Download Chapter 8—Earth`s Formative Stages and The Archean Eon

The Earth Through Time
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
B. Nebular Hypothesis
1.
2.
3.
accretion
4.
5.
IV.
Cold, rarified cloud of gas and dust particles
Dust cloud starts counterclockwise rotation
Eddies in dust cloud begin planetary development: cold, homogenous
Protoplanetary formation and graviational collapse forms Sun
Solar wind drives out lighter elements
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/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.
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
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 bandediron 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?
Chapter 8—Earth’s Formative Stages and the Archean Eon
CHAPTER OVERVIEW
This chapter opens with a short presentation of basic astronomy to place Earth in
context with the rest of the Universe. Our solar system is a small part of a much
larger aggregate of stars, planets, dust, and gases called a galaxy. Our galaxy,
the Milky Way, contains the sun and the nine planets that orbit around it.
The most widely proposed theories of the origin of the universe must conform to
an important astronomical observation called the “red shift.” The red shift theory
is examined along with the big bang, steady-state cosmology, and oscillating
universe cosmology theories. The development of the solar system is attributed
to the nebular hypothesis, which is explained in detail and verified by meteorites
found on Earth. This is followed by a solar system tour from center to fringe
explaining the composition and atmosphere of the nine planets. After the
comprehensive detailed review of the solar system, the differentiation of Earth is
discussed followed by an explanation of the development of the atmosphere and
the “hydrologic” cycle.
The structural features of the Archean, shields, platforms, and cratons, along with
the two major rock associations, granulites and greenstones, are discussed in
terms of their relationship to plate tectonics. The chapter concludes with a
detailed discussion of the origin of life on Earth. This includes the role of the
Earth’s mid-oceanic ridges and the ocean itself in this process. The earliest life
forms of Earth’s first two billion years are described, noting the associations of
the anaerobic, aerobic, prokaryotes, eukaryotes, heterotrophs, autotrophs, and
the fossil record left by these earliest forms. The Archean was the eon when
basic mechanisms governing geologic change were established and it was the
time when life appeared.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an
understanding of the following concepts:
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
Discuss the theories of the origin of the universe, solar system, and
planet Earth.
Explain the astronomical observation of the red shift in explaining the
origin of the universe.
Describe the role of meteorites in the formation of the Earth and the
other planets.
Briefly compare the composition of the nine planets and their
atmospheres.
Discuss how the Earth became differentiated into three distinct layers:
the core, the mantle, and the crust.
Discuss the evolution of the Earth’s atmosphere and the development
of the “hydrologic” cycle.
Define the dominant structural features of Archean: shields, cratons,
and platforms and how they relate to plate tectonics.
Locate the major Pre-Cambrian shield areas on a map: Canadian
Shield, Baltic shield, Patagonian shield, etc.
Discuss the formation of the two major rock associations known as
granulites and greenstones.
Describe characteristics that separate life forms of Archean:
anaerobic, aerobic, prokaryotes, eukaryotes, heterotrophs and
autotrophs.
Describe the fossil record during Archean and the importance of
stromatolites.
Explain what molecular fossils are and describe where they have been
found.
CHAPTER OUTLINE
I.
Earth in Context: a Little Astronomy
A. The Solar Nebula Hypothesis
B. Meteorites: Samples of the Early Solar System
II.
A Solar System Tour, From the Center to the Fringe
A. The Sun
1. Distance from the Sun
2. Earth’s Rotation
3. Earth’s Atmosphere
B. The Four Inner Planets
1. Mercury: Hot, Pockmarked and Swift
2. Venus: Nothing We Know Could Survive Here
3. Earth: The Best of All Possible Worlds
4. Earth’s Moon
5. Mars: Once Wetter, Warmer and Host to Life
6. The Asteroid Belt
C. The Five Outer Planets
1. Jupiter and Saturn: Giant Gas Balls
2. Uranus and Neptune: The Twin Planets
3. Pluto: An Outlier
III.
Following Accretion, Earth Differentiates
A. The Archean Crust
IV.
The Primitive Atmosphere – Virtually no Oxgen
A. Growing an Oxygen-Rich Atmosphere
B. Geologic Clues to Early Atmosphere
V.
The Primitive Ocean and the Hydrologic Cycle
VI.
