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Chapter 12
Earth’s Interior
Probing the Earth’s Interior
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P waves are always faster than S waves
Wave velocity increases with density and stiffness
Passing from one material to another causes waves to refract (bend)
P waves are able to propagate through liquids as well as solids.
S waves cannot pass through liquids
Discovering Earth’s Major Boundaries
 The core-mantle boundary
• Discovered in 1914 by Beno Gutenberg
• Based on the observation that P waves die out at 105 degrees from the
earthquake and reappear at about 140 degrees
• 35 degree wide belt is named the P-wave shadow zone
• P-wave and S-wave shadows zones
 Discovery of the inner core
• Predicted by Inge Lehmann in 1936
• P waves passing through the inner core show increased velocity suggesting
that the inner core is solid
 The Moho (Mohorovicic discontinuity)
• Discovered in 1909 by Andriaja Mohorovicic
• Separates crustal materials from underlying mantle
• Identified by a change in the velocity of P waves
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Seismic waves and Earth’s structure
 Abrupt changes in seismic-wave velocities that occur at particular depths helped
seismologists conclude that Earth must be composed of distinct shells
 Velocity increases at 410 km and 660 km depth are due to phases in the asthenosphere.
Below 410 km, olivine becomes spinel, and below 660 km, spinel becomes perovskite.
Crust
 Two parts
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Continental crust
• Average rock density about 2.7 g/cm3
• Average composition of granodiorite
• 30-70 km thick
Oceanic crust
• Density about 3.0 g/cm3
• Composed mainly of the igneous rock basalt
• 8-10 km thick, except at spreading ridges, where it is very thin
Mantle
 Contains 82% of Earth’s volume
 Solid, rocky layer
 Upper portion has the composition of the ultramafic rock peridotite
 Three parts
• Mantle lithosphere (uppermost mantle) ~70-180 km thick
• Asthenosphere (upper mantle) ~560 km thick
• Mesosphere (lower mantle) ~2240 km thick
Core
 Larger than the planet Mars
 Earth’s dense central sphere
 Two parts
• Outer core - liquid outer layer about 2270 km thick
• Inner core - solid inner sphere with a radius of 1216 km
 Density and composition
• Average density is nearly 11 g/cm3 and at Earth’s center approaches 14
times the average density of water
• Mostly iron, with 5% to 10% nickel and lesser amounts of lighter elements
 Earth’s magnetic field
• Electrically conductive inner and outer core
• Inner core rotates faster than the Earth’s surface and the axis of rotation is
offset about 10 degrees from the Earth’s poles. Makes one extra rotation
every 400 years.
• Magnetic field is generated by a “self-perpetuating dynamo.” Electric
current is generated when liquid iron circulating in the outer core moves
through the magnetic field. The electric current in turn generates a magnetic
field. The system is kept going by the circulation (convection) in the outer
core.
Origin of the layers: the “Great Catastrophe”
 Layers defined by physical properties
 Main layers of Earth’s interior are based on physical properties and hence
mechanical strength
 Layers defined by physical properties
 Lithosphere (sphere of rock)
 Earth’s outermost layer
 Consists of the crust and uppermost mantle
 Relatively cool, rigid shell
 Averages about 100 km in thickness, but may be 250 km or more
thick beneath the older portions of the continents
 Includes crust and upper mantle
 Asthenosphere (weak sphere)
 Beneath the lithosphere, in the upper mantle from depths of ~100 km
to ~ 600 km
 Small amount of melting in the upper portion mechanically detaches
the lithosphere from the layer below allowing the lithosphere to move
independently of the asthenosphere
 Mesosphere or lower mantle
 Rigid layer between the depths of 660 km and 2900 km
 Rocks are very hot and capable of very gradual flow
 Outer core
 Composed mostly of an iron-nickel alloy
 Liquid layer
 2270 km (1410 miles) thick
 Convective flow within generates Earth’s magnetic field
 Inner core
 Sphere with a radius of 3486 km (2161 miles)
 Behaves like a solid
Earth’s internal heat engine
 Earth’s temperature gradually increases with an increase in depth at a rate known as the
geothermal gradient
• Varies considerably from place to place
• Averages between about 20C and 30C per km in the crust (rate of increase
is much less in the mantle and core)
• Estimated temperature at the earth’s center is ~6700 °C
 Major processes that have contributed to Earth’s internal heat
• Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th),
and potassium (K). These elements are concentrated in the continental crust.
• Heat released as iron crystallized to form the solid inner core
• Heat released by colliding particles during the formation of Earth
• Compression from increasing pressure during accretion
 Heat flow in the crust
• Process called conduction
• Rates of heat flow in the crust varies – more in the continental crust than in
the oceanic crust.
 Mantle convection
• Mantle must have an effective method of transmitting heat from the core
outward
• Probably limited to the asthenosphere; the mesosphere is very viscous and
could only convect very slowly or not at all.