M_W_Chapter_19-2_Archean_and_Proterozoic_short_version -...

What is the Precambrian?
•
All rocks below the Cambrian
•
Adam Sedgwick called the “basement” or
unfossiliferous rocks beneath his Cambrian
rocks in Wales “pre-Cambrian”
•
Turns out to represent 88% of geologic time (4.6
Ga - 545 Ma) and contains an important record
of early life!

Subdivisions of Precambrian
•
Early concepts ( too simple!!! ):
–
Hadean Eon: 4.6 to 4.0 Ga
•
no surviving rocks, probably many meteorite
impacts
–
Archean Eon: 4.0 to 2.5 Ga
•
mostly metamorphic rocks (“basement” of
continents)
–
Proterozoic Eon: 2.5 to 0.5 Ga
•
early sediments, not obviously fossiliferous

Precambrian Time Scale and
Terminology
•
Keeps being modified! The time-scale
in your book (Figure 19.2) is far too
detailed for our needs and already out-
of-date!

“Genesis” and the “Hadean”
•
Time before preserved sedimentary
record (boundary could change with
new discoveries) – 4.6 Ga to about 3.85
Ga
•
Accretion and early differentiation
(oldest rocks dated are 4.28 Ga)
•
Moon-forming giant impact (4.51 Ga) as
boundary between Genesis and
Hadean

“Genesis”
•
Term now being used for first 50 million years of
Earth history (4.56-4.51 Ga)
•
Accretion and differentiation
•
4.56 Ga is age of oldest meteorites, considered also
to be birth of the Earth from cloud of planetesimals
•
4.51 age of moon – formed by impact between Earth
and Mars-sized body (NOT MARS!)

•
Growth of planet by
accretion:
•
gravitational energy
•
meteor bombardment
•
Earth homogenous mixture
of materials
•
Gravitational compression shrinks early Earth
•
Gravity concentrated denser materials (Ni & Fe) in center
•
Less densematerials (silicates) forced to outer layers
The Genesis Earth
Accretion

Compositional Zones
•
Crust
•
Continental
•
Oceanic
•
Mantle
•
Core
•
Outer (Liquid)
•
Inner (Solid)
The Genesis Earth
Differentiation

PRECAMBRIAN Evolution!
Atmosphere
Crust-Mantle-Core
Differentiation
Tectonics
Life

“Genesis” and Hadean
Bombardment
•
Meteorite impacts
and cratering
Protero
zoic
Archean
Hadean
Genesis
Formation of Moon
Lateheavy bombardment (Earth and Moon)
Oldest known rock 4.28 Ga

What was the Hadean like?
•
No rocks - no record
•
Accretion: heavily bombarded by meteorites
•
Hot: gravitation, decay of short-lived isotopes
•
Molten
•
Differentiation into crust, mantle, and core

What was theHadean like?
•
Poisonous atmosphere, no oxygen
•
No ozone layer: lethal UV radiation
•
No surface water
•
Moon was much closer than today
•
Intense volcanic activity
•
Somecrust existed (detrital zircon crystals dated at 4.3 to 4.4 Ga found in
Australia)

Where did our Atmosphere come from?