1. Chapter 8 HADEAN & ARCHEAN EONS
PRECAMBRIAN PART 1
Chapter 8
HADEAN & ARCHEAN EONS

2. TOPICS Formation & Evolution of: Rocks Atmosphere Hydrosphere
Life Forms

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5. Defining Characteristics of 3 Eons
Hadean: 4.6–4.0 bya
formation of Earth’s crust and main bombardment
Archean: 4.0–2.5 bya
first life appears
plate tectonics established
oxygen-poor atmosphere
Proterozoic: 2.5 bya–542 mya
first multicellular animals at end of interval
4 major mountain-building episodes
• oldest known glaciation

6. t08_03_pg226 PRECAMBRIAN EONS PROTEROZOIC EON ARCHEAN EON HADEAN EON
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HADEAN EON
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7. Formation of the Earth
Cold Acretion or Hot Acretion Models resulting in differentiation of layers
Formed from the coalescing and bombardment by objects from space ranging in size from dust particles to small planets (planetismals).
The impacts of large bodies and the decay of radioactive elements generated heat that melted the materials of the young Earth, creating the “hellish” conditions for which the Hadean Eon was named.
Evidence of bombardment is the craters on the moon that occurred at the same time (4.0 to 3.8 bya).
Heat on the Earth was generated through decay of radioactive elements and continued frequent bombardment by asteroids,
Earth lost heat to space and slowly cooled.
Earth eventually segregated into an iron core and silicate mantle..

8. Earth’s Crust
Once differentiation occurred, Earth's crust was dominated by iron and magnesium silicate minerals.
Covered by an extensive magma ocean in the Archean.
Magma cooled to form rocks called komatiites.
form at temperatures greater than those at which basalt forms (greater than 1100oC).
ultramafic rocks composed mainly of olivine and pyroxene. This rock formed Earth's Archean crust.
The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of rocks in the upper mantle.

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ARCHEAN
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10. Earth's Crust Today Earth has two types of crust today:
Oceanic Crust - dense, mafic (magnesium- and iron-rich) dominated by basalt.
Continental Crust - less dense, sialic (silicon- and aluminum-rich) dominated by granite.
Formed about 4 bya.

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12. Early Atmosphere

13. Evolution of the Atmosphere and Hydrosphere
Earth's first, primitive atmosphere lacked free oxygen.
The primitive atmosphere was derived from gases associated with the comets and meteorites which formed the Earth during accretion. The gases reached the Earth's surface through a process called outgassing.
Volatiles = substances easily driven off by heating.
Primitive Atmosphere
Earth's gases were originally derived from impacts of comets and meteorites.

14. Volcanic Outgassing
Outgassing = the release of water vapor and other gases from the Earth through volcanism.
Analysis of samples from Hawaiian eruptions include the following gases:
70% water vapor (H2O)
15% carbon dioxide (CO2)
5% nitrogen (N2)
5% sulfur (in H2S)
chlorine (in HCl)
hydrogen
argon

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Volcanic Outgassing
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17. The Early Anoxic Atmosphere
Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen or O2 gas)
The composition would have been similar to that of modern volcanoes, but probably with more hydrogen, and possibly traces of methane (CH4) and ammonia. If any free oxygen had been present, it would have immediately been involved in chemical reactions with easily oxidized metals such as iron.

18. Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere
Lack of oxidized iron in the oldest sedimentary rocks.
Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sedimentary rocks.
Rocks are commonly dark due to the presence of carbon, which would have been oxidized if oxygen had been present.
Archean sedimentary sequences lack carbonate rocks but contain abundant chert, presumably due to the presence of an acidic, carbon dioxide-rich atmosphere. In an acidic environment, alkaline rocks such as limestone do not form.

19. Oxygen 1) How... and 2) when did oxygen enter the scene?

20. Oxygen 1) How did oxygen enter the atmosphere and hydrosphere?
Answer: Cyanobacteria (e.g., the microorganisms comprising stromatolites)
and photosynthesis
Photosynthesis
6CO2 + 6H2O  C6H12O6 (“sugar”) + 6O2

21. Oxygen When? Rocks give us clues
Banded iron formations (BIFs) are marine rocks composed of Fe2O3 and silica. They date from 3.5 billion years, but became pervassive 3.1 billion years ago.

