Chapter 12 Earths Interior

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Chapter 12Earths Interior
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• How do we gain knowledge about the interior of
  the Earth?
• Drill a well obtain actual samples
• Distance from the surface to the center of the
  Earth is about 6456 km (4035 mi)
• Deepest well drilled so far reached the
  astonishing depth of 12.3 km (7.7 mi)
• Thus, not a viable method
• Volcanic activity brings samples from the depths
  however, only about 200 km (125 mi) deep
• Study of meteorites

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• Most information obtained by analysis of seismic
  (earthquake) waves
• Remember them?
• Measuring the travel times of P and S waves from
  an earthquake, nuclear explosion, or other large
  source, to various seismographic stations
• Nuclear tests and mining blasts are best, as the
  exact time and location of the source energy is
  known

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Nature of Seismic Waves

• Basics of wave (energy) propagation
• Builds on info presented in Chapter 10
• Seismic energy travels away from the source in
  all directions as waves (toss a pebble into a
  body of water and observe the waves)
• Danged physics
• Wave fronts are hard to visualize
• Instead, we use raypaths a line drawn
  perpendicular to the wavefront
• (a diagram is in order here)

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Figure 12.1
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Characteristics of seismic waves

• Velocity of seismic waves depends on the density
  and elasticity of the material
• Within a layer of the same composition, the
  velocity GENERALLY increases with depth (pressure
  increases, making the material more dense)

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• P waves (compressional waves) (particle vibration
  back forth in the direction of travel) travel
  through both solids and liquids
• Material behaves elastically, resists change in
  volume
• S waves (shear waves) (particles vibrate at right
  angles to direction of travel) pass through
  solids only
• Liquids have no shear strength
• When subjected to forces that can change
  materials shape, the material flows (apply
  pressure to ice, and do the same to water)

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• Always, P-wave velocity is faster than S-wave
  velocity
• When a seismic wave passes from one material to
  another, the wave path is refracted (bent) also,
  some of the energy is reflected

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If the Earth was homogeneous (all the same
throughout)

• (dont write this stuff down.)
• Seismic waves would spread in all directions
• Raypaths would be straight lines, with the waves
  travelling at a constant speed
• Therefore, the farther away a seismograph is from
  a source, the longer the travel time would be
  (both P- and S-waves)

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Figure 12.3
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• This is not what is seen on seismograms
• Some seismographs farther from the source record
  the wave arrival before seismographs closer to
  the source
• Due to increase in seismic velocity with depth
  (pressure)
• Assumed gradual change in seismic velocity
• Refraction of the wave energy
• Some waves not recorded at certain places (well
  cus this in a bit)

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Figure 12.4
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• Development of more sensitive seismographs (as
  well as more of them)
• Data indicate abrupt velocity changes at
  particular depths (physics of wave propagation)
• These changes detected world-wide, suggesting the
  Earth is composed of layers having varying
  compositions and/or mechanical properties

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Layers of the Earth

• Based on chemical composition
• From early melting denser elements sank, while
  less dense elements rose
• Three major regions
• Crust 3 to 70 km (2-40 mi) thick
• Mantle solid, rocky, magnesium-iron silicate
  shell to a depth of about 2900 km (1800 mi)
• Core iron-rich sphere

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Figure 12.6
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• Based on physical properties (derived from
  physics seismic data)
• Temperature, pressure, and density increase with
  depth
• These increases affect the physical properties
  mechanical properties of materials
• Increase in temperature weakens chemical bonds,
  reducing mechanical strength
• Increase in pressure increases rock strength
  also increases melting temperature because of
  confining pressure melting requires increase in
  volume

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• Five main layers lithosphere, asthenosphere
  (upper mantle), mesosphere (lower mantle), outer
  core, inner core
• Lithosphere crust uppermost mantle rigid
• Up to 250 km (156 mi) thick
• Asthenosphere base of lithosphere to about 660
  km (410 mi)
• Comparatively weak
• Temperatures/pressures in uppermost part results
  in small amount of melting
• Mechanically detached from lithosphere

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• Mesosphere (lower mantle) from 660 to 2900 km
  (410 to 1810 mi)
• Increased pressure counteracts higher
  temperatures
• Rocks become stronger with depth
• Still very hot, able to flow gradually
• Outer core
• About 2270 km (1420 mi) thick
• Mainly iron-nickel alloy
• Liquid
• Magnetic field generated here

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• Inner core
• Radius of about 1216 km (760 mi)
• Also iron-nickel alloy
• Higher pressures than outer core
• solid

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Boundaries in the Earth

• By analysis of seismic data from seismograph
  stations worldwide
• Laboratory studies determine properties of Earth
  materials under extreme temperature/pressure
  conditions
• Data continues to be collected and analyzed

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The Moho

• The long form Mohorovicic discontinuity
• A layer at about 50 km (31 mi) where seismic
  velocities abruptly increase
• Explanation not simple, will attempt in next
  slide
• Analogy of a driver taking the bypass route
  around a large city during rush hour (the author
  obviously doesnt live in Atlanta)

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Figure 12.7
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Core-mantle boundary

