THE EARTHS INTERIOR

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THE EARTHS INTERIOR Seismic Wave
Review 1. p-waves and s-waves 2. velocity
Distance / time 3. seismic waves travel at
different speeds through different materials
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• Geologic Discontinuities
• seismic waves reflect and refract at
  discontinuities or boundaries between geologic
  materials (ie. rocks)
• 2. velocity relationships

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Geologic Discontinuities 3. Paths of Seismic
Waves i. homogeneous composition straight
ii. non-homogeneous composition curved
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Composition and Structure of the Earths
Interior 1. Crust Oceanic and Continental
Crust i. Continental Crust (1) felsic
(granitic) 6 km / sec (2) 35 km
thick ii. Oceanic Crust (1) mafic (basalt)
7 km / sec (2) 5-10 km thick (3) denser
than continental crust
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Composition and Structure of the Earths
Interior 2. Isostasy i. continental crust
floats on top of oceanic curst
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3. Mantle i. Andrija Mohorovi i
(1909 (1) discovered a discontinuity at a depth
of about 30 km (2) Mohorovicic Discontinuity
(Moho) (a) p-waves below travel at 8km /
sec (b) p-waves above travel at 6.5km /
sec (3) the Moho separates the crust from the
mantle
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3. Mantle ii. low-velocity zone (1) 100 - 250
km deep (2) p- and s-wave velocities
decrease (3) corresponds to the
asthenosphere (a) a layer in which the rocks
are close to there melting point and are less
elastic (b) magmas are produced
here(?) (c) discontinuous (?)
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• 4. Core
• i. 1906 R.D. Oldham
• (1) discovered that seismic waves arrived later
  than expected at seismic stations more than 130
  degrees from an earthquake focus
• (2) he postulated the existence of a core that
  transmits seismic waves at a slower rate than
  shallower depths
• only very weak p-waves between 103-143 degrees
  from an earthquake focus P-wave shadow zone
• ii. Inge Lehman (1936)
• (1) postulated that the Earth has a solid inner
  core
• (2) explains the existence of weak p-waves in the
  shadow zone

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4. Core iii. Harold Jeffreys (1926) (1) discovere
d that s-waves were no just simply slowed by the
core but were blocked by it (2) large and
complete s-wave shadow at locations greater than
103 degrees
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Internal Heat and Heat Flow 1. heat is due to
meteor impact and radioactive decay 2. earth
cools by conduction and convection i. conduction
(1) hot particles transfer heat to cool
particles (2) very inefficient and cannot
explain the solid mantle (must be another
mechanism) ii. convection (1) heated liquid
expands and rises since it is less dense than
cool material around it
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Magnetic Field 1. William Gilbert (1600) realized
the earth is a big magnet 2. supports theory
that the outer core is liquid
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Magnetic Field 3. paleomagnetism i. the earths
magnetic field is recorded in cooling rocks that
contain material that will align to a magnetic
field - thermoremanent magnetism
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Magnetic Field 3. paleomagnetism iii. since
rocks that are 3.5 billion years old record a
magnetism, the earth had a magnetic field at that
time - fluid core iv. magnetic stratigraphy (1) ab
out ½ of the rocks on earth contain a magnetic
field opposite to today's field (2) patterns have
been recognized and we can use it as a
stratigraphy (a sequence) to tell approximate age
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