Igneous Rock Classification
Dana Desonie, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2014 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: December 14, 2014
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Igneous Rock Classification
1 Igneous Rock Classification
• Explain how igneous rocks are classified by composition and by cooling rate.
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 1.1). The table includes examples of minerals.
TABLE 1.1: Properties of Igneous Rock Compositions
Composition
Felsic
Color
Light
Density
Low
Intermediate
Intermediate
Intermediate
Mafic
Ultramafic
Dark
Very dark
High
Very high
Minerals
Quartz,
orthoclase
feldspar
Plagioclase feldspar, biotite, amphibole
Olivine, pyroxene
Olivine
1
www.ck12.org
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 1.2).
FIGURE 1.1
Is this an intrusive or an extrusive igneous
rock?
FIGURE 1.2
Is this an intrusive or an extrusive igneous
rock?
Pictured above are two pairs of igneous rocks ( Figures 1.1 and 1.2). 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 1.2).
TABLE 1.2: Silica Composition and Texture of Major Igneous Rocks
Type
Ultramafic
Mafic
Intermediate
Intermediate-Felsic
Felsic
Amount of Silica
<45%
45-52%
52-63%
63-69%
>69% SiO2
Extrusive
Komatiite
Basalt
Andesite
Dacite
Rhyolite
Intrusive
Peridotite
Gabbro
Diorite
Granodiorite
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.
2
www.ck12.org
Chapter 1. Igneous Rock Classification
Explore More
Use the resource below to answer the questions that follow.
• Igneous Rocks at http://www.youtube.com/watch?v=laVDypLGdbs (7:36)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/118818
1.
2.
3.
4.
5.
6.
7.
8.
9.
How does the rate of magma cooling affect crystal size?
How does an igneous rock get a glassy texture?
What is the name of the texture if extremely small crystals form? Where do rocks like this form?
If there is lot of water in the cooling magma what is the texture? Where might this happen?
What is a good example of a glassy rock? What feature does this rock have that is just like glass?
What does felsic mean? What are the features of felsic minerals?
What does mafic mean? What are the features of mafic minerals?
What does ultramafic magma produce? Why is this rock important? What is its main crystal?
How does a porphyry form?
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?
References
1. Rhyolite: Courtesy of the US Geological Survey and the Mineral Information Institute; Granite: B.W. Hallett,
V. F. Paskevich, L.J. Poppe, S.G. Brand, and D.S. Blackwood, US Geological Survey. Comparison between
rhyolite and granite. Public Domain
2. Images copyright Tyler Boyes, 2013. Comparison between basalt and gabbro. Used under licenses from
Shutterstock.com
3