Submarine volcanic eruptions primarily occur at spreading centers, where tectonic plates are moving away from each other. Most seafloor spreading centers lie at depths exceeding 2,000 meters (1.2 miles) and as a consequence, a majority of Earth’s volcanic activity occurs deep underwater where the effects cannot be seen from the surface of the ocean. Spreading centers typically produce a rock called basalt. Submarine basalt flows often have a distinctive “pillow” shape, but can exhibit smooth flows depending on the speed and viscosity of the erupting lava.
Submarine eruptions can also occur at subduction zones, where crustal plates collide and one plate is pushed beneath the other where it is re-melted. The primary rock type at subduction zones is andesite. This rock makes lava that has a high viscosity and high gas content, which produces very violent eruptions.
Hot spot volcanoes are the last type of submarine volcanoes. These take place where a magma plume rises through the Earth’s crust overlying an area of melting in the Earth’s mantle. These eruptions are also primarily basalt. They often produce chains of volcanic islands and seamounts.