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.