Volcanic bomb, a mass ejected from a volcano, often of molten lava having a rounded form. -- Volcanic cone, a hill, conical in form, built up of cinders, tufa, or lava, during volcanic eruptions. -- Volcanic foci, the subterranean centers of volcanic action; the points beneath volcanoes where the causes producing volcanic phenomena are most active. -- Volcanic glass, the vitreous form of lava, produced by sudden cooling; obsidian. See Obsidian. -- Volcanic mud, fetid, sulphurous mud discharged by a volcano. -- Volcanic rocks, rocks which have been produced from the discharges of volcanic matter, as the various kinds of basalt, trachyte, scoria, obsidian, etc., whether compact, scoriaceous, or vitreous.