned in How high/large can a mountain range get?.

  • 3
    $\begingroup$ Deccan or Siberian Traps, anyone? $\endgroup$ – Serban Tanasa Mar 25 '15 at 0:46
  • 1
    $\begingroup$ Why do you need the caldera to be large? Are you looking to get a massive eruption out of it? Or are there other reasons for the size? $\endgroup$ – Nick2253 Mar 25 '15 at 0:52
  • 1
    $\begingroup$ Then you don't actually need a caldera. The caldera forms after the explosion, when the crust sinks down into the mantle. Your setting could be before the first eruption of a previously unknown supervolcano that took however many millions of years to build up such an incredible pressure. $\endgroup$ – Nick2253 Mar 25 '15 at 0:58
  • 1
    $\begingroup$ I think Olympus Mons (mars) is probably your largest volcano that we can see...absolutely huge is the answer there, but plate tectonics would prevent that from fully happening on Earth. I assume Earthlike = plate tectonics, correct? $\endgroup$ – Twelfth Mar 25 '15 at 1:12
  • 1
    $\begingroup$ Makes for an interesting question, I'm curious to see answers...volcanic eruptions happen on a rather long time scale. On Earth, the plates move over this time and this tends to create a chain of volcanoes (Hawaii is a pretty good example, yellowstone as well). On Mars, the eruption happened from the same location over and over again instead making the volcano much larger. $\endgroup$ – Twelfth Mar 25 '15 at 1:16

10 Answers 10


Is this large enough?

siberian traps

If you've ever heard of the Permian–Triassic boundary, about 251 to 250 million years ago, the Siberian Traps are what caused it. Well, the Permian-Triassic boundary is such a bland term for it. A better word for it is the Great Dying or the Great Permian Extinction, Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. At 2.5 million sq. km. (!!!) in area and perhaps 3 million cu. km. in volume, this is the kind of fun stuff that could seriously ruin your day.

Too big? Try the Deccan Traps instead

The Deccan Traps are one of the largest volcanic provinces in the world. It consists of more than 2,000 m of flat-lying basalt lava flows and covers an area of nearly 500,000 square km (roughly the size of the states of Washington and Oregon combined) in west-central India. Estimates of the original area covered by the lava flows are as high as 1.5 million square km. The volume of basalt is estimated to be 512,000 cubic km (the 1980 eruption of Mount St. Helens produced 1 cubic km of volcanic material). Most of the basalt was erupted between 65 and 60 million years ago. Hmm, does that seem awfully close to another extinction event?

How does it work?

(Shamelessly ripped from here) In addition to plate tectonics, the Earth also has plume tectonics. Occasionally, an event at the boundary between the Earth's core and mantle sets a giant pulse of heat rising toward the surface as a plume. As it approaches the surface, the plume melts the crust to develop a flat head of basalt magma that can be 1000 km across and 100 km thick. Penetrating the crust, the plume generates enormous volcanic eruptions that pour hundreds of thousands of cubic kilometers of basalt ("flood basalts") out over the surface. If a plume erupts through a continent, it blasts material into the atmosphere as well.

  • 2
    $\begingroup$ That's what she said! $\endgroup$ – Aaru Mar 26 '15 at 2:17
  • 1
    $\begingroup$ I don't think this works for what the OP wants, though. It seems like s/he is looking for a single large erruption, and if I'm not mistaken, these flood basalt episodes are a series of events that happen over 10K-100K years, if not much longer. (And so likely would begin gradually, with small erruptions.) $\endgroup$ – jamesqf Mar 26 '15 at 4:20
  • $\begingroup$ those are not single volcanoes. A trap is a series of eruptions in relatively close proximity. Think the entire cascade range blowing over the period of a few decades, it would end up in the historic record as such by future geologists. $\endgroup$ – jwenting Apr 1 '15 at 3:33

As you've mentioned, you don't need a mountain. Just a volcano...preferably a mega-caldera like Yellowstone. Now we can ignore pesky gravity limitations and focus in on the geological issues

On an Earth-like planet, this is going to be hard. If you want a Caldera the size of Wyoming, you are going to need a similarly large hot spot to fuel it, or are going to need a really freaking weird tectonic layout.

The largest caldera on Mars is on Arsia Mons, and is merely 105km in diameter. Venus is a bit better with a max of around 150km.The largest caldera in the solar system is Tvashtar Paterae on Jupiter's moon Io at a diameter of 290km.

