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Okay, so I have an island where the story will be based in and around. It's actually more of an atoll, similar to Bikini or Kwajalein in the Marshalls, with the island parts being in somewhat of a ring around a deep-ish lagoon (bay?), but with more substance. I was mainly thinking of the caldera of an extinct volcano.

The island so far

As you can see, it's basically a ring of mountains and jungle sticking up out of the ocean. As it's currently drawn, the island is only about 2.5 miles (4.6ish km) across, lagoon and all.

The Main issue I'm having is scale. I'd like to keep the atoll on the small side, but I'd really like for the ring of mountains to be able to totally block low altitude aircraft (no oxygen required, so under 15000ft/4500m). Sadly I can't reasonably convince myself that a half kilometer wide strip of land could support that and still have room for the story I'm planning. So I'd like something in the 8-10k range. I could settle for smaller mountains and just toss a "reason" in as to why the locals couldn't get over it, but I'm already flipping physics the bird with their airships, so I'd like to avoid that.

In case it matters: The atoll is near the equator of the planet in question, which is mostly earth-like (only real exception to physics when it comes to geography, but not on this atoll). The locals use fantasy-style airships, but haven't figured out the oxygen/altitude thing yet. And a lot of the story will take place in and around the brownish "Arrival Clearing" so that has to stay at a decent size.

So, the question: What's a reasonable footprint for a 8,000-10,000ft (2400-3000ish m) mountain, and how much would this design need to be scaled in order to plausibly ring the atoll?

Thanks in advance!

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  • $\begingroup$ Soooo, quick terminology thing: "atoll" specifically refers to a ring of coral reefs which originally formed around a volcano or other island that has since eroded away. So your island might have a ring of reefs around it that might eventually become an atoll, but for now it'd just be called an island or archipelago. $\endgroup$ – Salda007 Jan 1 at 8:09
  • $\begingroup$ Also, reefs grow in agitated water, so you wouldn't find one forming in the central lagoon like that. They'd be growing on the outside of the ring. You could certainly have a shallow section of the lagoon, though, if the caldera collapsed unevenly. But it would just have a sandy or rocky bottom, not a reef. $\endgroup$ – Salda007 Jan 1 at 8:11
  • $\begingroup$ You simply cant have mountains that tall. However you did specify that what matters is that you cannot go over them without oxygen tanks. So make the mountains shorter, But make the atmosphere thinner. Then you may have a plausible setup for this. $\endgroup$ – Infinite Delta Jan 1 at 8:57
  • $\begingroup$ @Salda007 Good info, thanks! I didn't know that about reefs, and didn't think to look it up. I'll keep it and the terminology in mind as there will probably be a few other island/atoll like objects as my world progresses. $\endgroup$ – Teak Jan 1 at 16:58
  • $\begingroup$ @InfiniteDelta Thanks! Not sure if thinning the atmosphere is the workaround I'm looking for. That would mess with the flight mechanics of the main characters group (who have actual aircraft... plus helicopters). It could work, just not awake enough to do the math at the moment. Though I might just have to switch around the idea a bit after reading the answer from John and have them just set up a SAM site early. $\endgroup$ – Teak Jan 1 at 17:09
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too big, you can't have mountains that tall and anything like what you have drawn

The sides of the ring would have to be tens of miles wide, meaning the lagoon disappears.

Consider Mount Thor which is around half the desired height and one of the steepest mountains in existence (too steep for a a mountain in the ocean), and it is about two miles across at the narrowest at the base. And it is a glacial carved mountain which can be much much steeper than owns created by normal erosion, volcanic mountains in particular tend to be very wide relative to their height.

If you try to scale it up to large enough to get your lagoon it becomes too larger for the mountain ring to exist due to a volcano or any other reason.

To give you a better idea, your mountain is basically the same as the mountain that makes up the bulk of the island of Maui. So that's how big the sides of your lagoon ring have to be. the sides of your ring are 20+ miles across.
enter image description here

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  • $\begingroup$ Thanks for the info! 20 miles is way too big, I'd hoped to keep the whole atoll under 22, so curvature of the earth wouldn't get in the way when looking out to see whats in the surroundings. I know that volcanoes tend to be more conical, but what about just the caldera? I can't find it at the moment (hence the question) but I remember seeing something mentioning that they can be pretty steep. $\endgroup$ – Teak Jan 1 at 5:57
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    $\begingroup$ Calderas are generally only steep on the inner rim, though. They're formed when the magma pocket under the volcano empties after an eruption -- the empty space can no longer support the rock above it, and that rock collapses into it like a giant sinkhole. You'd basically take that map of Maui and punch out the peak with a massive cookie cutter. $\endgroup$ – Salda007 Jan 1 at 7:59
  • $\begingroup$ its not possible strictly, but with a little bit of handwaving it is believable. the caldera would be more like a shaft inside a a cone as Salda said. But the height means it will not connect to the outside except possibly through a tunnel. It also can't be very old geologically so no coral reef on the inside. the inside will barely get any sunlight. this also means no jungle and no beach on the inside, this would be true even as drawn. $\endgroup$ – John Jan 1 at 15:19
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    $\begingroup$ I'm not above handwaving something, though I do generally at least try to make it plausible before that. Thanks! $\endgroup$ – Teak Jan 1 at 17:16

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