Transcript CHAPTER 8 - Geological Sciences

Chapter 8
The Earliest Earth:
2,100,000,000 years of the
Archean Eon
The Archean Eon
The Archean Eon is the oldest unit on the
geologic time scale.
It began 4.6 billion years ago and ended 2.5
billion years ago.
The 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.
Earth's Oldest Rocks
There are no rocks on Earth that date back
to 4.6 billion years ago because the Earth
is geologically active, and the oldest rocks
have been recycled by plate tectonics or
by weathering and erosion as part of the
rock cycle.
Much of our knowledge of the Earth's
earliest history comes from indirect
evidence - meteorites.
The Precambrian
The Archean and
Proterozoic Eons
comprise the
Precambrian, which
spans 87% of the
geologic time scale.
Earth in Space
The origin, history, and characteristics of
Planet Earth need to be considered in the
context of the Universe.
Galaxies
• The Universe hosts billions of
galaxies. A galaxy is an
aggregate of stars, planets, dust,
and gases.
• Planet Earth orbits the Sun, a
dwarf star, that belongs to the
Milky Way galaxy.
Whirlpool galaxy. Image courtesy of NASA and The Hubble Heritage
Team (STScI/AURA)
The Solar System
The Sun and the planets, moons,
asteroids, comets and other objects that
orbit it, comprise the Solar System.
Origin of the Universe
•
•
The Earth is part of the Solar System;
the Solar System is part of the Milky Way
galaxy; and the Milky Way galaxy is part
of the Universe.
The story of the origin and history of the
Earth requires that the origin and history
of the Universe and Solar System must
be considered.
Origin of the Universe
Evidence to be considered when interpreting the
history of the Universe:
– Galaxies are rapidly moving apart (Hubble's
Law). Suggests that galaxies were closer
together in the past. Discovered by Edwin P.
Hubble in 1929.
– Observed temperature of the Universe today
(background microwave radiation) 3 degrees
above absolute zero.
– Present abundances of hydrogen and
helium.
Origin of the Universe
Interpretation:
– The Universe is expanding.
– Everything began together at a point.
– A big explosion occurred, which astronomers
call the Big Bang.
– This explosion caused everything in the
Universe to begin moving rapidly apart.
How do we know the galaxies
are moving apart?
• Red shift.
In 1914, W.M. Slipher first noted that
galaxies displayed the red shift.
Their light is shifted toward the red (or long
wavelength) end of the spectrum.
• Colors of the spectrum
ROY G BIV
What the Spectrum Reveals
The spectrum of a star reveals:
• The star's composition by means of
absorption lines. Various elements in the
star's atmosphere absorb parts of the light
of the spectrum.
• Whether it is moving toward or away from
the Earth (and at what speed).
How do we know the galaxies
are moving apart?
• Light reaching us from distant receding
galaxies has its absorption lines shifted
toward the red end of the spectrum. This
indicates that the galaxy is moving away
from the Earth.
• The red shift indicates that the universe is
expanding.
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.
Variations on the Big Bang Theory
• Steady-state cosmology - Universe will
continue to expand forever; new matter is
formed in spaces between galaxies at about the
same rate that older material is receding.
Density of matter in the universe remains
relatively constant.
• Oscillating Universe cosmology - The
Universe expands (like with the Big Bang) and
then contracts. Expansion and contraction
alternate.
Formation of the Solar System The Solar Nebula Hypothesis
Lines of evidence that 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
c. Pluto - on its side
5. Moons go CCW around planets (with a
few exceptions)
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 - Moon rocks and meteorites are as
old as 4.6 billion years
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.
Solar Nebula Hypothesis or
Nebular Hypothesis
4. Clumps of matter begin to form in the
disk.
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.
Solar Nebula Hypothesis or
Nebular Hypothesis
7. Ultimately hydrogen (H) atoms begin to
fuse to form helium (He) atoms,
releasing energy (heat and light). The
Sun "ignites".
8. The Sun's solar wind drives lighter
elements outward, causing observed
distribution of masses and densities in
the Solar System.
Solar Nebula Hypothesis or
Nebular Hypothesis
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.
The Nebular Hypothesis
How old is the Solar System?
Based on radiometric dates of moon rocks
and meteorites, the Solar System is about
4.6 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.
2.
3.
4.
5.
