Are there volcanoes in the ocean depths? If so, how deep? And how does an underwater, say Mt. St. Helens, manifested at the surface of the ocean? Any famous ones? How destructive?

rocky Jim Mulligan

Hi Mr. Mulligan,

The easiest answer to your question is yes, there are underwater volcanoes. The most obvious one is the system of mid-ocean ridges that sort of run around the globe like the seams on a baseball. These are where oceanic crust is formed. By definition, a volcano is a place where magma comes out of the earth's interior onto the surface, so the mid-ocean ridges can be thought of as a 60,000 km-long volcano!

Thinking more along the lines of "normal" volcanoes, certainly all present volcanic islands (such as those in Hawai'i, the Aleutians, Japan, the Marianas, the Azores, the Comoros, etc.) were once under-sea volcanoes since their bases are on the sea floor. This might make you think that in the vicinities of these presently emerged volcanoes there might be other volcanoes that haven't made it yet, and indeed there are. Lo'ihi is the youngest Hawaiian volcano, and its summit is presently about 1 km below sea level (it is already some 3-4 km high, depending on where you define its base). There is an undersea volcano in the Lesser Antilies named "Kick 'em Jenny" (I don't know how it got that name). Kick 'em Jenny erupts every few years or so. There are also reported undersea volcanoes near Japan, Tonga, the Cook Islands, and Iceland, to name a few.

Probably most undersea eruptions go un-noticed. Certainly nobody has ever wittnessed an eruption at a mid-ocean ridge. One reason for this is that it doesn't take very much water depth for the pressure to be high enough to keep explosions from occurring. The undersea eruptions that have been wittnessed have occurred in shallow water where the pressure cannot prevent strong explosions or where the volcano actually grows high enough during the eruption to breach the surface. Probably the most famous of these is Surtsey, which appeared off southern Iceland in 1963. Prior to 1963 the location where Surtsey appeared was known by fisherfolk to be shallow. The first part of the eruption was very violent due to the explosive mixing of hot lava and sea water without a deep overburden of water pressure. Soon the volcano built above sea level and as soon as it built enough of a structure to keep water out of the main vents, the eruption became much less violent, and more like a Hawaiian eruption.

What often happens is that an eruption will build a volcano above sea level, only to have subsequent wave action erode it away until the next eruption. Thus, reports of "disappearing islands" aren't so fanciful afterall. The most recent example of this is Metis Shoal in Tonga. It has had numerous reported eruptions since the mid 1800's many of which have produced floating "rafts" of pumice. There is a discussion of the most recent eruption (which started June 6) in the " Update on current volcanic activity " section of VolcanoWorld.

Hopefully this long-winded answer to your question will encourage you to delve further into the study of underwater volcanism. I don't know of any "Mt. Saint Helens-type" eruptions that have been recorded underwater, but sonar images indicate the presence of underwater volcanoes with steep slopes and what look to be large landslide scars, so it isn't that such eruptions are impossible.

Sincerely,

Scott Rowland, University of Hawaii 

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