Transcript No Slide Title

Chapter 8
Precambrian Earth and Life
History—The Hadean and
Archean
Archean
Rocks
• The Teton
Range
– is largely
Archean
– gneiss, schist,
and granite
– Younger rocks
are also
present
– but not visible
Grand Teton National Park,
Wyoming
Precambrian 4 Billion Years
• The Precambrian lasted for more than 4 billion
years!
– Such a time span is difficult for humans to
comprehend
Precambrian Time Span
• 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
– 545 million years ago
• No rocks are known for the first
– 640 million years of geologic time
Rocks Difficult to Interpret
• Precambrian rocks have been
•
•
•
•
•
altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare
the few fossils present are of little use in stratigraphy
• Eon Subdivisions
Archean and Proterozoic
Eons of the Precambrian
• The Archean Eon
– Start coincides with the age of Earth’s oldest
known rocks
• ~ 4 billion years old
– lasted until 2.5 billion years ago
– the beginning of the Proterozoic Eon
• Hadean is an informal designation
– for time preceding the Archean Eon
What Happened
During the Hadean?
– Earth accreted from planetesimals
– differentiated into a core and mantle
• and at least some crust
–
–
–
–
was bombarded by meteorites
ubiquitous volcanism
atmosphere formed
Ocean waters 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 comets and meteorites
with no continents, intense cosmic radiation
and widespread volcanism
Oldest Rocks
3.96-billion-year-old Acasta Gneiss in Canada
and other rocks in Montana
Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.2 billion years old
– so continental source rocks at least that old existed
during the Hadean
Hadean Crust
• Early Hadean crust was probably thin, unstable
– and made up of ultramafic rock
• those with comparatively little silica
• This ultramafic crust was disrupted
– by upwelling basaltic magma at ridges
– and consumed at subduction zones
• Hadean continental crust may have formed
–
–
–
–
–
by evolution of sialic material
Sialic crust contains considerable silicon, oxygen
and aluminum as in present day continental crust
Only sialic-rich crust, because of its lower density,
is immune to destruction by subduction
Second Crustal Evolution Stage
• Subduction and partial melting
– of earlier-formed basaltic crust
– resulted in the origin of andesitic island arcs
• Partial melting of lower crustal andesites,
– in turn, yielded silica-rich granitic magmas
– that were emplaced in the andesitic arcs
Second Crustal Evolution Stage
• Several sialic continental nuclei
– had formed by the beginning of Archean time
– by subduction and collisions
– between island arcs
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 platform make up a craton,
– a continent’s ancient nucleus and its foundations
• 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
metamorphism, igneous activity
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 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
Canadian Shield Rocks
• Gneiss, a metamorphic rock, Georgian Bay
Ontario, Canada
Canadian Shield Rocks
• Basalt (dark, volcanic) and granite (light, plutonic)
on the Chippewa River, Ontario
Amalgamated Cratons
• Actually the Canadian shield and adjacent
platform
– is made up of numerous units or smaller cratons
– that amalgamated along deformation belts
– during the Early Proterozoic
• Absolute ages and structural trends
– help geologists differentiate
– among these various smaller cratons
• Drilling and geophysical evidence indicate
– that Precambrian rocks underlie much
– of North America,
– in places exposed by deep erosion or uplift
Archean Rocks Beyond the Shield
Rocky
Mountains,
Colorado
• Archean metamorphic rocks found
– in areas of uplift in the Rocky Mtns
Archean Rocks Beyond the Shield
• Archean Brahma Schist in the deeply eroded
parts of the Grand Canyon, Arizona
Archean Rocks
• The most common Archean Rock associations
– are granite-gneiss complexes
• The rocks vary from granite to peridotite
– to various sedimentary rocks
– all of which have been metamorphosed
• Greenstone belts are subordinate in quantity
– but are important in unraveling Archean tectonism
Greenstone Belts
• An ideal 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
– greenish (chlorite)
Greenstone Belt Volcanics
• Abundant pillow lavas in greenstone belts
– indicate that much of the volcanism was
– under water, probably at or near a spreading ridge
• Pyroclastic
materials
probably
erupted
– where large
volcanic
centers built
above sea
level
Pillow lavas in Ispheming greenstone at
Marquette, Michigan
Ultramafic Lava Flows
• The most interesting rocks
– in greenstone belts cooled
– from ultramafic lava flows
• Ultramafic magma has less than 45% silica
– and 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 higher
– and the mantle was as much as 300 °C hotter