But a circle with the area of Wyoming would have a diameter of 568km. There isn't a caldera in the solar system that is anywhere close.

And, there is a further problem. On Earth, 60% of our volcanic activity is spread around plate tectonic lines. Only 40% is attributed to hot spots. If one hotspot is big enough to heft the entire state of Wyoming...imagine the volcanic activity of the rest of the planet.

So...what you would basically end up needing here is incredibly improbable, and even then, I can't find proof that it would work. Your region would likely need to be subducting multiple plates around itself...AND you would need a hotspot situated in the middle. This might, MAYBE give you enough geothermal activity to produce a sufficiently large volcanic area. And you'd need a lot of other stuff going on to prevent constant, running volcanic eruptions happening all over the place, allowing it to build up as a mega-caldera...such as the bedrock being a massive, impermeable sheet of basalt or granite...so that the lava ends up lifting the landmass, instead of leaking out in regular eruptions.

You would still have a lot of geothermal activity, like hot springs, geysers (think Yellowstone, but on the scale of an entire state). If this ever went off...expect total extinction on the planet.

  • $\begingroup$ It would be possible to have multiple overlapping calderas like Olympus Mons consists of, but additionally all the volcanic activity would lower the viscosity of the area reducing the size of any possible mountain-like formation. $\endgroup$ – rmoore Mar 25 '15 at 18:48
  • $\begingroup$ The snag there is that he wants it 'ready to erupt.' Even if you have multiple overlapping calderas, you still need a magma plume or subduction zone big enough to fuel the magma pools. $\endgroup$ – guildsbounty Mar 25 '15 at 18:49
  • $\begingroup$ Yea, I think it's a case of conflicting requirements that won't all be possible together. $\endgroup$ – rmoore Mar 25 '15 at 18:52
  • $\begingroup$ 100km diameter is not small, are we sure it isn't enough? $\endgroup$ – o0'. Aug 6 '15 at 16:41

For a single mountain/volcano on a Earth sized planet, (actual Earth) Mount Everest is calculated to be at about the maximum height that can exist. When it gets much larger than that gravity takes over and knocks it down, Mars has much smaller gravity and thus is able to support a much larger volcano.

Otherwise, if you are looking at just a 'large area' volcano then the supervolcano under Yellowstone is very large, something the size of Wyoming would pretty much be a planet killer, Yellowstone when it goes will likely be a mass extinction event.

here's a link that has some calculations for mountain height.

  • $\begingroup$ You are confusing a mountain's height with a volcano's size (area). These are very different things. Yellowstone's height has nothing to do with its great size. $\endgroup$ – RBarryYoung Mar 25 '15 at 15:14
  • $\begingroup$ @RBarryYoung actually no, I didn't confuse the two. I first talked about maximum height a volcano could stand on earth, then had some about 'size' as relates to area, Yellowstone is a very large caldera, but not particularly high. $\endgroup$ – bowlturner Mar 25 '15 at 15:18
  • 2
    $\begingroup$ @RBarryYoung: But the OP apparently wants a tall mountain, since the activity takes place on top of it. $\endgroup$ – jamesqf Mar 25 '15 at 18:00

Getting a Wyoming sized mountain peak wouldn't be possible on something like an Earth-Like planet.

The cosmic mountain-building recipe is simple: the weaker the gravity on the surface of an object, the higher its mountains can reach. Mount Everest is about as tall as a mountain on Earth can grow before the lower rock layers succumb to their own plasticity under the mountain's weight. - Neil deGrasse Tyson

Additionally, the current or impending volcanic activity is going to lower the viscosity of the area further reducing any possible maximum size.

But something else is going on that’s often not talked much about which is that mountains eventually tend to fall down, just through their own weight. And the important concept here is something called viscosity which if you like in the runniness of the rock that’s inside the mountains. A useful analogy is something like syrup. So, if you imagined that you created the Himalayas out of syrup, I think it would be easy to see that as soon as you start pushing the mountain up in the air, it also starts to fall down because the syrup will flow out sideways. - How high can a mountain be?

If we want a large, mountain-like volcano with lots of building space at the 'top' then a Shield Volcano might be the way to go, but that has the downside of not necessarily having 'cataclysmic' eruptions. If you went this route, allowing for slightly less gravity and slightly less active plate tectonics we could possibly look at Olympus Mons as an example. That shield volcano has an area of 290,000 km^2 which is larger then Wyoming but still the summit is only 50mi or so wide vs the 300 you're looking for.

