Unfinished Earth

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Unfinished Earth

• History and modern continuation of planetary
  accretion and
• The Origin of Crust

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Earths Early History

• Hadean Eon Earths 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

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Craters on the Moon Hadean and later bombardment
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Earth the water planet
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Earth the water planet

• Presently, Oceans cover 71 of Earths surface
• Odds of impact on land are 29
• Water and oxygen weather rocks and destroy
  evidence of impacts

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Crater Preservation

• Arid climate is more likely to preserve craters
  than humid climate
• Less weathering
• Less erosion

Australian Outback
Quebec, Canada
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Location of Craters on Earth
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Craters in North America
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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!

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Berringer Crater, Arizona

• Young impact structure
• Approximately 40,000 years old
• Arid climate excellent preservation
• Remnants of meteorite found in crater and in
  ejecta

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Planetary Accretion Today Shoemaker-Levy 9
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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

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A Focus on Earth
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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

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Earths Layered Structure

• Gravity caused Earths 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

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Evidence for Earths Internal Structure

• Refraction of seismic waves
• Density of the entire planet
• Gravity (mass) of entire planet
• Composition of meteorites
• Existence of magnetic field

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Earth The Unfinished Planet

• Earth continues to lose heat
• Volcanism brings material to Earths surface
• Other processes (subduction) return more dense
  material to interior
• Conclusion
  Earth is still under construction!

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Theory of Plate Tectonics

• Plates move as a result of heat inside the Earth
• Plates interact to cause
• Earthquakes
• Volcanos
• Mountain systems

• Earths lithosphere (crust uppermost mantle) is
  divided into plates

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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

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Plate Tectonics andthe Origin of Crust

• Direct link with formation of oceanic crust (sea
  floor spreading)
• Less obvious connections with continental crust
  (continental accretion)

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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