Archean Eon
Early History of the Solar System and Earth
Universe 14.6 Ga based on Red shift of galaxies
Big Bang produced mainly H and He
Gases condense to form stars. Stars form elements up to Fe, stars explode (supernova) and
heavier elements form.
Sun formed in the radial arm of the Milky Way Galaxy later from some of this material.
Gravitational attraction of matter, condenses into a disk, meteorites collide into planetessimals.
Volatile elements get blown out into outer reaches of solar system where they condense into gas
giant planets (colder). Heavier material left for inner, rocky planets.
Meteor bombardment in the early history of the earth (first 100s millions years)-record from the
moon and other “dead” planets.
Moon formed from collision of Mars-sized asteroid when earth was very hot (magma ocean).
Explains the relatively rapid rotation of the earth and its tilt.
Differentiation of material in the earth from excessive heat and subsequent melting led to the
formation of the Core (0-40 km [up to 70], Mantle (40-2890 km), and Crust (outer 2890-5150,
inner 5150-6370 km).
Moon
Maria (younger, recratering and basalt flows from melting) and Highlands (oldest)
No water-expelled during collision and melting-not kept in atmosphere due to small size
and lack of gravitational attraction
Small metallic core (earth’s is larger than expected)
Feldspar rick outer layer-ANORTHOSITE
Earth’s atmosphere: mainly the result of volcanic activity (gases) water, H, HCl, CO, CO2 and N
along with methane and ammonia (CH4) and (NH3)
Hot early earth and many meteorite impacts meant small “plates” that were remobilized
frequently.
Early Crust: ultramafic Komatiite (Peridotite) composition
Widespread volcanism, high heatflow
Major Groups of Archean Rocks:
Greenstone Belts-Mafic and Ultramafic volcanics and volcaniclastic sediments
Subduction zone related and sandwiched between felsic intrusions.
Granulites and Gneisses (Felsic)
High grade metamorphic rocks-vertical emplacement within mafic/ultramafic protocrust
Represent intial cores of stable cratons/continents
Formed from Partial Melting of more mafic material
Sedimentary Rocks
Greywackes-poorly sorted immature volcaniclastic sandstone
Dark mudstones
“First Cycle” sediments-much feldspar, mafic minerals, rock fragments, and clay
minerals
Rare Banded Iron Formations