1. Precambrian Earth and Life History—The Archean Eon
Chapter 8
Precambrian Earth and Life History—The Archean Eon

2. Archean Rocks The Beartooth Mountains
on the Wyoming and Montana border
consists of Archaean-age gneisses,
some of the oldest rocks in the US.

3. 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

4. Precambrian Time Span
88% of geologic time

5. Precambrian The term Precambrian is informal The Precambrian includes
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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Origin of Continental Crust
Andesitic island arcs
form by subduction
and partial melting of oceanic crust
The island arc collides with another

13. 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

14. 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

15. Distribution 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

16. 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

17. 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.

18. 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

19. 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

20. Archean Rocks
Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada

21. Archean Rocks
Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite

22. 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

23. Greenstone Belts and Granite-Gneiss Complexes
Two adjacent greenstone belts showing synclinal structure
They are underlain by granite-gneiss complexes
and intruded by granite

24. 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

25. 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

26. 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

27. 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

28. 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

29. Canadian Greenstone Belts
In North America,
most greenstone belts
(dark green)
occur in the Superior and Slave cratons
of the Canadian shield

30. 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

31. Evolution 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

32. 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

33. 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
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

34. Greenstone Belts—Intracontinental Rift Model
Ascending mantle plume
causes rifting
and volcanism

35. Greenstone Belts—Intracontinental Rift Model
Erosion of the rift flanks
accounts for sediments

36. Greenstone Belts—Intracontinental Rift Model
Closure of rift
causes compression
and deformation

37. 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

38. 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

39. Archean 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

40. 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

41. Southern Superior Craton Evolution
Greenstone belts (dark green)
Granite-gneiss complexes (light green
Geologic map
Plate tectonic model for evolution of the southern Superior craton
North-south cross section

42. Canadian 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

43. 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

44. Present-day Atmosphere Composition
Nonvariable gases
Nitrogen N %
Oxygen O
Argon Ar
Neon Ne
Others
in percentage by volume
Variable gases
Water vapor H2O 0.1 to 4.0
Carbon dioxide CO
Ozone O
Other gases Trace
Particulates normally trace

45. 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

46. Outgassing Once a magnetosphere was present Water vapor
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

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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
Heat production 4 billion years ago was 3 to 6 times as great as it is now
With less heat outgassing decreased

55. 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

56. 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

57. 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

58. 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

59. 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)

60. 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

61. 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

62. 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

63. 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

64. Proteinoids These concentrated amino acids Perhaps similar conditions
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

65. Proteinoid Microspheres
Proteinoid microspheres produced in experiments
Proteinoids grow and divide much as bacteria do

66. Protobionts
These proteinoid molecules can be referred to as protobionts
that are intermediate between
inorganic chemical compounds
and living organisms

67. 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

68. 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

69. 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

70. 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

71. 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

72. 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

73. 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

74. 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)

75. Stromatolites Different types of stromatolites include
irregular mats, columns, and columns linked by mats

76. 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

77. 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

78. 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

79. 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

80. 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

81. 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

82. 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

83. 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

84. 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

85. 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

86. 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

87. 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

88. 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

89. 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
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

90. 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

91. 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
some type of Archean plate tectonics occurred,
but plates probably moved faster
and igneous activity was more common
because Earth had more radiogenic heat

92. 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
an oxygen deficient atmosphere,
the necessary elements for organic molecules,
and energy to promote the synthesis
of organic molecules

93. Summary 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

94. 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