Geoscience Reference
In-Depth Information
Igneous Rock
Figure 12.18 Simplified view of the
rock cycle.
Over time, minerals
move from one rock phase to another
through a variety of processes, such
as heating, melting, weathering, and
recrystallization. This illustration does
not contain all forms of movement
within the cycle. For example, igne-
ous rocks can melt and re-form as
igneous rocks. Can you think of other
forms not shown here?
Heat, Pressure
Sedimentary Rock
Magma
Metamorphic Rock
rocks move in a
rock cycle
(Figure 12.18). Although movement
within this cycle is much slower than other cycles, often over mil-
lions of years, it nevertheless comprises a distinct series of steps
where rock gradually moves from one physical state or location
to another.
Let's examine the rock cycle from a holistic perspective by
examining Figure 12.18 and starting with the formation of an
extrusive igneous rock such as basalt. This rock originates from
magma that flows out of the ground, usually from a volcano,
but perhaps from just a fissure in the ground. This magma cools
and crystallizes, forming basalt that lies at or close to ground
level. With time, the basalt gradually weathers due to various
erosion processes, causing individual grains of the former rock
mass to be transported and deposited someplace else. As the
deposit of sediments thickens, those at the bottom of the stack
become compressed, causing lithification and formation of
sedimentary rocks. If these sedimentary rocks become deeply
buried over time, or come into contact with the intense heat of
an intrusive magma body, they will be altered to metamorphic
rock because the original sediments will be recrystallized, re-
combined, or replaced. These metamorphic rocks, in turn, may
be further heated so that they melt back to the magma state.
The Rock Cycle
Although the rock cycle moves very slowly, it is possible to ani-
mate the concept to capture its essence. Go to the
Geo Media
Library
and select
The Rock Cycle
. This animation describes
the ways in which rocks are created on Earth and how their
minerals move from one reservoir to another. It begins with the
formation of igneous rock and then describes how these rocks
can be altered to form new kinds of rock, whether it be a new
kind of igneous rock, sedimentary rock, or metamorphic rock. Af-
ter you complete the animation, be sure to answer the questions
at the end to test your understanding of this concept.
