unfinished earth history and modern continuation of planetary accretion and the origin of crust
TRANSCRIPT
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Unfinished Earth
History and modern continuation of planetary accretion and
The Origin of Crust
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Summary: Nebular Hypothesis
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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
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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 Earth’s 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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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
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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
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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!
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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
Earth’s lithosphere Earth’s lithosphere (crust +uppermost (crust +uppermost mantle) is divided into mantle) is divided into platesplates
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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