The exact same ways any volcano is formed - when magma escapes to the surface (of the sea floor in the case of an underwater volcano).

Formation of Underwater Volcanoes Underwater Volcanoes are formed as the surface of the Earth rips apart since the ocean surface is denser than the underlying mantle it creates pressure on the mantle and the surface cracks. The sea floor sinks down on the mantle and the pressure on the molten rock below decreases as a result of the crack. Due to the reduction in pressure there is a lot of buoyant liquid, which is molten, rock that is formed and it tries to come up. So when the liquid rock, which is called "magma", finally comes up through the fissure on the sea floor, it is called "eruption". Magma when it comes out onto the earth surface is called "lava". At times the lava blocks the fissure and also at times piles up in several layers to form underwater ridges. How does the presence of water distinguish Underwater Volcanoes from Surface Volcanoes ? The presence of water near these fissures greatly makes it different from surface volcanoes. The impact and the nature of eruption also change with the environment. The difference is because the pressure of water is more than the pressure of air. So the eruptions in case of underwater volcanoes are massive explosions and the results are as massive as an island. There are many islands that have been formed as a result of eruption of underwater volcanoes. The thermal conductivity of water is much more than earth or solid rock particles, as a result of which magma is transformed into glass much more faster than in case of volcanic eruption on land surface. As stated the pressure is more than in case of terrestrial eruption, as a matter of fact it is approximately two hundred and fifty times more. Thus the explosive boiling is considerably reduced between magma and the seawater. The lava that is formed in case of underwater eruption is also different from the lava of a terrestrial eruption. When lava comes in contact with the surrounding water a solid crust is formed and the lava flows in that crust consistently, thus forming a structure, which is called "pillow lava". Due to the immense heat that is produced during a volcanic eruption the ocean or the water body becomes really shallow where the volcano erupts. At times there are various chemicals dissolve in the water which come out under the earth’s surface. The biggest underwater volcano is now in Hawaii and if measured from the sea floor it is actually larger than Mount Everest. Scientists the activities of the volcano named Loihi. Since the underwater volcanoes