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Chapter 8
Precambrian Earth and Life History—
The Archean Eon
• Archean Rocks
The Beartooth
Mountains
on the Wyoming
and Montana
border
consists of
Archaean-age
gneisses,
some of the oldest
rocks in the US.
Precambrian
– This large time span is difficult for humans to
comprehend
• Suppose that a 24-hour clock represented
–
–
–
–
all 4.6 billion years of geologic time
then the Precambrian would be
slightly more than 21 hours long,
constituting about 88% of all geologic time
Precambrian Time Span
The Precambrian lasted
for more than 4 billion
years!
If Earth’s history were a
24 hour clock, the
Precambrian would use
21 hours!
• 88% of
geologic time
Precambrian
• The Precambrian includes
–
–
–
–
time from Earth’s origin 4.6 billion years ago
to the beginning of the Phanerozoic Eon
542 million years ago
all rocks below the Cambrian system
• No rocks are known for the first
– 600 million years of geologic time
– The oldest known rocks on Earth
– Are ~ 4.0 billion years old
Rocks Difficult to Interpret
• The earliest record of geologic time
– preserved in rocks is difficult to interpret
– because many Precambrian rocks have been
•
•
•
•
•
altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare, and
the few fossils present are not of any use in biostratigraphy
• Two eons for the Precambrian
– are the Archean and Proterozoic
– which are based on absolute ages
Eons of the Precambrian
• Eoarchean refers to all time
– from Earth’s origin to the Paleoarchean
– 3.6 billion years ago
• Earth’s oldest body of rocks
– the Acasta Gneiss in Canada
– is about 4.0 billion years old*
• We have no geologic record
– for much of the Archaen
What Happenedwas Earth like
During the Eoarchean?
• Although no rocks of Eoarchean age are
present on Earth,
– except for meteorites,
• we do know some events that took place then
– Earth accreted from planetesimals
– and differentiated into a core and mantle
• and at least some crust was present
– Earth was bombarded by meteorites
– Volcanic activity was occurring globally
– An atmosphere formed, quite different from
today’s
– Oceans began to accumulate
Hot, Barren, Waterless Early Earth
• about 4.6 billion years ago
• Shortly after accretion, Earth was
–
–
–
–
a rapidly rotating, hot, barren, waterless planet
bombarded by meteorites and comets
with no continents, intense cosmic radiation
and widespread volcanism
Oldest Rocks
• Continental crust was present by 4.0 billion
years ago
– Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.4 billion years old
– so source rocks at least that old existed
• The Eoarchean Earth probably rotated in as
little as 10 hours
– and the Earth was closer to the Moon
• By 4.4 billion years ago, the Earth cooled
sufficiently for surface waters to accumulate
Eoarchean Crust
• Early crust formed as upwelling mantle currents
– of mafic and ultramafic magma,
– and numerous subduction zones developed
– to form the first island arcs
• Eoarchean continental crust may have formed
– by collisions between island arcs
– as silica-rich materials were metamorphosed.
– Larger groups of merged island arcs
• protocontinents
– grew faster by accretion along their margins
Origin of Continental Crust
• Andesitic
island arcs
– form by
subduction
– and partial
melting of
oceanic
crust
• The island
arc collides
with another
Continental Foundations
• Continents have composition similar to that of
granite
• Continental crust is thicker
Ocean crust is more dense
– Composition of basalt and gabbro
• Precambrian shields
– consist of vast areas of exposed ancient rocks
– and are found on all continents
• Outward from the shields are broad platforms
– of buried Precambrian rocks
– that underlie much of each continent
Cratons
• A shield and its platform make up a craton,
– a continent’s ancient nucleus
• Along the margins of cratons,
– more continental crust was added
• Cratons have experienced little deformation
– since the Precambrian
Distribution of Precambrian Rocks
• Areas of
exposed
– Precambrian rocks
– constitute
the shields
• Platforms
consist of
– buried Precambrian
rocks
– Shields and adjoining platforms make up cratons
Canadian Shield
• The exposed part of the craton in North
America is the Canadian shield
– northeastern Canada
– large part of Greenland
– parts of the Lake Superior region
• in Minnesota, Wisconsin, and Michigan
– and the Adirondack Mountains of New York
Evolution of North America
• North America evolved by the amalgamation of
Archean cratons that served as a nucleus around
which younger continental crust was added.
