4.9 Igneous Rock Classification

Difficulty Level: At Grade | Created by: CK-12

Last Modified: Aug 02, 2016

lesson content

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

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

Explore More

Use this resource to answer the questions that follow.

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

Notes/Highlights

Color	Highlighted Text	Notes
	Please Sign In to create your own Highlights / Notes

Show Attributions

Show Details ▼

Image	Reference	Attributions
		License: CC BY-NC
		Credit: A) Courtesy of the US Geological Survey and the Mineral Information Institute; B) Mark A. Wilson (User:Wilson44691/Wikimedia Commons); C) User:Omphacite/Wikimedia Commons; D) User:GeoRanger/Wikipedia;(a) Kevin Walsh (Flickr: kevinzim); (b) User:deltalimatrieste/Wikimedia Commons; (c) Courtesy of the National Oceanic and Atmospheric Administration Source: A) http://commons.wikimedia.org/wiki/File:RhyoliteUSGOV.jpg; B) http://commons.wikimedia.org/wiki/File:GabbroRockCreek2.jpg; C) http://commons.wikimedia.org/wiki/File:Perid_SanCarlos.jpg; D) http://commons.wikimedia.org/wiki/File:KomatiiteCanada_682By512.jpg;(a) http://www.flickr.com/photos/86624586@N00/1125946364/; (b) http://commons.wikimedia.org/wiki/File:Pomice_di_veglia.jpg; (c) http://commons.wikimedia.org/wiki/File:Pillow_basalt_crop_l.jpg License: Public Domain; (A) CC BY 2.0; (B) Public Domain; (C) Public Domain