Geology Reference
In-Depth Information
Once Earth had cooled suffi ciently, water vapor fell
as precipitation, accounting for the origin of surface
waters.
During the Archean and Paleoproterozoic, only
single-celled bacteria existed.
During the Proterozoic, organisms that reproduced
sexually evolved and multicelled organisms appeared.
Several natural resources are found in Precambrian
rocks.
4.0 billion years ago with the oldest known rocks on Earth
except meteorites. The geologic record we do have for the
Precambrian, particularly the older part of the Precambrian,
is diffi cult to decipher because (1) many of the rocks have
been metamorphosed and complexly deformed; (2) in many
areas, they are deeply buried beneath younger rocks; and
(3) they contain few fossils of any use in biostratigraphy.
Because of the complexities of the rocks and the scarcity
of fossils, establishing formal subdivisions of the Precambrian
is complicated. In 1982, in an effort to standardize termi-
nology, the North American Commission on Stratigraphic
Terminology proposed two eons for the Precambrian, the
Archean , and the Proterozoic , both of which are now widely
used. More recent work in 2004 by the International Com-
mission on Stratigraphy (ICS) has refi ned the Precambrian
subdivision as shown in
INTRODUCTION
We think of time from the human perspective of years or
a few decades, but we have no frame of reference for time
measured in millions or billions of years. Accordingly, the
Precambrian, which lasted for more than 4.0 billion years, is
longer than we can even imagine, and Earth has existed for
4.6 billion years. Consider this. Suppose that 1 second equals
1 year. If that were the case, and you were to count to 4.6 bil-
lion, the task would take you and your descendants nearly
146 years! Or suppose that a 24-hour clock represented
4.6 billion years, in which case the Precambrian alone would
be more than 21 hours long and constitute about 88% of all
geologic time (
Figure 19.2. It is important to note
that the subdivisions in Figure 19.2 are based on absolute
ages rather than time-stratigraphic units, which is a depar-
ture from standard practice. Remember from Chapter 17
that the systems, such as the Cambrian System, are based on
stratotypes, whereas the Precambrian designations are not.
One important reason for studying Precambrian Earth
and life history is that it constitutes most of geologic time,
and many events that took place then set the stage for further
evolution of the planet and life. In Chapter 1, we introduced
the concept of interacting systems (see Figure 1.1); however,
it was during the Precambrian that these systems became op-
erative, although not all at the same time or necessarily in
their present form. Earth did not differentiate into a core
and mantle until millions of years after it formed. Once it
did, though, internal heat was responsible for moving plates
(plate tectonics) and the origin and continuing evolution
of the continents. Earth's early atmosphere evolved from a
carbon dioxide-rich one to one with free oxygen and an
ozone layer, organisms appeared at least 3.5 billion years ago,
and surface waters began to accumulate.
In short, Earth was very different when it fi rst formed
Figure 19.1).
Precambrian is a widely used term that refers to both
time and rocks. As a time term, it includes all geologic time
from Earth's origin 4.6 billion years ago to the beginning of
the Phanerozoic Eon 542 million years ago. The term also
refers to all rocks lying below Cambrian-age rocks. Unfortu-
nately, no rocks are known for the fi rst 640 million years of
geologic time, so our geologic record actually begins about
Mesozoic Era (4.0%)
Cenozoic Era (1.4%)
Paleozoic Era (6.3%)
(
Figure 19.3a), but during the Precambrian, it began to
evolve and became increasingly more like it is now. Evidence
of these changes is found in the geologic record —that is, the
record of physical and biological events preserved in rocks.
Our task in this and the following chapters is to investigate
the geologic record for the intervals of geologic time des-
ignated Precambrian (Archean and Proterozoic), Paleozoic,
Mesozoic, and Cenozoic.
24
23
1
22
2
21
3
4600
MYA
20
4
Archean Eon
(45.6%)
19
5
18
6
WHAT HAPPENED DURING
THE EOARCHEAN?
Eoarchean refers to all geologic time from Earth's origin
until the onset of the Paleoarchean 3.6 billion years ago
(Figure 19.2). Unfortunately, Earth's oldest known body of
rocks, the Acasta Gneiss in Canada, is 3.95 to 4.04 billion
years old (Figure 19.3b), so we have no geologic record for
much of the Archean. Nevertheless, it was during this time
that Earth accreted from planetesimals and differentiated
into a core and mantle (see Figures 1.8 and 1.9), and at
least some continental crust formed perhaps as much as
17
7
Proterozoic Eon
(42.5%)
16
8
9
15
14
10
13
11
12
Figure 19.1 Geologic Time Represented by a 24-Hour Clock If
24 hours represented all geologic time, the Precambrian would
be more than 21 hours long, more than 88% of the total.
 
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