Earth image Igneous Rocks

Dr. Pamela Gore
Georgia Perimeter College


  1. Identify igneous rocks by their physical and chemical properties.
  2. Tell which minerals are present in various types of igneous rocks.
  3. List and describe the characteristics of intrusive igneous rocks. including textures.
  4. List and describe the characteristics of extrusive igneous rocks, including textures.
  5. Explain the origin of various types of igneous rocks, including cooling history.
  6. Explain how igneous rocks are classified.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5b. Classify rocks by their process of formation.
  • S6E5e. Explain the effects of physical processes (plate tectonics, erosion, deposition, volcanic eruption, gravity) on geological features including oceans (composition, currents, and tides).

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • The earth is mostly rock. Three-fourths of its surface is covered by a relatively thin layer of water (some of it frozen), and the entire planet is surrounded by a relatively thin blanket of air. It is the only body in the solar system that appears able to support life. The other planets have compositions and conditions very different from the earth's.
  • Rock is composed of different combinations of minerals. Smaller rocks come from the breakage and weathering of bedrock and larger rocks. Soil is made partly from weathered rock, partly from plant remains-and also contains many living organisms.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Some changes in the solid earth can be described as the "rock cycle." Old rocks at the earth's surface weather, forming sediments that are buried, then compacted, heated, and often recrystallized into new rock. Eventually, those new rocks may be brought to the surface by the forces that drive plate motions, and the rock cycle continues.

Origin of Igneous Rocks

The word igneous means "fire-formed".

Igneous rocks form by cooling and crystallizing from hot molten magma or lava.

Igneous rocks that cool and crystallize beneath the Earth's surface are called intrusive igneous rocks.
Another name for intrusive igneous rocks is plutonic igneous rocks (named for Pluto, Roman god of the underworld).

Igneous rocks that cool and crystallize on the Earth's surface are called extrusive igneous rocks.
Another name for extrusive igneous rocks is volcanic igneous rocks (named for Vulcan, Roman god of the fire and forge).

Cooling Rates

Cooling rates influence the texture of the igneous rock:

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)


    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

Classification of Igneous Rocks:

Igneous rocks are classified or named on the basis of their texture and their composition.
Igneous rocks cannot be classified by their process of formation, because the processes are interpreted from the rocks.

Classification of rocks is always based on objective, observable, measurable data (such as grain size and the percentages of various minerals), and not on interpretations.

The igneous rocks classification diagram below shows varying percentages of minerals in each of the four major categories of igneous rocks (sialic, intermediate, mafic and ultramafic). The table below shows the rock names for the various textures, combined with the mineral information.

Aphanitic Rhyolite Andesite Basalt --
Phaneritic Granite Diorite Diabase
Pegmatitic Granite - Pegmatite Diorite - Pegmatite Gabbro - Pegmatite --
Porphyritic Aphanitic - Phaneritic Porphyritic Rhyolite
Porphyritic - Granite
Porphyritic - Andesite
Porphyritic - Diorite
Porphyritic - Basalt
Porphyritic - Gabbro
Porphyritic - Periodotite
Vesicular Pumice Pumice Vesicular Basalt
& Scoria
Glassy Obsidian -- -- --
Tuff (ash)
Volcanic Breccia
Tuff (ash)
Volcanic Breccia
(% Dark Minerals)
0 - 15
20 - 40
50 - 60
95 - 100

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created February 16, 2005
Links updated October 13, 2008