Volcano it is a vent or an opening through which the molten magma or lava comes out. Hot gases, superheated steam, pyroclast. Stones are thrown up. Volcanoes ma y be classified on the mode of eruption as (a) Central type, (b) fissure eruption.
Central type forms a conical hill. At the summit there is a depression called the crater. It is acid lava with high viscosity.
Fissure type forms a flat plateau or a series of steps. The lava spreads over several kilometers as the temperature is very high. It is basic lava. The lava flows through a crack or a fissure and flows quietly. E.g., Deccan plateaus.
Volcanoes may be classified on the basis of their activity as – (a) Active volcanoes: these erupt frequently, e.g., Stromboli, Cotapaxi, Mauna Kea.
(b) Dormant: these erupt after a long period of time say 20 years, 50 years, 100 years or more.
(c) Extinct: they had erupted in the pre-historic times and show no signs of eruption, e.g., Chimbrazo, Mount Kilimanjaro.
Extinct volcanoes must not be ignored as they can erupt suddenly, e.g., Mt. Vesuvius.