Igneous Rocks

Pamela J. W. Gore
Georgia Perimeter College

Objectives

  1. Contrast magma and lava.
  2. Describe how the rate of cooling influences the size of crystals in igneous rocks.
  3. List the different igneous rock textures (aphanitic, phaneritic, porphyritic, pegmatitic, vesicular, glass, and pyroclastic) and explain their origins.
  4. Discuss the contributions of N. L. Bowen to the understanding of igneous rocks. Be able to list the minerals of Bowen's Reaction series in order. Explain the significance of Bowen's Reaction Series to melting and crystallization.
  5. Discuss the various types of magma and how they relate to igneous rocks.
  6. List the various kinds of intrusive igneous bodies (dike, sill, laccolith, stock, batholith) and describe each in terms of the criteria used to classify plutons.


Origin

"fire-formed rocks"

Crystallize from molten material:

Lava cools more quickly because it is on the surface.


Cooling Rates

Cooling rates influence the texture if the igneous rock:


Igneous rocks are classified on their texture and their composition.


Igneous textures:

Pyroclastic rock


Composition of Igneous Rocks

Igneous rocks can be placed into four groups based on their chemical compositions:

  1. Sialic (or granitic or felsic)
    1. Dominated by silicon and aluminum (SiAl)
    2. Usually light in color
    3. Characteristic of continental crust
    4. Forms a stiff (viscous) lava or magma
    5. Rock types include:
      1. Granite
      2. Granite

      3. Rhyolite
      4. Rhyolite

    6. Minerals commonly present include:
      1. potassium feldspar (generally pink or white)
      2. Na-plagioclase feldspar (generally white)
      3. quartz (generally gray or colorless)
      4. biotite
      5. amphibole?
      6. muscovite?

  2. Intermediate (or andesitic)
    1. Intermediate in composition between sialic and mafic
    2. Rock types include:
      1. Andesite (aphanitic)
      2. Diorite (phaneritic)

      Diorite

    3. Minerals commonly present include:
      1. plagioclase feldspar
      2. amphibole
      3. pyroxene
      4. biotite
      5. quartz

  3. Mafic (or basaltic)
    1. Contains abundant ferromagnesian minerals (magnesium and iron silicates)
    2. Usually dark in color (dark gray to black)
    3. Characteristic of Earth's oceanic crust, Hawaiian volcanoes
    4. Forms a runny (low viscosity) lava
    5. Also found on the Moon, Mars, and Venus
    6. Rock types include:
      1. Basalt (aphanitic)
      2. Basalt

      3. Gabbro (phaneritic)
      4. Gabbro

      5. Diabase - texture intermediate between basalt and gabbro; characteristic of Early Mesozoic dikes in eastern North America.

    7. Minerals commonly present include:
      1. Ca-plagioclase feldspar
      2. pyroxene
      3. olivine
      4. amphibole

  4. Ultramafic
    1. Almost entirely magnesium and iron silicates (ferromagnesian minerals)
    2. Rarely observed on the Earth's surface
    3. Believed to be major constituent of Earth's mantle
    4. Commonly found as xenoliths in basaltic lavas
    5. Rock types include:
      1. Peridotite (phaneritic)
        1. dominated by olivine - the birthstone is Peridot, which gives its name to Peridotite
        2. Peridotite

    6. Minerals commonly present include:
      1. Olivine is dominant. (Olivine is olive green).
      2. may have minor amounts of pyroxene and Ca-plagioclase

Other types of igneous rock:

Syenite



A polished syenite called larvikite with centimeter- to inch-scale gray to blue plagioclase crystals. The industrial name for the rocks is "blue pearl". Photographed in an above-ground cemetery in New Orleans, LA

Bowen's Reaction Series

You have to know the series AND understand the concepts of how Bowen's Reaction Series relates to melting and to crystallization, and to the origin of igneous rocks of various composition.


Plutons

Subsurface igneous bodies


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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston, GA
October 2, 1995
Modified January 24, 1997
Modified September 21, 1998
Modified July 17, 1999
Links updated to facstaff, email updated September 6, 2008