A.
B.
CHAPTER 1 An Introduction to Geology
18
The Rock Cycle: One of Earth’ s Subsystems
An Introduction to Geology:
The Rock Cycle
Rock is the most common and abundant material on Earth. To a curious traveler, the variety
seems nearly endless. When a rock is examined closely, we find that it consists of smaller
crystals or grains called minerals. Minerals are chemical compounds (or sometimes single
elements), each with its own composition and physical properties. The grains or crystals
may be microscopically small or easily seen with the unaided eye.
The nature and appearance of a rock is strongly influenced by the minerals that compose it. In addition, a rock’ s texture—the size, shape, and/or arrangement of its constituent
minerals—also has a significant effect on its appearance. A rock’ s mineral composition and
texture, in turn, are a reflection of the geologic processes that created it (FIGURE 1.20).
The characteristics of the rocks in FIGURE 1.21 provided geologists with the clues they
needed to determine the processes that formed them. This is true of all rocks. Such analyses
are critical to an understanding of our planet. This understanding has many practical applications, as in the search for basic mineral and energy resources and the solution of environmental problems.
Geologists divide rocks into three major groups: igneous, sedimentary, and metamorphic. In Figure 1.21, the lava flow in northern Arizona is classified as igneous, the sandstone in Utah’ s Zion National Park is sedimentary, and the schist exposed at the bottom of
the Grand Canyon is metamorphic.
In the preceding section, you learned that Earth is a system. This means that our planet
consists of many interacting parts that form a complex whole. Nowhere is this idea better
illustrated than when we examine the rock cycle (FIGURE 1.22). The rock cycle allows us to
view many of the interrelationships among different parts of the Earth system. Knowledge
of the rock cycle will help you more clearly understand the idea that each rock
group is linked to the others by the processes that act upon and within
the planet. You can consider the rock cycle to be a simplified but
useful overview of physical geology. What follows is a
brief introduction to the rock cycle. Learn the
rock cycle well; you will be examining its
interrelationships in greater detail throughout
this book.
GEODe
ESSENTIALS
OF GEOLOGY
FIGURE 1.20 Texture and mineral composition
are important rock properties that allow
geologists to determine much about how and
where the rock formed. A. Gneiss is a
metamorphic rock and B. Granite porphyry is an
igneous rock. (Photos by E. J. Tarbuck)
C.
FIGURE 1.21 A. This fine-grained rock,
called basalt, is part of a lava flow from
Sunset Crater in northern Arizona. It
formed when molten rock erupted at
Earth’s surface hundreds of years ago and
solidified. (Photo by Dennis Tasa) B. This
rock is exposed in the walls of southern
Utah’s Zion National Park. This layer, known
as the Navajo Sandstone, consists of
durable grains of the glassy mineral quartz
that once covered this region with mile
after mile of drifting sand dunes. (Photo by
Jamie & Judy Wild www.DanitaDelimont.com)
C. This rock unit, known as the Vishnu
Schist, is exposed in the inner gorge of the
Grand Canyon. Its formation is associated
with environments far below Earth’s surface
where temperatures and pressures are high
and with ancient mountain-building
processes that occurred in Precambrian
time. (Photo by Dennis Tasa)
A.
B. B
B.
CHAPTER 1 An Introduction to Geology
18
The Rock Cycle: One of Earth’ s Subsystems
An Introduction to Geology:
The Rock Cycle
Rock is the most common and abundant material on Earth. To a curious traveler, the variety
seems nearly endless. When a rock is examined closely, we find that it consists of smaller
crystals or grains called minerals. Minerals are chemical compounds (or sometimes single
elements), each with its own composition and physical properties. The grains or crystals
may be microscopically small or easily seen with the unaided eye.
The nature and appearance of a rock is strongly influenced by the minerals that compose it. In addition, a rock’ s texture—the size, shape, and/or arrangement of its constituent
minerals—also has a significant effect on its appearance. A rock’ s mineral composition and
texture, in turn, are a reflection of the geologic processes that created it (FIGURE 1.20).
The characteristics of the rocks in FIGURE 1.21 provided geologists with the clues they
needed to determine the processes that formed them. This is true of all rocks. Such analyses
are critical to an understanding of our planet. This understanding has many practical applications, as in the search for basic mineral and energy resources and the solution of environmental problems.
Geologists divide rocks into three major groups: igneous, sedimentary, and metamorphic. In Figure 1.21, the lava flow in northern Arizona is classified as igneous, the sandstone in Utah’ s Zion National Park is sedimentary, and the schist exposed at the bottom of
the Grand Canyon is metamorphic.
In the preceding section, you learned that Earth is a system. This means that our planet
consists of many interacting parts that form a complex whole. Nowhere is this idea better
illustrated than when we examine the rock cycle (FIGURE 1.22). The rock cycle allows us to
view many of the interrelationships among different parts of the Earth system. Knowledge
of the rock cycle will help you more clearly understand the idea that each rock
group is linked to the others by the processes that act upon and within
the planet. You can consider the rock cycle to be a simplified but
useful overview of physical geology. What follows is a
brief introduction to the rock cycle. Learn the
rock cycle well; you will be examining its
interrelationships in greater detail throughout
this book.
GEODe
ESSENTIALS
OF GEOLOGY
FIGURE 1.20 Texture and mineral composition
are important rock properties that allow
geologists to determine much about how and
where the rock formed. A. Gneiss is a
metamorphic rock and B. Granite porphyry is an
igneous rock. (Photos by E. J. Tarbuck)
C.
FIGURE 1.21 A. This fine-grained rock,
called basalt, is part of a lava flow from
Sunset Crater in northern Arizona. It
formed when molten rock erupted at
Earth’s surface hundreds of years ago and
solidified. (Photo by Dennis Tasa) B. This
rock is exposed in the walls of southern
Utah’s Zion National Park. This layer, known
as the Navajo Sandstone, consists of
durable grains of the glassy mineral quartz
that once covered this region with mile
after mile of drifting sand dunes. (Photo by
Jamie & Judy Wild www.DanitaDelimont.com)
C. This rock unit, known as the Vishnu
Schist, is exposed in the inner gorge of the
Grand Canyon. Its formation is associated
with environments far below Earth’s surface
where temperatures and pressures are high
and with ancient mountain-building
processes that occurred in Precambrian
time. (Photo by Dennis Tasa)
A.
B. B