Transcript chapter8

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
• The Precambrian lasted for more than 4 billion
years!
– 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
• 88% of
geologic time
Precambrian
• The term Precambrian is informal
– but widely used, referring to both time and rocks
• The Precambrian includes
– time from Earth’s origin 4.6 billion years ago
– to the beginning of the Phanerozoic Eon
– 542 million years ago
• It encompasses
– 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
• Subdivisions of the Precambrian
– have been difficult to establish
• 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
• Precambrian eons have no stratotypes
– unlike the Cambrian Period, for example
What Happened
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 ubiquitous
– 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 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 consist of rocks
– with composition similar to that of granite
• Continental crust is thicker
– and less dense than oceanic crust
– which is made up 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
– as the continents took their present sizes and shapes
• Both Archean and Proterozoic rocks
–
–
–
–
are present in cratons and show evidence of
episodes of deformation accompanied by
igneous activity, metamorphism,
and mountain building
• 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
– which occupies most of northeastern Canada
– a large part of Greenland
– parts of the Lake Superior region
• in Minnesota, Wisconsin, and Michigan
– and the Adirondack Mountains of New York
• Its topography is subdued,
– with numerous lakes and exposed Archean
– and Proterozoic rocks thinly covered
– in places by Pleistocene glacial deposits
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.
North American Craton
• Drilling and geophysical evidence indicate
– that Precambrian rocks underlie much
– of North America,
– exposed only in places by deep erosion or uplift
Archean Rocks
• Only 22% of Earth’s exposed Precambrian crust is
Archean
• The most common Archean rock associations
– are granite-gneiss complexes
• Other rocks range from peridotite
– to various sedimentary rocks
– all of 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
• Pyroclastic
materials
probably
erupted
– where large
volcanic
centers built
above sea
level
Pillow lavas in Ispheming greenstone belt
at Marquette, Michigan
Ultramafic Lava Flows
• The most interesting rocks
– in greenstone belts are komatiites,
– cooled from ultramafic lava flows
• Ultramafic magma (< 45% silica)
– requires near surface magma temperatures
– of more than 1600°C
– 250°C hotter than any recent flows
• 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
Ultramafic Lava Flows
• As Earth’s production
– of radiogenic heat decreased,
– the mantle cooled
– and ultramafic flows no longer occurred
• They are rare in rocks younger
– than Archean and none occur now
Sedimentary Rocks of
Greenstone Belts
• Sedimentary rocks are found
– throughout the greenstone belts
– although they predominate
– in the upper unit
• 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
• Other sedimentary rocks are present, but not
abundant
– sandstone, conglomerate, chert, carbonates
• Iron-rich rocks, banded iron formations, are more
typical of Proterozoic 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 rising mantle plumes in intracontinental rifts
• As the plume rises beneath sialic crust
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–
–
–
–
–
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
• An episode of subsidence, deformation,
– 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
• Plate tectonic activity has operated
– since the Paleoproterozoic or earlier
• Most geologists are convinced
– that some kind of plate tectonic activity
– took place during the Archean as well
– but it differed in detail from today
• Plates must have moved faster
– with more residual heat from Earth’s origin
– and more radiogenic heat,
– and magma was generated more rapidly
Archean Plate Tectonics
• 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
• Also, ultramafic extrusive igneous rocks,
– komatiites,
– were more common
Archean World Differences
• The Archean world was markedly different
than later
– We have little evidence
of Archean rocks
– deposited on broad,
passive continental
margins
– Deformation belts
between colliding
cratons
– indicate that Archean
plate tectonics was
active
– but associations of
passive continental
margin sediments