Origin of Precambrian “Basement” Rocks
A. Where Can We See Precambrian Rocks
B. Continental Crust Appears Worldwide
C. The Earliest Plate Tectonics
1. Granulites and Greenstones
2. Archean Sedimentation
VII.
The Origin of Life
A. How Molecules Might Combine to Start Life
B. Pulling Together the Pieces of Life
C. Simulating the Origin of Life
D. Where Did Life Begin
E. Hyperthermophiles and Chemosynthesis
F. Life in Extremely Hostile Environments
G. Feeding Life on Earth
H. Prokaryotes and Eukaryotes
I. Archean Fossils
J. Molecular Fossils
VIII.
In Retrospect
KEY TERMS (pages given in parenthesis)
accretion (219): In the development of protoplanets, the process of
accumulation of bits of matter around an initial mass.
aerobic organism (243): An organism that uses oxygen in carrying out
respiratory processes.
anaerobic organism (243): An organism that does not require oxygen for
respiration, but rather makes use of processes such as fermentation to obtain its
energy.
Archean Eon (232): Division of Precambrian between 3800 million years to 2500
million years ago. Origin of life began in the eon.
autotroph (242): An organism that uses an external source of energy to produce
organic nutrients from simple inorganic chemicals.
banded iron formation (BIF) (231): A rock that consists of alternating bands of
iron-rich minerals, generally hematite, and chert or fine-grained quartz.
Canadian Shield (233): The most extensive exposure of Precambrian rocks in
North America. This geologically stable region extends across 3 million square
miles.
carbonaceous chondrite (220): Meteorites that take their name from spherical
bodies called chondrites which contain nitrogen, hydrogen, carbon, dark iron, and
magnesium silicates and water.
chemosynthesis (240): A means by which organisms derive their energy by
oxidizing such inorganic substances as hydrogen sulfide or ammonia.
chondrules (220): Spherical bodies that are solidified molten droplets splashed
into space during an impact that are found in chondrites.
craton (233): The long-stable region of a continent, commonly with Precambrian
rocks either at the surface or only thinly covered with younger sedimentary rocks.
differentiation (planetary) (228): The process by which a planet becomes
internally zoned, as when heavy materials sink toward its center and light
materials accumulate near the surface.
eukaryote (243): A type of living cell containing a true nucleus, enclosed within a
nuclear membrane, and having well-defined chromosomes and cell organelles.
felsic (235): Meaning rich in feldspars, quartz, and muscovite, as the continental
crust.
fermenter (243): The partial breakdown of organic compounds by an organism
in the absence of oxygen. The final product of fermentation is alcohol or lactic
acid.
fusion (219): The molten state of a substance, or the change it undergoes to
become molten.
granulite (236): A major rock association of the Archean cratons. Composed
largely of gneisses derived from strongly heated and deformed tonalities,
granodiorites, and granites, as well as layered intrusive gabbroic rocks called
anorthosites.
greenstone (234): A major rock association of the Archean cratons. Usually
occur in roughly trough-like or synclinal belts. Prominent features of Archean
terrains of all continents. Composed of basaltic, andesitic, and rhyolitic volcanic
rocks along with metamorphosed sediments derived by weathering and derosion
of the volcanics. Lavas of greenstone belts exhibit pillow structures that indicate
that they were extruded under water.
heterotroph (243): An organism that depends on an external source of organic
substances for its nutrition and energy.
hydrologic cycle (232): The continuous recirculation of water by evaporation
and precipitation—processes powered by the sun and gravity.
hyperthermophile (240): Literally, high-heat lovers.
iron meteorite (220): Iron nickel. Asteroids are the probable source for iron
meteorites.
komatiite (229): Ultramafic rocks that solidified from surface patches when the
magma ocean cooled. Formed at temperatures greater than 1100 degrees C,
which is required to produce basalt. They reflect the higher temperature
gradients that prevailed during the late Hadean.
lithotroph (242): Subterranean microbes that have been dubbed “rock
nourishment.” Most lithotrophs live off energy derived from hydrogen, iron,
magnesium, and sulfur.
lunar highlands (223): The lighter-hued craggy and heavily cratered regions of
the moon.
mafic (235): Rocks (or lava) dominated by dark iron and magnesium silicates as
is the oceanic crust.
magma ocean (229): The melting of the upper mantle during the early Archean
infancy that may have covered Earth’s surface.
