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.

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.

		Volcanic 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.
		Plutonic 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.
		Volcanic The presence of white plagioclase crystals in a glassy matrix identifies this as a volcanic rock.
		Plutonic You can tell by the mosaic of interlocking crystals—gray quartz, pink and white feldspar, black biotite, and silvery muscovite.