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A caldera is usually the remnant of a volcanic eruption powerful enough to collapse the structure surrounding the magma chamber. Many of our examples, like Yellowstone, Crater Lake, Toba and even Ngorongoro, are inland, hundreds of miles from any seawater. The only exception I can think of is Santorini. When it erupted some five thousand years ago, it must have measured 6 on the VEI scale, a colossal eruption, turning one island into several.

So with those examples in mind come this particular question. Inland calderas are one thing, but what about inshore calderas? Is it possible for a Ring of Fire eruption, like in the Andes or in Japan, to release a strong enough eruption to collapse many square miles of land to the bottom of the sea, thus creating a flooded crater?

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  • $\begingroup$ I can't think of any examples where a submarine volcano is inshore. $\endgroup$ – RonJohn May 3 '18 at 3:45
  • $\begingroup$ Also, this would be better asked on EarthScience.SE. $\endgroup$ – RonJohn May 3 '18 at 3:45
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    $\begingroup$ EarthScience doesn't accept what isn't true. $\endgroup$ – JohnWDailey May 3 '18 at 3:50
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    $\begingroup$ Like the harbor of Aden, the Aira Caldera and the Otago Harbour in Japan, or the Lyttelton Harbour in New Zealand? $\endgroup$ – AlexP May 3 '18 at 3:52
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    $\begingroup$ @RonJohn Like I said, all calderas are inland, too far from the sea to be considered coastline. $\endgroup$ – JohnWDailey May 3 '18 at 4:24
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Krakatoa

Krakatoa might satisfy your requirements. It was a large island till it erupted and split into three smaller islands. It's also one of the largest explosions in recorded history.

Finding off-shore caldera is tricky since they're usually covered in water immediately after erupting. Krakatoa is easier to find since it was a big island and smaller islands remain after its eruption.

Submerged caldera aren't common but they do exist. Wikipedia has a whole category devoted to them.

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Preatomagmatic blasts occur in shallow water where hot magma comes into direct contact with either lake or sea-water where the water pressure is not high enough to keep the water from flash boiling. These are extremely violent eruptions that often leave flat dishlike craters like the Emerald Lakes on Mount Tongariro in New Zealand. These can and often do form at or slightly below sea level as seen in the Tuff Rings and Maars of the Auckland Volcanic Field, and they're very similar to calderas in their form if not in their formation. But if you want a true caldera, from the collapse of a volcanic magma chamber, in deep water then odds are it formed above sea level and the sea rose rather than it forming on the sea floor. The pressure from a deep water column will keep the explosive index of an eruption low by preventing rapid out-gassing and thus prevent chamber exhaustion and collapse. You can also get semi-submerged calderas, Krakatoa and Thera(Santarini) are prominent examples, formed by the collapse of volcanic islands after major eruptions.

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