Volcano: A rupture in the crust of a planetary-mass object that allows lava, ash, and gases to escape from a magma chamber below the surface. Plate tectonics: The theory that Earth's outer shell is divided into several plates that glide over the mantle. Divergent boundary: A tectonic plate boundary where two plates are moving away from each other. Convergent boundary: A tectonic plate boundary where two plates are moving towards each other. Hotspot: A location on Earth's surface that has experienced active volcanism for a long period, thought to be fed by underlying mantle plumes. Mantle plume: A proposed mechanism of convection within the Earth's mantle that leads to volcanic activity at the surface. Supervolcano: A volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest value on the index.