Transcript 8.5 Hadean Eon

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
Cryptic
4.56 GA
Basin Groups
Lower Imbrian
4.1 GA
Early
Bombardment
Phase
Nectarian
3.8 GA
Late Bombardment Phase
The Cryptic “Era” – 4.56 to 4.1 Ga
Cryptic because not much geological evidence on Earth has
survived to the present day.
Correlative to the Early
Bombardment Phase of
Earth/Moon history
Late Bombardment
Phase 4.1 to 3.8 Ga
(beginning of Archaean
Eon).
Nebular Hypothesis for formation of our solar system
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).
Nebula
image taken by
Hubble Space
Telescope.
The Nebular Hypothesis:
• 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).
Nebular Hypothesis
Formation of Earth’s layered structure
As Earth formed, the decay of radioactive elements and heat from
high-velocity impacts (meteors and asteroids) caused the
temperature to increase.
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 olivine xls
Note Ca- and Al-rich minerals
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
- 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
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