maria (223): The darker areas of the moon. These darker areas form the floors
of immense basins that have been flooded with dark basaltic lava. (singular =
mare)
meterorite (220): Meteors that survive the heat and reach the Earth’s surface.
molecular fossils (247): Preserved organic molecules that only eukaryotes can
synthesize.
nebular hypothesis (219): The idea first suggested by German philosopher
Immanuel Kant in 1755, proposes that the solar system distilled from a rotating
cloud of dust particles and gases called the solar nebula.
ordinary chondrites (220): One of four meteorite compositions and are most
abundant and at 4.6 billion years old, are clearly Archean. Chondrites contain
spherical bodies called chondrules that are solidified molten droplets splashed
into space during an impact.
organelles (239): Bodies capable of performing specific function.
outgassing (230): The process by which water vapor and other gases are
released from the rocks that held them and then vented to the surface.
partial melting (230): The variation in melting that occurs in different minerals in
an original rock mass.
photoautotroph (242): Organisms that were capable of carrying on
photosynthesis, the unique capability of dissociating carbon dioxide into carbon
and free oxygen.
photochemical dissociation (230): The process of water molecules into
hydrogen and oxygen. The process occurs in the upper atmosphere when water
molecules are split by high-energy beams of ultraviolet light from the sun.
photosynthesis (230): The process of synthesizing carbohydrates from carbon
dioxide and water, utilizing the radiant energy of light captured by the chlorophyll
in plant cells.
platform (233): That part of a craton covered thinly by layered sedimentary rocks
and characterized by relatively stable tectonic conditions.
plutoid (228):
Precambrian (232): Term used to describe rocks that were older than Cambrian
Precambrian provinces (233): Divisions of the Canadian Shield based on
differences in the trends of faults and folds, the style of folding, and the ages of
component rocks.
prokaryote (243): Organisms that lack membrane-bounded nuclei and other
membrane-bounded organelles.
Proterozoic Eon (232): A younger divide of Precambrian time used to describe
rocks.
protoplanet (219): Large globules formed from swarms of accreting bodies,
dust, and gases. Enormously larger than present-day planets. Each rotated
somewhat like a miniature dust cloud, and each eventually swept away most of
the debris in its orbital path and was able to revolve around the central mass
without collision with other protoplanets. Formation required an estimated 10
million years.
shield (Precambrian) (233): Broadly upwarped, geologically stable regions of
continents. Every continent has one or more shields. The Canadian Shield
extends across 3 million square miles of northern North America.
solar nebula (219): The rotating cloud of dust particles and gases from which
the solar system was derived.
Solar System (216): In one galaxy of many located in the universe contains the
Milky Way. Among the Milky Way’s billions of stars is a small one, our sun.
Around the sun swirl nine planets, thousands of asteroids and comets, and
countless bits of space debris that comprise the solar system.
solar wind (219): The stream of radiation from the sun, which drove enormous
quantities of lighter elements and frozen gases outward into space. This solar
force is what causes a comet’s tail to show wavy streaming or to be bent away
from the sun. Also, this is the reason the planets closest to the sun have smaller
masses, but greater densities than the outer planets.
stony-iron meteorite (220): The least abundant of the meteorites. Derived from
a shattered asteroid, originating from the area in the asteroid that lies between
the iron core and the surrounding rocky shell. They are composed of silicate
minerals and iron nickel.
stromatolite (244): Distinctly laminated accumulations of calcium carbonate
having rounded, branching, or frondose shape and believed to form as a result of
the metabolic activity of marine algae. They are usually found in the high
intertidal to low supratidal zones. Associated with Cyanobacteria.
ultramafic (236): Rock or lava with extremely high concentrations of iron and
magnesium.
CHAPTER 8
Earth's Formative Stages and the Archean Eon
HADEAN
Hadean—An informal time units prior to the Archean which takes into account the
time before the preservation of the rock record (4.56 billion years ago to around 4.0
billion years ago)
Earth’s accretion was complete 4.56 billion years ago. The crust began to form about
4.0 billion years ago.
Geologic processes recycled, destroyed or altered the original rocks of the Earth
during the Hadean.
Much of our knowledge of the Earth's earliest history comes from indirect evidence:
meteorites.
ARCHEAN EON
Archean (Archean Eon) is the oldest unit on the geologic time scale.
It began about 4.0 billion years ago and ended 2.5 billion years ago.
