1 A centre of volcanic eruption, having the form typically of a conical hill or mountain, built of ashes and/or lava-flows, penetrated irregularly by dykes and veins of igneous rocks, with a central crater from which a pipe leads downwards to the source of magma beneath. Volcanoes may be active periodically, dormant, or extinct; the eruptions may involve violent explosions eg Krakatoa or the relatively quiet outpouring of lava, particularly in those cases where the lava is basaltic eg Hawaii. See figs. at plate tectonics panel and subduction zone. See lava. 2 A conical hill producing mud or sand. See mud volcano.
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