Archean Rocks
• Archean rock associations
– are granite-gneiss complexes
• Other rocks range from ultramafic peridotite
– To sedimentary rocks
– which have been metamorphosed
• Greenstone belts are subordinate in quantity,
– account for only 10% of Archean rocks
– but are important in unraveling Archean tectonic events
Archean Rocks
• Outcrop of Archean gneiss cut by a granite dike from
a granite-gneiss complex in Ontario, Canada
Archean Rocks
• Shell Creek in the Bighorn Mountains of Wyoming
has cut a gorge into this 2.9 billion year old granite
Greenstone Belts
• A greenstone belt has 3 major rock units
– volcanic rocks are most common
– in the lower and middle units
– the upper units are mostly sedimentary
• The belts typically have synclinal structure
– Most were intruded by granitic magma
– and cut by thrust faults
• Low-grade metamorphism
– makes many of the igneous rocks green
– Because they contain chlorite, actinolite, and epidote
Greenstone Belts and GraniteGneiss Complexes
• Two adjacent
greenstone
belts showing
synclinal
structure
• They are
underlain by
granite-gneiss
complexes
• and intruded
by granite
Greenstone Belt Volcanics
• Pillow lavas in greenstone belts
– indicate that much of the volcanism was
– subaqueous
Pillow lavas in Ispheming greenstone belt
at Marquette, Michigan
Ultramafic Lava Flows
• Ultramafic magma (< 45% silica)
– requires near surface magma temperatures
– of more than 1600°C
– 250°C hotter than any recent flows
• This is evidence that• During Earth’s early history,
– radiogenic heating was greater
– and the mantle was as much as 300 °C hotter
– than it is now
• This allowed ultramafic magma
– to reach the surface
• Earth cooled: They are rare in rocks younger
– than Archean and none occur now
Ultramafic Lava Flows
• As Earth’s production
– of radiogenic heat decreased,
– the mantle cooled
– and ultramafic flows no longer occurred
Sedimentary Rocks of
Greenstone Belts
• Many of these rocks are successions of
– graywacke
• sandstone with abundant clay and rock fragments
– and argillite
• slightly metamorphosed mudrock
Sedimentary Rocks of
Greenstone Belts
• Small-scale cross-bedding and graded bedding
– indicate an origin as turbidity current deposits
Canadian Greenstone Belts
• In North
America,
– most
greenstone
belts
– (dark green)
– occur in the
Superior and
Slave cratons
– of the
Canadian
shield
Evolution of Greenstone Belts
• Greenstone belts formed in several tectonic settings
• Models for the formation of greenstone belts
– involve Archean plate movement
• In one model, greenstone
belts formed
– in back-arc marginal
basins
Evolution of Greenstone Belts
• According to this model,
– There was an early stage of extension as the backarc marginal basin formed
– volcanism and
sediment
deposition
followed
Evolution of Greenstone Belts
• Then during closure,
– the rocks were compressed,
– metamorphosed,
– and intruded by
granitic magma
• The Sea of
Japan
– is a modern
example
– of a back-arc
basin
Another Model
• In another model accepted by some geologists,
– greenstone belts formed over a hot spot ==
– in intracontinental rifts
• As the plume rises beneath continental crust
–
–
–
–
–
–
it spreads and generates tensional forces
The mantle plume is the source
of the volcanic rocks in the lower and middle units
of the greenstone belt
and erosion of volcanic rocks and flanks for the rift
supply the sediment to the upper unit
• ,
metamorphism and plutonism followed
Greenstone Belts—Intracontinental
Rift Model
• Ascending mantle
plume
– causes rifting
– and volcanism
Greenstone Belts—Intracontinental
Rift Model
• Erosion of the
rift flanks
– accounts for
sediments
Greenstone Belts—Intracontinental
Rift Model
• Closure of rift
– causes
compression
– and
deformation
Archean Plate Tectonics
• Plates must have moved faster