Ordinary chondrites
Carbonaceous chondrites
Achondrites
Iron meteorites
Stony-iron meteorites
Meteorites: Ordinary Chondrites
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•
•
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: Achondrites
• Stony meteorites without chondrules,
resembling basalt
Meteorites: Iron Meteorites
•
•
•
Iron-nickel alloy
Coarse-grained
intergrown crystal
structure (Widmanstatten
pattern)
About 5% of all
meteorites
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 Solar System Tour, From
Center to Fringe
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 million oC 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.
2.
3.
4.
5.
6.
7.
8.
9.
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
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
Other:
Small
Low
density
Pluto
The Terrestrial Planets
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
Mercury
This mosaic of Mercury was taken by
the Mariner 10 spacecraft during its
approach on 29 March 1974. The
mosaic consists of 18 images taken
at 42 s intervals during a 13 minute
period when the spacecraft was
200,000 km (about 6 hours prior to
closest approach) from the planet.
Image courtesy of NASA NSSDC
Photo Gallery
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
Venus
Ultraviolet image of Venus' clouds as
seen by the Pioneer Venus Orbiter
(Feb. 5, 1979). Image courtesy of
NASA NSSDC
Topographic Map of Venus
from Pioneer Venus (Mercator
Projection). Image courtesy of
NASA NSSDC
Earth
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•
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Diameter = nearly 13,000 km (8000 mi)
Oceans cover 71% of surface
Atmosphere = 78% nitrogen and 21% oxygen
Surface temperature approx. -50 and +50 oC
Average density = 5.5 g/cm3
Surface rock density = 2.5-3.0 g/cm3
Earth - The Blue Marble. Credit: NASA Goddard Space
Flight Center Image by Reto Stöckli. Visible Earth.
Earth
• 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 - The Blue Marble. Credit: NASA Goddard Space
Flight Center Image by Reto Stöckli. Visible Earth.
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 terrane
– Lunar highlands
– Maria (singular = mare)
Lunar highlands
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•
•
•
Light-colored
Rough topography
Highly cratered
Rocks more than 4.2 billion years old
The Moon. Photograph taken by Apollo 11 in 1969.
Image courtesy of NASA National Space Science Data Center.
Maria
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•
•
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.
The Moon. Photograph taken by Apollo 11 in 1969.
Image courtesy of NASA National Space Science Data Center.
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
Mars
• Has volcanoes, canyons, channels, sand
dunes, and layered rock
Largest known volcano in the Solar
System, Olympus Mons, on Mars.
Summit caldera is 24 km above
surrounding plains. Surrounded by a
scarp 550 km in diameter and
several km high. Image courtesy of
NASA NSSDC.
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.
An Asteroid. Image courtesy of
NASA Jet Propulsion Laboratory
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
Image of Jupiter taken by Voyager in 1979. Ganymede,
Jupiter's largest satellite, can be seen to the lower left of
the planet. Image courtesy of NASA NSSDC.
Jupiter
• Has 63 or more moons
• Four largest moons are:
– Io- covered by sulfur volcanoes
– 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
Saturn
• Second largest planet
• Has prominent rings of debris encircling
planet in equatorial plane
NASA's Voyager 2 took this photograph of Saturn on July 21,
1981. The moons Rhea and Dione appear as blue dots to the
south and southeast of Saturn. Voyager 2 made its closest
approach to Saturn on Aug. 25, 1981. Image courtesy of
NASA NSSDC.
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 30 or more 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 27 or more moons
• Atmosphere of hydrogen, helium, and methane
• Has planetary ring system
Uranus. Image courtesy of NASA Jet Propulsion Laboratory
Neptune
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•
Similar in size and color to Uranus
Low density (density = 1.6 g/cm3)
Atmosphere = H, He, and methane
Has 13 or more moons
Has Great Dark Spot, a cyclonic storm
Has planetary ring system
Color image of Neptune showing its "Great Dark Spot".
Image courtesy of NASA NSSDC.
Pluto
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•
Smallest planet
Elliptical orbit crosses the orbit of Neptune
Orbit is inclined at 17o
Axis of rotation is sideways.
Has a moon, Charon
Atmosphere is methane and nitrogen
No rings
May be an escaped moon of Neptune, rather
than an original planet
Accretion and Differentiation
of the Earth
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.
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 1000 oC).
• 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.
The Archean Crust
The Archean Crust
• Once differentiation occurred, Earth's crust
was dominated by Fe and Mg silicate
minerals.
