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Igneous Rock Classification

Igneous rocks are grouped based on silica content and whether the rock is extrusive or intrusive.

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Igneous Rock Classification

Is this an intrusive or an extrusive igneous rock?

From this view the amazing structure of rocks that make up Devil's Tower doesn't really indicate whether the structure formed slowly or quickly. A close up view would show small crystals in a mafic rock, indicating a rapid cooling from a basalt lava. Cooling was slow enough that the hexagonal "posts" could form.

Igneous Rock Classification

Igneous rocks are first classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in Table below.

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

Second to composition in igneous rock classification is texture. Texture indicates how the magma that formed the rock cooled.

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% SiO2 Rhyolite Granite

Some of the rocks in Table above were pictured earlier in previous concepts about rocks. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the photos in Figure below:

Pictures of rhyolite, gabbro, peridotite, and komtiite

These are photos of A) rhyolite, B) gabbro, C) peridotite, and D) komatiite.

  

 

Summary

  • Composition is the first criteria on which to classify igneous rocks, with categories from felsic to ultramafic; color is a first order indicator of composition.
  • Texture is the second criteria for classifying igneous rocks because texture indicates how a rock cooled.
  • Igneous rocks are categorized in pairs with the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.

Review

  1. Describe the formation of the igneous rock pair gabbro-basalt. What makes the rocks the same and what makes them different?
  2. How does the composition of a rock affect its color?
  3. What are ultramafic rocks and where are they likely to be found?

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Use this resource to answer the questions that follow.

 

 

 

 

  1. What is one way that igneous rocks are classified?
  2. What are the two categories of igneous rocks based on composition?
  3. How is felsic igneous rock?
  4. What is mafic igneous rock?
  5. What is the same about granite and rhyolite? What is different?
  6. What is the same about gabbro and basalt? What is different?

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