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Chapter 11
The Archean Era
of Precambrian Time
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Guiding Questions
• When and how did Earth and its moon come into
being?
• How did the core, mantle, crust form?
• Where did Archean rocks form, and what is their
nature?
• When and why did large continents begin to form?
• Where did life arise and what kinds of life existed
at the end of Archean time?
• Why did relatively little free oxygen accumulate in
Earth’s atmosphere through Archean time?
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Precambrian Archean
• Precambrian
– Time prior to
Phanerozoic Era
– Archean Eon
• 4.6–2.5 billion
years ago
– Proterozoic Eon
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Precambrian Geology
• Cratons
– Large under-formed
portions of continents
– Primarily
Precambrian
• Precambrian shield
– Craton exposed at
surface
– Canadian Shield
exposed by glaciation
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Precambrian Geology
• Continental crust formed during Archean
• High heat flow required small continents
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Origin of the Universe
• Provide important
information
concerning age of
Earth
• Fragments of larger
bodies that have
undergone collision
and broken into
pieces
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Origin of the Universe
• Stony meteorites
– Rocky composition
• Iron meteorites
– Metallic composition
• Stony-iron meteorites
– Mixture of rocky and
metallic
– Proxy for core composition
• Most date around 4.6
billion years ago
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Origin of the Universe
• Stars cluster in
galaxies
– Organized in disks
• Milky Way
– Our galaxy of stars
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Origin of the Universe
• Expanding universe
– Galaxies move
apart
• Redshift
– Originally
concentrated into a
single point
• Big Bang
– 15 billion years ago
– Age of universe
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Origin of the
Universe
• Galactic matter is
concentrated
• Stars form
– Our Sun
• Supernova
– Exploding star
• Solar nebula
– Dense rotational
cloud
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Origin of the Solar System
• Planets formed near time of sun’s formation
– 4.6 billion years ago
• Planets far from sun are formed from volatile elements
• Planets close to sun are rocky
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Origin of the Solar System
• Rocky debris
– Collided to form
aggregates
– Aggregates collided
to form asteroids
• 40 km diameter
– Some coalesced to
form planets
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Origin of Earth and Moon
• Earth materials
differentiated
– Dense at center
– Less dense
silicates rose to
surface
• Magma
ocean
– Cooled to form
crust
• Meteorite impacts increased
concentrations of some elements in
upper Earth
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Origin of Earth and Moon
• Moon formed from
impact
– Mantle of
impacting body
– Proportions of Fe
and Mg differ from
Earth’s mantle
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Origin of Earth and Moon
• Early atmosphere
–Degassing from
volcanic emissions
–CH4 and NH3
abundant
–Little O2
•No
photosynthesis
• Earth’s oceans
– Volcanic emissions
cooled, condensed
– Salts
• Carried to sea by
rivers and
introduced at
ridges
• Approximately
constant through
time
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Origin of Earth and Moon
• Moon’s maria
– Originally thought
to be seas
– Craters formed by
asteroids
– Floored by basalts
– Craters
• 3.8–4.6 billion
years old
• Earth also impacted
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– Tilted Earth 23.5°
Origin of Earth and Moon
• Heat Flow
– Decreased through
time
• Indicates abundant
hot spots, small
lithospheric
fragments
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Origin of Continents
• First crust
– Basalt (oceanic)
• Felsics differentiated
– Formed nuclei of continental crusts
– Iceland
• Modern analogue
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Origin of Continents
• Small Archean
fragments
– High heat flow
limited continental
thickness
• Zircon crystals
– 4.1–4.2 B years old
– Weathered from
felsic rocks
• Canadian Shield
– 3.8–4.0 B years20 old
Origin of Continents
• Greenstone belts
– Weakly
metamorphosed
– Abundant chlorite
• Green color
– Nested in highgrade felsic
metamorphic rocks
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Origin of Continents
• Greenstone belts
contain igneous rocks
– Volcanics contain
pillow basalts
• Underwater extrusion
– Formation of sediments
in deep water
• Graywackes,
mudstones, iron
formations, volcanic
sediments
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Origin of Continents
• Banded iron
formations
– 3 Billion years old
– Isua, southern
Greenland
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Origin of Continents
• Continental accretion
– Deep water sediments accreted to continent
– Marine sediments form wedge between continental
masses
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Archean Life
• Earth is best suited
known planet
– Conditions right by
4.2 Billion years
• Western Australia
organic compounds
– 3.5 Billion years
• Mars
– Water flowed once
– Life may have
evolved separately
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Archean Life
• South African cherts
contain possible mold
of prokaryotic cell
– 3.4 Billion years
• Oldest unquestionable
life form
– 3.2 Billion years old
– Australia
– Intertwined filaments
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Archean Life
• Stromatolites
– 3.5 Billion years
– Suggest
photosynthesis
• Biomarkers for
cyanobacteria
– 2.7 Billion years
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Archean Life
• Miller and Urey
– Produced amino
acids found in
proteins
• Modeled primitive
atmosphere
• Added lightning
– Included oxygen
• Amino acids found
in meteorites
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Archean Life
• RNA world
– Nucleic acid
– Can replicate itself
– May have been
catalyst for
production of key
proteins
• Foundation for
DNA world
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Archean Life
• Mid-ocean ridges
• Sulfur reduction
– High heat
– S + H2 − > H2S + energy
– Chemosynthetic organisms • Methane production
• Hydrogen oxidation
– CO2 + 4H2 − > CH4 + 2H2O +
– 2H2 + O2 − > 2H2O + energy
energy
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Archean Life
• Ridges offer wide
range of temperatures
• Organic compounds
readily dissolve in
warm water
• Protection from
ultraviolet radiation
• Abundant phosphorous
• Contain metals
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• Contain clays
Archean Life
• Atmospheric Oxygen
– Low concentrations early on
– Later, O2 released through
photosynthesis
• Sink
– Reservoir that grows so as to
take up a chemical as it is
produced
• Early crust was sink for O2
– Pyrite (FeS2) transported but
not oxidized
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