– are widespread in
Proterozoic terrains
– but the ophiolites so
typical of younger
convergent plate
boundaries are rare,
– although Neoarchean
ophiolites are known
The Origin of Cratons
• Certainly several small cratons
– existed during the Archean
– and grew by accretion along their margins
• They amalgamated into a larger unit
– during the Proterozoic
• By the end of the Archean,
– 30-40% of the present volume
– of continental crust existed
• Archean crust probably evolved similarly
– to the evolution of the southern Superior craton of
Canada
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
Canadian Shield
• Deformation of the southern Superior craton
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–
–
–
–
was part of a more extensive orogenic episode
during the Mesoarchean and Neoarchean
that formed the Superior and Slave cratons
and some Archean rocks in Wyoming, Montana,
and the Mississippi River Valley
• By the time this Archean event ended
– several cratons had formed that are found
– in the older parts of the Canadian shield
Atmosphere and Hydrosphere
• Earth’s early atmosphere and hydrosphere
– were quite different than they are now
• They also played an important role
– in the development of the biosphere
• Today’s atmosphere is mostly
– nitrogen (N2)
– abundant free oxygen (O2),
• or oxygen not combined with other elements
• such as in carbon dioxide (CO2)
– water vapor (H2O)
– small amounts of other gases, like ozone (O3)
• which is common enough in the upper atmosphere
• to 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
• Earth’s very early atmosphere was probably
composed of
– hydrogen and helium,
• the most abundant gases in the universe
• If so, it would have quickly been lost into space
– because 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
• It was rich in carbon dioxide,
– and gases reacting in this early atmosphere
– probably formed
• ammonia (NH3)
• methane (CH4)
• This early atmosphere persisted
– throughout the Archean
Evidence for an
Oxygen-Free Atmosphere
• The atmosphere was chemically reducing
– rather than an oxidizing one
• Some of the evidence for this conclusion
– comes from detrital deposits
– containing minerals that oxidize rapidly
– in the presence of oxygen
• pyrite (FeS2)
• uraninite (UO2)
• But oxidized iron becomes
– increasingly common in Proterozoic rocks
– indicating that at least some free oxygen
– was present then
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 the early atmosphere
– and also 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
because the amount of 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
• Today, Earth’s biosphere consists
– of millions of species of archea, bacteria, fungi,
– protists, plants, and animals,
– whereas 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
• Reproduction ensures
– the long-term survival of a group of organisms
• whereas metabolism
– maintains the organism
• 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?
• To originate by natural processes,
– from non-living matter (abiogenesis), life must have
passed through a prebiotic stages
– in which 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
– probably promoted chemical reactions
– during which 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
– such as proteins
– and nucleic acids
• RNA (ribonucleic acid) and DNA (deoxyribonucleic acid)
– consisting of monomers linked together in a specific
sequence
• How did polymerization take place?
• Water usually causes depolymerization,
–
–
–
–
–
however, researchers synthesized molecules
known as proteinoids or thermal proteins
some of which consist of
more than 200 linked amino acids
when heating dehydrated concentrated amino acids
Proteinoids
• These concentrated amino acids
– spontaneously polymerized
– to form proteinoids
• Perhaps similar conditions
– for polymerization existed on early Earth,
– but the proteinoids needed to be protected
– by an outer membrane or they would break down
• 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
• The origin-of-life experiments are interesting,
– but what is their relationship to early Earth?
• 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
• Not much is known about the next critical step
– in the origin of life
• the development of a reproductive mechanism
• The microspheres divide
– and may represent a protoliving system
– but in today’s cells, nucleic acids,
• either RNA or DNA
– are necessary for reproduction
• 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
RNA World?
