Volcano: A geological formation through which magma, ash, and gases from below Earth's crust are expelled. Plate tectonics: A scientific theory that describes the large-scale motion of Earth's lithosphere. 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 volcanic activity thought to be fueled by a mantle plume. Mantle plume: A hypothetical upwelling of abnormally hot rock within the Earth's mantle. Shield volcano: A type of volcano with a broad, domed shape and gentle slopes, built by the eruption of fluid basalt lava. Cinder cone: A small, steeply-sloped volcano formed from volcanic ash, tephra, and bomb eruptions. Stratovolcano: A volcano built up of alternate layers of lava and ash, known for its steep profile and periodic explosive eruptions. Volcanic winter: A reduction in global temperatures caused by volcanic ash and sulfuric acid droplets in the atmosphere.