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Unfinished Earth

Unfinished Earth. History and modern continuation of planetary accretion and The Origin of Crust. Earth’s Early History. Hadean Eon – Earth’s first 500 Ma Intense bombardment by bolides (meteors, comets, etc.) No permanent crust – destroyed by impacts Little direct evidence on Earth

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Unfinished Earth

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  1. Unfinished Earth History and modern continuation of planetary accretion and The Origin of Crust

  2. Earth’s Early History • Hadean Eon – Earth’s first 500 Ma • Intense bombardment by bolides (meteors, comets, etc.) • No permanent crust – destroyed by impacts • Little direct evidence on Earth • Excellent evidence on Moon, Mars and other terrestrial planets

  3. Craters on the Moon – Hadean and later bombardment

  4. Earth – the water planet

  5. Earth – the water planet • Presently, Oceans cover 71% of Earth’s surface • Odds of impact on land are 29% • Water and oxygen weather rocks and destroy evidence of impacts

  6. Crater Preservation • Arid climate is more likely to preserve craters than humid climate • Less weathering • Less erosion Australian “Outback” Quebec, Canada

  7. Location of Craters on Earth

  8. Craters in North America

  9. Gene and Carolyn Shoemaker Gene Shoemaker, geologist Pioneer in study of impact structures worldwide Studied Berringer Crater, AZ Well-respected in geology, but not in astronomy Until Shoemaker-Levy 9!

  10. Berringer Crater, Arizona Young impact structure Approximately 40,000 years old Arid climate – excellent preservation Remnants of meteorite found in crater and in ejecta

  11. Planetary Accretion Today: Shoemaker-Levy 9

  12. Significance of Shoemaker-Levy 9 • Convinced astronomers that such events were still possible • Focused attention on near-Earth objects • Showed how little we know of such objects • Showed how completely unprepared we are

  13. A Focus on Earth

  14. Why the Gap: 4.6 – 3.98 Ga? • Asteroids, meteorites, comets, planetesimals collide as Earth accretes to form homogeneous protoplanet • Intense bombardment during continued accretion destroyed primeval crust (if there was any) • Inertial heating from impacts and radioactive decay may have liquified the entire planet • This afforded mobility for the start of density differentiation

  15. Earth’s Layered Structure • Gravity caused Earth’s interior to differentiate into layers • Layers are arranged according to density • Most dense material sank to the center – core • Density decreases outward • Density layering includes the hydrosphere (oceans) and atmosphere

  16. Evidence for Earth’s Internal Structure • Refraction of seismic waves • Density of the entire planet • Gravity (mass) of entire planet • Composition of meteorites • Existence of magnetic field

  17. Earth: The Unfinished Planet • Earth continues to lose heat • Volcanism brings material to Earth’s surface • Other processes (subduction) return more dense material to interior • Conclusion: Earth is still under construction!

  18. Theory of Plate Tectonics Plates move as a result of heat inside the Earth Plates interact to cause: Earthquakes Volcanos Mountain systems • Earth’s lithosphere (crust +uppermost mantle) is divided into plates

  19. What makes the plates move? • Uneven distribution of heat in the upper mantle (Asthenosphere) causes heat to rise in some places (e.g., Mid-ocean ridges) • Differences in density cause colder, more dense Lithosphere to sink back into the asthenosphere (subduction) • This Convection drives the motion of the plates

  20. Plate Tectonics andthe Origin of Crust • Direct link with formation of oceanic crust (sea floor spreading) • Less obvious connections with continental crust (continental accretion)

  21. Plate Tectonics andthe Origin of Crust • Archean convection may have caused earliest crust to float to surface as “scum” • Subduction returned more dense material to interior • Less dense components were scraped off or returned to surface via volcanism • Plate interactions added continental crust to margins of continents • Quantity of continental crust has increased through time

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