So from there I think it's going to be important to prioritize your wants and needs for this system and realize that it won't all work together.

  • Size of Settlement: How important is the size and placement of the settlement? Can it be scaled down at all, or possibly it's just the capital that is located on the peek / within the caldera?
  • Size & Distinctiveness of Volcano: Does it need to be something noticeable as a volcano? A lot of the larger calderas you wouldn't really think of as a volcano. (And I think in one of the previous links as well, it mentioned that if you were on something the size of Olympus Mons, it's so large in scale you wouldn't even notice the slope of the sides because it would be past the horizon!)
  • Power of the Volcano: Does the volcanic activity have to potentially encompass the whole settlement or can you get away with lesser volcanic activity? Like @guildsbounty brought up, it's going to be difficult to get to a volcano of that size with a Pompeii like eruption without it already have happened earlier along the way. Could the volcanic activity consist of smaller flows like those that go on in Hawaii instead of a singular eruption type event?
  • Sticking to Hard-Science - Can you get away with some hand-waving? Maybe the planet is more Mars-like, but we're just going to treat is as Earth-like anyway. Perhaps the planet's core has mysteriously moving hot-spots that just so happened to follow with the tectonic movements allowing for the larger volcano.
  • $\begingroup$ Do note that he never specified it had to be a volcanic mountain. It could be a large-caldera type volcano like Yellowstone is. :) Low elevation, huge diameter volcano. Erupts very infrequently $\endgroup$ – guildsbounty Mar 25 '15 at 19:16
  • $\begingroup$ I'd be fine with scaling it down a bit. It's not like I have to fit a mega-city in there. The volcano itself doesn't have to be noticeable at all - in fact, the les noticeable, the better. Yes, smaller events could be good, but they'd all have to happen roughly simultaneously. A slight delay is realistic, I'd guess, so I'd be fine with it. I'd prefer to stay away from handwaving, but if we have to go with more Mars-like in terms of tectonic activity, I'm okay with that. $\endgroup$ – HDE 226868 Mar 25 '15 at 20:14
  • $\begingroup$ @guildsbounty I think the confusion came in from mentioning the sides of the volcano in the note and comments since that implies it, as well as the mention of a steep incline in the comments on michaelpri's answer. $\endgroup$ – rmoore Mar 25 '15 at 20:54

Tamu Massif is the largest known Earth volcano.

It's about half the size of Olympus Mons, but that includes the entire thing, not just the summit. It's also bigger than Wyoming.

Now, that's probably not the largest volcano possible. But if the summit area is 10% of the total, you would need a volcano an order of magnitude larger to get what you're looking for. I think that's pretty iffy - I'll bet you start running into scaling issues - but I wouldn't rule it as entirely impossible either.

But what about a Ring of Fire type situation instead?

Set your location on a small tectonic plate fragment that's been pushed up. Due to activity in the past, it's surrounded by several supposedly dormant volcanos. An eruption of one could easily trigger eruptions in the other, leading to region-wide devastation.

  • $\begingroup$ Good catch. Though to be fair, Tamu Massif rises out of oceanic crust which is about four miles thinner than continental crust, so the generation of the shield and its large area is probably not directly comparable. Not sure if an equivalent shield could be created through a continent or if (more likely) it would spread out into a Yellowstone like formation. $\endgroup$ – RBarryYoung Mar 25 '15 at 15:22

I'm not sure that the volcano would have to be a Supervolcano. On Mars, the tallest mountain is Olympus Mons, a shield volcano. Mars is, at least to me, the most Earth-like planet in our solar system, so I don't think it would be that far-fetched for a volcano like Olympus Mons to form on Earth. It is about 26km (16mi) at its highest point and overall it is about the size of Arizona, which is a bit bigger than Wyoming, but not by much, so I'm sure it could be scaled down a bit. Also, it doesn't slope very much, so I don't think it would be hard for a people to live there (well, after they were able to climb it). Because it is a shield volcano, it wouldn't have an explosive eruption, but the effect of an eruption would still be devastating.

