R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 8
PRECAMBRIAN EARTH AND LIFE HISTORY—
THE ARCHEAN EON
OUTLINE
INTRODUCTION
WHAT HAPPENED DURING THE EOARCHEAN?
CONTINENTAL FOUNDATIONS—SHIELDS, PLATFORMS AND CRATONS
Archean Rocks
Greenstone Belts
Evolution of Greenstone Belts
PERSPECTIVE Earth’s Oldest Rocks
ARCHEAN PLATE TECTONICS AND THE ORIGIN OF CRATONS
THE ATMOSPHERE AND HYDROSPHERE
How Did the Atmosphere Form and Evolve?
Earth’s Surface Waters—The Hydrosphere
THE ORIGIN OF LIFE
Experimental Evidence and the Origin of Life
Submarine Hydrothermal Vents and the Origin of Life
The Oldest Known Organisms
ARCHEAN MINERAL RESOURCES
SUMMARY
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.
77
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 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
bacteria-like 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.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the determination of the subdivisions of the Precambrian Era
 the events—and evidence—for Earth during the Eoarchean
 the formation of the shields and platforms that make up the foundations of continents
 the difficulties in interpreting the Archean geological record
 the stratigraphic succession and models for the origin of greenstone belts
 the features of Archean plate tectonics and crustal evolution
 the origin and evolution of the atmosphere and hydrosphere
 the experimental evidence and models for the origin of life
 the nature of the Archean fossil record
 the major types and geologic associations of Archean age ore deposits
CHAPTER 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.
78
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
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
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
Enrichment Topic 1. Plate Tectonics—The Early Years
Some geologists argue that data supports modern-style subduction processes in plate
tectonics, possibly dating to the Hadean or Eoarchean. Mid-Archean island arc
volcanoes, subduction zone related volcanogenic sulfide deposits, and isotopic data from
the world’s oldest zircons are just some of the data supporting modern subduction
processes in the early Precambrian. Cawood, “Precambrian Plate Tectonics: Criteria and
Evidence,” GSA Today, July 2006 v.16, no.7, p.4-11.
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
79
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
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
Enrichment Topic 2. Replication of Miller’s Experiments
Researchers announced that they have uncovered vials from some of Stanley Miller’s
related experiments. After Miller died in 2007, the Scripps Institution of Oceanography
inherited his scientific possessions. Eleven vials were recovered from a second version of
the Miller experiment, which revealed better results than the original. The apparatus was
similar to the first experiment, except that a steam injector simulated water vapor from
early volcanoes. Testing of the vials’ contents revealed 22 amino acids, and researchers
concluded that volcanoes were important in the origin of life. Earth, December 2008, v
53 n. 12 p.24.
11. RNA molecules may have been the first molecules capable of self-replication.
However, the method whereby a reproductive system formed is not known.
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
Archean mineral resources include gold, chrome, zinc, copper, and nickel.
80
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Enrichment Topic 3. Minerals Evolved with Life
Robert Hazon, a geophysicist at Carnegie Institution for Science, studied the long-term
growth of Earth’s mineral complexity. Whereas interstellar dust contains only 12 or so
minerals, the accretion of interstellar dust and the formation of protoplanets produced
more minerals. Meteorite-size masses reveal about 60 minerals, whereas protoplanets of
200 km or more in diameter have 250 minerals.
The first life on Earth didn’t significantly increase the number of minerals. About 2.5
billion years ago the mineral number was about 1,500. With the arrival of cyanobacteria,
mineral types increased dramatically, and today there are about 4,300 minerals. Science
News, December 6, 2008, v. 174 n. 12 p. 10.
LECTURE SUGGESTIONS
Life in the Archean
1. To present a vivid image of the Precambrian world, have the students try to
envision a world with nothing living on the land surfaces. (Some of the recent
images of the stark Martian landscape may be appropriate to use.) An image of a
very stark landscape is easy to imagine; even modern deserts seem lush in
comparison. Reinforce this with a discussion of the role that plants play in
stabilizing the land. How would the processes of weathering and erosion compare
with modern-day rates?
2. Paleontologists deal with an incomplete fossil record and must recognize the
limitations of their data base. Should we expect the fossil record of the Archean to
be as complete as that of more modern times? What conditions would be
necessarily for the preservation of microscopic, single-celled, soft-bodied
organisms? The likelihood of these organisms being preserved and avoiding
destruction is very slim. Also discuss how some regular structures—the so-called
nannobacteria—are currently debated as early organic life forms.
3. Why do researchers look for signs of organic activity, as opposed to fossils of the
earliest organisms themselves? What products, or “signs” would living organisms
leave behind?
CONSIDER THIS
1. Archean tectonic processes are poorly understood. What evidence suggests that the
intracontinental rift model is valid for the formation of greenstone belts? Does the
modern analog of the Sea of Japan make the back-arc marginal basin model more
acceptable?
2. Can geologists easily distinguish between early life forms that are preserved in
rocks, and regular inorganic structures? What type of evidence and/or features must
accompany these structures before we can definitively accept them as fossils of
ancient life?
