The Earth Through Time, 10th Edition
by Harold L. Levin
CHAPTER 8—EARTH’S FORMATIVE STAGES AND THE
ARCHEAN EON
CHAPTER OUTLINE FOR TEACHING
I.
Earth’s Formative Interval: Archean
A. Duration of 2.1 Billion Years
B. For the Initial 560 Million Years, There is Little or No Record
II. Earth in Space
A. Third Planet from our Sun
B. Meteorites: Their Age is that of the Solar System
1.
2.
3.
4.
Ordinary chondrites
Carbonaceous chondrites
Iron meteorites
Stony-iron meteorites
C. Rocky (Terrestrial) Planet: density 5.5 g/cm3
III. Formation of the Solar System
A. Dynamic Constraints
1.
2.
3.
4.
5.
All planets revolve in same counterclockwise direction
All planets lie roughly in one plane
Nearly all planets and moons rotate counterclockwise
Density of planets roughly decreases away from Sun
Age of Earth and meteorites: 4.56 billion years
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B. Nebular Hypothesis
1.
2.
3.
4.
5.
Cold, rarified cloud of gas and dust particles
Dust cloud starts counterclockwise rotation
Eddies in dust cloud begin planetary development: cold, homogenous accretion
Protoplanetary formation and graviational collapse forms Sun
Solar wind drives out lighter elements
IV. Solar System
A. Sun
1. Energy source: atomic fusion
2. Ultimate source of energy for many geologic processes
B. Inner Planets
1. Mercury
a. Moon-like cratered surface
b. Moon-like smooth areas
2. Venus
a. volcano-dominated landscapes
b. vertical tectonic processes dominate
3. Earth
a. water stable on surface
b. atmosphere has 21% oxygen
4. Mars
a. heavy bombardment, then differentiation
b. outgassing and development of atmosphere and oceans
c. with global cooling, depletion of atmosphere and water
C. Earth’s Moon
1.
2.
3.
4.
Synchronous Rotation
Terrains: highlands and maria
Formed by impact event very early in Earth’s history
Density: 3.3 g/cm 3
D. Asteroid belt
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E. Four Outer Planets
1. Jupiter
a. giant, gaseous world
b. four inner satellites and many others
2. Saturn
a. giant, gaseous world
b. ring system and many satellites
3. Uranus
a. giant, gaseous world
b. numerous satellites
4. Neptune
a. giant, gaseous world
b. numerous satellites
V. Earth’s Earliest Stages
A. Accretion and Differentiation
1. Heating, partial melting, and solid diffusion
2. Ni and Fe migration to core
3. Mantle separation forming lighter crust
B. Source of Internal Heat
1. Accretionary heat of bombardment
2. Radioactive decay
C. Crustal Development
1. Crust formed by cooling magma ocean
2. Komatiites (ultramafic patches) formed early in Earth’s crust
3. Continental crust and water present as early as 4.36 billion years ago
D. Evolution of Atmosphere and Hydrosphere
1. Primitive atmosphere (4.56 to 3.8 billion years ago)
a. lacked oxygen
b. produced by outgassing
2. Transition atmosphere (3.8-1.8 billion years ago)
a. banded iron formations
b. cherts (bacterial fossils)
c. lack of carbonates
d. iron-sulfide compounds common
3. Oxygen-rich atmosphere: building since 3.85 billion years ago
a. photochemical dissociation: UV light + H2O
b. photosynthesis (and related evolution of plants)
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4. Origin of Oceans (after 4.4 billion years ago)
a. tied to onset of hydrologic cycle
b. salinity due to chemical weathering
VI. Archean Rocks
A. Age Distinguished by Radiometric Dating
1.
2.
3.
4.
5.
6.
4.5 billion years: first oceanic crust
4.4 billion years: first felsic crust
3.8 billion years: first known continental crust
3.46 billion years: first known soil formation
3.0 to 2.5 billion years: first protocontinents
2.6 billion years: first known glaciation
B. Earliest Plate Tectonics
1. Collision of steep-sided, small, protocontinents
2. Granulite and greenstone associations formed
a. granulites: mainly gneisses derived from metamorphism of granitic rocks
b. greenstones: volcanic rocks with metamorphosed sediments and submarine
(pillow) lavas; formed in trough-like basins
3. Archean sedimentation
a. coarse conglomerates
b. greywackes and dark shales
VII. Life of the Archean
A. Possible Origins of Life
1. Oceanic origin (the “rich organic broth”)
2. Hyperthermophiles of the ocean floor vents
a. chemosynthesis
b. mid-ocean ridges
3. Thermophiles of the subsurface crust
a. lithotrophs
b. subsurface bacteria
B. Feeding Archean Life
1.
2.
3.
4.
5.
6.
Fermenters
Autotrophs (sulfur bacteria and nitrifying bacteria)
Lithotrophs
Photoautotrophs (photosynthetic organisms)
Heterotrophs
Aerobic and anaerobic organisms
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C. Categories of Archean Life
1. Prokaryotes: living by chemical synthesis
2. Eukaryotes: symbiotic synthesis (engulfment)
D. Notable Archean Fossils
1. Cyanobacteria: “blue-green algae,” stromatolites
2. Microbially induced sedimentary structures
a. mats
b. filaments
c. films
3. Molecular fossils (preserved organic molecules)
Answers to Discussion Questions
1. From the Sun outward in our solar system, the planets are Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and Pluto. Our solar system resides in the Milky Way
Galaxy.