Archean lasted for 2.1 billion years (2,100,000,000 years).
EARTH'S OLDEST ROCKS
Earth's oldest rocks are found in Canada. They are about 4.04 billion years old.
But there are even older mineral grains. Sand-sized zircon grains in
metamorphosed sedimentary rocks from Australia are 4.4 billion years old.
PRECAMBRIAN
Archean and Proterozoic Eons comprise in interval of time informally called
Precambrian, which spans 87% of the geologic time scale.
FIGURE 8-1 The Precamrian eons span 87% of Earth history.
THE BIG BANG
Calculations indicate that the Big Bang occurred 18–15 billion years ago.
The Big Bang marked the instantaneous creation of all matter in the Universe
followed by an expansion.
THE SOLAR SYSTEM
The Sun and the planets, moons, asteroids, comets and other objects that orbit it,
comprise the Solar System.
FIGURE 8-4 Schematic view of the Solar System, showing orbits of the eight
planets, the planetoid Pluto, the asteroid belt, and a comet.
OBSERVATIONS THE MUST BE CONSIDERED FOR ANY HYPOTHESIS ON THE ORIGIN
OF THE SOLAR SYSTEM
1.Planets revolve around sun in same direction—counterclockwise (CCW)
2.Planets lie roughly within sun's equatorial plane (plane of sun's rotation)
3.Solar System is disk-like in shape
4.Planets rotate CCW on their axes, except for:
a.Venus: slowly clockwise
b.Uranus: on its side
5.Moons go CCW around planets (with a few exceptions)
OBSERVATIONS THE MUST BE CONSIDERED FOR ANY HYPOTHESIS ON THE ORIGIN
OF THE SOLAR SYSTEM
6.Distribution of planet densities and compositions is related to their distance from
sun
a.Inner, terrestrial planets have high density
b.Outer, Jovian planets have low density
7.Age—Meteorites are as old as 4.56 billion years
OBSERVATIONS THE MUST BE CONSIDERED FOR ANY HYPOTHESIS ON THE ORIGIN
OF THE SOLAR SYSTEM
SOLAR NEBULA HYPOTHESIS OR NEBULAR HYPOTHESIS
1.Cold cloud of gas and dust contracts, rotates, and flattens into a disk-like shape.
2.Roughly 90% of mass becomes concentrated in the center, due to gravitational
attraction.
3.Turbulence in cloud caused matter to collect in certain locations.
4.Clumps of matter begin to form in the disk.
FIGURE 8-6 The solar nebula hypothesis for the origin of the Solar System.
SOLAR NEBULA HYPOTHESIS OR NEBULAR HYPOTHESIS
5.Accretion of matter (gas and dust) around clumps by gravitational attraction.
Clumps develop into protoplanets.
6.Solar nebula cloud condenses, shrinks, and becomes heated by gravitational
compression to form Sun.
7.Ultimately hydrogen (H) atoms begin to fuse to form helium (He) atoms, releasing
energy (heat and light). The Sun "ignites."
FIGURE 8-6 The solar nebula hypothesis
for the origin of the Solar System.
SOLAR NEBULA HYPOTHESIS OR NEBULAR HYPOTHESIS
8.The Sun's solar wind drives lighter elements outward, causing observed distribution
of masses and densities in the Solar System.
9.Planets nearest Sun lose large amounts of lighter elements (H, He), leaving them
with smaller sizes and masses, but greater densities than the outer planets. Inner
planets are dominated by rock and metal.
10.Outer planets retain light elements such as H and He around inner cores of rock
and metal. Outer planets have large sizes and masses, but low densities.
FIGURE 8-6 The solar nebula hypothesis for the origin of the Solar System.
HOW OLD IS THE SOLAR SYSTEM?
Based on radiometric dates of moon rocks and meteorites, the Solar System is about
4.56 billion years old.
METEORITES: SAMPLES OF THE SOLAR SYSTEM
Meteors = "shooting stars." The glow comes from small particles of rock from space
being heated as they enter Earth's atmosphere.
