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The Precambrian
Archean Eon
Brad Hubeny
Tectonics: Origin of Continents
Types of Crust
• Original Mafic Crust around 4.5 Ga
• Felsic Crust comes in around 4.4 Ga
• At about 3.9 Ga BP, crustal block survival
increases… Why??
• Felsic Crust after ~3.9 Ga forms
protocontinents: Accretion!
Brown = Archean-Aged Crust
Continental Accretion
Pillow Structures
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Greenstone Belts
More Evidence for Growth of Cratons
Archean Atmosphere
Archean Atmosphere and Oxygen
• How do we know there was little oxygen?
– Iron minerals mostly sulfides (pyrite)
– Dark Color of Sed Rocks: Free Carbon!
– No carbonates: Mega Acid Rain!
• How do we get Oxygen?
f08_22_pg222
– Photochemical Dissociation
– Photosynthesis!
• How do we know when oxygen showed up?
– Banded Iron Formations!
Banded Iron Formations and Oxygen
Iron Oxides!
Archean Climate
• Not much evidence to draw from
• Evidence for extreme events
• Global Temperature appears to be about the
same as present
– Sun was probably about 33% as bright as at
present
– Mega Green House Situation!!
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Necessary Ingredients
Archean Life
Stanley Miller’s “Warm Little Pond”
• 1953
• The result: Brown
primordial soup
containing AMINO
ACIDS!
• Same amino acids
found in Murchison
meteorite (1969)
• Protein
– Composed of Amino
Acids
• Nucleic Acid
– DNA, RNA
• Organic
Phosphorous
Compounds
– Transfer Energy
• A Cellular Container
3.5 Ga BP
Problems with the Warm Little Pond
Early Life: Prokaryotes
• We now realize
that some
oxygen was
available during
the Hadean and
Archean
• The most
primitive
prokaryotes are
“warm-adapted”
Suggests Life Origin at Mid-Ocean Ridges!
Chemosynthesis!!
Direct Evidence of Early Life
• Cyanobacteria fossils ~3.5Ga observed
– This is only 300Ma after meteorite
bombardment ceased!
• Well-preserved Stromatolite deposits
through Archean (and Proterozoic) Eon(s)
indicate lack of herbivores for over 3Ga (or
until Early Cambrian
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Stromatolite Deposits
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