General features of the ocean basins.
Map courtesy of NASA and the Smithsonian Institution.

The most productive volcanic systems on Earth are hidden under an average of 8,500 feet (2,600 m) of water. Beneath the oceans a global system of mid-ocean ridges produces an estimated 75% of the annual output of magma. An estimated 0.7 cubic miles (3 cubic kilometers) of lava is erupted. The magma and lava create the edges of new oceanic plates and supply heat and chemicals to some of the Earth's most unusual and rare ecosystems.

 If an estimate of 4,000 volcanoes per million square kilometers on the floor of the Pacific Ocean is extrapolated for all the oceans than there are more than a million submarine (underwater) volcanoes. Perhaps as many as 75,000 of these volcanoes rise over half a mile (1 kilometer) above the ocean floor. Technology and hard work by a group of tenacious explorers/geologists have allowed us our first detailed glimpses of submarine volcanoes. The following pages outline some of the basic characteristics and features of submarine volcanoes.