1. The Archean Era of Precambrian Time
Chapter 11
The Archean Era
of Precambrian Time

2. Guiding Questions When and how did Earth and its moon come into being?
How did the core, mantle, crust form?
Where did Archean rocks form, and what is their nature?
When and why did large continents begin to form?
Where did life arise and what kinds of life existed at the end of Archean time?
Why did relatively little free oxygen accumulate in Earth’s atmosphere through Archean time?

4. Precambrian Archean Precambrian Time prior to Phanerozoic Era
Archean Eon
4.6–2.5 billion years ago
Proterozoic Eon

5. Precambrian Geology Cratons Precambrian shield
Large under-formed portions of continents
Primarily Precambrian
Precambrian shield
Craton exposed at surface
Canadian Shield exposed by glaciation

6. Precambrian Geology Continental crust formed during Archean
High heat flow required small continents

7. Origin of the Universe
Provide important information concerning age of Earth
Fragments of larger bodies that have undergone collision and broken into pieces

8. Origin of the Universe Stony meteorites Iron meteorites
Rocky composition
Iron meteorites
Metallic composition
Stony-iron meteorites
Mixture of rocky and metallic
Proxy for core composition
Most date around 4.6 billion years ago

9. Origin of the Universe Stars cluster in galaxies Milky Way
Organized in disks
Milky Way
Our galaxy of stars

10. Origin of the Universe Expanding universe Big Bang Galaxies move apart
Redshift
Originally concentrated into a single point
Big Bang
15 billion years ago
Age of universe

11. Origin of the Universe Galactic matter is concentrated Stars form
Our Sun
Supernova
Exploding star
Solar nebula
Dense rotational cloud

12. Origin of the Solar System
Planets formed near time of sun’s formation
4.6 billion years ago
Planets far from sun are formed from volatile elements
Planets close to sun are rocky

13. Origin of the Solar System
Rocky debris
Collided to form aggregates
Aggregates collided to form asteroids
40 km diameter
Some coalesced to form planets

14. Origin of Earth and Moon
Earth materials differentiated
Dense at center
Less dense silicates rose to surface
Magma ocean
Cooled to form crust
Meteorite impacts increased concentrations of some elements in upper Earth

15. Origin of Earth and Moon
Moon formed from impact
Mantle of impacting body
Proportions of Fe and Mg differ from Earth’s mantle

16. Origin of Earth and Moon
Early atmosphere
Degassing from volcanic emissions
CH4 and NH3 abundant
Little O2
No photosynthesis
Earth’s oceans
Volcanic emissions cooled, condensed
Salts
Carried to sea by rivers and introduced at ridges
Approximately constant through time

17. Origin of Earth and Moon
Moon’s maria
Originally thought to be seas
Craters formed by asteroids
Floored by basalts
Craters
3.8–4.6 billion years old
Earth also impacted
Tilted Earth 23.5°

18. Origin of Earth and Moon
Heat Flow
Decreased through time
Indicates abundant hot spots, small lithospheric fragments

19. Origin of Continents First crust Felsics differentiated
Basalt (oceanic)
Felsics differentiated
Formed nuclei of continental crusts
Iceland
Modern analogue

20. Origin of Continents Small Archean fragments Zircon crystals
High heat flow limited continental thickness
Zircon crystals
4.1–4.2 B years old
Weathered from felsic rocks
Canadian Shield
3.8–4.0 B years old

21. Origin of Continents Greenstone belts Weakly metamorphosed
Abundant chlorite
Green color
Nested in high-grade felsic metamorphic rocks

22. Origin of Continents Greenstone belts contain igneous rocks
Volcanics contain pillow basalts
Underwater extrusion
Formation of sediments in deep water
Graywackes, mudstones, iron formations, volcanic sediments

23. Origin of Continents Banded iron formations 3 Billion years old
Isua, southern Greenland

24. Origin of Continents Continental accretion
Deep water sediments accreted to continent
Marine sediments form wedge between continental masses

25. Archean Life Earth is best suited known planet
Conditions right by 4.2 Billion years
Western Australia organic compounds
3.5 Billion years
Mars
Water flowed once
Life may have evolved separately

26. Archean Life
South African cherts contain possible mold of prokaryotic cell
3.4 Billion years
Oldest unquestionable life form
3.2 Billion years old
Australia
Intertwined filaments

27. Archean Life Stromatolites Biomarkers for cyanobacteria
3.5 Billion years
Suggest photosynthesis
Biomarkers for cyanobacteria
2.7 Billion years

28. Archean Life Miller and Urey Modeled primitive atmosphere
Produced amino acids found in proteins
Modeled primitive atmosphere
Added lightning
Included oxygen
Amino acids found in meteorites

29. Archean Life RNA world Foundation for DNA world Nucleic acid
Can replicate itself
May have been catalyst for production of key proteins
Foundation for DNA world

30. Archean Life Sulfur reduction Mid-ocean ridges Methane production
S + H2 − > H2S + energy
Methane production
CO2 + 4H2 − > CH4 + 2H2O + energy
Mid-ocean ridges
High heat
Chemosynthetic organisms
Hydrogen oxidation
2H2 + O2 − > 2H2O + energy

31. Archean Life Ridges offer wide range of temperatures
Organic compounds readily dissolve in warm water
Protection from ultraviolet radiation
Abundant phosphorous
Contain metals
Contain clays

32. Archean Life Atmospheric Oxygen Sink Low concentrations early on
Later, O2 released through photosynthesis
Sink
Reservoir that grows so as to take up a chemical as it is produced
Early crust was sink for O2
Pyrite (FeS2) transported but not oxidized