Undersea Volcano / Underwater Volcano / Submarine Volcano are underwater fissures in the Earth’s surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as ocean ridges. Although most are located in the depths of seas and oceans, some also exist in shallow water, which can spew material into the air during an eruption. Hydrothermal vents, sites of abundant biological activity, are commonly found near submarine volcanoes.
The presence of water can greatly alter the characteristics of a volcanic eruption and the explosions made by these. For instance, the increased thermal conductivity of water causes magma to cool and solidify much more quickly than in a terrestrial eruption, often turning it into a volcanic glass. Below ocean depths of about 2200 meters where the pressure exceeds 218 atmospheres, the critical pressure of water, it can no longer boil; it becomes a supercritical fluid. Without boiling sounds, deep-sea volcanoes are difficult to detect at great distances using hydrophones.
The lava formed by submarine volcanoes is quite different from terrestrial lava. Upon contact with water, a solid crust forms around the lava. Advancing lava flows into this crust, forming what is known as pillow lava.
Underwater volcanoes form much like volcanoes on dry land, by a process known as subduction. This occurs as a result of the tectonic plates which form the top layer of the earth’s mantle, just below the earth’s crust. They support the weight of the continents and the combined water of the seas. This is not a completely solid layer though; they are broken up and float atop a layer of molten rock under intense pressure. The tectonic plates are on constant drift atop this layer of rock, occasionally two plates will pull just far enough apart for the molten rock to pass through and worm its way to the surface.
Underwater however, this occurs a bit differently. Without the presence of the tectonic plates to support the ocean floor, the floor caves in under the weight of the sea, creating a trench and bringing millions of gallons of seawater with it. From the trench arises a growing mound of rock, which continuously spews up from beneath the tectonic plates. The molten rock quickly cools upon contact with the chill seawater, forming a traditional volcano one brings to mind.
More and more magma builds up from the inside of the plug. A minor eruption can occur in which the pressure grows to sufficient levels to blast through the rock blockage. This happens all the time without anybody’s notice. Another possibility is that the magma within the top of the vent behind the blockage begins to cool as well, adding to the blockage. This may continue over a period of months or even years until such time as the pressure either breaks through the side of the volcano, forming a new secondary vent into which the magma passes, or it can blast the entire top of the volcano clean off, much like what happened with Mount Saint Helen’s in Washington. This throws magma up high from the ocean’s depths in such amount as to flash boil millions of gallons of water in minutes. This creates a huge roiling cauldron of water which rises to the surface of the ocean in the form of froth and furious bubbles stinking of sulfur. Any plant or sea life caught within the radius of this cloud of boiling water is killed quickly, adding to the mystique of the deeps as all sorts of dead things rise to the ocean surface to mystify the land dwellers.