– with more residual heat from Earth’s origin
– and more radiogenic heat,
– and magma was generated more rapidly
• As a result of the rapid movement of plates,
– continents grew more rapidly along their margins
– a process called continental accretion
– as plates collided with island arcs and other plates
Southern Superior Craton Evolution
Geologic map
• Plate tectonic model
for evolution of the
southern Superior
craton
• North-south cross
section
• Greenstone belts
(dark green)
• Granite-gneiss
complexes (light
green
Atmosphere and Hydrosphere
• Earth’s early atmosphere and hydrosphere
– were quite different than they are now
• Today’s atmosphere is mostly
– nitrogen (N2)
– free oxygen (O2),
• or oxygen not combined with other elements
• such as in carbon dioxide (CO2)
– water vapor (H2O)
– other gases, like ozone (O3)
• in the upper atmosphere block most of the Sun’s
ultraviolet radiation
Present-day
Atmosphere Composition
• Variable gases
• Nonvariable gases
Water vapor H2O 0.1 to 4.0
Nitrogen N2 78.08% Carbon dioxide CO 0.038
2
Oxygen O2 20.95 Ozone
O3
0.000006
Trace
Argon
Ar
0.93 Other gases
Neon
Ne
0.002
• Particulates
Others
0.001
normally trace
in percentage by volume
Earth’s Very Early Atmosphere
• composed of
– hydrogen and helium,
• the most abundant gases in the universe
• Earth’s gravity is insufficient to retain them
– because Earth had no magnetic field until its
core formed (magnetosphere)
• Without a magnetic field,
– the solar wind would have swept away
– any atmospheric gases
Outgassing
• Once a magnetosphere
was present
– Atmosphere began
accumulating as a result
of outgassing
– released during volcanism
• Water vapor
– is the most common
volcanic gas today
– but volcanoes also emit
– carbon dioxide, sulfur
dioxide,
– carbon monoxide, sulfur,
– hydrogen, chlorine, and nitrogen
Archean Atmosphere
• Archean volcanoes probably
– emitted the same gases,
– and thus an atmosphere developed
– but one lacking free oxygen and an ozone layer
• CO2 ammonia (NH3) methane (CH4)
Evidence for an
Oxygen-Free Atmosphere
• detrital deposits
– containing minerals that oxidize rapidly
– in the presence of oxygen
– These minerals are NOT bound to typical
abundances of oxygen
• pyrite (FeS2)
• uraninite (UO2)
Introduction of Free Oxygen
• Two processes account for
– introducing free oxygen into the atmosphere,
• one or both of which began during the Eoarchean.
1. Photochemical dissociation involves ultraviolet
radiation in the upper atmosphere
• The radiation disrupts water molecules and releases their
oxygen and hydrogen
• This could account for 2% of present-day oxygen
• but with 2% oxygen, ozone forms, creating a barrier
against ultraviolet radiation
2. More important were the activities of organisms
that practiced photosynthesis
Photosynthesis
• Photosynthesis is a metabolic process
– in which carbon dioxide and water
– to make organic molecules
– and oxygen is released as a waste product
CO2 + H2O ==> organic compounds + O2
• Even with photochemical dissociation
– and photosynthesis,
– probably no more than 1% of the free oxygen level
– of today was present by the end of the Archean
Oxygen Forming Processes
• Photochemical dissociation and photosynthesis
– added free oxygen to the atmosphere
– Once free
oxygen was
present
– an ozone
layer formed
– and blocked
incoming
ultraviolet
radiation
Earth’s Surface Waters
• Outgassing was responsible
– for some of Earth’s surface water
• the hydrosphere
– most of which is in the oceans
• more than 97%
• Another source of our surface water
– was meteorites and icy comets
• Numerous erupting volcanoes,
– and an early episode of intense meteorite and comet
bombardment
• accounted for rapid rate of surface water accumulation
Ocean Water
• Volcanoes still erupt and release water vapor
– Is the volume of ocean water still increasing?
