Chapter 6 - Archean: The First Two Billion Years

I.	Earth in Space
    	    A. Third planet from our Sun
    	    B. Rocky (terrestrial) planet:  Density. 5.5g cm3 

II.	Sun
	A. Energy source: Atomic fusion
	B. Ultimate source of energy for many geologic processes

III.	Origin of Universe
	A. Big Bang Hypothesis
	B. Steady State Cosmology
	C. Oscillating Universe

IV.	Birth of the Solar System
	A Dynamic Constraints
		1.  All planets revolve in same direction and lie in plane
		2.  All planets move counterclockwise (prograde)
		3.  Nearly all planets and moons rotate counterclockwise
		4. Density of planets roughly decreases away from sun
		5.  Planets in two groups
		    a.  Terrestrial
		    b.  Jovian
		6.  Age of Earth, moon, meteorites: 4.6 billion years
	B.  Hypotheses
		1.  Solar Nebula Concept
		    a.  cold, homogenous accretion
		    b.  solar ignition
		2.  Cold Accretion Concept
		    a. differentiation
		    b.  partial melting
		    c.  percolation
		3.  Hot Accretion Concept
		    a. layering during heterogenous accretion
		    b.  continued differdntiation

V.	Evolution of Atmosphere and Hydrosphere
	A.  Primitive Atmosphere:  Reducing
		1.  Earth’s volatiles:  H20, CO2, O2, etc.
		2.  Origin of volatiles:  Carbonaceous chondrites, comets
		3.  Outgassing
	B.  Oxygen-rich Atmosphere (3.5 billion years to present)
		1.  Photochemical cissociation: UV light + H2O
		2.  Photosynthesis
	C.  Transition Atmosphere (3.18 - 3.5 billion years ago)
		1.  Evidence
		    a.  banded iron formations
		    b.  cherts (bacterial fossils)
		    c.  lack of carbonates
		    d.  iron sulfide compounds common
		2.  Tied to biologic evolution of plants.
	D.  Origin of Oceans
		1.  Tied to onset of hydrologic cycle
		2.  Salinity due to chemical weathering

VI.	The Precambrian
	A.  Named for all rocks older than Cambrian (Sedgwick, 1835)
	B.  Subdivided by Logan (1850’s)
		1.  Proterozoic (younger)
		2.  Archean (older)
	C.  Pre-Archean:  Hadean
		1.  No direct rock reccord
		2.  Key events (4.6 - 3.9 billion years ago)
		    a.  separation of core from mantle
		    b.  oxygen - free atmosphere established
		    c.  meteoritic bombardment
		    d.  formation of lunar highlands
	D.  Protocontinents
		1.  Large size by end of Archean
		    a.  large shield areas (3.0 - 2.5 billion yr)
		    b.  Kenoran orogeny (2.7 - 2.6 billion yr)
		2.  Mountainous with little flat land (greywackes)
	E.  Fossil Record:  "Age of Prokaryotes" 
		1.  Beginning of life:  Inorganic elements, organic compounds organelles          then living cells
		2.  Time of first life:  3.5 billion yr. (bacteria)
		3.  Constraints of life
    		    a.  anaerobic (no oxygen)
     		    b.  no ozone shield
   		    c.  cell membrane needed
    		    d.  UV light or electrical discharge needed to synthesize organic
    		    compounds
		4.  Life synthesis experiments
    		    a.  Urey and Miller (U. Chicago, 1953)
     		    b.  S.W. Fox
   		    c.  H. G. Khorana (M.I.T., 1976)
		5.  Speculation of life origin and process
   		    a. heterotrophes (externally digest food)
    		    b.  fermentation to get food energy
    		    c.  autotrophs evolved:  Sulfur bacteria and nitrifying bacteria
    		    d.  photo autotrophs developed:  Used photosynthesis
   		    e.  ozone developed due to O buildup:  Diversification
     		    f.  aerobic organisms developed
		6.  Archean Fossil Record
    		    a.  Prokaryotes:  Chemical synthesis
    		    b.  Eukaryotes:  Symbiotic synthesis  (engulfment)
    		    c.  Cyanobacteria ("blue-green  algae"):  Stromatolites
    		    d.  Notable fossil sites:  Southern Africa, Australia (Warrawoona Group
    		    oldest fossils 3.5 B.Y.A.)
    		    e.  Notable fossil species:  Filamentous prokaryotes and spheroidal eukaryotes (Archaeosphaeroides)

VI.	Archean Mineral Wealth
	A.  Gold
		1.  Placer and vein deposits
		2.  Locales:  Africa, N. America, Asia, S. America, Australia
	B.  Other ones:  Fe, Ca, Cr, Ni
	C.  Diamonds:  Mantle-derived intrusions
	D.  Volcanogenic massive sulfide deposits:  Cu, Zn, Fe