archean eon friday 21 march 2008. the eoarchean earth?
TRANSCRIPT
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Archean EonFriday 21 March 2008
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The Eoarchean Earth?
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Archean rocksGreenstone belts:Typically found in belts which have a
synclinal form. • Show a sequential transition from ultramafic volcanics
at the bottom to felsic volcanics and capped by sediments.
• Mafic Volcanics - pillow lavas are common (environment?)
• Sedimentary Rocks - composed dominantly of graywackes ("dirty sandstones"), conglomerates and sandstones. - deposited in shallow water deltas, tidal flats, shallow marine shelf environments.
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Archean
• Making and keeping crustal fragments
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Arcs collide due to subduction to form larger land mass
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Land masses continue to grow
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Precambrian shields and cratons• Shield: area of exposed Precambrian rocks
• Craton: buried and exposed Precambrian rocks
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Superior Craton of Canada• Small fragments before 3.0 billion• Explosion of crustal growth 3.0-2.5 billion (end of
Archean)
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Model for assembly of cratons• Continental accretion: plates collide with
volcanic arcs and other plates.• Continents grow along margins
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Archean greenstone pillows
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Archean greenstone pillows
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Archean conglomerate: more evidence for water
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Archean sediment layers: more evidence for water
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Canadian Shield Gneiss
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• By end of Archean– Earth cooler– Easier to keep crust – Easier to make and
keep granite
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Proterozoic to the Cambrian explosion
Rocks, life and climate
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• ~1.8-2.6 Ga--first red Banded Iron Formations (BIF's) - red and gray zones of oxidized iron layers of silica.
• Responsible for world's most important iron deposits. As soluble iron was used up, O2 began to increase in atmosphere, and CO2 decreased
• continued until ~2 Ga=first appearance of red beds (and appearance of ozone!)
• Red beds = shallow-water, river, or soil deposits in which the iron has combined with O2 to form red iron oxide
Proterozoic rocks
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Banded iron formation
• 92% of BIF are Proterozoic (2.5-2.0 Ga)
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BIF’s
Banded Iron Formation, Alternating bands of red jasper and black hematite,about 2250 million years old (2.55 billion years old)
Jasper Knob, Ishpeming, Michigan
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Model for origin of BIF• Oxygen-rich upper
ocean• Oxygen-poor deep
ocean• Upwelling brings Fe
and Si-rich water up• Iron oxide formation
occurs (rust-like minerals form)
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•red beds=O2 increase in atmosphere
•BIFs decline, an indication that reducing compounds are disappearing from the oceans (
•weathering as we know it was probably set up at this time
Less CO2
more CO2
Proterozoic rocks
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Life in Proterozoic• Until about 1.9 Ga we only have evidence of the
simplest kinds of life - the PROKARYOTES – bacteria and blue green algae
• Requirements to be Life – self-replicating (DNA) – metabolism (chemical processes that convert food into
energy)
• But at about 1.9 billion years we start to see fossils of much larger cells. These cells belong to the Eukaryotes - of which we are members
• First sexual reproduction!
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Life in Proterozoic• ~650 mya (0.65 billion)
the first multicellular forms are present
• called the Ediacara assemblages
• seem to be elaborations of forms with – large surface areas– living in shallow relatively
high energy environments – often in red beds.
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Life in Proterozoic• Originally discovered in Pound Qtzt, Ediacara Hills, S.
Australia; later found worldwide (including Piedmont area of NC) at low paleolatitudes.0.59 - 0.7 by (590 - 700 my)
• impressions and molds of animals (associated with trace fossils)
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Mawsonites, similar to jellyfish, AustraliaSmithsonian Institution, Museum of Natural History
Dickensonia costata,segmented worm, from AustraliaSmithsonian Institution,Museum of Natural History
Unnamed "spindle-shaped organism"
from NewfoundlandSmithsonian Institution, Museum of Natural History
Tribrachidium heraldicum, Echinoderm?,from AustraliaSmithsonian Institution, Museum of Natural History
EDIACARA FAUNA
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Cambrian explosion—545mya
What happened at the P-Tr boundary? What about all of the other “dips” in the diversity?
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Paleogeography and climate
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• Animals with hard-shells appeared in great numbers for the first time during the Cambrian. The continents were flooded by shallow seas. The supercontinent of Gondwana had just formed and was located near the South Pole.
NA=Laurentia
N.Europe=Baltica
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Proterozoic climate
• Cold multiple times!!• Evidence:
– Till
– Polished, striated bedrock
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N.A. Orogenies
• 8 major Archean-aged crustal blocks: Hearne, Nain, Penokean, Rae, Slave, Superior, Wopmay, and Wyoming.
• Collisions between these blocks resulted in orogenic (mountain building) events. - continent-continent collisions
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