• If Earth experienced heating and partial
melting, it may have been covered by an
extensive magma ocean in the Archean.
• Magma cooled to form rocks called
komatiites.
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, sialic (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 sialic 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).
Earth’s Oldest Rocks
• 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 Enderby Land, 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 in the 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
Continental Crust
First appearance About 4.5 b.y. ago
About 4.4 b.y. ago
Where formed
Mid-ocean ridges
Subduction zones
Composition
Komatiite & basalt
Lateral extent
Widespread
How formed
Partial melting of
ultramafic rocks in
upper mantle
Tonalite &
granodiorite, and
later, granites
Local
(few 100 km or mi)
Partial melting of
wet, sedimentcovered mafic
rocks in
subduction zones
Evolution of Earth’s Atmosphere
and Hydrosphere
Evolution of Earth’s Atmosphere
and Hydrosphere
• Earth's first, primitive atmosphere lacked
free oxygen.
• The primitive atmosphere was derived
from gases associated with the comets
and meteorites which formed the Earth
during accretion.
• The gases reached the Earth's surface
through a process called outgassing.
Gases Associated with Comets
• Comets are made of frozen gases, ice and dust.
• Halley's comet is composed of:
– 80% water ice
– Frozen carbon dioxide (dry ice)
– Hydrogen cloud surrounds comet
– Dust near the nucleus contains iron, oxygen,
silicon, magnesium, sodium, sulfur, and carbon
Gases Associated with Meteorites
• Carbonaceous chondrites are mainly
composed of silicate minerals, but also
contain:
– Nitrogen
– Hydrogen
– Water
– Carbon in the form of complex organic
molecules (proteins and amino acids)
Water and gaseous elements would have
been released from the newly accreted
Earth by the heat associated with
bombardment and accretion, or by melting
and volcanism accompanying later
differentiation.
Volcanic Outgassing
• Outgassing = release of water vapor and other
gases from Earth through volcanism.
• Gases from Hawaiian eruptions consist of:
– 70% water vapor (H2O)
– 15% carbon dioxide (CO2)
– 5% nitrogen (N2)
– 5% sulfur (in H2S)
– chlorine (in HCl)
– hydrogen
– argon
Volcanic Outgassing
Most of the water on the surface of the Earth
and in the atmosphere was outgassed in
the first billion years of Earth history.
We know this because there are 3.8 b.y.-old
marine sedimentary rocks, indicating the
presence of an ocean by 3.8 billion years
ago.
Formation of the Hydrosphere
Once at the Earth's surface, gases and other
volatile elements underwent a variety of
changes.
1. Water vapor condensed and fell as rain.
2. Liquid water probably began to fall on the
Earth's surface as early as 4.4 billion years
ago.
3. Rain water accumulated in low places to form
seas. The seas were originally freshwater
(rain).
Formation of the Hydrosphere
4. Carbon dioxide and other gases dissolved in
the rain made the water more acidic than today.
Carbon dioxide and water combine to form
carbonic acid.
5. Acid waters caused rapid chemical weathering
of the exposed rocks, adding Na, Ca, K, and
other ions to seawater.
6. A change to more alkaline water may have
occurred rapidly as large amounts of Ca, Na,
and Fe were introduced by submarine
volcanism, neutralizing the acid.
Formation of the Hydrosphere
7. Ions accumulated in the seas, increasing the
salinity. Sea salinity is relatively constant today
because salts are precipitated at about the
same rate they are supplied to the sea. Sodium
remains in sea water due to its high solubility.
8. Later, when the seas became less acidic, Ca
ions bonded with CO2 to form shells of marine
organisms and limestones (CaCO3).
9. The presence of marine fossils suggests that
sodium has not varied appreciably in sea water
for at least the past 600 million years.
Hydrologic Cycle
Today Earth's water is continuously
recirculated through the hydrologic cycle
(evaporation and precipitation, powered by
the sun and by gravity).
Evolution of the Atmosphere
Note gases released by volcanoes, condensation
of water vapor, precipitation, and accumulation of
liquid water, photochemical reactions in the
atmosphere, and formation of carbonate rocks
(limestones) later, after the seas became less
acidic.
The Early Anoxic Atmosphere
Earth's early atmosphere was strongly reducing
and anoxic (lacked free oxygen or O2 gas), and
probably consisted primarily of:
– Water vapor (H2O)
– Carbon dioxide (CO2)
– Nitrogen (N2)
– Carbon monoxide (CO)
– Hydrogen sulfide (H2S)
– Hydrogen chloride (HCl)
The Early Anoxic Atmosphere
The atmosphere composition would have
been similar to that of modern volcanoes,
but probably with more hydrogen, and
possibly traces of methane (CH4) and
ammonia.
