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The Primordial Earth: Hadean and Archean Eons

The Primordial Earth: Hadean and Archean Eons

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The Primordial Earth: Hadean and Archean Eons

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  1. The Primordial Earth: Hadean andArchean Eons CHAPTER 6

  2. Earth’s Formative Interval (b.y. = billion years) • Hadean: 4.6–3.96 b.y. ago • Archean: 3.96–2.5 b.y. ago

  3. Milky Way Galaxy

  4. Figure 6-2 (p. 198)Schematic view of the solar system, showing orbits of the planets.

  5. The Sun • The sun is a star. • Composition: about 70% hydrogen and 27% helium, and 3% heavier elements. • Temperature: may exceed 20oC million in the interior. • Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen atoms are fused together to form helium. Excess mass is converted to energy.

  6. Figure 6-4 (p. 200)Example of two fusion reactions. (n = neutron).

  7. Examples of Solar Energy in Geology • Evaporation of water to produce clouds which cause precipitation which causes erosion • Uneven heating of the Earth's atmosphere causes winds and ocean currents • Variations in heat from sun may trigger continental glaciations • Sun and moon influence tides

  8. Origin of the Universe Big Bang Theory • Evidence: • The galaxies are rapidly moving apart (Hubble's Law); indicates that galaxies were closer together in the past (This was discovered in 1929 by Edwin P. Hubble. • In 1914, W.M. Slipher first noted the red shift. • Observed temperature of the universe today (background microwave radiation) 3 degrees above absolute zero • Present abundances of hydrogen and helium.

  9. Formation of the Solar System Dynamic Constraints • All planets revolve in same counterclockwise (prograde) direction • All planets lie roughly in one plane • Nearly all planets and moons rotate counterclockwise • Density of planets roughly decreases away from Sun • Planets in two groups • terrestrial • Jovian • Age of Earth and meteorites: 4.6 billion years

  10. Formation of the Solar System Solar Nebula Hypothesis • Cold cloud of gas and dust particles • Dust cloud starts counterclockwise rotation • Eddies in dust cloud begin planetary development: cold, homogeneous accretion • Protoplanetary formation and gravitational collapse form Sun • Solar wind drives out lighter elements

  11. The Hadean Earth Accretion and Differentiation • Heating, partial melting, and solid diffusion • Ni and Fe migration to core • Mantle separation forming lighter crust Source of Internal Heat • Accretionary heat of bombardment • Radioactive decay • Gravitational compression

  12. Earth’s Differentiation • Differentiation = segregated into layers of differing composition and density • Early Earth was probably uniform • Molten iron and nickel sank to form the core • Lighter silicates flowed up to form mantle and crust

  13. The Hadean Earth Development of Magnetic Field and Magnetosphere: product of movement within liquid core Crustal Development • Crust formed by cooling magma ocean • Komatiites (ultramafic patches) formed early in Earth’s crust • Continental crust and water present as early as 4.4 b.y. ago

  14. Evolution of Atmosphere and Hydrosphere Primitive Atmosphere: reducing (before 3.8 b.y. ago) Earth's volatiles: H2O, CO2, O2, etc. Origin of volatiles: carbonaceous chondrites, comets,volcanic outgassing Atmosphere began to accumulate oxygen (3.5 b.y. ago to present) • Photochemical Dissociation: UV light + H2O • Photosynthesis

  15. No Free Oxygen • Lack of oxidized iron in the oldest sedimentary rocks. • Uraninite and pyriteare readily oxidized today, but are found unoxidized in Precambrian sediments • Archean sedimentary 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. • Carbon dioxide and water combine to form carbonic acid. • In such an acidic environment, alkaline rocks such as limestone do not develop.

  16. BIF’s • Banded iron formations are cherts with alternating laminations of red oxidized iron and gray unoxidized iron. • Formation: • UV light • Bacteria • Hydrothermal Vents

  17. Formation of the Oceans • Water vapor (from volcanic outgassing) condensed in the atmosphere and fell as precipitation. • Condensation and collection start @ 4.4 Bya • The rains accumulated in the low places on the Earth's surface to form oceans.

