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Chapter 8
Precambrian Earth and Life History—
The Archean Eon
• Archean Rocks
The Beartooth
Mountains
on the Wyoming
and Montana
border
some of the oldest
rocks in the US.
gneiss
Precambrian Time Span
The Precambrian lasted
for more than 4 billion
years!
If Earth’s history were a
24 hour clock, the
Precambrian would use
21 hours!
• 88% of
geologic time
Precambrian
• from Earth’s origin 4.6 billion years ago
– to the beginning of the Phanerozoic Eon
• 542 million years ago
• all rocks older than the Cambrian system
• No rocks are known for the first 600 million
years of geologic time
• The oldest known rocks on Earth
• 4.0 billion years old
Two eons for the Precambrian
– Archean and Proterozoic
• which are based on absolute ages from igneous and metamorphic
rocks
Eoarchean?
– What we know:
– Earth accreted from planetesimals
– differentiated into a core and mantle
• some crust was present
– bombarded by meteorites
– Volcanic activity was occurring globally
– An atmosphere formed, quite different from
today’s
– Oceans began to accumulate
Hot, Barren, Waterless Early Earth
• 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
Eoarchean evidence shows:
• Continental crust was present by 4.0 billion
years ago
– Evidence:
Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.4 billion years old
• so source rocks at least that old existed
• Eoarchean Earth probably rotated in as little as
10 hours
– and the Earth was closer to the Moon
• By 4.4 billion years ago, the Earth cooled
sufficiently for surface waters to accumulate
Eoarchean Crust
– mafic and ultramafic magma: Temperatures >
1600C
– and numerous subduction zones developed
– Small island arcs
• Eoarchean continental crust may have formed
– by collisions between island arcs
– as silica-rich materials were metamorphosed.
– Larger groups of merged island arcs
• protocontinents
• grew faster by accretion along their margins
Origin of Continental Crust
• Intermediate
• (Andesitic)
island arcs
– form by
subduction
– and partial
melting of
oceanic crust
• The island
arc collides
with another
Continental Foundations
• Continents: composition similar to granite
(same density as silica or quartz)
• Continental crust is thicker
• Ocean crust: Composition of basalt and gabbro
(higher density )
Ocean Crust is thinner
Cratons
• Cratons have experienced little deformation
– since the Precambrian
• Precambrian shields are exposed ancient rocks
on all continents
• Covered by platforms
– Sedimentary rocks that overlie the shield
– Shield + Platform = Craton
Distribution of Precambrian Rocks
•
The exposed craton in
North America is the
Canadian shield
– northeastern
Canada
– large part of
Greenland
– parts of the Lake
Superior region
• in Minnesota,
Wisconsin,
and Michigan
– and the
Adirondack
Mountains of
New York
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.
Archean Rocks
• granite-gneiss complexes
– ultramafic igneous peridotite
– sedimentary rocks had been metamorphosed
• Greenstone belts are 10% of Archean rocks
– Help unravel Archean tectonic events
Archean Rocks
• Outcrop of Archean gneiss cut by a granite dike from
a granite-gneiss complex in Ontario, Canada
Archean Rocks
• Shell Creek in the Bighorn Mountains of Wyoming
has cut a gorge into this 2.9 billion year old granite
Greenstone Belts
• A greenstone belt has distinct rock units
– volcanic rocks are most common
– Sedimentary
– Intruded by granitic magma
• Low-grade metamorphism
– Makes igneous rocks green
– chlorite, actinolite, and epidote
Greenstone Belts and GraniteGneiss Complexes
• Two adjacent
greenstone
belts showing
synclinal
structure
• They are
underlain by
granite-gneiss
complexes
• and intruded
by granite
Greenstone Belt Volcanics
• Pillow lavas in greenstone belts
– indicate that much of the volcanism was
– subaqueous
Pillow lavas in Ispheming greenstone belt
at Marquette, Michigan
Ultramafic Lava Flows
• Ultramafic magma (< 45% silica)
– requires near surface magma temperatures
of more than 1600°C
– 250°C hotter than any recent flows
• Why then, not now?