Meteorites = chunks of rock from the Solar System that reach Earth's surface. They
include fragments of:
Asteroids
Moon rock
Planets, such as Mars (i.e., "Martian meteorites")
TYPES OF METEORITES
1.Ordinary chondrites
2.Carbonaceous chondrites
3.Iron meteorites
4.Stony-iron meteorites
METEORITES: ORDINARY CHONDRITES
Most abundant type of meteorite
About 4.6 billion years old
May contain chondrules: spherical bodies that solidified from molten droplets
thrown into space during Solar System impacts
METEORITES: CARBONACEOUS CHONDRITES
Contain about 5% organic compounds, including amino acids—the building blocks
of proteins, DNA, and RNA
May have supplied basic building blocks of life to Earth
Contain chondrules
METEORITES: IRON METEORITES
Iron-nickel alloy
Coarse-grained inter-grown crystal structure
About 5% of all meteorites
(c) AP/Wide World Photos
METEORITES: STONY-IRON METEORITES
Composed partly of Fe, Ni and partly of silicate minerals, including olivine (like
Earth's mantle).
About 1% of all meteorites. Least abundant type.
THE SUN
The Sun is a star
Composition:
70% hydrogen
27% helium
3% heavier elements
Size: About 1.5 million km in diameter
Contains about 98.8% of the matter in the Solar System
THE SUN
Temperature: may exceed 20 millionoC in the interior
Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen
atoms are fused together to form helium, releasing energy
The Sun's gravity holds the planets in their orbits
SUN'S ENERGY IS THE FORCE BEHIND MANY GEOLOGIC PROCESSES ON EARTH
Evaporation of water to produce clouds, which cause precipitation, which causes
erosion.
Uneven heating of the Earth's atmosphere causes winds and ocean currents.
Variations in heat from Sun may trigger continental glaciations or change forests to
deserts.
Sun and moon influence tides which affect the shoreline.
THE PLANETS
1.Mercury
2.Venus
3.Earth
4.Mars
5.Jupiter
6.Saturn
7.Uranus
8.Neptune
THE PLANETS
Terrestrial planets:
Small
Dense (4 - 5.5 g/cm3)
Rocky + Metals
Mercury, Venus, Earth, Mars
Jovian planets:
Large
Low density (0.7 - 1.5 g/cm3)
Gaseous
Jupiter, Saturn, Uranus, Neptune
MERCURY
Smallest of the terrestrial planets
Revolves rapidly around the sun; its year is 88 Earth days
Densely cratered
Thin atmosphere of sodium and lesser amounts of helium, oxygen, potassium and
hydrogen
Weak magnetic field and high density suggest an iron core
No moons
VENUS
Similar to Earth in size, mass, volume, density and gravity
No oceans or liquid water
Very high atmospheric pressure
Atmosphere is 98% carbon dioxide
Dense clouds of sulfuric acid droplets in atmosphere
Greenhouse effect causes temperature on planet's surface to reach 470°C, hot
enough to melt lead
VENUS
Rotates once on its axis (one day on Venus) in 243 Earth days
Rotates on axis in opposite direction to other planets, possibly due to collision with
other object
Has volcanoes
Has craters
Surface rocks resemble basalt
No moons
EARTH
Diameter = nearly 13,000 km (8000 mi)
Oceans cover 71% of surface
Atmosphere = 78% nitrogen and 21% oxygen
Surface temperature approx. –50 and +50oC
Average density = 5.5 g/cm3
Surface rock density = 2.5-3.0 g/cm3
Core about 7000 km in diameter
Mantle surrounds core. Extends from base of crust to depth of 2900 km
Geologically active. Plate tectonics
Only body in the Universe known to support life
EARTH'S INTERNAL LAYERED STRUCTURE
The Earth is internally layered, with a basic structure consisting of:
Crust
Mantle
Inner and outer core
The Earth's internal structure may be primary (formed initially as the Earth formed),
or secondary due to later heating.
FACTORS THAT MAKE EARTH HOSPITABLE FOR LIFE
Distance from Sun maintains temperatures in the range where water is liquid.
Temperature relatively constant for billions of years.
Rotation allows all sides of Earth to have light and heat.
Atmosphere absorbs some heat from the Sun and reflects some solar radiation
back to space.
Magnetic field protects life from dangerous high energy particles and radiation in
the solar wind.
EARTH'S MOON
Diameter = about 1/4 that of Earth.
Density = about 3.3 g/cm3 (similar to Earth's mantle).
Rotates on its axis at same rate as it revolves around Earth (29.5 days). Results in
same side of Moon always facing Earth.
Far side of moon is more densely cratered
No atmosphere.
Ice is present at the poles.
