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Igneous Rock Classification

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Is this igneous rock intrusive or extrusive?

You can't really tell from this view. A close up view would show small crystals in a dark colored rock. This indicates rapid cooling from a basalt lava. The most interesting feature you can see are the hexagonal "posts." For them to form, cooling must not have been too rapid. This amazing feature is Devil's Tower in Wyoming.

Igneous Rock Classification

Igneous rocks are classified by composition and texture. The composition can be felsic, intermediate, mafic, or ultramafic. The composition depends on the minerals the rock includes. A felsic rock will contain felsic minerals.

The mineral compositions are listed below ( Table below ). The table includes examples of minerals.

Properties of Igneous Rock Compositions
Composition Color Density Minerals
Felsic Light Low Quartz, orthoclase feldspar
Intermediate Intermediate Intermediate Plagioclase feldspar, biotite, amphibole
Mafic Dark High Olivine, pyroxene
Ultramafic Very dark Very high Olivine

Texture indicates how the magma cooled. Magma that cools rapidly has small or no visible crystals. Magma that cools slowly has large crystals; reference the table above for the type of rock, the amount of silica it has, and examples ( Table below ).

Is this an intrusive or an extrusive igneous rock?

Is this an intrusive or an extrusive igneous rock?

Pictured above are two pairs of igneous rocks ( Figures above and above ). The first pair is two rocks that formed from a felsic magma. The rhyolite cooled rapidly, and the granite cooled slowly. The second pair is two rocks that formed from mafic magma. The basalt cooled rapidly, and the gabbro cooled slowly. The color of the rocks and sizes of the crystals are your clues. There are other rock pairs that you could find listed below ( Table below ).

Silica Composition and Texture of Major Igneous Rocks
Type Amount of Silica Extrusive Intrusive
Ultramafic <45% Komatiite Peridotite
Mafic 45-52% Basalt Gabbro
Intermediate 52-63% Andesite Diorite
Intermediate-Felsic 63-69% Dacite Granodiorite
Felsic >69% SiO 2 Rhyolite Granite

Summary

  • Igneous rocks are classified first by composition. Categories go from from felsic to ultramafic. Color can indicate composition.
  • Texture is second when classifying igneous rocks. Texture indicates how a rock cooled.
  • Igneous rocks are categorized in pairs. The two rocks in a pair have the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.

Practice

Use the resource below to answer the questions that follow.

  1. Describe andesite.
  2. What is basalt?
  3. What is diorite?
  4. Describe gabbro.
  5. What is granite?
  6. How is obsidian formed? What is it?
  7. What is pegmatite?
  8. What is peridotite composed of?
  9. What is pumice?
  10. What is rhyolite?
  11. Describe scoria. How does it differ from pumice?
  12. What is tuff?

Review

  1. Describe the formation of the igneous rock pair gabbro-basalt. Compare and contrast the two rock types.
  2. How does the composition of a rock affect its color?
  3. What are ultramafic rocks? What is their color?

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