Laboratory Manual for Introductory Geology 4e

EXERCISE 5.6
Interpreting Igneous Cooling Histories from Textures
Name:
Course:
Section:
Date:
Examine the igneous rocks provided by your instructor. Apply what you have learned about the origins of igneous textures
to fill in the “cooling history” column in the study sheets at the end of the chapter. Use the following questions as a guide
to your interpretation.
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Which specimens cooled quickly? Which cooled slowly?
Which specimens cooled very rapidly at the Earth’s surface (i.e., are extrusive)?
Which specimens cooled slowly beneath the surface (i.e., are intrusive)?
Which specimens cooled from a magma rich in gases?
Which specimens experienced more than one cooling rate?
5.3 Igneous Rock Classification and Identification
You now know how to determine the conditions under which igneous rocks form by
examining their textures. Chemical composition is the key to answering other questions
about igneous rocks because it helps reveal how magma forms and why some
igneous rocks occur in specific plate-tectonic settings. In this section, we look first at
how geologists use chemical composition to classify igneous rocks and then at how
igneous rocks are named by their composition and texture.
5.3.1 Igneous Rock Classification: The Four Major
Compositional Groups
In Exercise 5.1, you saw that some of the igneous rocks in your set are relatively dark
colored and that others are light colored. You also saw that some have high specific
gravities, others relatively low specific gravities. A rock’s color and specific gravity are
controlled mostly by its minerals, which are, in turn, determined by its chemical composition.
Oxygen and silicon are by far the two most abundant elements in the lithosphere,
so it is not surprising that nearly all igneous rocks are composed primarily of
silicate minerals, such as quartz, feldspars, pyroxenes, amphiboles, micas, and olivine.
There are many different kinds of igneous rock, but they all fit into one of four
compositional groups—felsic, intermediate, mafic, and ultramafic—defined by how
much silicon and oxygen (silica) they contain and by which other elements are most
abundant (TABLE 5.2).
TABLE 5.2 The four major compositional groups of igneous rocks
Igneous rock
group
Approximate %
silica (SiO2) by
weight
Other major elements
Felsic >66 Aluminum (Al), potassium (K),
sodium (Na)
Intermediate ∼52–66 Al, Na, calcium (Ca), iron
(Fe), magnesium (Mg)
Most abundant
minerals
K-feldspar, Naplagioclase,
quartz
Ca–Na-plagioclase,
amphibole, pyroxene
Mafic ∼45–52 Al, Ca, Mg, Fe Ca-plagioclase,
pyroxene, olivine
Ultramafic <45 Mg, Fe Olivine, pyroxenes
5.3 IGNEOUS ROCK CLASSIFICATION AND IDENTIFICATION
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