If any free oxygen had been present, it
would have immediately been involved in
chemical reactions with easily oxidized
metals such as iron.
Evidence for a Lack of Free Oxygen
in Earth's Early Atmosphere
1. Lack of oxidized iron in the oldest sedimentary
rocks. (Instead, iron combined with sulfur to
form sulfide minerals like pyrite. This happens
only in anoxic environments.)
2. Urananite and pyrite are readily oxidized today,
but are found unoxidized in Precambrian
sedimentary rocks.
3. Archean sedimentary rocks are commonly dark
due to the presence of carbon, which would
have been oxidized if oxygen had been
present.
Evidence for a Lack of Free Oxygen
in Earth's Early Atmosphere
4. Archean sedimentary sequences lack
carbonate rocks but contain abundant
chert, presumably due to the presence of
an acidic, carbon dioxide-rich atmosphere.
In an acidic environment, alkaline rocks
such as limestone do not form.
Banded Iron Formation
5. Banded iron formations (BIF) appear in the
Precambrian (1.8 - about 3 b.y.).
Cherts with alternating laminations of red
oxidized iron and gray unoxidized iron.
Formed as precipitates on shallow sea floor.
Some iron probably came from weathering of
iron-bearing rocks on continents. Most Fe was
probably from submarine volcanoes and
hydrothermal vents (hot springs).
Great economic importance; major source of
iron mined in the world.
Banded Iron Formation
Polished specimen of banded iron from
Australia. A common name for this type of
banded iron is "Tiger Iron". Metric ruler for scale.
Photo courtesy of Pamela Gore.
Additional Evidence for
an Anoxic Atmosphere
6. The simplest living organisms have an
anaerobic metabolism. They are killed by
oxygen.
Includes some bacteria (such as botulism),
and some or all Archaea, which inhabit
unusual conditions.
7. Chemical building blocks of life (such as
amino acids, DNA) could not have formed
in the presence of O2.
Formation of an
Oxygen-rich Atmosphere
The change from an oxygen-poor to an
oxygen-rich atmosphere occurred by the
Proterozoic, which began 2.5 billion years
ago, at the end of the Archean.
Formation of an
Oxygen-rich Atmosphere
The development of an oxygen-rich
atmosphere is the result of:
1. Photochemical dissociation - Breaking up
of water molecules into H and O in the upper
atmosphere, caused by ultraviolet radiation
from the Sun (a minor process today)
2. Photosynthesis - The process by which
photosynthetic bacteria and plants produce
oxygen (major process).
Evidence for Free Oxygen in the
Proterozoic Atmosphere
1. Red beds - Sedimentary rocks with iron
oxide cements (including shales,
siltstones, and sandstones), appear in
rocks younger than 1.8 billion years old.
This is in the Proterozoic Eon, after the
disappearance of the banded iron
formations (BIF).
Evidence for Free Oxygen in the
Proterozoic Atmosphere
2. Carbonate rocks (limestones and
dolostones) appear in the stratigraphic
record at about the same time that red
beds appear.
This indicates that CO2 was less abundant
in the atmosphere and oceans so that the
water was no longer acidic.
The Precambrian
Overview
of the
Precambrian
The Precambrian
The Precambrian covers about 4 billion
years (and 87%) of Earth history.
The Precambrian is divided into 2 eons:
– Proterozoic Eon 2.5 - 0.542 billion years ago
(or 2500 - 542 million years ago)
– Archean Eon 4.6 - 2.5 billion years ago
(lower limit not defined)
Table of time
divisions of the
Precambrian
The Precambrian is not well known
or completely understood. Why?
• Precambrian rocks are often poorly exposed.
• Many Precambrian rocks have been eroded or
metamorphosed.
• Most Precambrian rocks are deeply buried
beneath younger rocks.
• Many Precambrian rocks are exposed in fairly
inaccessible or nearly uninhabited areas.
• Fossils are seldom found in Precambrian rocks;
only way to correlate is by radiometric dating.
Areas where Precambrian rocks are exposed are shown
in yellow, as well as in the red areas in orogenic belts.
