IGNEOUS ROCKS

PART II - Plutonic (Intrusive) Rocks

PERIDOTITE

Peridotite is an intrusive ultramafic plutonic rock. It is the major component of the mantle. Extrusive equivalents of peridotite are uncommon because ultramafic magmas rarely reach the surface before solidification.
  • Texture: phaneritic
  • Composition: ultramafic (extremely enriched in iron and magnesium)
  • Mineral Content: Mostly olivine with some pyroxene
  • Color: green
  • Cooling Rate: slow, intrusive
  • Extrusive Equivalent: rare
  • Other Characteristics: green, olivine-rich composition and granular texture

    • GABBRO

    Gabbro is an intrusive mafic plutonic rock. It forms the lower layer of oceanic crust and many continental intrusions.
  • Texture: phaneritic
  • Composition: mafic (enriched in iron and magnesium)
  • Mineral Content: Ca-plagioclase and pyroxene/amphibole
  • Color: black or dark gray
  • Cooling Rate: slow, intrusive
  • Extrusive Equivalent: basalt
  • Other Characteristics: reflective cleavage surfaces on the visible minerals distinguish gabbro from basalt.

    • DIORITE

    Diorite is an intrusive plutonic rock with a composition that is intermediate between gabbro and granite. Many samples of diorite display a 'salt-and-pepper' texture due to the near-equal mixture of light- and dark-colored minerals.
  • Texture: phaneritic
  • Composition: intermediate
  • Mineral Content: 50:50 mixture of ferromagnesium and non-ferromagnesium minerals (plagioclase and ampibole/pyroxene/biotite)
  • Color: salt-and-pepper mixture of light and dark minerals
  • Cooling Rate: slow, intrusive
  • Extrusive Equivalent: andesite
  • Other Characteristics:

    • GRANITE

    Granite is an intrusive felsic (silicic) plutonic rock. The high silica and low iron and magnesium contents result in crystallization of mostly non-ferromagnesium silicate minerals (K-feldspar, Na-plagioclase and quartz). Granite is a common intrusive on continents, often forming erosional remnants like Stone Mountain, GA.
  • Texture: phaneritic
  • Composition: felsic (silicic), enriched in silica and depleted in iron and magnesium
  • Mineral Content: K-feldspar, Na-plagioclase, quartz with minor micas and amphibole
  • Color: light-colored but variable with pink and white varieties most common
  • Cooling Rate: slow, intrusive
  • Extrusive Equivalent: rhyolite
  • Other Characteristics:

    • Click Here to Return to Part I - Extrusive (Volcanic) Igneous Rocks

    Click Here to Return to Course Materials Homepage