A volcanic eruption is the expulsion of magma and gas from the Earth's interior. Most current volcanic eruptions are the result of one of three eruption mechanisms.
Magmatic eruptions occur as pressure releases from magma inside the Earth. The depressurization forces magma out through cracks in the Earth's crust, usually in the seafloor. Most of these eruptions involve basalt magma.
Phreatomagmatic eruptions, which are typically underwater, are the result of tectonic plates plunging downwards into Earth's mantle. This type of eruption is the opposite of a magmatic eruption in that it involves the addition of pressure to magma, rather than the release of pressure. Phreatomagmatic eruptions are responsible for the formation of most of earth's volcanoes.
Phreatic eruptions happen when steam superheats upon contact with magma. These eruptions often have little to do with the edges of tectonic plates, and instead, they occur at hotspots. These account for approximately 10 percent of the Earth's volcanoes.
Flood volcanism no longer occurs, but it is responsible for such landforms as the Columbia River Basalts and various underwater plateaus. In flood volcanism, basalt lava issues from fissures in the Earth's crust, sometimes covering over a thousand square miles. Tar and mud volcanoes also exist, neither of which involve magma.