Chapter 12 Earths Interior

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Chapter 12 Earths Interior

• Most of what we know about the Earths interior
  is from the study of seismic waves.
• From Chapter 11, remember that Body Waves occur
  as P-waves (Compressional Waves) and S-waves
  (Shear Waves).
• In all materials, P-waves travel faster than
  S-waves. Liquids do not transmit S-waves.
• Velocity is dependent on density and elasticity
  of Earth materials. Pressure and compaction
  causes waves to travel faster with depth. When
  waves cross an interface between materials,
  refraction reflection occurs. (See slide 9).

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• The Earths principle layers as defined by
  Composition iron-rich core, mantle, and crust.
• The Earths layers as defined by Physical
  Properties.
• From the center Inner core of is solid
  because of intense pressure and has a radius of
  3486 km. Outer core is a thick liquid that
  maintains a convective flow that generates the
  Earths magnetic field. Outer core is 2270 km
  thick and both parts consist of an Fe-Ni alloy.
• Mesosphere Lower mantle semi-solid from 660
  km to 2900 km below surface.

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• Asthenosphere is the outer portion of the
  mantle from just below the crust to approx. 660
  km (mesosphere boundary).
• The upper portion of the asthenosphere is
  semi-molten and structurally weak and is thought
  to host slow-moving vertical convection currents
  and isolated plumes.
• The lithosphere, the crust, averages approx. 100
  km. in thickness, but may reach 250 km. beneath
  large mountain ranges.

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• Major Earth boundaries were revealed by the
  refraction and reflection of seismic waves as
  they crossed boundaries (interfaces). Because of
  the precise location of post-WWII nuclear tests,
  locations were precisely known.
• The Moho boundary was discovered by evidence of
  deep seismic waves (below 50 km) traveling faster
  than shallow seismic waves (see pp. 366 368).
• The refraction of P-waves through the Earth, that
  produced a zone of No P-waves from 105 to 140
  degrees revealed the Mantle Core boundary.

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• The Lehmann boundary (see Box article 12.1)
  marks the boundary between the inner and outer
  cores.
• The Mantle (pp. 370 371) text explains more
  about vertical changes within the mantle due to
  varying pressure conditions with increasing
  depth.
• The origins of the compositional layers is
  dicussed in pages 372 373. It is thought that
  while the Earth was molten, the more dense Fe-Ni
  sank to the core, while the other materials were
  separated by density contrasts as well. It is
  thought that the inner core is growing as it
  cools.

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• Box article 12.2 (p. 374) contains
  informa-tion about the current understanding as
  to the origins of the Earths magnetic field.
• When magnetic materials are heated, they lose
  their magnetism, thus there must be another
  reason for the Earths magnetic field, beyond the
  Fe-Ni core.
• The current concept is that interactions between
  the solid core and the slowing churning outer
  core, both of which are composed of an Fe-Ni
  alloy, contribute to an electrical field that
  reinforces the existing magnetic field.

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• Through study of the iron-bearing minerals in
  the basaltic seafloor and their magnetic
  orientations, we have learned that the Earths
  magnetic polarity reverses itself about every one
  million years. How or why this reversal takes
  place is unknown.
• Recent calculations suggest that the magnetic
  field has weakened about 5 in the last 100
  years. At this rate, in about 1500 years, the
  magnetic field would become weak or non-existent
  (p. 374). Whether this is the prelude to a
  reversal is unknown.

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• The Earths internal heat is thought to be
  from
• Radioactive decay of Uranium, Thorium, and
  Potassium isotopes.
• Heat released during the crystallization of the
  iron core.
• Heat released by colliding particles during
  Planetary Accretion.
• Heat from first two is less than in geologic
  past, thus Earth is getting progressively cooler.

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• Crustal Heat Flow is related to the
• Thickness of the crust.
• Presence of young, shallow intrusions.
• Quantity of radioactive minerals undergoing
  decay.
• Mantle convection of heat is thought to drive
  Plate Tectonics. Though outer mantle is
  semi-molten, it does transmit S-waves due to
  confining pressure. The outer core does not
  transmit S-waves.
• Model on Fig. 12.14 (p. 375) is Deep Con-vection
  Model. Other models suggest more shallow mantle
  convection.