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In order for the plot and character development I have planned to work the people need to be living on an island continent about 200,000km (England/New Zealand/Japan size, but oval/round shaped) with such steep impassable mountains (and/or cliffs to the ocean) on every side so that everyone is essentially locked in from the ocean world.

At first I was thinking this could be created by a massive volcano with the caldera being the sea in the middle, but after researching the biggest in the solar system: 624 km (Mars: Olympus Mons) I had to laugh at how too big that would to be for a realistic volcano on an earth sized planet.

So then I was thinking it could be an impact crater from an asteroid that could also possibly deliver many of the resources the civilization would be able to use and the salt water sea in the middle could be created by just a collection over time - but mine would also be magnitudes bigger than the largest impact crater 4880km (Mercury: Caloris)

Then, I thought, maybe my island continent is on its own tectonic plate; the smallest on earth is 250,000km (Juan de Fuca) and all of the surrounding ones are pushing up the mountains...But that would likely create so many volcanoes and constant earthquakes that it would be terrifying to live there. (My poor people!)

My question: Can a tectonic plate ever STOP moving, at least long enough for a civilization to develop?

If not, can anyone think of another way to have this scenario come to be?

Bonus - is there any way it could have a salt sea in the middle that is alive with some smaller ocean creatures for fishing - maybe even connected to the ocean way underground so there can be tides? That's probably asking too much...

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    $\begingroup$ "Is there any way it could have a salt sea in the middle that is alive with some smaller ocean creatures for fishing": you mean, like the Caspian Sea in the middle of Eurasia? (Note that in the real world the Caspian all by itself covers 371,000 km² or 143,200 sq mi. An island of 200,000 km² is not a continent by any stretch of imagination. It is smaller than Romania, and my country is mid-sized even among European countries. I would say that the 3,000,000 km² of India are the smallest acceptable size for a continent.) $\endgroup$
    – AlexP
    Feb 8, 2022 at 15:40
  • $\begingroup$ @AlexP Continents don't have to be above water though. If we look at the tectonics and all of that, New Zealand is its own continent. It just appears as a smaller island chain because only the highest elevations are above sea level. If he's willing to have a large continental shelf, there is no reason his land mass can't be the size he wishes it. I'd say the mountain barrier ringing it is the more difficult issue. $\endgroup$
    – John O
    Feb 8, 2022 at 16:00
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    $\begingroup$ @JohnO: The question says "continent sized land formation". Yes I know that geologically Zealandia is a continent; and alongside New Zealand, New Caledonia also belongs to it. But New Caledonia and New Zealand's islands are still islands, just like Great Britain. $\endgroup$
    – AlexP
    Feb 8, 2022 at 16:18
  • $\begingroup$ AlexP - I will now refer to it as an island, or consider making it bigger. Thank you! I should do some more research on how big and advanced civilizations can grow in confined spaces. $\endgroup$ Feb 9, 2022 at 2:38

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It is the same as Earth. There is just more water.

tibetan plateau

Here is the Tibetan Plateau from our world. It is bigger than what you want, but is here for the principle. Everything around it is underwater. In your world there is a lot more water. The oceans are deeper. It is the Earth we know but it is covered with water except for the Tibetan Plateau and a couple of other scattered tall mountains here and there, which in your world are little islands. The roof of the world is the only place where you find a dry land mass of any area.

Earth's tectonic activity makes lots of landforms under the ocean. We just don't see them because they are under the ocean. You can have a world that works the same as ours and site your story in the highest elevation landmass that world has produced. It is just not that high above sea level because your oceans are very deep!

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  • $\begingroup$ This is good to know, thank you! What it looks like under the water doesn't matter too much for the majority of what I wanted to do, but knowing this allows for some flexibility for one aspect of the story I hadn't figured out yet. Based on the Mordor comment above, now I want to know what the weather is like there. hehe $\endgroup$ Feb 9, 2022 at 2:35
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I don't think that tectonic plate would be a problem, it is so slow that a civilization should not be affected during its developmente. After all Earth has a still active tectonic plate and developed civilizations.

A way you can explain an island with this shape is a series of volcanoes that created the island (think the fire belt on Earth) and are now extinct so no more eruptions and a pretty stable place for a civilization to develop.

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  • $\begingroup$ This will work! Thank you! $\endgroup$ Feb 9, 2022 at 2:31
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Impassable mountains all over the perimeter of a very tiny "continent" ?

On top of that you also add a watermass in the center?

Did you notice you basically created Mordor?

Places like that exist on earth and they are living nightmares, basically there's lighting storms almost 365 days a year.

So yeah, it exists in real life so probably it is realistic in your story, and those places are populated, even though they are basically Mordor.

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  • $\begingroup$ I had not realized it was Mordor, no. LOL I would be fine with lightning storms every day in some places, but over the entire island (I'm agreeing it's too small to be a continent) seems like it would be a lot to have to factor in for the story line. I only have 2 places when I was going to write a rain storm and I'd feel I need to include it more. Maybe I could contrive some way to direct the storms, or even just the lightning in some way for it not to be a factor. Thank you! $\endgroup$ Feb 9, 2022 at 2:29

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