(Precambrian time = Archean eon + Proterozoic eon)

 Rocks: only 20 % of outcrops,
 Archean rocks destroyed by            erosion
                                                         metamorphism
                                                         or covered by younger rocks

life: started here!  unicellular prokaryotes => few index fossils
         => correlation based on radiometric dating

Cratons: large portions of the continents not deformed since Pc or early
 Paleozoic
 Shield:     exposed (outcropping) craton
                 Felsic crust evolved towards the end of the Archean
Very primitive rocks do not outcrop => use meteorites of the same age to calculate the age of the Earth

 Meteorites as representatives of the primitive Earth. (Broken fragments of previous bodies)
                         stony = mantle
                         iron   = core                 4.6 b.y. old

 Universe: originated during the Big Bang, expanding (15 - 18 b.y.a.)
 Solar system: Sun similar age as Earth

 Solar nebula theory:

•     nebula material rotates and flattens
•      concentration in rings => asteroids
•      asteroids coalesce by gravity

 Earth = accretion of asteroids => heat from

• accretion
•  collisions                          => Earth melted
•  radioactive decay

 Differentiation: layered structure

•     Fe - Ni core
•      dense silicate mantle
•      magma ocean at the surface = basalt => first crust

•  asteroid core sank, its mantle exploded & accreted into the moon
•  no water in the moon minerals
•  primitive moon crust: feldspars crystallized from a magma ocean (a result of the heat of its formation)

Earth's Atmosphere

•  out gassed when Earth was liquid
•  out gassing weaker today (volcanoes)
•  composition: H2O, H2, CO, C02, N2 (very little O2)

Oceans     condensation of degassed H2O
                 comet contribution                                                 no salt

Salinity     leached out from the continents
                 increased first but now in equilibrium by evaporite precipitation

Origin of the Continents
            Primitive crust: solidified magma ocean (basalt)
             Earliest continental crust => in hot spots

 Process:

• felsic pods from basalt
• magma ascending through faults melts pods
• molten pods ascend
• magma also from isostatic sinking

•   continents remained small (a lot of convection), rifting and subduction
•   Oldest crust
• 4.1 or 4.2 b.y.a (detrital zircon in sandstone)
• NW of Canadian shield: 3.8 - 4.0 (felsic rock)
• Growth of continental crust to present volume at 2.5 b.y.a., produced by melting, destroyed by erosion & subduction (oceanic crust only)
•  Suturing of smaller landmasses = larger continents
• Lithologies (rock types):
• volcanic arcs

   dark igneous rocks
   dark sedimentary rocks
                deep sea                     turbidites
                                                   muds
                                                   cherts
                                                   banded iron formations
• greenstone belts among sutured protocontinents

Large metamorphic episode at the end of the Archean => cratonization
 

ARCHEAN LIFE

 Earth: only planet in our solar system with water coexisting in the liquid, solid and vapor state.

Achean Life:             Kingdoms                 Archeobacteria
                                                                    Eubacteria                            Both prokaryotes

Oldest fossils (in cherts)                          filamentous cyanobacteria? (3.4 b.y.)
                                                                     Photosynthesis started with them
                                                                    stromatolites (3.2 b.y. in the Proterozoic)

Early Life: The RNA world...
 

•   Nucleic acids RNA and DNA are basic for life because they carry the information that allows life to replicate itself and therefore continue through generations. They must have been the structures to perform such functions in earliest life forms

 

RNA:

• Simple, single strand structure
•  Carries information to manufacture proteins
•  Ferries amino acids to the sites where proteins are to be made
•  Acts as a catalyst for protein formation
•  Can duplicate itself

DNA:

• Double strand structure
•  Contains chemically coded genetic information

DNA evolved from RNA through natural selection on occasional mutations

Organisms must have developed a semipermeable membrane to...

•   protect the organisms chemical system
•   allow specific kinds of compounds in and out

Life Origins
Old idea: electrical discharges => amino acids

   Amino acids brought by meteorites?
   Life in a warm little pond

 New view: origin along midoceanic ridges close to underwater volcanic hot springs
                  Bacteria harness the energy of chemical reactions: chemosynthesis