ADJECTIVE (3)

1. relating to or produced by or consisting of volcanoes;
- Example: "volcanic steam"
- Example: "volcanic islands such as Iceland"
- Example: "a volcanic cone is a conical mountain or hill built up of material from volcanic eruptions"

2. explosively unstable;
- Example: "a volcanic temper"

3. igneous rock produced by eruption and solidified on or near the earth's surface; rhyolite or andesite or basalt;
- Example: "volcanic rock includes the volcanic glass obsidian"

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.
      [1913 Webster]

   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
    adj 1: relating to or produced by or consisting of volcanoes;
           "volcanic steam"; "volcanic islands such as Iceland"; "a
           volcanic cone is a conical mountain or hill built up of
           material from volcanic eruptions"
    2: explosively unstable; "a volcanic temper"
    3: igneous rock produced by eruption and solidified on or near
       the earth's surface; rhyolite or andesite or basalt;
       "volcanic rock includes the volcanic glass obsidian"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "volcanic":
   agitable, bursting, combustible, detonating, edgy, emotional,
   emotionally unstable, eruptive, excitable, explodable, explosible,
   explosive, fierce, fiery, fulminating, furious, hasty,
   high-mettled, high-spirited, high-strung, highly emotional, hot,
   hot-tempered, hotheaded, impetuous, inflammable, irascible,
   irritable, mettlesome, nervous, passionate, peppery, perturbable,
   prickly, quick, quick-tempered, sensitive, short-tempered,
   simmering, skittish, spunky, startlish, stormy, tempestuous,
   tornadic, touchy, vehement, violent, wild