22. Oxygen
BIFs are thought to have been formed through oxygenation of sea water containing Fe2+ (reduced iron).
Fe  Fe3+
Soluble Insoluble

23. Oxygen What about the atmosphere?
Around the world, we see a shift in color of river floodplain shale
Green to Red
Fe  Fe3+
At 1.8 GA

24. Oxygen
So the big change (oxygenation) culminated in Archean oceans (c. 3.1.GA) courtesy of cyanobacteria.
The atmosphere became oxidizing by 1.8 GA and reached near current levels by the Ordovician.

25. Hydrosphere

26. Formation of the Hydrosphere
Source:
1) Volcanoes &
2) Comets
Most of the water on the surface of the Earth and in the atmosphere was outgassed in the first billion years of Earth history. We know this because there are 3.8 billion-year-old marine sedimentary rocks, indicating the presence of an ocean by 3.8 billion years ago.
Hydrologic Cycle initiated

27. Formation of the Hydrosphere
Seas were originally freshwater (rain).
Water more acidic than today.
Ions accumulated in the water, increasing the salinity.
Much later, when the seas became less acidic, calcium ions bonded with carbon dioxide to form shells of marine organisms and limestones.
The presence of marine fossils suggests that sodium has not varied appreciably in sea water for at least the past 600 million years.

28. Earth’s Hydrosphere
All water on, in and over the Earth is recycled via the hydrologic cycle

29. Rock Deposits
Precambrian Rocks

30. Precambrian Rock Deposits
Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.
Various Precambrian provinces can be delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.
3 types of Precambrian Deposits
Shields – Base precambrian rock.
Platforms – Layer of sedimentary rock that covers the shields.
Cratons – Combination of the Shields and Platforms.

31. Precambrian Rocks Precambrian rocks are poorly exposed.
Many Precambrian rocks have been eroded or metamorphosed.
Most Precambrian rocks are deeply buried beneath younger rocks and exposed in fairly inaccessible or nearly uninhabited areas.
Fossils are seldom found in Precambrian rocks; only way to correlate is by radiometric dating.

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34. Granulites and Greenstones
The major types of Archean rocks on the cratons are:
Granulites – Formed from highly metamorphosed crust and sediments in subduction zones.
Greenstones - Metamorphosed volcanic rocks and sediments derived from the weathering and erosion of the volcanic rocks. Greenstone volcanic rocks commonly have pillow structures, indicating extrusion under water. There is a specific sequence of rock types in greenstone belts. These include:
Ultramafic volcanic rocks near the bottom (komatiites)
Mafic volcanic rocks (basalts)
Felsic volcanic rocks (andesites and rhyolites)
Sedimentary rocks at the top (shales, graywackes, conglomerates, and sometimes BIF), deposited in deep water environments adjacent to mountainous coastlines.

35. Plate Tectonics

36. Origin of Plate Tectonics
By about 4 b.y. ago, the Earth had probably cooled sufficiently for plate formation.
Once plate tectonics was in progress, it generated crustal rock that could be partially melted in subduction zones and added to the continental crust.
Continents also increased in size by addition of microcontinents along subduction zones.
Greater heat in Archean would have caused faster convection in mantle, more extensive volcanism, more midoceanic ridges, more hot spots, etc.
Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.

37. Glaciation

38. Glaciation
By 2.8 billion years ago, Earth had cooled sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.

39. Archian Life

40. How Did Life Get Started?
1953: two very clever biochemists (Stanley Miller and Harold Urey) conducted some experiments that duplicated the composition of the Earth’s atmosphere 3 or 4 billion years ago.
They added water (oceans), and electricity (lightning) and made it a closed system.
The result…. Organic chemical reactions

41. Life of the Archean - The Fossil Record
The earliest evidence of life occurs in Archean sedimentary rocks.
Evidence of Archean life consists of:
Stromatolites - An organo-sedimentary structure built by photosynthetic cyanobacteria or blue-green algae. They are not true fossils.
Stromatolites form through the activity of cyanobacteria in the tidal zone. The sticky, mucilage-like algal filaments of the cyanobacteria trap carbonate sediment during high tides. Modern stromatolites are found today in isolated environments with high salinity, such as Shark Bay, western Australia.

42. The Earliest “Visible” Fossils
The earliest fossils that you can see in rocks are called stromatolites. They are colonies of photosynthetic prokaryotes called cyanobacteria.
450 MA stromatolites from Newfoundland

43. Stromatolites
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44. Other evidence of Archean life:
Oldest direct evidence of life b.y.
Indirect evidence of life in older rocks b.y.
Algal filament fossils b.y.
Spheroidal bacterial structures b.y.
Molecular fossils b.y.

45. THE END