EARTHS CRUST

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• EARTHS CRUST
• and
• INTERIOR

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Different thickness
Crustal thickness Continental crust 1570
km Oceanic crust 7 km
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Different seismic velocities
10
20
30
40
Velocity
What can we infer about the relationship between
density and velocity? thickness of plates?
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Different seismic velocities
10
20
30
40
Velocity
Average seismic (P-) wave velocitiesContinental
crust 6 km/sOceanic crust 7 km/sLithospheric
mantle 8 km/s
Velocity is related to density  oceanic crust is
denser
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Different strengths
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What do we have so far?
OCE CON Physical properties Thickness 7
km 15-70 km Seismic velocity 7 km/s 6
km/s Heat flow predictable not Strength unifor
m, high variable
What else do we know?
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Ages of oceanic (inset) and continental crust
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Pacific Ocean depth profile
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Continents show a wide variety of geologic origins
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OCE CON Physical properties Thickness 7
km 15-70 km Seismic velocity 7 km/s 6
km/s Heat flow vs age predictable not Strength u
niform, high variable Other properties Elevation
vs age predictable not Composition uniform varia
ble
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Oceanic crust formation
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Formation of continental crust
Original formation of ancient continent
cores Accretion of new terranes
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Cratons old continental cores
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Cratons
Old and cold continental crust Formed
between 3.3 and 1.7 Ga  Products of the first
partial melting(s) of the bulk Earth
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Putting it together
Oceanic crust uniform strength and
composition globally, predictable thickness, heat
flow, age and water depth, high seismic
velocity  Formed by a single process
 mid-ocean ridge magmatism
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Putting it together
Continental crust variable strength and
composition globally, variable thickness, heat
flow, age and elevation, lower seismic
velocity  Formed by multiple processes
originally by partial melting, and subsequently
by various accretion processes
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Quirky viewpoints you could take
The original differentiation of the Earth is
most important (low density prevents continents
subducting)  Mantle convection is most
important (mixes up the MORB source)  Presence
of surface water most important (leads to the
creation of more felsic melts in subduction
zones)