• Also called the Gutenberg discontinuity, after
  its discoverer
• P-waves diminish disappear about 105º away from
  an earthquake focus
• About 140º away, they reappear but about 2
  minutes later than expected based on distance
  traveled
• The P-wave shadow zone

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Figure 12.8
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• P-wave velocity decreases by about 40 on
  entering the core
• Also, S-waves are not observed over 105º away
  from the focus
• These data combined suggest change from a solid
  to a liquid

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Figure 12.9
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Inner core boundary

• Also called the Lehmann discontinuity after its
  discoverer
• Discovery based on both the reflection and
  refraction of seismic waves
• Reflection of seismic waves is much akin to the
  formation of echos

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• Original estimates of depth werent accurate
• Underground nuclear tests in the early 1960s
  allowed refinement of estimates
• Data also indicated that P waves passing through
  this inner core traveled much faster than those
  passing through the outer core

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Figure 12.10
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Some details on the layers of Earth

• Crust
• Averages less than 20 km (12 mi) thick ranges
  from 3 km (1.9 mi) to over 70 km (40 mi) thick
• Continental rocks average 2.7 g/cm3, and are up
  to 4 b.y. old
• Average composition of upper continental crust
  is that of granodiorite (a mix of granite and
  diorite)
• Lower continental crust is likely closer to basalt

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• Oceanic rocks are more dense, 3.0 g/cm3, rather
  young (less than 200 m.y.)
• Composition predominantly basalt

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Some alternate ideas about the crust
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Mantle

• Both layers of the mantle comprise over 82 of
  Earths volume
• What is known is based on experimental data and
  examining material brought to the surface by
  volcanic activity
• i.e., kimberlite pipes likely from about 200 km
  depth, roughly peridotite in composition
• Overall composition thought to be that of the
  mineral olivine

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• Break between the upper lower mantle defined by
  an abrupt increase in seismic velocity
• About 410 km (256 mi) depth
• Result of a phase change
• Crystal structure of a mineral changes as a
  result of changes in temperature and/or pressure
• The mineral olivines structure is changed to one
  that resembles that of the mineral spinel
• A denser packing results in an increase in
  seismic velocity
• Another velocity change at about 660 km (412 mi)
• Another phase change
• olivine converts to perovskite structure

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Figure 12.11
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• Lowermost 200 km (125 mi) of mantle
• Called the D layer
• P-wave velocities show a sharp decrease (and,
  this is looking at the data REALLY closely)
• Explanation this zone is partially molten, at
  least in places

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The Core

• Overall, larger than the planet Mars
• 1/6 of Earths mass, but 1/3 of its density
• Average density is about 11 g/cm3
• Meteorites helped decipher this
• Assumed to represent samples of the material from
  which Earth formed
• Composition mostly iron-nickel, to stony ones
  resembling peridotite

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How did the core form?

• Dont know that for sure
• Best idea
• During accretion, heat released as material
  collided with proto-Earth
• At some point, internal temp high enough to melt
  materials
• Metal-rich material, being denser, collected and
  sank toward the center, while less dense material
  floated upward

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• Entire core may have been molten initially
• As a material crystallizes, heat is released
• This is part of the source of heat within the
  Earth

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The Magnetic Field

• Earth acts as if there is a large, permanent
  magnet at the center
• Some stuff about magnets
• Need a material that can be magnetized
• Something very hot will not retain magnetism
• Ideas for Earth
• Core material needs to be able to conduct
  electricity
• The material needs to be mobile
• And, this gets deeply into physics

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• Dynamo theory
• This takes place in the OUTER core
• Moving, molten metal generates electric fields
• When you have an electric field, you also have an
  associated magnetic field
• The magnetic field generates an electric field,
  which then generates a magnetic field
• And so on
• And that is the simple explanation

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Figure 12.C
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Heat in the Earth

• Geothermal gradient the gradual increase in
  temperature as you go deeper in the Earth
• Varies considerably place to place, especially in
  the crust increase thought to be less in the
  mantle and core (we cant measure it directly)
• 3 main sources
• Decay of radioactive isotopes
• Heat of crystallization of iron in the core
• Kinetic energy due to collisions during formation

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Heat flow in the crust

• Heat is transferred through matter by molecular
  activity
• Put a metal spoon in a hot pan, come back a few
  minutes later, then pick it up
• Rock materials are poor conductors of heat
• Crust acts as an insulator (cool on top, hot on
  bottom)

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Heat flow in the mantle

• Models of the mantle need to explain temperatures
  calculated for the specific layers (note the
  underlines)
• Temperatures increase with depth in the mantle,
  but more gradually than in the crust

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Figure 12.13
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• Rocks are poor conductors of heat how is the
  heat distributed?
• Convection is the best explanation (transfer of
  heat by circulation)
• Hot rock rises, pushing cooler rock downward
• Suggests the rocks are not solid, yet they
  transmit S-waves
• Indicates the rocks are plastic behave as both
  a solid and a liquid
• Ex. taffy hit a piece with a hammer, it
  shatters slowly pull it apart, it flows

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Figure 12.14
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• End of chapter