WordWeb Online

1. Explosively unstable
   "a volcanic temper"
    
2. (geology) having igneous rock produced by eruption and that solidified on or near the earth's surface
   "volcanic rock includes the volcanic glass obsidian"
    
3. 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"

See also: extrusive, unstable

Encyclopedia: Volcanic