Title: Precambrian Earth and Life History
1Chapter 8
Precambrian Earth and Life HistoryThe Archean Eon
2Archean Rocks
- The Beartooth Mountains
- on the Wyoming and Montana border
- consists of Archaean-age gneisses,
- some of the oldest rocks in the US.
3Precambrian
- 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
4Precambrian Time Span
5Precambrian
- The term Precambrian is informal
- but widely used, referring to both time and rocks
- The Precambrian includes
- time from Earths 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
6Rocks 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
7Eons of the Precambrian
- Eoarchean refers to all time
- from Earths origin to the Paleoarchean
- 3.6 billion years ago
- Earths 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
8What 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
todays - Oceans began to accumulate
9Hot, 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
10Oldest 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
11Eoarchean 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
12Origin of Continental Crust
- Andesitic island arcs
- form by subduction
- and partial melting of oceanic crust
- The island arc collides with another
13Continental 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
14Cratons
- A shield and its platform make up a craton,
- a continents 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
15Distribution of Precambrian Rocks
- Areas of exposed
- Precam-brian rocks
- constitute the shields
- Platforms consist of
- buried Pre-cambrian rocks
- Shields and adjoining platforms make up cratons
16Canadian 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
17Evolution of North America
- North America evolved by the amalgamation of
Archean cratons that served as a nucleus around
which younger continental crust was added.
18North American Craton
- Drilling and geophysical evidence indicate
- that Precambrian rocks underlie much
- of North America,
- exposed only in places by deep erosion or uplift
19Archean Rocks
- Only 22 of Earths 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
20Archean Rocks
- Outcrop of Archean gneiss cut by a granite dike
from a granite-gneiss complex in Ontario, Canada
21Archean Rocks
- Shell Creek in the Bighorn Mountains of Wyoming
has cut a gorge into this 2.9 billion year old
granite
22Greenstone 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
23Greenstone Belts and Granite-Gneiss Complexes
- Two adjacent greenstone belts showing synclinal
structure
- They are underlain by granite-gneiss complexes
24Greenstone 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
25Ultramafic Lava Flows
- The most interesting rocks
- in greenstone belts are komatiites,
- cooled from ultramafic lava flows
- Ultramafic magma (lt 45 silica)
- requires near surface magma temperatures
- of more than 1600C
- 250C hotter than any recent flows
- During Earths 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
26Ultramafic Lava Flows
- As Earths 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
27Sedimentary 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
28Sedimentary 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
29Canadian Greenstone Belts
- In North America,
- most greenstone belts
- (dark green)
- occur in the Superior and Slave cratons
- of the Canadian shield
30Evolution 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
31Evolution of Greenstone Belts
- According to this model,
- There was an early stage of extension as the
back-arc marginal basin formed - volcanism and
sediment
deposition
followed
32Evolution 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
33Another 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
- 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
34Greenstone BeltsIntracontinental Rift Model
- Ascending mantle plume
- causes rifting
- and volcanism
35Greenstone BeltsIntracontinental Rift Model
- Erosion of the rift flanks
- accounts for sediments
36Greenstone BeltsIntracontinental Rift Model
- Closure of rift
- causes compression
- and deformation
37Archean 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 Earths origin
- and more radiogenic heat,
- and magma was generated more rapidly
38Archean 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
39Archean World Differences
- The Archean world was markedly different than
later
- but associations of passive continental margin
sediments - are widespread in Proterozoic terrains
- We have little evidence of Archean rocks
- deposited on broad, passive continental margins
- but the ophiolites so typical of younger
convergent plate boundaries are rare, - although Neoarchean ophiolites are known
- Deformation belts between colliding cratons
- indicate that Archean plate tectonics was active
40The 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
41Southern Superior Craton Evolution
- Greenstone belts (dark green)
- Granite-gneiss complexes (light green
- Plate tectonic model for evolution of the
southern Superior craton - North-south cross section
42Canadian Shield
- Deformation of the southern Superior craton
- 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
43Atmosphere and Hydrosphere
- Earths early atmosphere and hydrosphere
- were quite different than they are now
- They also played an important role
- in the development of the biosphere
- Todays 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 Suns ultraviolet radiation
44Present-day Atmosphere Composition
- Nonvariable gases
- Nitrogen N2 78.08
- Oxygen O2 20.95
- Argon Ar 0.93
- Neon Ne 0.002
- Others 0.001
- in percentage by volume
- Variable gases
- Water vapor H2O 0.1 to 4.0
- Carbon dioxide CO2 0.038
- Ozone O3 0.000006
- Other gases Trace
- Particulates normally trace
45Earths Very Early Atmosphere
- Earths 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 Earths 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
46Outgassing
- 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
47Archean 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
48Evidence 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
49Introduction 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
50Photosynthesis
- 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 gt 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
51Oxygen 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
52Earths Surface Waters
- Outgassing was responsible
- for the early atmosphere
- and also for some of Earths 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
53Ocean 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
54Decreasing 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
- Heat production 4 billion years ago was 3 to 6
times as great as it is now
- With less heat outgassing decreased
55First Organisms
- Today, Earths 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
56What 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
57What 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
58How 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
59Elements of Life
- All organisms are composed mostly of
- carbon (C)
- hydrogen (H)
- nitrogen (N)
- oxygen (O)
- all of which were present in Earths early
atmosphere as - carbon dioxide (CO2)
- water vapor (H2O)
- nitrogen (N2)
- and possibly methane (CH4)
- and ammonia (NH3)
60Basic 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
61Experiment 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
62Experiment 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
63Polymerization
- 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
64Proteinoids
- 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
65Proteinoid Microspheres
- Proteinoid microspheres produced in experiments
- Proteinoids grow and divide much as bacteria do
66Protobionts
- These proteinoid molecules can be referred to as
protobionts - that are intermediate between
- inorganic chemical compounds
- and living organisms
67Monomer 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
68Next 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 todays 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
69RNA 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
70Much 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
71Submarine 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
72Submarine 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
73Oldest 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
74Oldest 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)
75Stromatolites
- Different types of stromatolites include
- irregular mats, columns, and columns linked by
mats
76Stromatolites
- 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
77Other 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
78Earliest 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
79Earliest 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
80Fermentation
- 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
81Photosynthesis
- 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
82Fossil Prokaryotes
- Photomicrographs from western Australias
- 3.3- to 3.5-billion-year-old Warrawoona Group,
- with schematic restoration shown at the right of
each
83Archean 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 worlds 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
84Archean 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
85Chrome
- About 1/4 of Earths 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
86Chrome 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
87Iron
- Banded Iron formations are sedimentary rocks
- consisting of alternating layers
- of silica (chert) and iron minerals
- About 6 of the worlds
- 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 worlds major source of iron
88Pegmatites
- 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
89Summary
- Precambrian encompasses all geologic time
- from Earths 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
90Summary
- 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
91Summary
- 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
- some type of Archean plate tectonics occurred,
- but plates probably moved faster
- and igneous activity was more common
- because Earth had more radiogenic heat
92Summary
- 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 - an oxygen deficient atmosphere,
- the necessary elements for organic molecules,
- and energy to promote the synthesis
- of organic molecules
93Summary
- The first naturally formed organic molecules
- 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
94Summary
- 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