One interesting fact about underwater volcanoes is that they deposit about 75 percent of the annual magma output on Earth. The molten magma and hardened lava shape the ocean floor and create the edges of the new plates on the ocean floor.
The largest underwater volcano is the Krakatau, a volcanic island between Sumatra and Java in Indonesia. It is mostly a submerged caldera, a basin-shaped volcanic depression formed from a previous powerful volcanic eruption. Located in the Sundra Strait, Krakatau is approximately 5.5 miles long and 3 miles wide.
Underwater or submarine volcanoes are commonly found in ocean ridges or areas where tectonic plates move away or towards each other. Because most of these volcanoes are submerged, they do not produce the same spectacular volcanic eruption like some of their terrestrial counterparts. Their constant activity, however, greatly transforms the majority of the oceanic crust.
It is also an interesting fact that the chain of islands in the Pacific, specifically the Hawaiian Islands, is formed through continuous volcanic eruptions. The Hawaiian hot spot, which sits near the Pacific Plate, is responsible for the formation of the Big Island in Hawaii. The largest island in the state is also home to five volcanoes, of which three are active. Other hot spots are found in other parts of the world, such as Galapagos Islands and Iceland.