Geoscience Reference
In-Depth Information
Figure 12.24 Humans moving rocks
and dirt.
(a) An obvious place where
people move a lot of Earth is a mine.
Although many mines are located far
below ground, a large number of them
are strip mines such as this one that
took off the top of a hill. (b) This scene
of a bulldozer moving Earth is replayed
in countless places at any given point
in time.
(a)
(b)
excavated, mines enlarged, roads built, hillslopes bulldozed,
and agricultural fields shaped over much of Earth's surface. So
much of this activity occurs today in our complex industrialized
world that humans are now the primary shapers of Earth's land-
scapes, with over 100 gigatons of sediment and rock estimated
to be moved every year (Figure 12.25). This amount is believed
to be greater than the combined amount of sediment moved by
wind, glaciers, streams, and ocean currents. The total amount
of Earth moved so far by humans is thought to be sufficient to
build a 4000-m-high (~13,000-ft-high) mountain range, 40 km
(25 mi) wide and 100 km (~60 mi) long!
In this context, imagine for a moment if geologists from
another solar system came to study Earth millions of years in
the future, after humans became extinct. They would discover
that dinosaurs disappeared after an asteroid impact at a point
in deep geological time and that other significant geological
events occurred at distinct moments in Earth history. And, from
their view in the distant future, one of those key moments would
certainly be when humans began to dramatically shape the en-
vironment at the onset of our Industrial Revolution. No wonder
that contemporary geologists are strongly considering making
the Anthropocene a formal epoch in the geological timescale.
KEY COnCEPTs TO REMEMbER AbOUT
ROCKs, gEOLOgIC TIME, AnD OIL
80
1.
Three kinds of basic rocks occur: igneous, sedimentary,
and metamorphic.
60
Unintentional
(agriculture)
2.
Igneous rocks form when magma cools within Earth
(forming intrusive igneous rocks) or on Earth's surface
(forming extrusive igneous rocks).
40
3.
Sedimentary rocks consist of large layers of formerly
loose sediments that slowly compacted and lithified.
20
Intentional
4.
Metamorphic rocks form when igneous, sedimentary,
or other metamorphic rocks are altered by intense heat
and pressure.
10
5000
4000
3000
2000
1000
0
5.
The age of rocks can be confidently dated because radio-
active isotopes decay at a known rate, called the half-life.
Years before present
Figure 12.25 Assessing the amount of Earth moved by hu-
mans in the past 5000 years.
The red line represents the
amount of Earth intentionally moved by people in activities such
as mining, road constructing, and bulldozing, whereas the green
line reflects the amount of sediment that has moved indirectly as a
result of agricultural activities. A good example of such an indirect
impact is sediment eroded from a plowed field because plants are
no longer present to protect the soil. Notice the dramatic increase
in the amount of Earth moved in gigatons in the very recent past.
(
Source
: R. LeB. Hooke, On the history of humans as geomorphic
agents. Geology 2000; 28:843-846.)
6.
The geologic timescale involves a concept of deep
time that transcends the normal confines of the human
timescale.
7.
Our current economy depends a great deal on fossil
fuels for energy. Many scientists are concerned that we
are reaching the peak in global oil production.
8.
As a result of human activities, Earth scientists are con-
sidering formal adoption of the Anthropocene as a new
epoch on the geological timescale.
