Geoscience Reference
In-Depth Information
layers of sedimentary rock. Between about 25 million and
14 million years ago, tremendous quantities of magma began
to intrude into this portion of the Earth's crust, creating a giant
chamber within the sedimentary rock body and filling smaller
cracks in that same mass of rocks.
Sometime after 14 million years ago the intruded magma
cooled to form a granite batholith with an associated pair of
stocks at the top. Extending outward several kilometers from
the stocks were
radial dikes
that also consisted of granite.
In the past 5 million years, the sedimentary rocks that were
once present were completely eroded away from this part of
Colorado, exposing the granite stocks and dikes on the surface
of Earth. The igneous bodies remained intact because these
kinds of rocks are much harder and more resistant to erosion
than sedimentary rocks. The end result is that the Spanish Peaks
now tower over the landscape.
With the formation of the Spanish Peaks in mind, think about
the broader questions related to geologic time and the rock cycle.
In the context of the rock cycle, note that the sediments once con-
tained within the layers of sandstone, limestone, and shale, which
in turn covered the stocks and radial dikes, have been transported
to other locations where new rock formations developed or are
now developing. What is particularly impressive is that these lay-
ers of sedimentary rock must have collectively been greater than
2134 m (7000 ft) thick! Otherwise, the magma that formed the
batholith, stocks, and radial dikes could not have been intruded
within them. It is even more impressive that the process of erosion
subsequently removed
all
the sedimentary rock above the stocks.
In the time since the stocks and dikes were exposed, they were
being slowly worn down by erosion. The resulting liberated sedi-
ments have also been transported elsewhere to form new rock.
In the context of deep time, the entire sequence of events
that led to the development of what we now call the Spanish
Peaks took over 75 million years, beginning with the deposi-
tion of more than 2134 m (7000 ft) of sand, calcium carbon-
ate, and clay in a marine environment. These sediments slowly
lithified to form rock. Next, the formation of the Sangre de
Cristo Mountains lasted 50 million years and caused large
fractures to develop in the sedimentary rocks. These fractures,
in turn, filled with magma some time after 25 million years
ago that intruded from a pair of developing stocks associated
with an underlying batholith. This magma then cooled, form-
ing granite. Then
all
the sedimentary rocks were eroded away,
exposing the granite stocks and dikes, and leaving a pair of
mountains. Imagine how much time
that
process took. And
perhaps the most impressive thing of all in this history is
that the past 75 million years is encompassed within the last
2 weeks in the calendar analogy used to represent geologic
time (see Table 12.2). In other words, the Spanish Peaks have
existed for only a very short period of time within the geologic
timescale.
Think about that!
Human Interactions with Geologic
Time: The Anthropocene
As you can see, the subdivisions of the geologic timescale
are based on significant events that occurred at various times
in Earth history. The onset of the Paleozoic Era, for example,
is marked by the emergence of the first fish about 540 million
years ago. Similarly, the boundary between the Paleozoic and
Cenozoic Eras at 65 million years ago was set because the dino-
saurs became extinct at that time. More recently, the beginning
of the Holocene Epoch is set at 10,000 years ago because that
is considered to mark the end of the last great ice age and the
onset of the general climate conditions we know today.
In the context of significant time markers in Earth history,
the geologic community is currently considering the addition of
a new epoch to the timescale, one that reflects the dramatic im-
pact that humans are now having on the environment. This new
epoch is informally called the
Anthropocene
and is based on
the recognition that humans have become the foremost agent of
change on the Earth's surface. The term was first used in print in
2000, and momentum has since grown within the scientific com-
munity to make the epoch a formal part of the geologic timescale.
A source of debate centers on when the beginning of the Anthro-
pocene should be marked. Some believe that the onset of this
proposed epoch should be set as far back as 8000 years ago when
the origins of human agriculture occurred. Others argue that the
most logical time to begin the Anthropocene is the beginning of
the Industrial Revolution in the mid-18th century. This latter time
marker may make the most sense because it shows up clearly in
the ice core record as elevated levels of pollutants.
Regardless of when the beginning of the Anthropocene is
defined, we are clearly living at a time when the human foot-
print on the planet is pervasive and growing. As we saw in
Chapter 9, the evidence strongly suggests that increased con-
centrations of atmospheric CO
2
related to human industrial
activity (Figure 9.26) are causing global climate change. In
Chapter 10, we saw that the human footprint on vegetation pat-
terns around the globe is extensive (Figure 10.26), with defor-
estation, commercial forestry, and agriculture dominating over
much of Earth's land surface (see Figures 10.29 and 10.33).
Only the most remote places remain in what can be considered
their natural state. In future chapters of this text, you will see
the extensive impact that humans are having on groundwater
supplies (Chapter 15), stream behavior (Chapter 16), melting
ice caps and glaciers (Chapter 17), amount of windblown dust
(Chapter 18), and the world's coastlines (Chapter 18).
One way to view this issue is simply by assessing the
amount of dirt, soil, and rock moved by people as they ad-
just Earth's surface to meet their needs (Figure 12.24). At any
given point in the present time, countless trenches are being
Radial dikes
A long, wall-like feature of intrusive igneous
rock that forms when magma is injected into thin cracks within
older rock and cools.
Anthropocene
A proposed new epoch in the geological
timescale based on the recognition that humans are now the
foremost agent of change on the Earth's surface.
