Submarine volcanoes

Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. A large number of submarine volcanoes are located near areas of tectonic plate movement, known as mid-ocean ridges. The volcanoes at mid-ocean ridges alone are estimated to account for 75% of the magma output on Earth. Although most submarine volcanoes are located in the depths of seas and oceans, some also exist in shallow water, and these can discharge material into the atmosphere during an eruption. The total number of submarine volcanoes is estimated to be over 1 million, of which some 75 000 rise more than 1 km above the seabed.

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 of underwater volcanoes in comparison to those on land.

For instance, water causes magma to cool and solidify much more quickly than in a terrestrial eruption, often turning it into a volcanic glass. 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.

Below ocean depths of about 2200 m, where the pressure exceeds the critical pressure of (22.06 MPa or about 218 atmospheres for pure water), it can no longer boil; it becomes a supercritical fluid. Without boiling sounds, deep-sea volcanoes can be difficult to detect at great distances using hydrophones.

The values of the critical temperature and pressure increase in solutions of salts, which are normally present in the seawater. The actual composition of aqueous solution in the vicinity of hot basalt, and circulating within the conduits of hot rocks, is expected to differ from that of bulk water (i.e., of sea water away from the hot surfaces). One estimation is that the critical point is 407 °C and 29.9 MPa, while the solution composition corresponds to that of approximately 3.2% of NaCl.

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