GEOLOGY OF THE MOON
Dominant rock type is anorthosite (related to gabbro; rich in Ca plagioclase
feldspar).
Basalt is also present.
Two types of terranes

Maria (singular = mare)
LUNAR HIGHLANDS
Lunar Highlands
Light-colored
Rough topography
Highly cratered
Rocks more than 4.2 billion years old
LUNAR MARIA
Lunar Maria
Large, dark areas
Immense basins covered with basaltic lava flows
Age of basalt is 3.8 to 3.2 billion years
Maria have few craters. This indicates a decrease in meteorite bombardment after
about 3.8 billion years ago.
ORIGIN OF THE MOON
Moon may have formed as a result of an impact of a large body with Earth about
4.4 billion years ago.
Debris from the impact was thrown into orbit around Earth and collected to form
the Moon.
Heat from impacts led to melting and differentiation (or segregation of materials of
different density; low density materials rose and high density materials sank).
MARS
Has white polar caps made of frozen carbon dioxide ice
Has seasonal changes. Polar ice caps expand and contract
Rusty orange color due to iron oxides on surface
Heavily cratered due to early bombardment by meteorites and asteroids
MARS
Diameter is about half that of Earth
Mass is only about 10% of Earth's mass, so gravity is much less
Thin atmosphere (less than 1% as dense as Earth's).
Dominant gas is carbon dioxide; small amounts of nitrogen, oxygen and carbon
monoxide. No greenhouse effect.
MARS
Previously had a denser atmosphere
Evidence of abundant liquid water in the past.
An ocean once existed, at least 0.5 km deep and larger than all 5 U.S. Great Lakes.
Temperatures range from –85oC to 21oC (21oC is about room temperature)
MARS
Lower density than other terrestrial planets
Little to no magnetic field, suggesting only a small iron-rich core.
Lack of magnetic field exposed planet to solar winds which swept away
atmosphere and liquid water.
Two small moons, Phobos and Deimos
ASTEROID BELT
Thousands of asteroids, primarily between Mars and Jupiter.
Asteroids are composed of rocks and metal (Fe & Ni).
Size of asteroids ranges from a few km in diameter to about 1/10 the size of Earth.
JUPITER
Largest planet in the Solar System. (Diameter 11 times greater than Earth)
Low density. (Density is about 1/4 that of Earth)
Most of planet's interior is probably liquid metallic hydrogen.
Rotates on axis rapidly. One day on Jupiter is 10 hours on Earth.
Rotation causes bands in atmosphere
JUPITER
Note Great Red Spot, a cyclonic storm
Atmosphere composed of H, He, with lesser amounts of methane and ammonia
Has a faint ring of debris which encircles the planet
Courtesy NASA
JUPITER
Has more than 60 moons
Four largest moons are:

Europa: has sea of liquid water beneath an icy surface
Ganymede: planet-sized body larger than Mercury. Cratered with sinuous ridges;
has sea of liquid water beneath an icy surface
Callisto: highly cratered; has sea of liquid water beneath an icy surface
Courtesy NASA
SATURN
Second largest planet
Has prominent rings of debris (mostly ice with minor silicates) encircling planet in
equatorial plane. Debris ranges from microns to meters.
SATURN
Density is less than that of water; it could float. (Density = 0.7g/cm3)
Mostly H and He; also contains methane, ammonia, and water; may have iron core
Has magnetic field, radiation belts, and internal heat source
Has more than 30 moons.
URANUS
About 4 x larger than Earth
Low density (density = 1.3 g/cm3)
Axis of rotation is tipped on its side, possibly due to collision with another Solar
System object
Has more than two dozen moons
Atmosphere of hydrogen, helium, and methane
Has planetary ring system
E. Karkoschka, University of Arizona/Space Telescope Science Institute
NEPTUNE
Similar in size and color to Uranus
Low density (density = 1.6 g/cm3)
Atmosphere = H, He, and methane
Has more than a dozen moons
Has Great Dark Spot, a cyclonic storm
Has planetary ring system
Courtesy NASA
SOLAR NEBULA HYPOTHESIS OR COLD ACCRETION MODEL (SECONDARY
DIFFERENTIATION)
Earth formed by accretion of dust and larger particles of metals and silicates.
Earth was originally homogeneous throughout - a random mixture of space debris.
Origin of layering requires a process of differentiation.
