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When a volcano is under the sea, but close to the coastline, it creates islands. I have a volcanic island near (less than 20km away) a river mouth.

  • Is this feasible? I couldn't find any real example.
  • If it is, would the shape of the island be affected by the currents/sediments from the river? I want the island to be diamond-shaped, so I need to know that.
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  • $\begingroup$ You seems to be essentially asking "will the volcano build the island faster than the sea will erode it?" Maybe yes, maybe no - it's your decision if it's a high-volcanic-activity period or not. "less than" 20 km from a river mouth makes the river and it's sediments seem irrelevant unless it's a huge river - the sea has currents and waves and tides. Shape is trivial to handwave with a few minutes of research on geology - my favorites are "basalt" and "sandstone" $\endgroup$ – user535733 Oct 11 '18 at 23:20
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I don't see why it wouldn't be feasible. Off the top of my head, Mt. Sakurajima in Japan fits the description of a volcanic island (well, not since the eruption a few years ago turned it into an archipelago, but it used to be an island) near a river mouth (well mouths, since there's a number of small rivers that terminate in the bay there.)

If you want a diamond shaped island, just make it diamond shaped. It's entirely plausible that the island just formed in that shape during the last eruption, and the rivers have nothing to do with it one way or the other.

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  • $\begingroup$ Thanks, I just didn't know if I was missing something, because I wasn't able to find real examples of volcanic islands in similar places. $\endgroup$ – scrp Oct 12 '18 at 0:13

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