  18. Formation of the Oceans (cont) • Oceans may have been acidic originally (rainwater absorbs CO2 from atmosphere). • Became more basic with input of calcium and iron. • Salts from dissolution of rocks (enhanced when acidic). • Today Earth's water is continuously re-circulated through the hydrologic cycle.

  19. Table 6-2 (p. 219)Time Divisions for the Precambrian.

  20. Precambrian (Archean and Proterozoic) • The Precambrian world was almost certainly as diverse and complex a place as today's world. • The Precambrian is not well known or completely understood. Because: • Many Precambrian rocks have been eroded or metamorphosed • Most Precambrian rocks are deeply buried beneath younger rocks • Many Precambrian rocks are exposed in fairly inaccessible or nearly uninhabited areas. • Fossils are seldom found in Precambrian rocks.

  21. Figure 6-27 (p. 220)Major areas of exposed Precambrian rocks (shown in yellow).

  22. Archean Geology • Craton: • shield • platform • Craton: group of provinces • Seven provinces in North America • collisional orogens between provinces

  23. Figure 6-29 (p. 221)Precambrian provinces of North America.(Adapted from Hoffman, P. F. 1989. Precambrian geology and tectonic history of North America, in Geology of North America, Vol. A, Ch. 16. Boulder, CO: Geological Society of America.)

  24. Expansion of the Continental Crust

  25. Two Major Crustal Rock Associations in Archean • Granulites (granite/gneiss comlexes): gneisses of tonalites, granodiorites, granites, and layered intrusive gabbros • Greenstones: basaltic, andesitic, and rhyolitic volcanic rocks with metamorphosed sediments and basaltic pillow lavas (sequential transition)

  26. Archean Protocontinental Crust • Felsic rocks present • Graywacke and dark shales= deep water near coasts • No broad interior shallow areas because no well-sorted cross-bedded sandstones • No shelf environment

  27. Tectonics • Begin @ 4 Bya • Have evidence of crustal rock movement and ancient subduction zones • Convection faster= since internally very hot still

  28. Life of the Archean: "Age of Prokaryotes" • Beginning of life: inorganic elements, organic compounds, then living cells • Time of first life: 3.5 billion years (bacteria) • Constraints of life • Anaerobic (no oxygen) • No ozone shield • UV light or electrical discharge needed to synthesize organic compounds • Cell components

  29. The Origin of Life Four essential components of life: • Proteins (chains of amino acids linked together), used to build living materials and as catalysts in chemical reactions in organisms. • Nucleic acids • Example- DNA or RNA • Organic phosphorus, used to transform light or chemical fuel into energy required for cell activities. • A cell membrane to enclose the components within the cell.

  30. Experiment on the Origin of Life • During the late 1950s (1953) • Stanley Miller • synthesized several amino acids • by circulating gases approximating the early atmosphere • in a closed glass vessel

  31. Experiment on the Origin of Life • This mixture was subjected to an electric spark • to simulate lightning • In a few days it became cloudy • Analysis showed that several amino acids typical of organisms had formed • Since then, scientists have synthesized all 20 amino acids found in organisms

  32. Ocean Origin • Salts for life • Excellent circulation for chance encounters of compounds • No O2 or organisms

  33. Deep Sea Origins • Origin of life may have been associated with deep sea hydrothermal vents along the mid-oceanic ridge spreading centers. • Evidence: • Hyperthermophiles or microbes at these vents thrive in seawater hotter than 100oC , and can live in fissures deep below the seafloor. • These microbes derive energy by chemosynthesis, without light, rather than by photosynthesis • Hyperthermophiles are Archaea, with DNA different from bacteria

  34. Deep Sea Tube Worm

  35. Archean Fossil Record • Archean fossil evidence of life consists of: • Stromatolites (cyanobacteria) • Organosedimentary structure (not a true fossil) • Forms through the activity of cyanobacteria/blue green algae in the tidal zone. The sticky, mucilage-like algal filaments trap carbonate sediment during high tides • Oldest are 3.5 By old, Warawoona Group, Australia • More abundant later in Poterozoic rocks, but are rarer today

  36. Present-day stromatolites Shark Bay, Australia

  37. Stromatolites • Different types of stromatolites include • irregular mats, columns, and columns linked by mats