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Chapter 8
Precambrian Earth and Life
History—The Eoarchean and
Archean
Time check
• The Precambrian lasted for more than 4
billion years!
– Such a time span is almost impossible for us
comprehend
• If a 24-hour clock represented all 4.6 billion
years of geologic time
– the Precambrian would be slightly more than 21
hours long,
– It constitutes about 88% of all geologic time
Precambrian Time Span
Precambrian
• The term Precambrian is informal term referring to both
time and rocks
• It 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
– The oldest known rocks on Earth are 3.96 billion years old
Rocks of the Precambrian
• 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
the few fossils present are of little use in stratigraphy
• Because of this subdivisions of the Precambrian
have been difficult to establish
• Two eons for the Precambrian
– the Archean and Proterozoic
Eons of the Precambrian
• The onset of the Archean Eon coincides with the age of
Earth’s oldest known rocks
• approximately 4 billion years old
• lasted until 2.5 billion years ago (the beginning of the
Proterozoic Eon)
• The Eoarchean is an informal designation for the time
preceding the Archean Eon
• Precambrian eons have no stratotypes
– the Cambrian Period, for example, which is based on the
Cambrian System, a time-stratigraphic unit with a
stratotype in Wales
– Precambrian eons are strictly terms denoting time
US Geologic Survey Terms
• Archean and Proterozoic are used in our discussions
of Precambrian history, but the U.S. Geological Survey
(USGS) uses different terms
• Precambrian W begins within the Early Archean and ends at the
end of the Archean
• Precambrian X corresponds to the Early Proterozoic, 2500 to
1600 million years ago
• Precambrian Y, from 1600 to 800 million years ago, overlaps with
the Middle and part of the Late Proterozoic
• Precambrian Z is from 800 million years to the end of the
Precambrian, within the Late Proterozoic
The Hadean?
• Except for meteorites no rocks of Eoarchean
age are present on Earth, however we do know
some events that took place during this period
– Earth was accreted
– Differentiation occurred, creating a core and
mantle and at least some crust
Earth beautiful
Earth….
about 4.6 billion years ago
• Shortly after accretion, Earth was a rapidly rotating, hot,
barren, waterless planet
–
–
–
–
bombarded by comets and meteorites
There were no continents,
intense cosmic radiation
widespread volcanism
Oldest Rocks
• Judging from the oldest known rocks on Earth, the
3.96-billion-year-old Acasta Gneiss in Canada some
continental crust had evolved by 4 billion years
ago
• Sedimentary rocks in Australia contain detrital zircons
(ZrSiO4) dated at 4.2 billion years old
• so source rocks at least that old existed
• These rocks indicted that some kind of Hadean crust
was certainly present, but its distribution is unknown
Hadean Crust
• Early Hadean crust was probably thin, unstable and made
up of ultramafic rock
» rock 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
Crustal Evolution
• A second stage in crustal evolution began as Earth’s
production of radiogenic heat decreased
• 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 silicarich granitic magmas that were emplaced in the andesitic arcs
Crustal Evolution
• Several sialic continental nuclei had formed by
the beginning of Archean time by subduction
and collisions between island arcs
Dynamic Processes
• During the Hadean, various dynamic systems similar to
ones we see today, became operative,
– not all at the same time nor in their present forms
• Once Earth differentiated into core, mantle and crust,
– internal heat caused interactions among plates
– they diverged, converged and slid past each other
– Continents began to grow by accretion along convergent
plate boundaries
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
– Occupies most of northeastern Canada, a large part of
Greenland, parts of the Lake Superior region in
Minnesota, Wisconsin, Michigan, and the Adirondack
Mountains of New York
• It’s topography is subdued, with numerous lakes and
exposed Archean and Proterozoic rocks thinly
covered in places by Pleistocene glacial deposits
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
• The Canadian shield and adjacent platform consists of
numerous units or smaller cratons that were welded
together along deformation belts during the Early
Proterozoic
– Absolute ages and structural trends help geologists
differentiate among these various smaller cratons
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
• 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 40% 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
higher and the mantle was as much as 300 °C hotter
than it is now
• This allowed ultramafic magma to reach the surface
Sedimentary Rocks of
Greenstone Belts
• Sedimentary rocks are found
throughout the greenstone belts
– Mostly found 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
– 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
• Plate tectonic activity has operated since the Early
Proterozoic or earlier
• Most geologists are convinced that some kind of
plate tectonics took place during the Archean as well
but it differed in detail from today
• Plates must have moved faster
– residual heat from Earth’s origin
– more radiogenic heat
• 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
– 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 Late Archean
ophiolites are known
The Origin of Cratons
• Certainly several small cratons existed by the
beginning of the Archean
• During the rest of that eon they amalgamated into a
larger unit
– during the Early 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
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)
• oxygen not combined with other elements
• such as in carbon dioxide (CO2)
– water vapor (H2O)
– ozone (O3)
• which is common enough in the upper atmosphere to block most of
the Sun’s ultraviolet radiation
Present-day Atmosphere
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
Carbon dioxide
CO2
Ozone
O3
Other gases
•
Particulates
0.1 to 4.0
0.034
0.0006
Trace
normally 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.
– Also because Earth had no magnetic field until its core
formed the solar wind would have swept away any
atmospheric gases
Outgassing
• Once a core-generated magnetic field
protected Earth, gases released
during volcanism began to accumulate
– Called outgassing
• Water vapor is the most common
volcanic gas today
– also emitted
• carbon dioxide
• sulfur dioxide
• Hydrogen Sulfide
•
•
•
•
carbon monoxide
Hydrogen
Chlorine
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)
• Oxidized iron becomes increasingly common in
Proterozoic rocks
Introduction of Free Oxygen
• Two processes account for introducing free oxygen
into the atmosphere,
1. Photochemical dissociation involves ultraviolet
radiation in the upper atmosphere
• The radiation breaks up water molecules and releases
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
Earth’s Surface Waters
• Outgassing was responsible for the early
atmosphere and also for Earth’s surface water
– the hydrosphere
• Some but probably not much of our surface water was
derived from icy comets
• At some point 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?
• Much of volcanic water vapor today is recycled surface water
First Organisms
• Today, Earth’s biosphere consists of millions of
species of bacteria, fungi, protistans, plants, and
animals,
– 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
• it showed signs of living organisms but was not truly living
• In 1924 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
• 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
• During the late 1950s
– Stanley Miller synthesized
several amino acids by
circulating gases
approximating the early
atmosphere in a closed
glass vessel
Polymerization
• The molecules of organisms are polymers
– proteins
– nucleic acids
• RNA-ribonucleic acid and DNA-deoxyribonucleic acid
– consist 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
– are bounded by cell-like membranes
– 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 evaporationand 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
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)
– 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)
Fossil Prokaryotes
• Photomicrographs from western Australia’s 3.3- to 3.5billion-year-old Warrawoona Group
– with schematic restoration shown at the right of each