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TITLE: Rocks Transform

Inside their cocoons, caterpillars change into butterflies. This is called metamorphism, from a Latin word that means “to transform.” Inside Earth, heat and pressure change a rock’s minerals from one set to another. This, too, is called metamorphism. In each case, the basic ingredients stay the same. They’re just reassembled into a new form.

  • Home-Cooked Minerals
  • Variations on a Metamorphic Theme
  • [Illustration: Photos of basalt, greenstone and eclogite on illustrated background.  Basalt at top left, greenstone in center, and eclogite at bottom right.  Illustration labeled 'Deeper' on left with arrow pointing down, and 'Hotter' at top left with arrow pointing right.]

    These three specimens show how the volcanic rock basalt can change both its minerals and its appearance as it is buried deeper and deeper in the Earth’s crust. Read more below...

    [Photo: Microscopic view of basalt.]

    The basalt pictured above is made up of brown glass and the mineral olivine. As it erupted and cooled, escaping gas bubbles created its spongelike texture.


    [Photo: Microscopic view of greenstone.]

    If basalt is buried to 2,000-4,000 atmospheres pressure, equivalent to depths of 7-15 km, and heated to 350C (660F), its minerals and glass react to form new minerals-epidote and albite. The result is a metamorphic rock: greenstone.


    [Photo: Microscopic view of eclogite showing clinopyroxene and garnet.]

    If greenstone is buried to 18,000 atmospheres, equivalent to a depth of 60 km, and heated to 700C (1,290F), its minerals convert to other ones, including garnet and clinopyroxene. The result is a new metamorphic rock: eclogite.

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    Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits