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A volcanic eruption is measured in two constants--gas and viscosity. For clearance, low viscosity is like squirting water off a nozzle, whereas high viscosity is like squirting caramel off a nozzle, which takes more effort to do, which makes it more dangerous.

In geology, there are four different kinds of volcanic eruptions:

  1. Low Gas + Low Viscosity = A quiet lava flow. enter image description here
  2. High Gas + Low Viscosity = A fire fountain eruption. enter image description here
  3. Low Gas + High Viscosity = A dome-building eruption. (This sort of eruption doesn't reach critical right away.) enter image description here
  4. High Gas + High Viscosity = A Ring of Fire explosion. enter image description here

These sorts of eruption are impressive enough on land, but in the event of someone doing some serious worldbuilding, this question puts three of the four types underwater. Type #1 has long been confirmed to happen underwater, but the oceanic differences in temperature and pressure have produced a different result called "pillow lava".

enter image description here

Is it possible for eruption types numbers 2, 3 and 4 to occur underwater? If yes, then would the oceanic differences in temperature and pressure make them look and act differently from how they have acted above sea level?

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    $\begingroup$ Please, attribute authors of images you are using, and make sure that they are CC-compatible. $\endgroup$ – Mołot Apr 26 '18 at 7:59
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    $\begingroup$ I'm voting to close this question as off-topic because it's a question purely about geology. $\endgroup$ – StephenG Apr 26 '18 at 13:07
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    $\begingroup$ @StephenG, How can a question that is literally about worldbuilding be off-topic? Please site a reason from the help center to justify the VTC. $\endgroup$ – JBH Apr 26 '18 at 15:50
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    $\begingroup$ Author's choice people. Feel free to suggest migrating to another site but being on-topic elsewhere doesn't make it off-topic here. $\endgroup$ – James Apr 26 '18 at 16:12
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    $\begingroup$ @StephenG, you just disqualified about 30% of the questions that are asked here. I asked you to point to a help center condition that made this question off-topic and you did not do so. If you want to bring your opiniion to the Community for review, please post a question in Meta, but this question is not off-topic. $\endgroup$ – JBH Apr 27 '18 at 0:04
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Underwater volcanoes have basaltic lava, which is poor in silicates. This means their lava is low viscosity, so you are pretty much limited to type #2.

However, since a low viscosity magma is also less capable of capturing gases on the long term, the only way to have low viscosity and high gas is a late mixing of water and magma with the resulting formation of high pressure steam.

Being underwater strongly reduce the bursting of steam, as water pressure can increasingly counter the steam pressure with increasing the depth.

That's why most of the observed type #2 eruptions happen closer to the surface, like in the Isola Ferdinandea or Graham Island.

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First, for (2) and (4), the key element is decompression - volatiles (Mostly CO2 and H2O) coming out of solution as the pressure decreases. However, the solubility increases exponentially with depth (unfortunately all the good links for this are pay-walled). So in any significant depth of water (more than a few hundred meters), these eruptions can't really happen because the magma can't decompress sufficiently.

For (4) as well, many of the effects of the eruption come from a column of superheated gas and ash that rises into the air in an Pelean Eruption. This then collapses to give pyroclastic flows. Obviously this won't happen underwater. In shallower water, you'd get a hot mix of water and ash, and some submarine pyroclastic flows.

(3) is interesting; but the presence of water would lead to explosive fracturing, and I suspect a block and ash flow as described above.

Overall you will see fewer if any actual explosions, but a lot of ash/block density-driven flows.

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  • $\begingroup$ I've never seen a block-and-ash flow. $\endgroup$ – JohnWDailey Apr 27 '18 at 4:30

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