Download The Hadean Outline •Theories on Formation of Solar System, Universe

The Hadean
Outline
•Theories on Formation of Solar System, Universe
–The Nebular Hypothesis
–The Big Bang Theory
•The PreCambrian
•Hadean
–Layering of Earth
–Formation of Continents and Moon
–Origin of Oceans and Atmosphere
Nebular Hypothesis
•Planets started forming ~ 5 billion years ago
•Solar system condensed from gaseous nebula drawn by gravity around sun
•Gravity draws matter inward
•Generating spin
•Condensing core material
•Planets formed, materials that compose them separated
•Denser metallic elements sank toward centers
•Lighter elements migrated toward surfaces
•Process =“chemical differentiation”
The Big Bang Theory
•Doppler effect
•Change in wavelength (of light) emitted by object due to motion
•Movement away stretches wavelength
•Longer wavelength
•Light appears redder
•Movement toward “squeezes” wavelength
•Shorter wavelength
•Light shifted toward the blue
•Most galaxies exhibit red Doppler shift
• Discovered 1929, Edwin Hubble
•Hubble's Law: recessional speed of galaxies proportional to their distance
•Must account for galaxies moving away
•States Universe once confined to "ball" that was
•Supermassive
•Dense
•Hot
•Upon explosion, matter shot out in all directions
•Eventually gravitational attraction caused assembly into galaxies
•Marks inception of universe
–~13.8 billion years old (may be older)
–From existence/structure of cosmic microwave background, large
scale structure in distribution of galaxies, abundances of H, He, Li,
acceleration of expansion of universe by light from
galaxies/supernovae
–Matter=continually moving outward
•Fate of universe, two possibilities:
–Universe will expand forever
–Outward expansion will stop; gravitational contraction will follow
•Final fate depends on average density of universe
•Density=more than “critical” density, universe contracts
•Density=less than critical, universe expand
•Current estimates point to ever-expanding, or open, universe
The PreCambrian:
• Hadean Eon –
– Earth’s formation prior to solidification of crust (4.6-4.0 Bya/ga)
• Archean Eon –
– After Hadean
– ~4+ billion years ago
– Ends at beginning of 2.5 billion years ago (Proterozoic Eon)
• Proterozoic Eon –
– Begins 2.5 billion years ago
– Ends at 542 million years ago (beginning of Phanerozoic Eon)
The PreCambrian
•Crucial events occurred:
–Initial formation of Earth
–Solidification of crust & development of cratons
–Initiation of plate tectonics
–Formation of oceans and atmosphere
–First prokaryotic and eukaryotic life
–Early icehouse-greenhouse cycles
Hadean=Layering of Earth
• Earth shows
– Layers of differing composition
• iron-nickel-rich core
• silicate-rich mantle
• silicate crust
– Zones of differing rock strength:
• cool, rigid lithosphere (on outside of planet)
• hot, more “plastic” asthenosphere
• Hot, but strong, mesosphere
• Supported by Nebular Hypothesis
Hadean=Formation of Moon
• Most accepted hypothesis today= moon formed after Earth was impacted by body
about size of Mars
• Two extraterrestrial sources of information about age of Earth:
• Meteorites
• Moon rocks
• Rocks allow us to:
• Determine absolute ages for solar system/ moon
• Using isotopes/radiometric dating
• Understand composition of earth’s deep interior
•
•
Provides clusters ~4.5-4.6 billion years
Terrestrial rocks dated by Zircons
Hadean=Origin of the Atmosphere & Ocean
• Early atmosphere formed when Earth
– Coalesced
– Big enough to trap gases
• Volitles from Magma Lake
– Easily escaped to surface
– Condensed=form liquid water
• Comets
• Salt from chemical weathering of rocks:
– on land
– shoreline
Early Continental Crust
•Large rock bodies welded along metamorphic zones (“greenstone belts”)
•Podlike bodies
–Mostly high-grade metaigneous rocks
• Probably felsic crust of Archean protocontinents
•Greenstones connecting them =metavolcanic, metasedimentary
•Rich in chlorite
Hadean--Early Continental Crust
• Felsic protocontinents from magma ocean
– Chemical differentiation occurred
• Early pieces=mafic-ultramafic rock
• Partial melting occurred,
– remelted portion, chemical differentiation
– separating out lighter (felsic) from denser (mafic)
The Archean
Outline
•Early Continental Crust
•Banded Iron Formations (BIFs)
•Life in Archean
•The Oxygen Revolution
Archean=Formation of Continents
• Craton - core of continent
– Part of crust attained relative stability
– Received little deformation for ~1 billion years
Banded Iron Formation (BIFs)
• Sedimentary rock made of
• Thin chert bands
• Iron oxide minerals
• Two main depositional environments:
• Submarine volcanic zones
•
Rift basins
Life in Archean
•Earliest fossils
–Simple microbes
–Preserved molds with carbon in chert
•Cyanobacteria –
–Blue-green eubacteria
–Most are photosynthetic
•Photosynthesis
Life in the Archean-Stromatolites
• Thin-layered biogenic-sed. structures
• Formation: binding layers of by photosynthetic cyanobacteria
• Persistent in Archean, Proterozoic rock record
– oldest known (from South Africa) ~3.2 billion years old
Life in the Archean-Amino Acids
• Building blocks of life
– Composed of carbon, hydrogen, oxygen and nitrogen
• Found on some meteorites
• Can form from inorganic settings without free oxygen
• Miller and Urey demonstrated this
Life in the Archean-Archaeabacteria
• Some require extreme conditions to live
• Many require iron, phosphorus, zinc and nickel
– Dissolved from rocks along MORs
• Live through chemosynthesis
• Gives insight earliest life on Earth
Life in the Archean
•Rich in spreading centers/subduction zones
•Mid-ocean ridges
–Anoxic environment
–Amino acids organize into more complex genetic, organic materials
•Have no fossil record from Archean hydrothermal vents
–Only in younger strata/modern MOR environments
–Both formed in chemically different ocean
•Origin of life on earth =handful of hypotheses
•Life elsewhere in Solar System?
–Early Earth “seeded” with life forms (panspermia)?
–Not impossible
•Search for fossilized life from elsewhere=same criteria as Earth
•Search=fruitless
The Oxygen Revolution
•Early atmosphere=no free oxygen/no ozone layer
•Early oceans
–rich in dissolved iron/silica
•present
in rare amounts today
•Today
–Silica extracted from ocean at almost same rate as tests dissolved
–Iron = only dissolved in oxygen-poor water
• e.g. Red Sea
•where only bacteria can survive
The Oxygen Revolution=BIFs
•Can be traced for hundreds of miles
•Laid down uniformly & continuously
•Can only be formed by oxidation reaction
•Early atmosphere had no free oxygen
–Volcanoes do not produce pure oxygen
The Oxygen Revolution
•Formation of BIFs/composition of present atmosphere require appearance of free
oxygen in atmosphere
•Cyanobacteria (producers of stromatolites)
–Present
–Photosynthesizing
–Generating free oxygen
•Presence of free oxygen
–new niche to be filled: iron-oxidizing bacteria
•No ozone layer, UV radiation was very high
•Free oxygen in ocean, inorganic precipitation could occur
The Oxygen Revolution
• With free oxygen in ocean and atmosphere,
– new ecological niche could be filled=respiration
• Free oxygen in atmosphere:
– UV produced the ozone layer
– Now blocks most UV radiation
• Uraninite, not found in rocks younger than 2.3 billion years ago
• 1.8 billion of years ago, BIFs = become extremely rare
• 1.7 billion years ago, sedimentary copper deposits appear in rock record