2. Mercury, Venus, Earth, and Mars area rocky planets of the inner solar system. Mercury,
Venus, and Earth have mean densities in the range of 5.2 to 5.5 gm/cm 3, but Mars has
slightly less (3.9 gm/cm 3). Mercury and Mars are rather smaller than Earth and Venus (which
are about the same size). Mercury and Mars have little atmosphere, but Earth and Venus
have relatively dense atmospheres. Earth and Venus have active volcanoes, whereas
Mercury and Mars apparently do not. Impact craters are quite common on Mercury and
relatively common on Mars, but are rather few on Venus and Earth. Venus and Mercury are
relatively quite hot (but for different reasons), and Earth is much cooler (fortunately) and Mars
cooler still. Water was a key factor in the early history of Earth and Mars, but apparently not
so with Venus and Mercury. Earth and Venus have rather high rates of surficial change (for
different reasons), and Mercury and Mars do not.
3. The Archean crust of Earth has been recycled (melted or metamorphically changed) by a
combination of erosional and tectonic processes, which is not the case on the Moon.
4. A more elliptical orbit for a planet like Earth would likely result in more profound seasonal
changes during the year.
5. The Archean record of heavy bombardment is preserved well on the Moon, but not so on
Earth. Rocks from the Moon show evidence of a large impact upon the Earth, which formed
the Moon. The Earth itself lacks such a record.
6. Internal heat of the Earth (from accretionary impacts and later from radioactive decay) causes
lighter materials to rise and heavier materials (e.g., Fe and Ni) to sink. Plate-tectonic
reprocessing of the Earth’s crust also move lighter material to the surface.
7. Meteorites are either chondrites (ordinary or carbonaceous), achondrites (or stony), iron, or
stony iron. The distinction between chondrite and other kinds of meteorites is made on
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whether or not the meteorite possesses small spherical structures called chondrules.
Carbonaceous chondrites contain organic compounds including dozens of inorganically
produced amino acids.
8. Precambrian shields are broadly up-warped, geologically stable regions of continents. The
shields form stable platforms for blankets of sedimentary strata, and the regions where such
strata overlie shields are called platforms. The platform of a continent, together with its
shield, constitutes a continental craton.
9. The terms mafic and felsic are adjectives used to describe the mineralogic composition of
igneous rocks. Mafic refers to rocks dominated by dark iron and magnesium silicates. Felsic
refers to rocks dominated by feldspars, quartz, and muscovite. An example of a mafic
extrusive igneous rock is basalt; of felsic, rhyolite.
10. Patches of felsic crust could have been derived by partial melting of lighter mineral
components of subducted mafic oceanic crust. During Archean, the rate of subduction was
likely greater than now due to higher internal Earth temperatures.
11. In greenstone belts, the structural configuration is as a syncline, or trough-like feature.
Greenstone's general vertical sequence (from base) is: ultramafic igneous rocks including
komatiites; basalts with low-grade metamorphic minerals like chlorite and hornblende; felsic
volcanics; and sedimentary rocks (including shales, greywackes, and conglomerates (in
some places banded-iron formations top the sequence).
12. The komatiites are denser than other mafic rocks. They are typically found in the lowermost
(oldest) layers of the greenstone sequential zonation. As komatiites crystallize at 1100o C,
their presence indicates a crust cooler than that temperature.
13. The early atmosphere must have gradually accumulated some oxygen by about 2.5 billion
years ago as indicated by the occurrence of banded-iron formations, a type of chert that
contains iron-oxide rich layers.
14. Archean rocks are rarely preserved due to tectonic and weathering processes over billions of
years. Correlation is thus difficult because of lack of exposure. Further, fossil content is very
low and microscopic. Most Archean rocks are dated using radiometric methods.
15. Symbiotic synthesis has a prokaryote engulfing a primitive eukaryote to produce a respiratory
prokaryote. A respiratory organelle may be the result of this engulfment.
16. Hyperthermophiles live in exceedingly high temperature environments. If life can exist in
such conditions there is hope that it may exist in other worlds where temperature ranges
might otherwise be viewed as abiotic.
17. Autotrophs manufacture their own food, whereas heterotrophs who must scavenge food from
their environment. Anerobic organisms do not depend upon oxygen to survive (and cannot
survive in oxygen), whereas aerobic organisms thrive in oxygen. Prokaryotes do not have a
nucleus and must reproduce asexually, whereas eukaryotes have a nucleus with well-defined
chromosomes and they have organelles.
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18. e
19. d
20. d
Chapter Activities
Student activities for in-depth learning:
1. The origin of the Moon has been a matter for debate for many years. When samples were
returned from the Moon, starting in 1969, they did not support any of the existing hypotheses.
Take a look at the discussion on the Lunar and Planetary Laboratory web page at the University
of Arizona (http://www.lpl.arizona.edu/outreach/origin/) and write a brief review of the impact
hypothesis for the origin of the Moon. What facts from the lunar samples and meteorites does this
hypothesis address? What do the computer simulations show about an impact forming the
Moon? What is your view of this hypothesis?
2. Using web pages at the University of California’s Museum of Paleontology
(http://www.ucmp.berkeley.edu/paleo/fossils/molecu.html), take a look at the description of
molecular fossils. After reviewing what this web page has to offer, use the resources there to
answer briefly these questions. What are the four main organic compounds that form molecular
fossils? What conditions are necessary for the formation and preservation of molecular fossils?
What can we learn from molecular fossils?
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