Volcano: A rupture in the planetary crust where magma, ash, and gases can escape from below the surface. Plate tectonics: The theory that Earth's outer shell is divided into several plates that glide over the mantle. Mantle plume: A hypothesized upwelling of hot rock from the Earth's mantle. Hotspot: A volcanic region thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Subduction: The process by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge. Tephra: Rock fragments and particles ejected by a volcanic eruption. Lahar: A destructive mudflow on the slopes of a volcano. Supervolcano: A volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index.