Submarine volcanoes 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 mid-ocean ridges. A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by Although most are located in the depths of 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. A hydrothermal vent is a Fissure in a planet's surface from which geothermally heated Water issues
The presence of water can greatly alter the characteristics of a volcanic eruption and the explosions made by these eruptions. For instance, the increased thermal conductivity of water causes magma to turn into glass much more quickly than in a terrestrial eruption. Below ocean depths of about 2243 meters where the pressure exceeds 218 atmospheres, the critical pressure of water, it can no longer boil; it becomes a supercritical fluid. In Physical sciences standard conditions for temperature and pressure are Standard sets of conditions for experimental measurements to allow comparisons to be made The critical temperature, Tc of a material is the Temperature above which distinct Liquid and Gas phases do not exist A supercritical fluid is any substance at a Temperature and Pressure above its Thermodynamic critical point. Without boiling sounds, deep-sea volcanoes are difficult to detect at great distances from hydrophones. A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening
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.
Scientists still have much to learn about the location and activity of underwater volcanoes. The Kolumbo underwater volcano in the Aegean Sea was discovered in 1650 when it burst from the sea and erupted, killing 70 people on the nearby island of Santorini. More recently, NOAA's Office of Ocean Exploration has funded missions to explore submarine volcanoes. In the United States the Office of Ocean Exploration ( OE) is a division of the National Oceanic and Atmospheric Administration (NOAA run under the auspices Most notably, these have been the Ring of Fire missions to the Mariana Arc in the Pacific Ocean. Using Remote Operated Vehicles, scientists studied underwater eruptions, ponds of molten sulfur, black smoker chimneys and even marine life adapted to this deep, hot environment.
Many submarine volcanoes are usually found as seamounts. A seamount is a Mountain rising from the Ocean Seafloor that does not reach to the water's surface ( Sea level) and thus is not an Island These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000 - 4,000 meters depth. They are defined by oceanographers as independent features that rise to at least 1,000 meters above the seafloor. Oceanography (from the greek words Ωκεανός meaning Ocean and γράφω meaning to write also called oceanology or The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. The deep sea, or deep layer, is the lowest layer in the Ocean, existing below the Thermocline, at a depth of 1000 fathoms or more  An estimated 30,000 seamounts occur across the globe, with only a few having been studied. However, some seamounts are also unusual. For example, while the summits of seamounts are normally hundreds of meters below sea level, the Bowie Seamount in Canada's Pacific waters rises from a depth of about 3,000 meters to within 24 meters of the sea surface. The Bowie Seamount is a Submarine volcano, located 180 kilometers west of the Queen Charlotte Islands, British Columbia, Canada.