CHAPTER 1 An Introduction to Geology
20
The Basic Cycle
We begin at the top of Figure 1.22. Magma
is molten material that forms inside Earth.
Eventually magma cools and solidifies. This
process, called crystallization, may occur
either beneath the surface or, following a
volcanic eruption, at the surface. In either
situation, the resulting rocks are called
igneous rocks (ignis = fire).
If igneous rocks are exposed at the surface, they will undergo weathering, in which
the day-in and day-out influences of the
atmosphere slowly disintegrate and decompose rocks. The materials that result are
often moved downslope by gravity before
being picked up and transported by any
of a number of erosional agents, such as
running water, glaciers, wind, or waves.
Eventually these particles and dissolved
substances, called sediment, are deposited.
Although most sediment ultimately comes
to rest in the ocean, other sites of deposition include river floodplains, desert basins,
swamps, and sand dunes.
Next the sediments undergo lithification,
a term meaning “conversion into rock.”
Sediment is usually lithified into
sedimentary rock when compacted by
the weight of overlying layers or when
cemented as percolating groundwater
fills the pores with mineral matter.
If the resulting sedimentary rock is
buried deep within Earth and involved in
the dynamics of mountain building or
intruded by a mass of magma, it will be
subjected to great pressures and/or intense
heat. The sedimentary rock will react to the
changing environment and turn into the
third rock type, metamorphic rock. When
metamorphic rock is subjected to additional pressure changes or to still higher
temperatures, it will melt, creating magma,
which will eventually crystallize into
igneous rock, starting the cycle all
over again.
Although rocks may seem to be
unchanging masses, the rock cycle shows
that they are not. The changes, however,
take time—great amounts of time. The rock
cycle is operating all over the world, but in
different stages. Today new magma is
forming under the island of Hawaii, while
the Colorado Rockies are slowly being
worn down by weathering and erosion.
Some of this weathered debris is eventually
understood when viewed in the context of
much earlier events in Earth history.
Origin of Planet Earth
This section describes the most widely
accepted views of the origin of our solar
system. The theory summarized here
represents the most consistent set of ideas
available to explain what we know about
our solar system today.
Our scenario begins about 13.7 billion
years ago with the Big Bang, an incomprehensibly large explosion that sent all matter
of the universe flying outward at incredible
speeds. In time, the debris from this explosion, which was almost entirely hydrogen
and helium, began to cool and condense
into the first stars and galaxies. It was in
one of these galaxies, the Milky Way,
that our solar system and planet Earth
took form.
Earth is one of eight planets that, along
with several dozen moons and numerous
smaller bodies, revolve around the Sun.
The orderly nature of our solar system
leads most researchers to conclude that
Earth and the other planets formed at
essentially the same time and from the
same primordial material as the Sun. The
nebular theory states that the bodies of our
solar system evolved from an enormous
rotating cloud called the solar nebula
(FIGURE 1.23). Besides the hydrogen and
helium atoms generated during the Big
Bang, the solar nebula consisted of microscopic dust grains and the ejected matter of
long-dead stars. (Nuclear fusion in stars
converts hydrogen and helium into the
other elements found in the universe.)
Nearly 5 billion years ago this huge
cloud of gases and minute grains of heavier
elements began to slowly contract due to
the gravitational interactions among its
particles (FIGURE 1.24). Some external influence, such as a shock wave traveling from a
catastrophic explosion (supernova), may
have triggered the collapse. As this slowly
spiraling nebula contracted, it rotated faster
and faster for the same reason ice skaters
do when they draw their arms toward their
bodies. Eventually the inward pull of
gravity came into balance with the outward
force caused by the rotational motion of the
nebula (Figure 1.23). By this time the once
vast cloud had assumed a flat disk shape
carried to the Gulf of Mexico, where it is
deposited, adding to the already substantial
mass of sediment that has accumulated
there.
Alternative Paths
The paths shown in the basic cycle are not
the only ones that are possible. To the
contrary, other paths are just as likely to be
followed as those described in the preceding section. These alternatives are indicated
by the blue arrows in Figure 1.22.
Igneous rocks, rather than being
exposed to weathering and erosion at
Earth’ s surface, may remain deeply buried.
Eventually these masses may be subjected
to the strong compressional forces and high
temperatures associated with mountain
building. When this occurs, they are transformed directly into metamorphic rocks.
Metamorphic and sedimentary rocks,
as well as sediment, do not always remain
buried. Rather, overlying layers may be
stripped away, exposing the once buried
rock. When this happens, the material is
attacked by weathering processes and
turned into new raw materials for
sedimentary rocks.
Where does the energy that drives
Earth’ s rock cycle come from? Processes
driven by heat from Earth’ s interior are
responsible for forming igneous and
metamorphic rocks. Weathering and the
movement of weathered material are
external processes powered by energy
from the Sun. External processes produce
sedimentary rocks.
C O N C E P T C H E C K 1 . 8
Sketch and label a basic rock cycle. Make
sure your sketch includes alternative paths.
Early Evolution
of Earth
Recent earthquakes caused by displacements of Earth’ s crust, along with lavas
erupted from active volcanoes, represent
only the latest in a long line of events by
which our planet has attained its present
form and structure. The geologic processes
operating in Earth’ s interior can be best
1
20
The Basic Cycle
We begin at the top of Figure 1.22. Magma
is molten material that forms inside Earth.
