Post on 28-Dec-2015
transcript
Hadean Eon & the formation of Earth
The Hadean Eon
Basal part of the former Precambrian Era
Hadean Eon subdivided into several unofficial “eras” based on lunar geology
Basin GroupsCryptic
4.56 GA 4.1 GA
Early Bombardment
Phase
Nectarian Lower Imbrian
3.8 GA
Late Bombardment Phase
Cryptic because not much geological evidence on Earth has survived to the present day.
The Cryptic “Era” – 4.56 to 4.1 Ga
Correlative to the Early Bombardment Phase of
Earth/Moon history
Late Bombardment Phase 4.1 to 3.8 Ga (beginning
of Archaean Eon).
Nebulaimage taken by Hubble Space
Telescope.
It is currently believed that the
Earth and the other planets formed at
essentially the same time from a cloud of
gas – mostly hydrogen and
helium (nebula).
Nebular Hypothesis for formation of our solar system
• 15-18 Ga Big Bang occurs – instantaneous formation of all matter in our universe.
• ~ 5 billion years ago, swirling mass of the nebula began to contract.
• A flat, disk-shaped cloud with thickened center forms.
• The inner planets begin to accrete from metallic and rocky clumps (protoplanets). ~ 10Ma
• Material at center coalesces, shrinks, becomes heated to several millions of degrees; hydrogen atoms begin to fuse (sun).
The Nebular Hypothesis:
Nebular Hypothesis
As Earth formed, the decay of radioactive elements and heat from high-velocity impacts (meteors and asteroids) caused the
temperature to increase.
Formation of Earth’s layered structure
Iron and nickel began to melt and sink toward the
center.
Lighter rocky components migrate outward toward
the surface.
Gaseous material escapes from Earth’s interior, producing
primitive atmosphere.
Formation of Earth’s inner layers
Structure of the Moon
c) Impactor hypothesis– A Mars-sized object collides with a young Earth .
– Material ejected from the collision of the impactor and from the Earth forms the Moon.
– The differences in chemical composition between the Earth and the Moon is explained
• Earth’s high density from contribution of the impactor’s core.• Moon’s lower density because it originated from Earth’s crust.
– Mechanics of collision explain Earth-Moon orbit.
Meteorites – fragments of shattered planets
Fe - Ni
Stony
Note Ca- and Al-rich mineralsNote olivine xls
Mars-size object collides approx. 60 Ma after formation of Earth – results in a planet with a larger than normal core and the formation of the Moon.
Earth(South Pole)
moon
Theia
Meanwhile back on Earth
4.5 – 4.4 Ga: Magma seas, komatiite crust, and liquid water
Hadean Factoids
• First Earth crust destroyed during formation of the Moon• Subsequent crust formed from a molten “ocean” of magma (much
higher temps than today’s magmas).• Composition of first crust was ultramafic komatiite (4.5 Ga).
Oldest zircons from Australia indicate granitic rocks were
already forming 4.4 Ga
komatiite
Oldest sedimentary rocks in Greenland 3.8 Ga contain banded iron and possible organic carbon – alternating bands of oxygen-rich and oxygen poor iron layers
- Rock vapor atmosphere forms after moon-forming collision - vapor condenses & results in CO2 atmosphere with H and H20 vapor- liquid water even though surface temps over 250deg C – due to high CO2 pressure