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of Earth were at one time igneous rocks. Today, about two-thirds the minerals on the left side of the chart is granite. If you look
of the outer layer, or crust, is made up of igneous rocks. Th is is closely at the surface of a freshly broken piece of granite, you
not apparent in many locations because the surface is covered by will note that it is coarse-grained with noticeable particles of dif-
other kinds of rocks and rock materials (sand, soil, etc.). ferent size, shape, and color. The vitreous, white particles are
As a magma cools, atoms in the melt begin to lose kinetic probably orthoclase feldspar, which makes up about 45 percent
energy and come together to form the orderly array of a crystal of the particles. The clear, glassy-looking particles are probably
structure. How rapidly the cooling takes place determines the quartz crystals, which make up about 25 percent of the total
texture of the igneous rock being formed. In general, a coarse- sample. The remaining particles of black specks are ferromag-
grained texture means that you can see mineral crystals with nesian minerals —biotite and/or amphibole.
the unaided eye. The texture is said to be fine-grained if you Rocks with the chemical composition of granite make up
need a lens or a microscope to see the crystals. The presence the bulk of Earth’s continents, and granite is the most common
of a fine-grained or coarse-grained texture tells you something intrusive rock in the continental crust. As shown in the chart,
about the cooling history of a particular igneous rock. rhyolite and obsidian are the chemical equivalents of granite,
How rapidly a magma cools and hardens is generally except they have a different texture (Figure 17.14). Rhyolite is
determined by its location. Magma that cools slowly deep below fine-grained and obsidian is a translucent volcanic glass.
the surface produces coarse-grained intrusive igneous rocks.
Below the surface, the magma loses heat slowly, and the atoms
have sufficient time to produce large crystals. Lava that cools
rapidly above the surface produces fine-grained extrusive
igneous rocks. Rapid cooling does not result in sufficient time
for large crystals to form so extrusive rocks are fine-grained.
Very rapid cooling results in no time for any crystals to form,
and a volcanic glass is produced as a result. Glass does not have
an orderly arrangement of atoms and is therefore not a crystal.
A general classification scheme for igneous rocks is given
in Figure 17.13. Igneous rocks are various mixtures of minerals,
and this scheme names the rocks according to (1) their min-
eral composition and (2) their texture. Note that the mineral
composition changes continuously from one side of the chart to
the other. There are many intermediate types of igneous rocks
possible, but this chart identifies only the most important ones.
FIGURE 17.14 This is a piece of obsidian, which has the
Igneous rocks on the left side of Figure 17.13 are blends
same chemical composition as the granite shown in Figure 17.12.
of nonferromagnesian minerals. Thus, these rocks are com- Obsidian has a different texture because it does not have crystals
paratively light in density and color, appearing to be light gray, and is a volcanic glass. The curved fracture surface is common in
white, or ivory-colored. The most common igneous rock with noncrystalline substances such as glass.

Coarse-grained GRANITE DIORITE GABBRO
Fine-grained RHYOLITE ANDESITE BASALT Rocks
100%
Biotite
75% Amphibole Olivine
Ferromagnesians
50% Potassium feldspar Pyroxene Minerals
(orthoclase)
Sodium-rich plagioclase
25% Plagioclase Calcium-rich
Quartz
0%
feldspar plagioclase
Increasing silica
75% SiO 2 45% SiO 2
Increasing K 2O and Na 2O

Increasing CaO, FeO and MgO

FIGURE 17.13 Igneous rock classification scheme based on mineral composition and texture. There are other blends of minerals with
various textures, many of which have specific names.

442 CHAPTER 17 Rocks and Minerals 17-10