Studying the Past
Identifying ancient orogens (mountain belts) – look for igneous activity, deformation and metamorphism. A mountain belt can be substantially eroded in 50 myr.
Radiometric dating. Foreland basins give clues to original mountains.
Dating continental crust by radiometric dating.
Studying sedimentary rocks – clues about depositional environment and whether global sea level was rising or falling. Recognizing past positions of continents using apparent polar wander path and paleomagnetic anomalies.
Recognizing past climates – fossil, rock, and geochemical evidence
Recognizing life evolution – examining the fossil record
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Studying the Past
Identifying ancient orogens!
Apparent polar wander paths!
Studying the Past
Magnetic anomalies on the ocean floor!
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Enormity of Precambrian time
Enormity of Precambrian time
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Hadean Eon
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Greek Hades, god of underworld. ca 4.6 to 3.8 billion years ago (Ga) from formation of Earth by planetesimals to creation of preservable crust
Differentiation of Earth’s interior
Formation of Moon – by impact of small planet (Moon was just 20,000 km away
(384,000 km today)
Surface – an ocean of magma.
Atmosphere –H2O, methane, ammonia, hydrogen, nitrogen, CO2, SO2 etc (no oxygen). Perhaps 250 times denser than today.
Oldest evidence – a 4.4 Ga detrital zircon in Australia. Oldest whole rock, 4.3 Ga.
Major bombardment of meteorites 4.0–3.9 Ga (lunar evidence) - crustal rocks?!
Archean Eon
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Archean = Greek, “ancient”. 3.8–2.5 Ga progressively complete record of Earth’s crust, cessation of intense bombardment
Substantial increase in area of Earth’s surface covered in continental crust.
Uncertainty whether “modern” plate tectonics operated at this time (due to heat)
First cratons (long-lived continental crust) developed at 2.7 Ga
By end of Archean, ca. 80% of present-day continental crust had developed
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Archean Eon
Plate tectonics but, possibly hot-spot
(“plume”) driven, rather than spreading ridges. Plates smaller than today
Archean cratons have 5 principal rock types:
Gneiss (formed in collisional zones)
Greenstone (metamorphosed oceanic crust)
Granite (from partial melting of crust)
Graywacke (sedimentary rock - mixture of sand and clay deposited in ocean)
Chert (precipitation of silica in the deep sea)
Clastic grains occasionally rounded – evidence of rivers
Oceans formed and became salty (corrosive rain leaching salts out of volcanic rocks
Life in the Archean
3.2 Ga bacteria from S. America
Fossil stromatolite
Life formed during the Archean
Isotopic evidence since at least 3.5 Ga (increased amount of 12C to 13C)
Oldest body fossils are bacteria from 3.2 Ga.
Stromatolites also begin about 3.2 Ga (cyanobacteria)
Biomarkers for photosynthesis from 2.7 Ga – important for establishing O2
Life may have started at hot-water vents in early
