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...