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Accretion and Layering of Terrestrial Planets
Following condensation of planetary embryos in
solar system disk:
„
EmbryoEmbryo-embryo collisions, giant impacts for 100 m.y.
– Redistributed and segregated elements chaotically
– Led to incremental development of FeFe-rich core
„
Sinking of dense metallic elements (Fe, Ni) released
gravitational energy
„
ShortShort-lived radioisotopes = heating
„
These energy sources combined to heat interior to temperature
of 7000 K at earth’s center, 5500 K at corecore-mantle boundary
Accretion and Layering of Terrestrial Planets
„
Heat loss at surface due to
radiation
– thin chilled crust forms
– Insulates thick subsurface
magma ocean
„
Coalescing blobs of Fe sink
through convecting magma
ocean
„
Result is a layerd Earth
– Thin rocky (silcate) crust
» Feldspar+quartz+pyroxene
– Thick rocky mantle
» Olivine+pyroxene (Fe & Mg –
rich silicate minerals)
– Metallic (Fe(Fe-rich) core
The Earth’s Interior
„
Earth’s interior
is largely
inaccessible
The Earth’s Interior
„
„
How do we know that the Earth comprises:
– crust
– mantle
– metallic core (solid and liquid portions) ?
How deep have we drilled?
continental crust
Kola, Russia
12.2 km deep hole
15 years to drill
ocean crust
East Pacific
2.2 km deep hole
10 years to drill
The Earth’s Interior
„
Most of what we know is from geophysics
„
Geophysics is basically “remote sensing” of
Earth’s interior
– seismic waves
– magnetic field
– gravity
– heat
Evidence from Seismic Waves
„
Seismic reflection
– return of seismic energy to surface
– rock layers of different density
» boundary reflects energy like a mirror
» time since earthquake gives depth to boundary
„
Seismic refraction
– bending of seismic waves passing from one rock
to another
– similar to light rays passing through eyeglasses
– generates curved ray paths
Evidence from Seismic Waves
„
Seismic reflection
– return of seismic energy to surface
– rock layers of different density
» boundary reflects energy like a mirror
» time since earthquake gives depth to boundary
„
Seismic refraction
– bending of seismic waves passing from one rock
to another
– similar to light rays passing through eyeglasses
– generates curved ray paths
Earth’s Internal Structure
„
„
Established using seismic reflection, refraction
The crust
–
Continental
»
»
–
Oceanic
»
»
„
Less dense
20-70 km thick
more dense
5-10 km thick
Mohorovičic discontinuity
–
–
Boundary separating crust from mantle
defined by increase in P-wave velocity (to 8 km/sec)
Earth’s Internal Structure
„
The Mantle
– Ultramafic Rock
– Lithosphere
»
–
Asthenosphere
»
»
–
Crust & uppermost mantle
Low velocity zone
lubrication for plate
tectonics
Lower mantle
»
»
»
boundaries at 400 & 670 km
Pressure increases with
depth
more dense mineral
structures
Earth’s Internal Structure
„
The Mantle
– Ultramafic Rock
– Lithosphere
» Crust & uppermost mantle
– Asthenosphere
» Low velocity zone
» lubrication for plate
tectonics
– Lower mantle
» boundaries at 400 & 670 km
» Pressure increases with
depth
» more dense mineral
structures = faster seismic
wave velocities
Earth’s Internal Structure
„
The Core
– P-wave Shadow Zones
– S-wave Shadow Zone
–
What satisfies these observations?
»
A solid inner core, molten liquid outer core
Earth’s Internal Structure
„
A “static” picture
Earth’s Internal Structure
„
Composition of the Core
– Overall density of Earth = 5.5 g/cm3
– Core must be dense to offset
mantle+crust (<3 g/cm3)
– Made of metallic iron
(+Nickel, Oxygen, Sulfer?)
»
»
„
required to generate magnetic field
iron meteorites = planetary interior
– Molten convecting outer/solid inner parts
The Core-Mantle Boundary
– Convection in both core and mantle
– Ultra Low Velocity Layer
– D” layer
A Dynamic Mantle!
„
Seismic Tomography
– “CAT Scan” of mantle
– Seismic waves fastest in:
» Dense cold rock (blue)
» Old, cold, subducted ocean
plates ?
– Waves slowest in:
» Less dense, warmer rock
(red)
» Upwelling hot plumes,
heated by core?
–
Upwelling of deepest
mantle “plumes” coupled
with sinking of subducted
plates (“slabs”)
»
Mantle convection
animation