Download Planetary Accretion and the Origin of Crust

Unfinished Earth
History and modern continuation of
planetary accretion and
The Origin of Crust
Earth’s Early History
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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
Craters on the Moon – Hadean
and later bombardment
Earth – the water planet
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
Crater Preservation
Australian “Outback”
Quebec, Canada
• Arid climate is more
likely to preserve craters
than humid climate
• Less weathering
• Less erosion
Location of Craters on Earth
Craters in North America
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!
Berringer Crater, Arizona
• Young impact structure
• Approximately 40,000
years old
• Arid climate – excellent
preservation
• Remnants of meteorite
found in crater and in
ejecta
Planetary Accretion Today:
Shoemaker-Levy 9
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
A Focus on Earth
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
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
Evidence for Earth’s Internal Structure
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Refraction of seismic waves
Density of the entire planet
Gravity (mass) of entire planet
Composition of meteorites
Existence of magnetic field
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!
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
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
Plate Tectonics and
the Origin of Crust
• Direct link with formation of oceanic crust
(sea floor spreading)
• Less obvious connections with continental
crust (continental accretion)
Plate Tectonics and
the 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