Early Earth: The First Two Billion Years

Question 1

How do we know about Earth in the Precambrian?

Answer 1

Precambrian rocks are poorly exposed -> eroded or metamorphosed or deeply buried beneath younger rocks
Fossils are seldom found in Precambrian rocks
Long time ago - Does “Uniformitarianism” apply?
87% of Earth history is poorly known – a fragmented record

Question 2

when was the Hadean?

Answer 2

4.55-4.0 Ga

no rocks are preserved from this time

Question 3

when was the Archean?

Answer 3

4.0-2.5 Ga

Question 4

what are the great precambrian events?

Answer 4

4. 0 Ga - oldest known rocks
5. 5 Ga - oldest known fossils
6. 0-1.6 Ga - transition to oxygen atmosphere and approx origin of eukaryotes
7. 54 Ga (540 Ma) - oldest animal fossils

Question 5

what happened in the first 50 million years with regards to earths core and layering?

Answer 5

Layering requires process of differentiation
Initial heating, partial melting: ‘magma ocean’
Fe metal sinks to form core
Less dense ‘silicate’ melt (Si,O, remaining other elements) forms lighter mantle & crust

Question 6

Why was early Earth hot?

Answer 6

Heat from gravitational contraction
Accretionary heat from asteroid impacts
Radioactive decay

Question 7

what is a possible explanation for the formation of the moon?

Answer 7

(4.5 Ga)

Giant Impact Hypothesis (the Big Splash) -> Theia crashes into earth -> moon forms

Question 8

what is the evidence for liquid water in the early hadean?

Answer 8

Earth’s oldest minerals
Jack Hills, Australia
4.4 billion yr-old zircons
Isotope data
-> Magma in which the zircons formed included melt from crustal material that must have interacted with liquid water
-> Liquid water present during early Hadean

Question 9

when are the earths oldest preserved rocks from?

Answer 9

the Archean

Question 10

where are precambrian rocks found today?

Answer 10

archean rocks: form cores of continents -cratons

- metamorphosed granite, volcanic & sedimentary rocks in ‘belts’

Question 11

what are the earths oldest rocks?

Answer 11

Acasta Gneiss, NWT, Canada
3.962 Ga
Oldest rock age (igneous origin)

Question 12

what is evidence for liquid water from the Archean?

Answer 12

Isua, West Greenland
3.8 Ga
Deformed pillow lavas - Implication?
- subaqueous eruption
- ‘ocean water’ existed

Question 13

what is the ‘faint young sun’ paradox

Answer 13

Early Earth Surface Temperature Should be Freezing (no liquid water)
4.5 Ga:
Sun’s output only 70% that of today (Stellar evolution)
Too cold to maintain a liquid ocean -> refuted by geologic evidence

Question 14

Why are there so few rocks older than ~3.8 Ga?

Answer 14

Late Heavy Bombardment
‘Impact zones’ & impact melt rocks on the Moon
Ages peak at 3.8 Ga
Consequence for Earth’s surface nearby? -> obliterated

Question 15

what were plate tectonics like in the Archean?

Answer 15

Higher internal temperature of the Earth
Faster plate motion?
Many small, mobile plates
Early crust oceanic –continents formed later

Question 16

what is some evidence for continents in the Archean (4-2.5 Ga)?

Answer 16

Oldest rocks (Acasta Gneiss, 3.96 Ga; Isuapillow lavas, 3.8 Ga)
3.46 Ga fossil soil (paleosol): Pilbara region, Australia
Liquid oceans -> warm greenhouse atmosphere
Subaerial weathering, soil formation
Continents above sea level

Question 17

what was the early atmosphere like?

Answer 17

Initial H, He lost to space
Volcanic outgassing -> water vapour, CO2, SO2, H2S, CH4.
Meteorites/comets -> water, nitrogen
Convection in core -> magnetic field deflects solar wind, so gases/atmosphere can accumulate
Early atmosphere dense, very hot
Mostly water vapour, CO2, nitrogen
Very little oxygen -> Earth inhospitable to most forms of life as we know it today

Question 18

what are most Archean fossils?

Answer 18

stromatolites and single cells

Question 19

what is a stromatolite?

Answer 19

dome-like layered structure formed from mat-like colonies (of single-celled cyanobacteria) that trap sediment and calcium carbonate
Oldest stromatolites: 3.5 Ga (W. Australia)

Question 20

where are modern stromatolites found?

Answer 20

extreme environments: e.g., high-salinity Shark Bay, Australia

Question 21

what are the archean single-celled micro-organisms?

Answer 21

Simplest form of modern carbon-based life
Lack DNA-packaging nuclei
Only life on Earth for next 2 billion years

Infer: Photosynthesis: occurring by 3.0 Ga, possibly as early as 3.5 Ga -> oxygen

Question 22

what is needed for the origins of life?

Answer 22

Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulfur
Proteins (chains of amino acids): build living materials, catalysts for reactions in organisms
Nucleic acids (DNA, RNA)
Organic phosphorus: transforms light/chemical fuel -> energy
Cell membrane: encloses cell components

Question 23

how were amino acids formed in a simulated early earth atmosphere?

Answer 23

Miller and Urey experiment (1950’s):
Formed amino acids (building blocks of life)from: H2, CH4(methane), NH3(ammonia), H2O (steam) gases & sparks (simulated lightning)

Question 24

in what type of environment did early organisms form in?

Answer 24

Developed in presence of an oxygen-free atmosphere (anaerobic- no oxygen for respiration)
No oxygen -> no ozone shield(O3) against harmful ultraviolet radiation

Question 25

where did early organisms live?

Answer 25

Deep-sea hydrothermal vents
Hyperthermophiles or microbes thrive in seawater hotter than 100 C
Derive energy by chemosynthesis, not by photosynthesis
Hyperthermophiles are Archaea, different from bacteria (also single-celled)
Possible environment for origin of life