Shields and Cratons
Most information on the Precambrian is
from cratons - large portions of continents
which have not been deformed since
Precambrian or Early Paleozoic time.
Shields and Cratons
• The most extensive exposures of
Precambrian rocks are in geologically
stable regions of continents called
shields.
• Example = Canadian shield in North America.
Mostly igneous and metamorphic rocks; few
sedimentary rocks. Overlying sedimentary rocks
were scraped off by glaciers during last Ice Age.
Shields and Cratons
• Stable regions of the craton where shields
are covered by sedimentary rocks are
called platforms.
• Precambrian rocks are often called
basement rocks because they lie
beneath a covering of fossil-bearing
sedimentary strata.
North American
craton, shield,
platform, and
orogenic belts.
Precambrian Provinces
Various Precambrian provinces can be
delineated within the North American
continent, based on radiometric ages of
rocks, style of folding, and differences in
trends of faults and folds.
Precambrian provinces in North
America, with dates
– Oldest (Archean)
rocks are shown
in orange.
– Younger
(Proterozoic)
rocks are shown
in green.
Origin of Plate Tectonics
• By about 4 b.y. ago, the Earth had
probably cooled sufficiently for plate
formation.
• Once plate tectonics was in progress, it
generated crustal rock that could be
partially melted in subduction zones and
added to the continental crust.
Origin of Plate Tectonics
• Continents also increased in size by
addition of microcontinents along
subduction zones.
• Greater heat in Archean would have
caused faster convection in mantle, more
extensive volcanism, more midoceanic
ridges, more hot spots, etc.
• Growth of volcanic arcs next to subduction
zones led to formation of greenstone belts.
Granulites and Greenstones
The major types of Archean rocks on the
cratons are:
– Granulites
– Greenstones
Granulites
• Granulites - Highly metamorphosed
gneisses (metamorphosed tonalites,
granodiorites, and granites) and
anorthosites (layered intrusive gabbroic
rocks).
Granulites formed from partially melted
crust and sediments in subduction zones.
Metamorphism altered the rocks to form
granulites.
Greenstones
• Greenstones - Metamorphosed volcanic rocks
and sediments derived from the weathering and
erosion of the volcanic rocks.
Greenstone volcanic rocks commonly have
pillow structures, (called pillow basalts),
indicating extrusion under water.
The green color is the result of low-grade
metamorphism, producing green minerals such
as chlorite and hornblende.
Greenstones
• Mostly found in trough-like or synclinal belts.
• Sequence of rock types :
– Ultramafic volcanic rocks near the bottom
(komatiites)
– Mafic volcanic rocks (basalts)
– Felsic volcanic rocks (andesites and rhyolites)
– Sedimentary rocks at the top (shales,
graywackes, conglomerates, and sometimes
BIF), deposited in deep water environments
adjacent to mountainous coastlines.
Generalized cross-section through two
greenstone belts. Note sequence of rock types
and relationships between granulites and the
greenstones. Granulites are present between
greenstone belts.
Earth's Earliest Glaciation
By 2.8 billion years ago, Earth had cooled
sufficiently for glaciation to occur. Earth's
earliest glaciation is recorded in 2.8 billion
year-old sedimentary rocks in South
Africa.
Overview
of the
Precambrian
Life of the Archean –
The Archean Fossil Record
Earliest Evidence of Life
The earliest evidence of life occurs in
Archean sedimentary rocks.
– Stromatolites
– Microscopic cells of prokaryotes
– Algal filaments
– Molecular fossils
Stromatolites
• An organo-sedimentary structure built by
photosynthetic cyanobacteria or bluegreen algae.
• Stromatolites form through the activity of
cyanobacteria in the tidal zone. The sticky,
mucilage-like algal filaments of the
cyanobacteria trap carbonate sediment
during high tides.
Stromatolites
Modern stromatolites,
Shark Bay, western
Australia
Sketch of stromatolites,
showing top and side
views. Sketch courtesy
of Pamela Gore.
Stromatolites
• More abundant in Proterozoic rocks than
in Archean rocks.
Examples:
– Oldest are 3.5 b.y. old, Warrawoona Group,
Australia's Pilbara Shield
– 3 b.y. old Pongola Group of southern Africa
– 2.8 b.y. old Bulawayan Group of Australia
Stromatolites
• Stromatolites are scarce today because
microorganisms that build them are eaten by
marine snails and other grazing invertebrates.
• Stromatolites survive only in environments that
are too saline or otherwise unsuitable for most
grazing invertebrates.