– Perhaps it is, but if so, the rate has decreased
considerably
– -- heat needed to generate magma has diminished
Decreasing Heat
• Ratio of radiogenic heat production in the past
to the present
– The width of
the colored
band
indicates
variations in
ratios from
different
models
• With less heat
outgassing
decreased
• Heat production
4 billion years
ago was 3 to
6 times as great
as it is now
First Organisms
• Only bacteria and archea are found in Archean rocks
• We have fossils from Archean rocks
– 3.5 billion years old
• Chemical evidence in rocks in Greenland
– that are 3.8 billion years old
– convince some investigators that organisms were present
then
What Is Life?
• Minimally, a living organism must reproduce
– and practice some kind of metabolism
• The distinction between
– living and nonliving things is not always easy
• Are viruses living?
– When in a host cell they behave like living
organisms
– but outside they neither reproduce nor metabolize
What Is Life?
• Comparatively simple organic (carbon based)
molecules known as microspheres
– form spontaneously
– can even grow and
divide in a somewhat
organism-like fashion
– but their processes are
more like random
chemical reactions, so
they are not living
How Did Life First Originate?
– from non-living matter (abiogenesis), life must have
passed through a prebiotic stages
– it showed signs of living
– but was not truly living
• The origin of life has 2 requirements
– a source of appropriate elements for organic
molecules
– energy sources to promote chemical reactions
Elements of Life
• All organisms are composed mostly of
–
–
–
–
carbon (C)
hydrogen (H)
nitrogen (N)
oxygen (O)
• all of which were present in Earth’s early
atmosphere as
–
–
–
–
–
carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)
Basic Building Blocks of Life
• Energy from
• Lightning, volcanism,
• and ultraviolet radiation
– C, H, N, and O combined to form monomers
• such as amino acids
• Monomers are the basic building blocks
– of more complex organic molecules
Experiment on the Origin of Life
• Is it plausible that monomers
– originated in the manner postulated?
– Experimental evidence indicates that it is
• During the late 1950s
– Stanley Miller
– synthesized several
amino acids
– by circulating gases
approximating
– the early atmosphere
– in a closed glass
vessel
Experiment on the Origin of Life
• This mixture was subjected to an electric spark
– to simulate lightning
• In a few days
– it became cloudy
• Analysis showed that
– several amino acids
– typical of organisms
– had formed
• Since then,
– scientists have
synthesized
– all 20 amino acids
– found in organisms
Polymerization
• The molecules of organisms are polymers
• consisting of monomers linked together in a specific
sequence
• RNA (ribonucleic acid) and DNA (deoxyribonucleic acid)
• How did polymerization take place?
• Water usually causes depolymerization,
– however, researchers synthesized molecules
– known as proteinoids when heating dehydrated
concentrated amino acids
Proteinoids
• These concentrated amino acids spontaneously
polymerized to form proteinoids
• Experiments show that proteinoids
– spontaneously aggregate into microspheres
– which are bounded by cell-like membranes
– and grow and divide much as bacteria do
Proteinoid Microspheres
• Proteinoid
microspheres
produced in
experiments
• Proteinoids grow
and divide much as
bacteria do
Protobionts
• These proteinoid molecules can be referred to
as protobionts
– that are intermediate between
– inorganic chemical compounds
– and living organisms
Monomer and Proteinoid Soup:
Model for abiogenesis
• Monomers likely formed continuously and by
the billions
– and accumulated in the early oceans into a “hot,
dilute soup”
– The amino acids in the “soup”
– might have washed up onto a beach or perhaps
cinder cones
– where they were concentrated by evaporation
– and polymerized by heat
• The polymers then washed back into the ocean
– where they reacted further
Next Critical Step
• The microspheres divide
– and may represent a protoliving system
– but in today’s cells, nucleic acids,
• either RNA or DNA
– are necessary for reproduction Dead End???