Early Earth:
– radiogenic heating was greater
– and the mantle was as much as 300 °C hotter
• Earth cooled: They are rare in rocks younger
– than Archean and none occur now
To summarize…
Ultramafic Lava Flows
• As Earth’s production
– of radiogenic heat decreased,
– the mantle cooled
– and ultramafic flows no longer occurred
Sedimentary Rocks of
Greenstone Belts
• Many of these rocks are successions of
– graywacke
• sandstone with abundant clay and rock fragments
– and argillite = shale
Sedimentary Rocks of
Greenstone Belts
• Small-scale cross-bedding and graded bedding
– indicate an origin as turbidity current deposits
Canadian Greenstone Belts
• In North
America,
– most
greenstone
belts
– (dark green)
– occur in the
Superior and
Slave cratons
– of the
Canadian
shield
Evolution of Greenstone Belts
• Greenstone belts formed in several tectonic settings
• Models for the formation of greenstone belts
– involve Archean plate movement
• In one model, greenstone
belts formed
– in back-arc marginal
basins
Evolution of Greenstone Belts
• According to this model,
– There was an early stage of extension as the backarc marginal basin formed
– volcanism and
sediment
deposition
followed
Evolution of Greenstone Belts
• Then during closure,
– the rocks were compressed,
– metamorphosed,
– and intruded by
granitic magma
• The Sea of
Japan
– is a modern
example
– of a back-arc
basin
Archean Plate Tectonics
• Plates moved faster
– heat from Earth’s origin
– more radiogenic heat (radioactive decay),
– Magma generated more rapidly
• Therefore…
– continents grew rapidly
– By continental collision and accretion with other
plates and islands
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
• Today’s atmosphere:
– nitrogen (N2) 78%
– free oxygen (O2) 21%
• or oxygen in compounds (CO2)
– water vapor (H2O) varies 0.1 – 04%
– other gases, like ozone (O3) ~ < 2%
• block most of the Sun’s ultraviolet radiation
Earth’s Very Early Atmosphere
– hydrogen and helium,
• the most abundant gases in the universe
• Earth’s gravity is insufficient to retain them
• Earth had no magnetic field until its core formed
(magnetosphere)
• Without a magnetic field,
– the solar wind would have swept gases away
Outgassing
• After magnetic field
forms:
– Atmosphere accumulated
from outgassing
• during volcanism
• Water vapor
–
–
–
–
–
–
common volcanic gas
volcanoes also emit
carbon dioxide
sulfur dioxide
Methane
Nitrogen oxide
Archean Atmosphere
• Archean volcanoes
– emitted the same gases,
– atmosphere developed
– ! lacking free oxygen and an ozone layer
•
Oxygen in compounds:
CO2, ammonia (NH3) methane (CH4)
Evidence for a lack of
Free Oxygen Atmosphere
• detrital deposits
– contain minerals that oxidize rapidly
– These minerals are NOT bound to typical
abundances of oxygen
• pyrite (FeS2)
• uraninite (UO2)
Introduction of Free Oxygen
• Two processes account for
1. Photochemical dissociation
• radiation breaks up water molecules upper atmosphere
– releases their oxygen and hydrogen
– 2% of present-day oxygen
• with 2% oxygen, ozone forms, creating a barrier against
ultraviolet radiation
2. More important were the activities of organisms
that practiced photosynthesis
Photosynthesis
• Photosynthesis is a metabolic process
– carbon dioxide and water make organic molecules
– and oxygen is released as a waste product
CO2 + H2O ==> organic compounds + O2
– 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
• Volcanic Outgassing
Meteorites and icy comets
Rapid rate of accumulation of water
– Most of hydrosphere in the oceans -- more than 97%
•
– Today: water vapor still emitted
the rate of volcanism has decreased
considerably
– -- heat needed to generate magma has diminished
Decreasing Heat
• Ratio of radiogenic heat production in the past
to the present
• With less heat
outgassing
decreased
• Heat production
4 billion years
ago was 3 to
6 times as great
as it is now
First Organisms
• We have fossils from Archean rocks 3.5 billion yrs
– Only bacteria and archea are found in Archean rocks
• Chemical evidence in rocks in Greenland
– that are 3.8 billion years old
– convince some investigators that organisms were present
then
What Is Life?