• The decline of stromatolites is associated with
the evolutionary appearance of new groups of
marine invertebrates in the early Paleozoic.
Oldest direct evidence of life
• Microscopic cells and filaments of prokaryotes.
• Associated with stromatolites
• Similar to cyanobacteria living today, which
produce oxygen.
• Fossiliferous chert bed associated with the Apex
Basalt
• Found in Warrawoona Group, Pilbara
Supergroup, western Australia
• 3.460-3.465 billion years old
Other evidence of Archean life
• Indirect evidence of life in older rocks
Found in banded iron deposits in
Greenland.
Carbon-13 to carbon-14 ratios are similar
to those in present-day organisms.
3.8 b.y.
Other evidence of Archean life
• Algal filament fossils
Filamentous prokaryotes preserved in
stromatolites.
Found at North Pole, western Australia.
3.4-3.5 b.y.
• Spheroidal bacterial structures
Found in rocks of the Fig Tree Group,
South Africa (cherts, slates, ironstones,
and sandstones).
Prokaryotic cells, showing possible cell
division.
3.0 - 3.1 b.y.
Other evidence of Archean life
• Molecular fossils
Preserved organic molecules that only
eukaryotic cells produce.
Indirect evidence for eukaryotes.
In black shales from northwestern
Australia.
2.7 b.y.
Origin of eukaryotic life is pushed back to
2.7 b.y.
The Origin of Life
The basic materials from which microbial
organisms (i.e., life) could have developed
initially. May have arrived on Earth during
the Archean in meteorites called
carbonaceous chondrites, which contain
organic compounds.
Life requires these elements:
• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Phosphorus
• Sulfur
Each of these is abundant in the Solar System.
Four essential components of life:
1. Proteins - Chains of amino acids. Proteins are
used to build living materials, and as catalysts in
chemical reactions in organisms.
2. Nucleic acids - Large complex molecules in cell
nucleus.
1. DNA (carries the genetic code and can replicate itself)
2. RNA
3. Organic phosphorus compounds - Used to
transform light or chemical fuel into energy
required for cell activities.
4. Cell membrane to enclose the components within
the cell.
• The earliest organisms developed in the
presence of an atmosphere which lacked
oxygen. The organisms must have been
anaerobic (i.e., they did not require oxygen for
respiration).
• Organic molecules could not assemble into
larger structures in an oxygenated environment.
Oxidation and microbial predators would break
down the molecules.
• Because the atmosphere lacked oxygen, there
was no ozone shield to protect the surface of
the Earth from harmful ultraviolet (UV) radiation.
Origin of amino acids
UV radiation can recombine atoms in
mixtures of water, ammonia and
hydrocarbons, to form amino acids.
(The energy in lightning can do the same
thing.)
Miller-Urey Experiment
Lab simulation experiments by Miller and
Urey in the 1950's formed amino acids
from gases present in Earth's early
atmosphere:
– H2,
– CH4 (methane),
– NH3 (ammonia), and
– H2O (water vapor or steam),
along with electrical sparks (to simulate
lightning).
Miller-Urey Experiment
This was the first laboratory synthesis
of amino acids. A liquid was produced
that contained a number of amino acids
and other complex organic compounds
that comprise living organisms. A main
requirement was the lack of free oxygen.
Joining Amino Acids
to Form Proteins
Amino acids are monomers and have to be
joined together to form proteins, which are
polymers (or chains).
This requires:
– Input of energy
– Removal of water
Joining Amino Acids
to Form Proteins
How could this occur?
1. Heating (volcanic activity)
2. At lower temperatures in the presence of
phosphoric acid
3. Evaporation
4. Freezing
5. Involve water in a dehydration chemical
reaction
Joining Amino Acids
to Form Proteins
6. On surface of clay particles, which have
charged surfaces, and to which polar
molecules could attach. Metallic ions on clays
could concentrate organic molecules in an
orderly array, causing them to align and link
into protein-like chains.
7. On pyrite, which has a positively charged
surface to which simple organic compounds
can become bonded. Formation of pyrite yields
energy which could be used to link amino acids
into proteins.
Proteinoids
• Proteinoids are protein-like chains
produced in the lab by Fox from a mixture
of amino acids. Considered to be possibly
like the transitional structures leading to
proteins billions of years ago.
• Similar proteinoids are also found in
nature around Hawaiian volcanoes.
