What is "volcanic foci"

Volcanic \Vol*can"ic\, a. [Cf. F. volcanique, It. vulcanico.]

1. Of or pertaining to a volcano or volcanoes; as, volcanic heat.

2. Produced by a volcano, or, more generally, by igneous agencies; as, volcanic tufa.

3. Changed or affected by the heat of a volcano.

   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.