• living organism must reproduce
– and practice some kind of metabolism
• 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?
• Carbon based molecules known as
microspheres
– form spontaneously
– can even grow and
divide in a somewhat
organism-like fashion
– but their processes are
more like random
chemical reactions, so
they are not living
How Did Life First Originate?
– from non-living matter (abiogenesis), life must have
passed through a prebiotic stages
– it showed signs of living
– but was not truly living
• The origin of life has 2 requirements
– 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)
– SPONCH
• all of which were present in Earth’s early
atmosphere as
–
–
–
–
–
carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)
Basic Building Blocks of Life
• Energy from
• Lightning, volcanism,
• ultraviolet radiation
– C, H, N, and O combined to form monomers
• such as amino acids
• Monomers are the basic building blocks
– of more complex organic molecules
Experiment on the Origin of Life
• Is it plausible that monomers
– originated in the manner postulated?
– Experimental evidence indicates that it is
• 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
• Produced
– 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
• consisting of monomers linked together in a specific
sequence
• RNA (ribonucleic acid) and DNA (deoxyribonucleic acid)
• Problem:
• Water usually causes depolymerization,
– however, researchers synthesized molecules
– known as proteinoids when heating dehydrated
concentrated amino acids
Proteinoid Microspheres
• Proteinoid
microspheres
produced in
experiments
• Proteinoids grow
and divide much as
bacteria do
Protobionts
• These proteinoid molecules can be referred to
as protobionts
– that are intermediate between
– inorganic chemical compounds
– and living organisms
Monomer and Proteinoid Soup:
Model for abiogenesis
• Monomers likely formed continuously and by
the billions
– accumulated in the early oceans into a “hot, dilute
soup”
– The amino acids in the “soup” dried (might have
washed up onto a beach or perhaps cinder cones)
– they were polymerized by heat
• The polymers then washed back into the ocean
– where they reacted further
Next Critical Step
• The microspheres divide
– and may represent a protoliving system
– but in today’s cells, nucleic acids,
• either RNA or DNA
– are necessary for reproduction Dead End???
• 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
Much Remains to Be Learned
• Many researchers believe that
– the earliest organic molecules were synthesized
from atmospheric gases
– but some scientist suggest that life arose instead
– near hydrothermal vents on the seafloor
Submarine Hydrothermal Vents
• Seawater seeps into the crust near spreading
ridges, becomes heated, rises and discharges
hot waters
• Black smokers
• Mineral-rich fluids
Submarine Hydrothermal Vents
• Several minerals containing zinc, copper, and iron
precipitate around them
• Communities of organisms
– previously unknown to
science, are supported here.
– Necessary elements, sulfur,
and phosphorus are present in
seawater
– Polymerization can take
place on surface of clay
minerals
– Protocells were deposited on
the ocean floor
Oldest Known Organisms
• The first organisms were archaea and bacteria
– both of which consist of prokaryotic cells,
– cells that lack an internal, membrane-bounded
nucleus and other structures
Oldest Know Organisms
• stromatolites,
– not until 1954 were they shown
– to be products of organic activity
Present-day stromatolites (Shark Bay, Australia)
Stromatolites
• Different types of stromatolites include
– irregular mats, columns, and columns linked by mats
Stromatolites
• Present-day stromatolites form and grow
– as sediment grains are trapped
– on sticky mats
– of photosynthesizing cyanobacteria
• The oldest known undisputed stromatolites
– are found in rocks in South Africa
– that are 3.0 billion years old
Other Evidence of Early Life
• The oldest known cyanobacteria
– were photosynthesizing organisms: COMPLEX!!
• A simpler type of metabolism
– must have preceded it
• No fossils are known of these earliest organisms
Earliest Organisms
• The earliest
– tiny anaerobic bacteria
– required no oxygen
• totally depended on
– external source of nutrients
– Heterotrophic
• They all had only prokaryotic cells
• The earliest organisms, then,
– were anaerobic, heterotrophic prokaryotes
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