Hot aqueous solutions of proteinoids will
cool to form microspheres, tiny spheres
that have many characteristics of living
cells:
– Film-like outer wall
– Capable of osmotic shrinking and swelling
– Budding similar to yeast
– Divide into daughter microspheres
– Aggregate into lines to form filaments, as in
some bacteria
– Streaming movement of internal particles, as
in living cells
Where Did Life Originate?
Early life may have avoided UV radiation by
living:
– Deep beneath the water
– Beneath the surface of rocks (or below
sediment - such as stromatolites)
Life probably began in the sea, perhaps in
areas associated with submarine
hydrothermal vents or black smokers.
Evidence for life beginning in the
sea near hydrothermal vents:
1. Sea contains salts needed for health and
growth.
2. Water is universal solvent, capable of
dissolving organic compounds, producing a
"rich organic broth" or primordial soup.
3. Ocean currents mix these compounds, leading
to collisions between molecules, leading to
combination into larger organic molecules.
Evidence for life beginning in the
sea near hydrothermal vents:
4. Microbes at vents are hyperthermophiles
that thrive in seawater hotter than boiling
point (100oC).
5. These microbes derive energy by
chemosynthesis, without light, rather than
by photosynthesis (suggests origin in deep
water in absence of light).
6. Hyperthermophiles are Archaea, with DNA
different from bacteria.
Feeding Life on Earth –
Obtaining Nutrients
Examples of types of feeding modes:
1. Fermenters - digest chemicals, such as sugar,
in the absence of oxygen, to obtain energy.
Produce CO2 and alcohol. Example: Yeast
2. Autotrophs - manufacture their own food.
Examples: sulfur bacteria, nitrifying bacteria,
and photoautotrophs (such as plants and
photosynthetic bacteria) that use photosynthesis
3. Heterotrophs - can't make their own food, so
they must find nutrients in the environment to
eat. Example: Animals.
Evolution of Early Life
• The earliest cells had to form and exist in anoxic
conditions (in the absence of free oxygen).
Likely to have been anaerobic bacteria or
Archaea.
• Some of the early organisms became
photosynthetic, possibly due to a shortage of
raw materials for energy.
Produced their own raw materials. Autotrophs.
Photosynthesis was an adaptive advantage.
• Oxygen was a WASTE PRODUCT of
photosynthesis.
Consequences of Oxygen Buildup
in the Atmosphere
1. Ozone layer which absorbs harmful UV
radiation, and protected primitive and
vulnerable life forms.
2. End of banded iron formations which only
formed in low, fluctuating O2 conditions
3. Oxidation of iron, leading to the first red beds.
4. Aerobic metabolism developed. Uses oxygen
to convert food into energy.
5. Development of eukaryotic cell, which could
cope with oxygen in the atmosphere.
Prokaryotes vs. Eukaryotes
• Prokaryotes reproduce asexually by simple cell
division. This restricts their genetic variability.
Prokaryotes have shown little evolutionary
change for more than 2 billion years.
• Eukaryotes reproduce sexually through the
union of an egg and sperm. This combines
chromosomes from each parent and leads to
genetic recombination and increased variability.
Many new genetic combinations. Led to a
dramatic increase in the rate of evolution.
Prokaryotes vs. Eukaryotes
Prokaryotes vs. Eukaryotes
The Earliest Eukaryotes
Earliest large cells that appear to be eukaryotes
appear in the fossil record about 1.6 - 1.4 b.y.
ago (in the Proterozoic).
Eukaryotes diversified around the time that the
banded iron formations disappeared and the red
beds appeared, indicating the presence of
oxygen in the atmosphere.
Origin of eukaryotic life was probably around 2.7
b.y., based on molecular fossils.
Endosymbiotic Theory for the
Origin of Eukaryotes
• Billions of years ago, several prokaryotic cells
came together to live symbiotically within a host
cell as protection from (and adaptation to) an
oxygenated environment.
• These prokaryotes became organelles.
• Evidence for this includes the fact that
mitochondria contain their own DNA.
• Example - a host cell (fermentative anaerobe) +
aerobic organelle (mitochondrion) + spirochaetelike organelle (flagellum for motility).
Endosymbiotic
Theory for the
Origin of
Eukaryotes
Eukaryotes
The appearance of eukaryotes led to a
dramatic increase in the rate of evolution,
and was ultimately responsible for the
appearance of complex multicellular
organisms.
Overview
of the
Precambrian