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by weathering and erosion. Seas advance and retreat over the Weathering
and erosion
continents as materials are cycled from the atmosphere to the
land and from the surface to the interior of Earth and then
back again. Rocks are transformed from one type to another
through this continual change. There is not a single rock on
Earth’s surface today that has remained unchanged through Lithification
Earth’s long history. The concept of continually changing Solidification
rocks through time is called the rock cycle (Figure 17.22).
The rock cycle concept views an igneous, a sedimentary, or a

metamorphic rock as the present but temporary stage in the
ongoing transformation of rocks to new types. Any particular
rock sample today has gone through countless transforma- Metamorphism
Melting
tions in the 4.6 billion year history of Earth and will continue
to do so in the future.
FIGURE 17.22 A schematic diagram of the rock cycle
concept, which states that geologic processes act continuously to
produce new rocks from old ones.

SUMMARY

The elements silicon and oxygen make up 75 percent of all the ele- rocks. The most abundant igneous rocks are the ferromagnesian-rich
ments in the outer layer, or crust, of Earth. The elements combine to basalt and the silicon-rich and ferromagnesian-poor granite.
make crystalline chemical compounds called minerals. A mineral is Sedimentary rocks are formed from sediments, accumulations of
defined as a naturally occurring, inorganic solid element or compound weathered rock materials that settle out of the atmosphere or out of
with a crystalline structure. water. Sedimentary rocks from clastic sediments, or rock fragments,
About 92 percent of the minerals of Earth’s crust are composed of are named according to the size of the sediments making up the rock:
silicon and oxygen, the silicate minerals. The basic unit of the silicates conglomerate, sandstone, and shale, in decreasing sediment size. Chem-
is a tetrahedral structure that combines with positive metallic ions or ical sediments form from precipitation, crystallization, or the action
with other tetrahedral units to form chains, sheets, or an interlocking of plants and animals. Limestone is the most common sedimentary
framework. rock from chemical sediments. Sediments become sedimentary rocks
The ferromagnesian silicates are tetrahedral structures com- through lithification, a rock-forming process that involves both com-
bined with ions of iron, magnesium, calcium, and other elements. paction and cementation of the sediments.
The ferromagnesian silicates are darker in color and more dense than Metamorphic rocks are previously existing rocks that have been
other silicates. The nonferromagnesian silicates do not have iron or changed by heat, pressure, or hot solution into a different kind of rock
magnesium ions, and they are lighter in color and less dense than the without melting. Increasing metamorphism can change the sedimentary
ferromagnesians. The nonsilicate minerals do not contain silicon and rock shale to slate, which is then changed to schist, which can then be
are carbonates, sulfates, oxides, halides, sulfides, and native elements. changed to gneiss. Each of these stages has a characteristic crystal size
A rock is defined as an aggregation of one or more minerals that and alignment known as foliation. Quartzite and marble are examples
have been brought together into a cohesive solid. Igneous rocks formed of two nonfoliated metamorphic rocks.
as hot, molten magma cooled and crystallized to firm, hard rocks. The rock cycle is a concept that an igneous, a sedimentary, or a
Magma that cools slowly produces coarse-grained intrusive igneous metamorphic rock is a temporary stage in the ongoing transformation
rocks. Magma that cools rapidly produces fine-grained ex trusive igneous of rocks to new types.

KEY TERMS

Bowen’s reaction series (p. 440) lava (p. 440)
cementation (p. 446) magma (p. 440)
chemical sediments (p. 445) metamorphic rocks (p. 446)
clastic sediments (p. 444) mineral (p. 435)
compaction (p. 446) rock (p. 441)
extrusive igneous rocks (p. 442) rock cycle (p. 449)
foliation (p. 446) sedimentary rocks (p. 444)
granite (p. 442) sediments (p. 444)
igneous rocks (p. 441) silicates (p. 436)
intrusive igneous rocks (p. 442)

17-17 CHAPTER 17 Rocks and Minerals 449