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In the world I'm building, there is a single continent. However, in the deeper parts of the ocean, there are hydrothermal vents. They're pretty necessary to my story, but I can't figure out why they would be there as to my knowledge they usually need to be near cracks in the tectonic plates.
I don't know if it's worth noting, but the core of my world is very small and there is little magma to be found. The hydrothermal vents are also about 16 500 ft down instead of around 16 000 feet.
Hotspots, Hawaii and Yellowstone are two of the most active geothermal/volcanic areas on Earth. Both are sited in the middle of tectonic plates rather than along the edges where most volcanism occurs and both occur at fixed locations that are fed heat and magma from great depths. Such locations could be prime candidates for subsurface geothermal vents.
Alternately you could have a "double-ring" plate system in which spreading zones, along which your thermal vents would be, could feed crust into subduction zones all around the continent. This will mean that there are tectonically and volcanically active mountains all around your one continent. This is not a situation that is stable over geological time, but could be maintained for a long time. This would be like the modern pacific basin only more so, with the ocean being around the continent instead of the continent around the ocean.
You can have tectonic plates even on a water world, that is still "land" under there, you just have enough water to cover it all.
In your case, any tectonic plates carrying enough land to break through water are all adjacent to each other; just like our continents were once all adjacent. Look at early Earth and Pangaea for an example.
With multiple plates you can still have mountains, valleys, etc. No problems with it, just as you have it. Any tectonic plate joins under the ocean are just plates that don't carry enough land to be seen above water.
No need for the single continent (let's hypothetically call it "Pangea" -- all earth) to have always existed, or for it to always exist in the future.
https://en.wikipedia.org/wiki/Pangaea
Pangaea or Pangea ( /pænˈdʒiːə/) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from earlier continental units approximately 335 million years ago, and it began to break apart about 175 million years ago.
Those hydrothermal vents are part of the undersea mid-ocean ridges which pushed the earlier continents together...
By en:User:Kieff - File:Pangaea continents.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8161694
Natural nuclear reactor.
Nuclear fission makes a lot of heat, which is how we use uranium to generate power. Naturally occurring concentrations of uranium can undergo nuclear chair reactions naturally, and generate a lot of heat. The Wikipedia article details a naturally occurring fission chain reaction at a site called Oklo.
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor
Mechanism of the reactors The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials no longer could sustain a chain reaction.
In your world there are multiple such deposits. If they are undersea that is good because it will be harder for the groundwater to boil away. The reaction will continue until the fissile materials are depleted.
