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I have a world which has a drip-and-plume tectonics, meaning that there are no plate divisions. Instead, plumes of magma rise to the surface to create more scattered volcanism, coronae and gradually sloping continents.

I also have coral life in the oceans, which inhabit a global range (thanks to much warmer temperatures and shallower oceans), instead of only being in certain latitudes like on Earth.

How differently might coral islands and atolls (think Polynesia, Maldives, Bahamas etc) form in this drip-and-plume tectonic regime instead of plate tectonics?

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  • $\begingroup$ keep in mind to get big atolls relative sea level needs to change, on earth this can be caused by tectonic sinking of the plate the island is on or by sea level change due to climatic effects. also without moving plates you won't have chains of islands, just super huge volcanoes as the magma from each plume erupts all in the same place. $\endgroup$
    – John
    Commented Sep 11 at 11:40

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It really depends on how densely distributed are these drip and plume spots, and how long they live before fading out.

With dense and short lived spots, you will have a lot of atolls going through the known atoll life cycle, ensuring a thriving environment (all the rest being the same) for the coral reef.

On the other end, sparse and long lived spots will make for very rare atolls, with subsequent less available niches for corals to grow. This might be a more problematic environment for corals to develop, because the further away a suitable spot is, the more difficult it is for life forms to hop from here to there.

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Rings atolls exist because the volcanic mountains they form around have sunk into the crust and dropped below the water line. Drip and plume volcanism should generally result in longer dwell time for volcanic islands, the thicker crust doesn't deform as rapidly so the islands sink far less after they stop erupting. Thus you're not likely to see ring atolls, the sea mounts never completely retreat and the reef defends it from eroding flat. Coupled with the global range of corals you will have large offshore reefs around every coastline that has been above water for any length of time.

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They should be much the same. Polynesia & Micronesia are mostly made from volcanic activity. Coral will form anywhere that the conditions are right and the rest follows.

The larger Islands like Fiji and Samoa started as just a whole bunch of volcanoes.

Reef building corals just need sunlight and a decent temperature. Their spawning can be synchronised over thousands of km. There's also cold water corals that don't need much light.

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  • $\begingroup$ those formed from a whole bunch of volcanoes at multiple subduction zones, which this planet does not have. tectonics is what caues the islands ot be clustered. $\endgroup$
    – John
    Commented Sep 11 at 11:42
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We've seen on Mars that plumes tend to create nothing but massive mountains. Olympus Mons, the largest volcano in the solar system, was created because of a plume and its 3x the size of Mount Everest. If these atolls are being created via plumes, and there's no tectonics to move the forming islands elsewhere, its likely that the atoll will eventually just become an ever-widening mountain.

In other words, you're not going to have chains of islands. At some point, you'll have one nice-sized island, which will grow until its bigger than Hawaii.

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