• 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
• a sandstone with abundant clay and rock
fragments
– and argillite
• a slightly metamorphosed mudrock
Sedimentary Rocks of
Greenstone Belts
• Small-scale cross-bedding and
– graded bedding indicate an origin
– as turbidity current deposits
• Quartz sandstone and shale,
– indicate delta, tidal-flat,
– barrier-island and shallow marine
deposition
Relationship of Greenstone Belts
to Granite-Gneiss Complexes
• Two adjacent
greenstone
belts showing
synclinal
structure
• They are
underlain by
granite-gneiss
complexes
• and intruded
by granite
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
• Models for the formation of greenstone belts
– involve Archean plate movement
• In one model, plates formed volcanic arcs
– by subduction
– and the greenstone
belts formed
– in back-arc
marginal basins
Evolution of Greenstone Belts
• According to this model,
– volcanism and sediment deposition
– took place as
the basins
opened
Evolution of Greenstone Belts
• Then during closure,
– the rocks were compressed, deformed,
– cut by faults,
– and intruded by
rising magma
• The Sea of
Japan
– is a modern
example
– of a back-arc
basin
Archean Plate Tectonics
• Most geologists are convinced
– that some kind of plate tectonics
– took place during the Archean
• BUT, Plates must have moved faster
– with more residual heat from Earth’s origin
– and more radiogenic heat,
– Thus, magma was generated more rapidly
Archean Plate Tectonics
• As a result of the rapid movement of plates,
–
–
–
–
continents, no doubt,
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
– were more common
– due to the higher temperatures
Archean World Differences
• The Archean world was markedly different
than later
– We have little evidence
of Archean rocks
deposited on broad,
passive continental
margins
– but associations of
passive continental
margin sediments
are widespread in
Proterozoic terrains
– Deformation belts
between colliding
cratons indicate that
Archean plate
tectonics was active
– but the ophiolites so
typical of younger
convergent plate
boundaries are rare,
– although Late Archean
ophiolites are known
The Origin of Cratons
• Certainly several small cratons
– existed by the beginning of the Archean
– and grew by periodic continental accretion
• 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
• This deformation was
– the last major Late Archean event in North America
– and resulted in the formation of several sizable
cratons now in the older parts of the Canadian shield
Present-day
Atmosphere Composition
• Variable gases
• Nonvariable gases
Water vapor H2O
Nitrogen N2 78.08% Carbon dioxide CO
2
Oxygen O2 20.95 Ozone
O3
Argon
Ar
0.93 Other gases
Neon
Ne
0.002
• Particulates
Others
0.001 normally trace
in percentage by volume
0.1 to 4.0
0.034
0.0006
Trace
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
• Wthout a magnetic field,
– the solar wind would have swept away
– any atmospheric gases
Outgassing
• Once a core-generated
magnetic field
– protected the gases released
during volcanism
• called outgassing
– they began to accumulate to
form a new atmosphere
• Water vapor
– is the most common
volcanic gas today
– but volcanoes also emit
– carbon dioxide, sulfur
dioxide,
– carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen
Hadean-Archean Atmosphere
• Hadean 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 Hadean
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 organism
that practiced photosynthesis
Photosynthesis
• Photosynthesis is a metabolic process
– in which carbon dioxide and water
– combine into 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 Earth’s surface water
• the hydrosphere
– most of which is in the oceans
• more than 97%
• However, some but probably not much
– of our surface water was derived from icy comets
• Probably at some time during the Hadean,
– the Earth had cooled sufficiently
– so that the abundant volcanic water vapor
– condensed and began to accumulate in oceans
• Oceans were present by Early Archean times
Ocean water
• The volume and geographic extent
– of the Early Archean oceans cannot be determined
• Nevertheless, we can envision an early Earth
– with considerable volcanism
– and a rapid accumulation of surface waters
• 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
• Much of volcanic water vapor today
– is recycled surface water
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 4 to
6 times as great
as it is now
First Organisms
• Today, Earth’s biosphere consists
– of millions of species of bacteria, fungi,
– protistans, plants, and animals,
– whereas only bacteria are found in Archean rocks
• We have fossils from Archean rocks
– 3.3 to 3.5 billion years old
• Carbon isotope ratios in rocks in Greenland
– that are 3.85 billion years old
– convince some investigators that life was present
then
What Is Life?
