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This is a question for geology people... Imagine an exoplanet with a thin crust, intense tectonic activity, but with weak, fragmented crust plates that buckle and sink as well as subduct beneath each other.

This generates volcanic islands constantly emerging above the sea level of the global ocean, but with sinking plates, these islands are pulled under as fast as new ones emerge. I'm thinking a turn-over rate on the order of centuries or millennia.

Human colonists can farm the nutrient-rich islands, but no stable population centers can truly establish themselves before the islands sink. Are there known geologic factors preventing this scenario, or would the heat necessary render the planet a molten pressure-cooker?

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    $\begingroup$ If you don't mind a curious question ... why don't the colonists build raft-cities on the water as their islands start to sink, and move the soil to them? $\endgroup$
    – Anon
    Dec 16, 2021 at 6:29
  • $\begingroup$ Millennia is good enough for quite large population centers. Since humans got there somehow, I assume they brought enough knowledge and technology to learn about what's actually happening and predict with more that 50% of success on certain islands stability for quite handy number of centuries. What stops them from doing so? $\endgroup$
    – Adrian Colomitchi
    Dec 16, 2021 at 6:39
  • $\begingroup$ Please consider editing your question. It's not clear if on this planet there are continents at all. Subduction happens when the thinner ocean crust is pressing against the much thicker continental crust. The ocean crust gets bended and sinks. However this is very slow on Earth compared to volcanic activity and small islands formation. Consider also that if volcanic activity / continental drift is so much more active then human life would be in jeopardy due to atmospheric changes. $\endgroup$
    – Duncan Drake
    Dec 16, 2021 at 9:58
  • $\begingroup$ Human colonists might very well be able to predict it, and would need creative rafting options for civilization. The big question is whether buckling and sinking smaller tectonic plates are possible in a way that would cause islands to rise and sink with regularity on this ocean planet? $\endgroup$
    – DAVID RICKS
    Dec 17, 2021 at 7:40

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We have examples of islands appearing and disappearing due to volcanic activity, so what you describe it's perfectly plausible.

However I have some doubts that a world structurally made in that way would allow for humans to develop.

Based on our best knowledge, homo developed in a limited region in Africa and then migrated to colonize the whole planet. Most of that migration happened over thousands of years on bare feet, while ships and navigation came much later.

I doubt humanity could have reached as far as in our case on a planet where the first hominids would sink into the ocean in few hundreds of years.

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  • $\begingroup$ in this case, Humans came along - found the planet and colonized it, they didn't evolve there - native life are aquatic non-sentients. Maybe a species of flying amphibians? Ooooh.... I should write that down... $\endgroup$
    – DAVID RICKS
    Dec 17, 2021 at 7:13
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The most likely factor to prevent this from developing is Mantle Convection.

Heat from the center of the earth traveling up forms convection cells that are somewhat regularly spaced around the globe. These large scale formations would always tend to cause thicker continental plates to form and from there any plate movement or subduction would only thicken the plates and raise the edges of the plates higher to form mountain ranges or island chains depending on the ocean depth.

In order to get a planet with no thicker continental crust would require something to interfere with this normal heat transfer phenomenon. In general a planet with the level of volcanism you describe would likely have a huge amount of water vapor in the atmosphere (I wouldn't be surprised if there was no liquid ocean) and be very hot and not very viable for human habitation. Whatever is interfering with the mantle convection cells would likely make the planet even less livable as it would likely be adding more energy into the system.

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  • $\begingroup$ I was worried about something similar - whether the energy necessary would vaporize the ocean and turn into a pressure cooker? What if you had abundant island chains, but regular Krakatoa-scale eruptions at a rate of once a century? Making dry land unreliable for the sea-faring human colonists? $\endgroup$
    – DAVID RICKS
    Dec 17, 2021 at 7:28
  • $\begingroup$ @DAVIDRICKS making it an ocean world with only small island chains is perfectly possible. Unusually high or difficult to predict long period tides would work, or volcanic or earthquake induced tsunamis would also be a threat on a mostly ocean world. $\endgroup$
    – Josh King
    Dec 17, 2021 at 15:02
  • $\begingroup$ I'm imagining intense lunar-induced tidal compression triggering Krakatoa or Thera-scale eruptions, leading to tsunamis and rising sea levels on other islands... and the effects such impermanence would have on human colonists... $\endgroup$
    – DAVID RICKS
    Dec 18, 2021 at 8:45

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