Transcript GEOL3025, Section 030 Lecture #11 31 August 2007

Chapter 12: Earth’s Interior
Propagation of Seismic Waves
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Velocity of seismic
waves depends on
density (ρ) & elasticity of
material
Velocity of waves
increases with depth b/c
pressure (P) makes rocks
more elastic
Refraction of waves
occurs when they pass
from one material to
another
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P waves travel through both
liquids & solids
S waves travel only through
solids
P waves travel faster than S
waves in all materials
P waves
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P waves
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Compressional
First to arrive (primary)
Travel through both
liquids & solids
S waves
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S waves
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Shear waves
Second to
arrive
Travel
through
solids only
Propagation of Waves
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Through Uniform
Solid
Constant velocity with
depth
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Through Solid of
Increasing P
Increasing velocity
with depth
Wave Propagation Through Earth
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Because Earth has
differentiated,
waves travel at
different speeds
through the
different layers
Layers Defined by Composition
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Crust:
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Mantle:
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3-70 km thick outer skin
70-2900 km solid silicate-rich shell
Core:
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Fe-rich sphere with 3486 km radius
Layers Defined by Physical
Properties (Seismic Waves)
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Lithosphere:
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3-250 km thick crust &
uppermost mantle
Cold and rigid
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250-660 km lower upper mantle
Warm and soft
Mesosphere: (lower mantle)
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660-2900 km lower mantle
More rigid than asthenosphere
due to increased P
Inner Core:
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Asthenosphere:
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2270 km thick
Liquid b/c s-waves don’t
penetrate
Convection produces Earth’s
magnetic field
Outer Core:
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Fe-Ni metallic sphere with radius
of 3486 km
Solid because of increased P
Earth’s
Layers
Earth’s Major Boundaries
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The Mohorovičič discontinuity (Moho)
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Divides Crust from Mantle
Seismic velocities higher in Mantle so deeper
waves arrive earlier
Core-Mantle Boundary
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P-wave shadow zone 105°-140°
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Due to high refraction of waves
S-waves do not penetrate at all
Inner-Outer Core
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P-waves reflected off surface of solid inner Core
P-waves faster in inner Core than in outer Core
The
Moho
CoreMantle
Boundary
Core-Mantle Boundary
Discovery of
Inner Core
The Crust
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Averages <20 km thick
Deep oceanic crust compositionally different
from continents
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Continental Crust
ρ ~ 2.7 g/cm3
 Older (up to 4 Ga)
 Felsic (enriched in K, Na and Si)
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Oceanic Crust
ρ ~ 3.0 g/cm3
 Younger (< 180 Ma)
 Mafic (enriched in Mg, Fe)
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The Mantle
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>82% (by volume)
Knowledge from xenoliths and experiments
410 km discontinuity
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660 km discontinuity
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Abrupt increase in wave velocities
Phase change from olivine to spinel
Divides Asthenosphere and Mesosphere
Spinel changes into perovskite
D’’ layer
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Deepest 200 km
Sharp decrease in P-waves velocities
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Implies partially molten
Hot spot plumes may originated from this layer
The Mantle
Phase change
Core-Mantle
Boundary
Origin of Hot Spot Plumes ?
The Core
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Larger than Mars
1/6 of volume and 1/3 of mass
Temperature > 6700°C
Average density 11-14 g/cm3
Knowledge from Meteorites
Probably began all liquid
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Solidifying with cooling of Earth
Earth’s Magnetic Field
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Inner core rotates about 1° faster per year
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From west to east (same as surface)
Axis of rotation offset ~10°
Earth’s Internal Heat Engine
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Geothermal gradient:
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Heat Sources:
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20°-30° per km
Rate of increase much lower in Mantle & Core
Radioactive decay
Heat of crystallization (core)
Accretional heat
Crust: conduction
Mantle: convection
Mantle Convection