Volcanoes are openings in the Earth's crust through which magma, gases, and ash erupt, classified by the shape of the cone, the nature of the lava (viscous or runny), and the style of eruption.
- By activity: active (erupting or likely to erupt, like Etna), dormant (quiet but not extinct, like Vesuvius), and extinct or dead (no record of eruption, like Aconcagua).
- By form and lava type: shield volcanoes (broad, gently sloping, built by runny basaltic lava, like Mauna Loa in Hawaii); composite or strato volcanoes (steep, layered ash and lava, explosive, like Fuji, Stromboli, and Mount St Helens); cinder cones (small, steep, from fragmented ejecta); and calderas (huge collapse craters after a violent eruption).
- Flood basalt or fissure eruptions are not single cones but vast outpourings along cracks, building lava plateaus such as the Deccan Trap of India and the Columbia plateau of the USA.
- Eruption styles (in rising violence) include Hawaiian, Strombolian, Vulcanian, Pelean, and Plinian.
- Barren Island in the Andaman Sea is India's only confirmed active volcano; Narcondam nearby is treated as dormant.
The active or dormant or extinct distinction, the shield versus composite versus caldera forms, the link between fissure eruptions and the Deccan Trap, and Barren Island are recurring facts.
Shield (gentle, runny basalt) versus composite (steep, explosive, viscous); dormant (could erupt again) versus extinct (no future eruption expected); caldera (collapse crater) is much larger than an ordinary crater.
By activity (active, dormant, extinct) and by form (shield, composite, cinder cone, caldera, fissure-fed lava plateau).