• The problem is that nucleic acids
–
–
–
–
cannot replicate without protein enzymes,
and the appropriate enzymes
cannot be made without nucleic acids,
or so it seemed until fairly recently
Much Remains to Be Learned
• Scientists agree on some basic requirements
– for the origin of life,
– but the exact steps involved
– and significance of results are debated
• Many researchers believe that
– the earliest organic molecules were synthesized
from atmospheric gases
– but some scientist suggest that life arose instead
– near hydrothermal vents on the seafloor
Submarine Hydrothermal Vents
• Seawater seeps into the crust near spreading
ridges, becomes heated, rises and discharges
• Black smokers
– Discharge water
saturated with
dissolved
minerals
– Life may have
formed near these
in the past
Submarine Hydrothermal Vents
• Several minerals containing zinc, copper, and iron
precipitate around them
• Communities of organisms
– previously unknown to
science, are supported here.
– Necessary elements, sulfur,
and phosphorus are present in
seawater
– Polymerization can take
place on surface of clay
minerals
– Protocells were deposited on
the ocean floor
Oldest Known Organisms
• The first organisms were archaea and bacteria
– both of which consist of prokaryotic cells,
– cells that lack an internal, membrane-bounded
nucleus and other structures
• Prior to the 1950s, scientists assumed that life
– must have had a long early history
– but the fossil record offered little to support this idea
• The Precambrian, once called Azoic
– (“without life”), seemed devoid of life
Oldest Know Organisms
• Charles Walcott (early 1900s) described structures
– from the Paleoproterozoic Gunflint Iron Formation of Ontario,
Canada
– that he proposed represented reefs constructed by
algae
• Now called
stromatolites,
– not until 1954
were they
shown
– to be products
of organic
activity
Present-day stromatolites (Shark Bay, Australia)
Stromatolites
• Different types of stromatolites include
– irregular mats, columns, and columns linked by mats
Stromatolites
• Present-day stromatolites form and grow
– as sediment grains are trapped
– on sticky mats
– of photosynthesizing cyanobacteria
• The oldest known undisputed stromatolites
– are found in rocks in South Africa
– that are 3.0 billion years old
Other Evidence of Early Life
• Chemical evidence in rocks 3.85 billion years old
– in Greenland indicate life was perhaps present then
• The oldest known cyanobacteria
– were photosynthesizing organisms: COMPLEX!!
• A simpler type of metabolism
– must have preceded it
• No fossils are known of these earliest organisms
Earliest Organisms
• The earliest organisms must have resembled
– tiny anaerobic bacteria
– meaning they required no oxygen
• They must have totally depended
– on an external source of nutrients
– that is, they were heterotrophic
– as opposed to autotrophic organisms
• that make their own nutrients, as in photosynthesis
• They all had prokaryotic cells
Earliest Organisms
• The earliest organisms, then,
– were anaerobic, heterotrophic prokaryotes
Photosynthesis
• A very important biological event
– occurring in the Archean
– was the development of
– the autotrophic process of photosynthesis
• This may have happened
– as much as 3.5 billion years ago
• These prokaryotic cells were still anaerobic,
– but as autotrophs they were no longer dependent
– on preformed organic molecules
– as a source of nutrients
Fossil Prokaryotes
• Photomicrographs from western Australia’s
– 3.3- to 3.5-billion-year-old Warrawoona Group,
– with schematic restoration shown at the right of each
Archean Mineral Resources
• A variety of mineral deposits are of Archean-age
– but gold is the most commonly associated,
• jewelry,
– monetary standard,
– ` glass making, electric circuitry, and chemical industry
• About half the world’s gold since 1886
– has come from Archean and Proterozoic rocks
– in South Africa
• Gold mines also exist in Archean rocks
– of the Superior craton in Canada
Archean Sulfide Deposits
• Archean sulfide deposits of
• zinc,
• copper
• and nickel
– occur in Australia, Zimbabwe,
– and in the Abitibi greenstone belt
– in Ontario, Canada
• Some, at least, formed as mineral deposits
– next to hydrothermal vents on the seafloor,
– much as they do now around black smokers
Chrome
• About 1/4 of Earth’s chrome reserves
– are in Archean rocks, especially in Zimbabwe
• Chrome is needed in the steel industry
• The United States has very few chrome deposits
– so must import most of what it uses
Pegmatites
• Pegmatites are very coarsely crystalline igneous
rocks,
– Archean pegmatites contain minerals mined
– for lithium, beryllium, rubidium, and cesium