For a steeper incline, you'll probably want a stratovolcano. The tallest stratovolcano on Earth is Ojos del Salado. If the volcano has a high eruption rate (or at least did for a long time) then it can be much bigger. If the crust of your planet is more like Mars than Earth, then the volcanoes can also be taller. The volcano will have to be a hot spot volcano though. The moving plates on Earth cause the lava to get spread out over many volcanoes, but on Mars the plates stay stationary, causing a lot of lava to pile up on one volcano. The eruption would probably be explosive too, because it is a stratovolcano.

Also, the gravity on your planet would have to be a lot lower because as bowlturner said in a comment, Mount Everest is about as tall as a mountain can get without being pulled down by gravity.

  • $\begingroup$ I didn't make this clear in the question, but I'm looking for the setting to be at the very top of the volcano, not just on its slopes. $\endgroup$ – HDE 226868 Mar 25 '15 at 1:21
  • $\begingroup$ @HDE226868 They could be anywhere on the volcano, the top might even be better $\endgroup$ – michaelpri Mar 25 '15 at 1:24
  • $\begingroup$ But the top of Olympus Mons is nowhere near as big as I'd like. It's the very tip that I'm interested in. $\endgroup$ – HDE 226868 Mar 25 '15 at 1:24
  • $\begingroup$ @HDE226868 Oh, so you want a steep incline $\endgroup$ – michaelpri Mar 25 '15 at 1:25
  • 2
    $\begingroup$ Olympus Mons is too big for earth, Mount Everest is at about the maximum height any mountain can be on Earth, it has to do with gravity pulling the mountain down. $\endgroup$ – bowlturner Mar 25 '15 at 1:43

Check out this paper: http://pirlwww.lpl.arizona.edu/~jani/radebaugh-pateraeio-jgr01.pdf Which talks about Paterae on Io, which are a type of Caldera. The max size given for those are 200 km diameter; with a mean size of 41 km, which is close to the mean diameters for caldera's for Mars (47.7 km) and Venus (68 km). On earth it lists that ash flow calderas are mean sized at 18.7 km.

The largest Caldera on earth is possibly the first phase Island Park Caldera associated with the Yellowstone hotspot. Note however that the hotspot wasn't associated with that area prior to the first phase Island Park eruption, but was further west and I don't believe that it would have been known that there was a caldera there prior to the initial eruption. Given the initial eruption the hotspot tends to stay in roughly the same place so if they built in something like the Island Park Caldera one of the other eruptions could happen. That is 50 miles by 40 miles, which is massive, but Paterae are bigger. Note though that the Island park ashfall covered from California to the Mississippi and would have had a global climate impact.

The Deccan traps wouldn't seem to work for what you describe as looking for. While such events are the largest examples of volcanism, they aren't properly volcanoes as they happen with multiple fissures and over long time spans.


Perhaps the largest volcanic eruption in human history was the Toba catastrophe @ 77,000 years ago.

The eruption gouged out a 100km X 30 Km caldera complex and is estimated to have ejected up to 3000 cubic kilometres of rock into the atmosphere, causing a "nuclear winter" at least 6 years long and cooling the climate for perhaps 1000 years.

While not the size of Wyoming, this is thought to have caused a genetic bottleneck in the human population and was possibly close to leading to the extinction of the species.

Even "small" volcanic eruptions like Mount Tambora in 1816 can cause a year without summer, and Krakatoa created tsunamis which killed an estimated 40,000 people.

How much more apocalyptic do you want?


If you're looking for an instance of a city built on top of a volcano, Naples in Italy is a pretty good example. It's built right next to the caldera of a large volcano, with some of the suburbs inside. http://en.wikipedia.org/wiki/Phlegraean_Fields


One could have a huge volcano-like event occur as a consequence of an exit wound from a meteor impact on the opposite side of the planet. Certainly anything small that hits something big could come right thru and out the other side, or make a shockwave which produces an eruption of interior material out the other side, or both.

It has been posited that this has happened before on Earth. http://earthepochs.blogspot.com/2014/10/600-mile-wide-impact-crater-in-pacific.html

Wyoming is a square roughly 300 miles each side so this is the same order of magnitude.

I could imagine a scenario where inhabitants live on a nonrotating planet, shielded by the planet's mass from meteors which strike the far side regularly. They feel secure until one of these exit wound type meteor strikes empties a sea in proximity to them. They then realize that their own position is not as secure as they thought.

This scenario is also nice because if one identified the incoming asteroid likely to produce such an event you could send Bruce Willis to go sort it out.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.