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TITLE: Race to the Surface

Magma’s upward journey is a race: Will it erupt at the surface? Or will it cool and crystallize below ground? Depending on the outcome, two kinds of igneous rocks form.

[Photo: Volcanic rock with close-up detail showing glassy matrix.]

Volcanic Rocks
If magma ascends faster than it cools, it erupts and solidifies at the surface to form a fine-grained volcanic rock. The magma erupts from volcanoes, which are named after Vulcan, the Roman god of fire. The grains of this volcanic rock are enclosed in a glassy matrix—evidence that the magma cooled within minutes to days, turning the liquid portion to glass.

[Photo: Plutonic rock with close-up detail showing large crystals that fit closely together.]

Plutonic Rocks
If magma cools faster than it ascends it becomes a mush of crystals and liquid that stops in its tracks underground to form a coarse-grained plutonic rock. Plutonic rocks are named after Pluto, the Roman god of the underworld. Like all plutonic rocks, this one is made completely of crystals, with no glass or gas bubbles. The large crystals fit closely together—evidence that the rock cooled underground over tens to hundreds of thousands of years.

Volcanic or Plutonic?
These specimens have similar compositions and minerals. Some are volcanic rocks and some are plutonic rocks. Can you tell which is which?

Look for:

  • larger crystals in plutonic rocks,smaller crystals in volcanic rocks;
  • the presence of glass and gas bubbles in volcanic rocks.
[Photo: Volcanic rock with close-up detail showing glass, plagioclase and olivine.]

The presence of green olivine crystals in a glassy matrix is one clue to this rock’s origin. Another is its rough, spongy appearance—the result of gas escaping during eruption.

[Photo: Plutonic rock with close-up detail showing plagioclase and pyroxine.]

The large black pyroxene and white plagioclase crystals—and the absence of glass or gas cavities—are give-aways that this rock cooled slowly underground.

[Photo: Volcanic rock with close-up detail showing glass, plagioclase and biotite.]

The presence of white plagioclase crystals in a glassy matrix identifies this as a volcanic rock.

[Photo: Plutonic rock with close-up detail showing microcline, quartz, plagioclase, muscovite and biotite.]

You can tell by the mosaic of interlocking crystals—gray quartz, pink and white feldspar, black biotite, and silvery muscovite.

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