Differentiation is the result of heating and at least partial melting.
POSSIBLE SOURCES OF HEAT FOR MELTING:
1.Accretionary heat from bombardment (meteorite impacts)
2.Heat from gravitational compression as material accumulated
3.Radioactive decay
DIFFERENTIATION AFTER ACCRETION
Iron and nickel sink to form core.
Less dense material (silicon and oxygen combined with remaining iron and other
metals) forms mantle and lighter crust (dominated by silicon and oxygen).
Presence of volatile gases on Earth indicates that complete melting did not occur.
Earth was repeatedly partly melted by great impacts, such as the Moon-forming
impact.
AN ALTERNATIVE MODEL: HOT ACCRETION (PRIMARY DIFFERENTIATION)
Internal zonation of planets is a result of hot heterogeneous accretion.
Hot solar nebula (over 1000oC).
Initial crystallization of iron-rich materials forms planet's core.
With continued cooling, lower density silicate materials crystallized.
WHICH MODEL?
Solar Nebula Hypothesis also known as the
Cold Accretion Model (secondary differentiation)
OR
Hot Accretion Model (primary differentiation) ???
Parts of both models may have been in operation.
ARCHEAN CRUST
Once differentiation occurred, Earth's crust was dominated by Fe & Mg silicate
minerals.
If Earth experienced heating and partial melting, it may have been covered by an
extensive magma ocean during Archean.
Magma cooled to form rocks called komatiites.
Archean rocks form South Africa indicate that the Earth had a magnetic field.
The existence of a magnetic field is important to the development of life on Earth
KOMATIITES
Komatiites are ultramafic rocks composed mainly of olivine and pyroxene.
Komatiites form at temperatures greater than those at which basalt forms (greater
than 1100oC).
This rock formed Earth's Archean crust.
ORIGIN OF MAFIC CRUST
The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of
rocks in the upper mantle.
EARTH'S CRUST TODAY
Earth has two types of crust today:
1.Dense, mafic (Mg- and Fe-rich) oceanic crust dominated by basalt.
2.Less dense, silicic (Si- and Al-rich) continental crust dominated by granite.
ORIGIN OF CONTINENTAL CRUST
Continental crust developed after the initial mafic to ultramafic crust.
Continental crust is silicic or felsic (such as granite). Dominated by light-colored
minerals such as quartz and feldspar.
Felsic crust began forming around 4.4 billion years ago.
ORIGIN OF CONTINENTAL CRUST
Felsic crust formed in subduction zones where descending slabs of crust partially
melted.
The early-melting, less dense components of the melt rose to the surface where
they cooled to form continental crust.
EARTH'S OLDEST ROCKS
One of the oldest dated felsic Earth rocks is the 4.04 billion year old Acasta Gneiss
from northwestern Canada. Dates are from zircon grains in tonalite gneisses.
(Tonalite gneiss is metamorphosed tonalite, a rock similar to diorite, with at least
10% quartz).
The Amitsoq Gneiss from Greenland is another old tonalite gneiss (3.8 b.y. old).
Patches of old felsic crust have also been found in Antarctica (3.9 b.y. old).
EARTH'S OLDEST LAND SURFACE
A 3.46 b.y. old fossil soil zone (or paleosol) associated with an unconformity in the
Pilbara region of Australia indicates that Archean continents stood above sea level.
This paleosol represents the oldest land surface known, and provides evidence
that subaerial weathering, erosion, and soil formation processes were at work during
Archean.
THE OLDEST MINERAL GRAINS
The oldest zircon grains are 4.4 b.y. old.
Found in quartzite in western Australia.
Sedimentary structures in the quartzite resemble those in modern stream deposits.
Interpreted as fluvial (river) deposits.
Derived from weathering of granitic rocks (some of the earliest continental crust),
and deposited above sea level, indicating the presence of both liquid water and
continental crust by 4.4 b.y. ago.
Oceanic Crust
First appearance
Where formed
Composition
Lateral extent
How formed
Continental Crust
About 4.5 b.y. ago
About 4.4 b.y. ago
Mid-ocean ridges
Subduction zones
Komatiite & basalt
Tonalite & granodiorite,
and later, granites
Widespread
Local
(few 100 km or mi)
Partial melting of
Partial melting of wet,
ultramafic rocks in upper sediment-covered mafic
mantle
rocks in subduction
zones