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