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Plagioclase
feldspar
High
temperature
Ca
Olivine

Augite

Hornblende
Na

Biotite Low
temperature
Potassium feldspar
Muscovite
Quartz

FIGURE 17.11 Bowen’s reaction series. Minerals at the top of the series (olivine, augite, and calcium-rich plagioclase) crystallize
at higher temperatures, leaving the magma enriched in silica. Later, the residual magma cools, and lighter-colored, less dense minerals
(orthoclase feldspar, quartz, and white mica) crystallize.

as more and more iron and magnesium are removed. Mineral first step in forming igneous rocks is the creation of some very
separation does not take place in nature to the extent that Bowen high temperature, hot enough to melt rocks. Recall that a mass
envisioned from his experiments, but his work did lead to other of melted rock materials is called magma. Magma may cool and
theories on the behavior of magmas. crystallize to solid igneous rock either below or on the surface of
Earth. Earth has had a history of molten materials, and all rocks
17.4 ROCKS
Elements are chemically combined to make minerals. Minerals
are physically combined to make rocks. A rock is defi ned as an
aggregation of one or more minerals and perhaps other materi-
als that have been brought together into a cohesive solid. Th ese
materials include volcanic glass, a silicate that is not considered

a mineral because it lacks a crystalline structure. Thus, a rock
can consist of one or more kinds of minerals that are somewhat
“glued” together by other materials such as glass. Most rocks
are composed of silicate minerals, as you might expect since
most minerals are silicates. Granite, for example, is a rock that is
primarily three silicate minerals: quartz, mica, and feldspar. You
can see the grains of these three minerals in a freshly broken
surface of most samples of granite (Figure 17.12).
There is a classification scheme that is based on the way
the rocks were formed. There are three main groups: (1) igne-
ous rocks formed as a hot, molten mass of rock materials cooled
and solidified; (2) sedimentary rocks formed from particles or
dissolved materials from previously existing rocks; and (3) met-
amorphic rocks formed from rocks that were subjected to high
temperatures and pressures that deformed or recrystallized the
rock without complete melting.

IGNEOUS ROCKS
FIGURE 17.12 Granite is a coarse-grained igneous rock
The word igneous comes from the Latin ignis, which means “fire.” composed mostly of light-colored, light-density, nonferromagnesian
This is an appropriate name for igneous rocks, which are rocks minerals. Earth’s continental areas are dominated by granite and by
formed from a hot, molten mass of melted rock materials. Th e rocks with the same mineral composition as granite.

17-9 CHAPTER 17 Rocks and Minerals 441