Eventually magma cools and solidifies. This
process, called crystallization, may occur
either beneath the surface or, following a
volcanic eruption, at the surface. In either
situation, the resulting rocks are called
igneous rocks (ignis = fire).
If igneous rocks are exposed at the surface, they will undergo weathering, in which
the day-in and day-out influences of the
atmosphere slowly disintegrate and decompose rocks. The materials that result are
often moved downslope by gravity before
being picked up and transported by any
of a number of erosional agents, such as
running water, glaciers, wind, or waves.
Eventually these particles and dissolved
substances, called sediment, are deposited.
Although most sediment ultimately comes
to rest in the ocean, other sites of deposition include river floodplains, desert basins,
swamps, and sand dunes.
Next the sediments undergo lithification,
a term meaning “conversion into rock.”
Sediment is usually lithified into
sedimentary rock when compacted by
the weight of overlying layers or when
cemented as percolating groundwater
fills the pores with mineral matter.
If the resulting sedimentary rock is
buried deep within Earth and involved in
the dynamics of mountain building or
intruded by a mass of magma, it will be
subjected to great pressures and/or intense
heat. The sedimentary rock will react to the
changing environment and turn into the
third rock type, metamorphic rock. When
metamorphic rock is subjected to additional pressure changes or to still higher
temperatures, it will melt, creating magma,
which will eventually crystallize into
igneous rock, starting the cycle all
over again.
Although rocks may seem to be
unchanging masses, the rock cycle shows
that they are not. The changes, however,
take time—great amounts of time. The rock
cycle is operating all over the world, but in
different stages. Today new magma is
forming under the island of Hawaii, while
the Colorado Rockies are slowly being
worn down by weathering and erosion.
Some of this weathered debris is eventually
understood when viewed in the context of
much earlier events in Earth history.
Origin of Planet Earth
This section describes the most widely
accepted views of the origin of our solar
system. The theory summarized here
represents the most consistent set of ideas
available to explain what we know about
our solar system today.
Our scenario begins about 13.7 billion
years ago with the Big Bang, an incomprehensibly large explosion that sent all matter
of the universe flying outward at incredible
speeds. In time, the debris from this explosion, which was almost entirely hydrogen
and helium, began to cool and condense
into the first stars and galaxies. It was in
one of these galaxies, the Milky Way,
that our solar system and planet Earth
took form.
Earth is one of eight planets that, along
with several dozen moons and numerous
smaller bodies, revolve around the Sun.
The orderly nature of our solar system
leads most researchers to conclude that
Earth and the other planets formed at
essentially the same time and from the
same primordial material as the Sun. The
nebular theory states that the bodies of our
solar system evolved from an enormous
rotating cloud called the solar nebula
(FIGURE 1.23). Besides the hydrogen and
helium atoms generated during the Big
Bang, the solar nebula consisted of microscopic dust grains and the ejected matter of
long-dead stars. (Nuclear fusion in stars
converts hydrogen and helium into the
other elements found in the universe.)
Nearly 5 billion years ago this huge
cloud of gases and minute grains of heavier
elements began to slowly contract due to
the gravitational interactions among its
particles (FIGURE 1.24). Some external influence, such as a shock wave traveling from a
catastrophic explosion (supernova), may
have triggered the collapse. As this slowly
spiraling nebula contracted, it rotated faster
and faster for the same reason ice skaters
do when they draw their arms toward their
bodies. Eventually the inward pull of
gravity came into balance with the outward
force caused by the rotational motion of the
nebula (Figure 1.23). By this time the once
vast cloud had assumed a flat disk shape
carried to the Gulf of Mexico, where it is
deposited, adding to the already substantial
mass of sediment that has accumulated
there.
Alternative Paths
The paths shown in the basic cycle are not
the only ones that are possible. To the
contrary, other paths are just as likely to be
followed as those described in the preceding section. These alternatives are indicated
by the blue arrows in Figure 1.22.
Igneous rocks, rather than being
exposed to weathering and erosion at
Earth’ s surface, may remain deeply buried.
Eventually these masses may be subjected
to the strong compressional forces and high
temperatures associated with mountain
building. When this occurs, they are transformed directly into metamorphic rocks.
Metamorphic and sedimentary rocks,
as well as sediment, do not always remain
buried. Rather, overlying layers may be
stripped away, exposing the once buried
rock. When this happens, the material is
attacked by weathering processes and
turned into new raw materials for
sedimentary rocks.
Where does the energy that drives
Earth’ s rock cycle come from? Processes
driven by heat from Earth’ s interior are
responsible for forming igneous and
metamorphic rocks. Weathering and the
movement of weathered material are
external processes powered by energy
from the Sun. External processes produce
sedimentary rocks.
C O N C E P T C H E C K 1 . 8
Sketch and label a basic rock cycle. Make
sure your sketch includes alternative paths.
Early Evolution
of Earth
Recent earthquakes caused by displacements of Earth’ s crust, along with lavas
erupted from active volcanoes, represent
only the latest in a long line of events by
which our planet has attained its present
form and structure. The geologic processes
operating in Earth’ s interior can be best
1