The Earth Through Time, 10Th Edition

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. Ordinary chondrites

2. Carbonaceous chondrites

3. Iron meteorites

4. Stony-iron meteorites

C. Rocky (Terrestrial) Planet: density 5.5 g/cm3

III. Formation of the Solar System

A. Dynamic Constraints

1. All planets revolve in same counterclockwise direction

2. All planets lie roughly in one plane

3. Nearly all planets and moons rotate counterclockwise

4. Density of planets roughly decreases away from Sun

5. Age of Earth and meteorites: 4.56 billion years

B. Nebular Hypothesis

1. Cold, rarified cloud of gas and dust particles

2. Dust cloud starts counterclockwise rotation

3. Eddies in dust cloud begin planetary development: cold, homogenous accretion

4. Protoplanetary formation and graviational collapse forms Sun

5. 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. Synchronous Rotation

2. Terrains: highlands and maria

3. Formed by impact event very early in Earth’s history

4. Density: 3.3 g/cm3

D. Asteroid belt

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)

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. 4.5 billion years: first oceanic crust

2. 4.4 billion years: first felsic crust

3. 3.8 billion years: first known continental crust

4. 3.46 billion years: first known soil formation

5. 3.0 to 2.5 billion years: first protocontinents

6. 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. Fermenters

2. Autotrophs (sulfur bacteria and nitrifying bacteria)

3. Lithotrophs

4. Photoautotrophs (photosynthetic organisms)

5. Heterotrophs

6. Aerobic and anaerobic organisms

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/cm3, but Mars has slightly less (3.9 gm/cm3). 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 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.

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?