• Now we know that small RNA molecules
– can replicate without the aid of protein enzymes
• Thus, the first replicating systems
– may have been RNA molecules
• Some researchers propose
– an early “RNA world”
– in which these molecules were intermediate between
• inorganic chemical compounds
• and the DNA-based molecules of organisms
• How RNA was naturally synthesized
– remains an unsolved problem
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
although now they are restricted
to environments where snails cannot live
• The oldest known undisputed stromatolites
–
–
–
–
–
are found in rocks in South Africa
that are 3.0 billion years old
but probable ones are also known
from the Warrawoona Group in Australia
which is 3.3 to 3.5 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
– but photosynthesis is a complex metabolic process
• 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
• Their nutrient source was most likely
–
–
–
–
adenosine triphosphate (ATP)
from their environment
which was used to drive
the energy-requiring reactions in cells
• ATP can easily be synthesized
– from simple gases and phosphate
– so it was available
– in the early Earth environment
Fermentation
• Obtaining ATP from the surroundings
– could not have persisted for long
– because more and more cells competed
– for the same resources
• The first organisms to develop
– a more sophisticated metabolism
– probably used fermentation
– to meet their energy needs
• Fermentation is an anaerobic process
– in which molecules such as sugars are split
– releasing carbon dioxide, alcohol, and energy
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,
– although it is also found
– in Proterozoic and Phanerozoic rocks
• This soft yellow metal is prized for jewelry,
– but it is or has been used as a monetary standard,
– in 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
• These ore deposits are found in
–
–
–
–
–
the volcanic units of greenstone belts
where they appear to have formed
when crystals settled and became concentrated
in the lower parts of plutons
such as mafic and ultramafic sills
• Chrome is needed in the steel industry
• The United States has very few chrome deposits
– so must import most of what it uses
Chrome and Platinum
• One chrome deposit in the United States
– is in the Stillwater Complex in Montana
• Low-grade ores were mined there during war
times,
– but they were simply stockpiled
– and never refined for chrome
• These rocks also contain platinum,
– a precious metal, that is used
• in the automotive industry in catalytic converters
• in the chemical industry
• for cancer chemotherapy
Iron
• Banded Iron formations are sedimentary rocks
– consisting of alternating layers
– of silica (chert) and iron minerals
• About 6% of the world’s
– banded iron formations were deposited
– during the Archean Eon
• Although Archean iron ores
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are mined in some areas
they are neither as thick
nor as extensive as those of the Proterozoic Eon,
which constitute the world’s major source of iron
Pegmatites
• Pegmatites are very coarsely crystalline igneous
rocks,
– commonly associated with granite plutons
• Some Archean pegmatites,
– such in the Herb Lake district in Manitoba, Canada,
– and Rhodesian Province in Africa,
– contain valuable minerals
• In addition to minerals of gem quality,
– Archean pegmatites contain minerals mined
– for lithium, beryllium, rubidium, and cesium
Summary
• Precambrian encompasses all geologic time
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from Earth’s origin
to the beginning of the Phanerozoic Eon
The term also refers to all rocks
that lie stratigraphically below Cambrian rocks
• The Precambrian is divided into two eons
– the Archean and the Proterozoic,
– which are further subdivided
• Rocks from the latter part of the Eoarchean
indicate crust must have existed,
– but very little of it has been preserved
Summary
• All continents have an ancient stable nucleus
– or craton made up of
• an exposed shield
• and a buried platform
• The exposed part of the North American
craton
– is the Canadian shield,
– and is make up of smaller units
– delineated by their ages and structural trends
• Archean greenstone belts are linear,
– syncline-like bodies found within
– much more extensive granite-gneiss complexes
Summary
• Greenstone belts typically consist of
– two lower units dominated by igneous rocks
– and an upper unit of mostly sedimentary rocks
• They probably formed in back-arc basins
– and in intracontinental rifts
• Many geologists are convinced
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some type of Archean plate tectonics occurred,
but plates probably moved faster
and igneous activity was more common
because Earth had more radiogenic heat
Summary
• The early atmosphere and hydrosphere
– formed as a result of outgassing,
– but this atmosphere lacked free oxygen and
– contained abundant water vapor and carbon dioxide
• Models for the origin of life by natural
processes require
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–
an oxygen deficient atmosphere,
the necessary elements for organic molecules,
and energy to promote the synthesis
of organic molecules
Summary
• The first naturally formed organic molecules
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were probably monomers,
such as amino acids,
that linked together to form
more complex polymers such as proteins
• RNA molecules may have been
– the first molecules capable of self-replication
– However, how a reproductive mechanism evolved
is not known
Summary
• The only known Archean fossils
– are of single-celled, prokaryotic bacteria or
cyanobacteria
– but other chemical evidence may indicate presence
of archaea
• Stromatolites formed by photosynthesizing
bacteria
– are found in rocks as much as 3.5 billion years old
• Archean mineral resources include gold,
chrome, zinc, copper, and nickel