#volcanic_rock  extrusive igneous rock solidified near or on the surface of the Earth
  supertype:  #igneous_rock__igneousrock  rock formed by the solidification of molten magma
  subtype:  #basalt  the commonest type of solidified lava; a dense dark gray fine-grained igneous rock composed chiefly of calcium-rich plagioclase feldspar and pyroxene
  subtype:  #volcanic_glass  a kind of natural glass produced when molten lava cools very rapidly
     subtype:  #obsidian  acid or granitic glass; usually dark, but transparent in thin pieces
     subtype:  #pitchstone  dark acid granitic glass
     subtype:  #tachylite  a basic or basalt glass
  subtype:  #lava  rock that in its molten form (as magma) issues from volcanos; lava is what magma is called when it reaches the surface
     subtype:  #pillow_lava  lava that hardened in rounded shapes suggestive of pillows; believed to result from underwater eruptions
  subtype:  #tuff__tufa  hard volcanic rock composed of compacted volcanic ash

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