• Minimally, a living organism must reproduce
– and practice some kind of metabolism
• Reproduction insures
– the long-term survival of a group of organisms
• whereas metabolism
– such as photosynthesis, for instance
– insures the short-term survival of an individual
• 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
– show greater
organizational
complexity
– than inorganic objects
such as rocks
– 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,
– life must have passed through a prebiotic stage
• in which it showed signs of living organisms
• but was not truly living
• In 1924, the great Russian biochemist,
– A.I. Oparin, postulated that life originated
– when Earth’s atmosphere had little or no free oxygen
• Oxygen is damaging to Earth’s
– most primitive living organisms
• Some types of bacteria must live
– where free oxygen is not present
How Did Life First Originate?
• With little or no oxygen in the early
atmosphere
– and no ozone layer to block ultraviolet radiation,
– life could have come into existence from nonliving
matter
• 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
• and ultraviolet radiation
– probably promoted chemical reactions
– during which C, H, N and O combined
– to form monomers
• comparatively simple organic molecules
• 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
some of which consist of
more than 200 linked amino acids
when heating dehydrated concentrated amino acids
Proteinoids
• The heated dehydrated 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
• Protobionts are intermediate between
– inorganic chemical compounds
– and living organisms
• Because of their life-like properties
– the proteinoid molecules can be referred to
– as protobionts
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” (J.B.S. Haldane, British biochemist)
• 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 and unsolved problem
Much Remains to Be Learned
• The origin of life has not been fully solved
–
–
–
–
but considering the complexity of the problem
and the fact that scientists have been experimenting
for only about 50 years
remarkable progress has been made
• Debate continues
• 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
Azoic (“without life”)
• Prior to the mid-1950s, scientists
– had little knowledge of Precambrian life
• They assumed that life of the Cambrian
– must have had a long early history
– but the fossil record offered little
– to support this idea
• A few enigmatic Precambrian fossils
– had been reported but most were dismissed
– as inorganic structures of one kind or another
• The Precambrian, once called Azoic
– (“without life”), seemed devoid of life
Oldest Know Organisms
• Charles Walcott (early 1900s) described structures
– from the Early Proterozoic 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 blue-green algae (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
• Carbon isotopes 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
– meaning they lacked a cell nucleus
– and lacked other internal cell structures typical of
eukaryotic cells (to be discussed later in the term)
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 doubtless 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
which is used by most living prokaryotic cells
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
• These anaerobic, autotrophic prokaryotes
– belong to the Kingdom Monera,
– represented today by bacteria and cyanobacteria
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 Archean
– but gold is the most notably Archean,
– 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 various 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
time,
– 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
–
–
–
–
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,
– composed of quartz and feldspars
• Some Archean pegmatites,
– such in the Herb Lake district in Manitoba, Canada,
– and Zambia 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
–
–
–
–
from Earth’s origin
to the beginning of the Phanerozoic Eon
The term also refers to all rocks
that lie stratigraphically below Cambrian rocks
• Terms for Precambrian time include
– an informal one, the Hadean,
– followed by two eons, the Archean and Proterozoic
• Some Hadean crust must have existed,
– but none of it has been preserved
• By the beginning of the Archean Eon,
– several small continental nuclei were present
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 by plate movements
– responsible for opening
– and then closing back-arc marginal basins
• Widespread deformation took place
– during the Late Archean
– as parts of the Canadian shield evolved
Summary
• Many geologists are convinced
– some type of Archean plate tectonics occurred,
– but it probably differed
– from the tectonic style of the present
• For one thing, Earth had more heat
– and for another, plates probably moved faster
• 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
Summary
• Models for the origin of life
–
–
–
–
–
by natural processes require
an oxygen deficient atmosphere,
the appropriate elements for organic molecules,
and energy to promote the synthesis
of organic molecules
• The first naturally formed organic molecules
–
–
–
–
were probably monomers,
such as amino acids,
that linked together to form
more complex polymers such as proteins
Summary
• RNA molecules may have been
– the first molecules capable of self-replication
– However, how a reproductive mechanism evolved is
not known
• The only known Archean fossils
– are of single-celled, prokaryotic bacteria
– or blue-green algae (cyanobacteria)
• Stromatolites formed by photosynthesizing
bacteria
– are found in rocks as much as 3.5 billion years old
• Carbon isotopes indicate
– life was present even earlier
Summary
• The most important
– Archean mineral resources are
– gold, chrome, zinc, copper, and nickel