Historical Geology
Chapter 8 Precambrian Earth and Life History
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 8:
 Precambrian time, which accounts for most of geologic time, is divided into two eons: the
Archean and the younger Proterozoic.
 The Archean geologic record is difficult to interpret because many of the rocks are
metamorphic, deformed, deeply buried, and they contain few fossils.
 Each continent has at least one area of exposed Precambrian rocks called a shield and a
buried extension of the shield known as a platform. A shield and its platform make up a
craton.
 All cratons show evidence of deformation, metamorphism, and emplacement of plutons, but
they have been remarkably unaffected by these activities since Precambrian time.
 The two main associations of Archean rocks are granite-gneiss complexes, which are by far
the most common, and greenstone belts that consist of igneous rocks but sedimentary rocks
are present primarily in their upper parts.
 Greenstone belts likely formed in several tectonic settings, but many appear to have evolved
in back-arc marginal basins and in rifts within continents.
 Plate tectonics was taking place during the Archean, but plates probably moved faster, and
igneous activity was more common then because Earth possessed more heat from radioactive
decay.
 Gases released during volcanism were responsible for the origin of the hydrosphere and
atmosphere, but this early atmosphere so formed had little or no free oxygen.
 The oldest known fossil organisms are single-celled bacteria and chemical traces of bacterialike organisms known as archaea. Bacteria known as blue-green algae produced irregular
mats and moundlike structures called stromatolites.
 Resources found in Archean rocks include gold, platinum, copper, zinc, and iron.
CHAPTER 8 SUMMARY
1. All geologic time from Earth’s origin to the beginning of the Phanerozoic Eon is included
in the Precambrian. Precambrian also refers to rocks lying stratigraphically below
Cambrian-aged rocks.
Figure 8.1
Geologic Time Represented by a 24-hour Clock
2. The Precambrian is divided into two eons, the Archean and the Proterozoic, each of which
has further subdivisions.
3. Rocks from the latter part of the Eoarchean indicate that crust existed then, but very little
of it has been preserved.
Figure 8.2
The Precambrian Geologic Time Scale
Figure 8.3
Earth As It May Have Appeared Soon After It Formed
Historical Geology
Chapter 8 Precambrian Earth and Life History
4. All continents have an ancient craton made up of an exposed shield and a buried platform.
In North America, the Canadian shield is made up of smaller units delineated by their ages
and structural trends.
Figure 8.4
Origin of Granitic Continental Crust
Figure 8.5
The Distribution of Precambrian Rocks
Figure 8.6
The Geologic Evolution of North America
5. Archean greenstone belts are linear, syncline-like bodies of rock found within much more
extensive granite-gneiss complexes.
Figure 8.7
Archean Rocks in North America
Figure 8.8
Greenstone Belts and Granite-Gneiss Complexes
Figure 8.9
Greenstone Belts in North America
6. An ideal greenstone belt consists of two lower units of mostly igneous rocks and an upper
sedimentary unit. They probably formed in back-arc basins and in intracontinental rifts.
Figure 8.10 Origin of a Greenstone Belt in a Back-Arc Marginal Basin
Figure 8.11 Origin of a Greenstone Belt in an Intracontinental Rift
7. Many geologists are convinced that Archean plate tectonics took place, but plates probably
moved faster, and igneous activity was more common then because Earth had more
radiogenic heat.
Figure 8.12 Origin of the Southern Superier Craton
8. Outgassing was probably responsible for the early atmosphere and the hydrosphere.
However, the atmosphere so formed lacked free oxygen but contained abundant carbon
dioxide and water vapor.
Table 8.1
Composition of Earth’s Present-Day Atmosphere
Figure 8.13 Outgassing and Earth’s Early Atmosphere
Figure 8.14 Evolution of the Atmosphere
Figure 8.15 Ratio of Radiogenic Heat Production in the Past Compared to the Present
9. Models for the origin of life by natural processes require an oxygen-deficient atmosphere,
the necessary elements for organic molecules, and energy to promote the synthesis of
organic molecules.
Figure 8.16 Stanley Miller’s Experimental Apparatus
10. The first naturally formed organic molecules were probably monomers, such as
amino acids, that linked together to form more complex polymers, including nucleic acid
and proteins.
Figure 8.17 Experimental Production of Thermal Proteins and Microspheres
11. RNA molecules may have been the first molecules capable of self-replication. However,
the method whereby a reproductive system formed is not known.
Historical Geology
Chapter 8 Precambrian Earth and Life History
12. Submarine hydothermal vents were probably more common during the Archean. Several
minerals containing zinc, copper, and iron precipitate around them, and they support
communities of organisms. These environments may represent the environments in which
life evolved.
Figure 8.18 Submarine Hydrothermal Vents
13. The only known Archean fossils are of single-celled prokaryotic bacteria such as
cyanobacteria, but chemical compounds in some Archean rocks may indicate the
presence of archaea.
14. Stromatolites that formed by the activities of photosynthesizing bacteria are found in
rocks as much as 3.5 billion years old. .
Figure 8.19 Stromatolites
Figure 8.20 Probable Archean Microfossils
15. Archean mineral resources include gold, chrome, zinc, copper, and nickel.