The Collaborative International Dictionary of English v.0.48:

Volcanic \Vol*can"ic\, a. [Cf. F. volcanique, It. vulcanico.]
   1. Of or pertaining to a volcano or volcanoes; as, volcanic
      heat.
      [1913 Webster]

   2. Produced by a volcano, or, more generally, by igneous
      agencies; as, volcanic tufa.
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   3. Changed or affected by the heat of a volcano.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

volcanic glass
    n 1: a kind of natural glass produced when molten lava cools
         very rapidly