81
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
IMPORTANT TERMS
abiogenesis
anaerobic
autotrophic
back-arc marginal basin
black smoker
Canadian shield
continental accretion
craton
granite-gneiss complex
greenstone belt
heterotrophic
monomer
outgassing
photochemical
dissociation
photosynthesis
platform
Polymer
Precambrian shield
prokaryotic cell
Stromatolite
submarine hydrothermal
vent
SUGGESTED MEDIA
Videos
1. Exploring Space: The Quest for Life, PBS Home Video
2. Journey of Life: Seas of Life, Discovery Channel
3. Miracle Planet: Episode 1 The Violent Past, Ambrose Video
4. The Cosmic Link: The Origin of Life on Earth, National Film Board of Canada
5. The Origin of Life: Researching the Possibilities, BBC
6. Unearthing Life: A Deep History of a Living Planet, Classroom Encounters, LLC
Software and Demonstration Aids
1. Life of the Precambrian and Lower Paleozoic, JLM Visuals
2. Origins of Life, slide set, Educational Images, Ltd.
3. Evolution of Life on Earth, slide set, Educational Images, Ltd.
CHAPTER 8 – ANSWERS TO QUESTIONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
b
c
e
a
5.
6.
7.
8.
a
d
c
a
9. d
10. b
Short Answer Essay Review Questions
11. Black smokers discharge water saturated with dissolved minerals. Several minerals
containing zinc, copper, and iron are precipitated around them. Biologically, they
support unique communities of organisms, and it is proposed that the first selfreplicating molecules came into existence near these vents on the seafloor.
12.
Continents grow at their margins in a process called continental accretion. This
happens as plates collide with island arcs and other plates.
82
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
13.
In photosynthesis, water and carbon dioxide are combined to make organic
molecules and molecular oxygen is a byproduct. In photochemical dissociation, UV
light breaks up water molecules into H2 and O2 components. Of the two processes,
photosynthesis is more important in adding O2 to the atmosphere.
14.
Monomers are simple organic molecules such as amino acids, and polymers are
more complex molecules consisting of monomers linked together in a specific
sequence. In Miller’s and subsequent experiments, all 20 common amino acid
monomers have been produced by putting energy into a mixture of simple gases
thought to mimic the early atmosphere. Proteinoids, which consist of more than
200 linked amino acids, were synthesized when dehydrated concentrated amino
acids were heated. Monomers probably formed continuously and by the billions
and accumulated in the early oceans, while polymers may have been formed when
monomers were washed up on to a beach, or a cinder cone. They may have been
concentrated by evaporation and polymerized by heat.
15.
Greenstone belts have volcanic rocks, including thick accumulations of pillow lava,
in their lower and middle parts; they are topped by sedimentary rocks. They include
ultramafic lava flows, which require temperatures far in excess of those found in
lava flows today. The rocks typically have synclinal structures and most have been
intruded by granitic magma and cut by thrust faults. Greenstone belts likely formed
in back-arc marginal basins, as lava flows and their overlying sediments, that
opened and closed during the extremely hot Archean.
16. Graywacke is a sandstone with abundant clay and rock fragments, while argillite is
a slightly metamorphosed mudrock. Small-scale cross-bedding and graded bedding
indicate these rocks represent turbidity current deposition. These rocks are found
within the sedimentary rocks of a greenstone belt, usually in the upper unit.
17.
Stromatolites represent reefs constructed by algae. They form and grow as
sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria.
They are significant in that they represent early life forms that photosynthesized.
18.
Archean rocks are extremely old. They have been altered and are not in their
original state. The stratigraphic relationships that may once have existed are now
destroyed. There are no fossils, so Archean rocks can’t be used as an indicator of
relative age or position.
19.
The earliest cells depended on an external food source (heterotrophic), existed in an
environment without oxygen (anaerobic), and were simple cells lacking a nucleus
or other complex structures (prokaryotic).
83
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Apply Your Knowledge
1. The oldest known rocks on Earth, the 3.96-billion-year Acasta Gneiss in Canada
and the 3.8-billion-year-old rocks from Montana and Greenland indicate some crust
had evolved by Eoarchean time. In addition, some sedimentary rocks in Australia
contain 4.4-billion-year-old detrital zircons, so source rocks at least that old must
have existed. Plate tectonics operates on Earth, so much of the early crust would
have been recycled during subduction.
2. Ultramafic lavas cannot form today because Earth has cooled significantly since the
Precambrian. The extremely high temperatures needed to form ultramafic rocks
have not been seen on Earth since the Precambrian. If Earth were to get hot enough,
ultramafic lavas would form again.
3. The processes of volcanism, seismicity, plate movements, and mountain building
are associated with tectonic activity. (Hot spots, representing intraplate tectonism,
are associated with stationary mantle plumes.) Plate tectonics is driven by the
convection cells in the mantle, although the specific mechanism is debated.
Convection cells are produced by temperature differences. The heat of Earth came
from gravitational compression, meteorite bombardment, and radiogenic heating.
The residual heat from gravitational compression is dissipated, and meteorite
bombardment is (fortunately) much reduced now. The process that is still yielding
internal heat is radiogenic heating. However, Earth had all its radioactive elements
at its formation, and with each half-life, the amount of parent decreases by half.
Therefore, eventually, the remaining radioactivity will be insufficient to drive
tectonics. Earth produced 3-6 times more heat than it does today
4. Precambrian subdivisions are based on absolute ages, as opposed to the
Phanerozoic rocks that are based on stratotypes. The principles of superposition,
original horizontality, and inclusions are used to decipher a set of sedimentary rock
layers, but most of the Precambrian rocks are deformed and highly metamorphosed.
Therefore, relative dating is not of much use in Precambrian age rocks.
84