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GEOLOGY 12
STUDY GUIDE / NOTES
CHAPTER 11 EARTH'S INTERIOR
Name _____________________
Major Concepts
1.
We obtain samples of the deep crust and uppermost mantle in xenoliths. (see p. 227)
2.
The cosmic abundance curve is based on spectral analyses of the sun and other stars and
chemical analyses of meteorites. Since the abundance of elements are similar we assume the
composition of the earth to be similar. Therefore, elements not in the earth's outer layers can be
expected to occur in the interior. (see p. 228)
3.
To study depths below which we have no means of investigating directly, geoscientists use the
paths and travel times of body waves generated by large earthquakes. These seismic waves
allow scientists to deduce properties such as density and physical state.
4.
In general, body waves travel faster through more dense mediums. Differing densities will
cause the waves to be bent or refracted. (see p.229)
5.
The Earth's core is believed to be neither pure iron nor an iron-nickel alloy because the density
determined by seismic waves is lower than would be expected for either iron or iron-nickel at
those pressures.
6.
The earth's magnetic field cannot be due to a lump of solid iron in the core because the iron in
the core would be far above its Curie temperature, and so would have lost its magnetism.
However, motions in an electrically conducting fluid can generate a magnetic field. Therefore,
the magnetic field may be produced by the molten iron-rich outer core.
7.
The Moho (Mohorovicic Discontinuity) is the crust-mantle boundary, where there is a sharp
increase in seismic-wave velocities (7.8 km/sec or greater) as the waves pass into the ultramafic
mantle. At seismograph stations near an earthquake, seismic waves traveling only through the
crust arrive ahead of similar waves refracted through the mantle along part of their path. (see
p.230)
8.
The Low-Velocity Layer extends from the base of the lithosphere (50-100 km) to 175-250 km. It
is caused by the partial melting in the asthenosphere of the upper mantle. The plastic behavior
of the rocks transmits P-waves and S-waves less efficiently and the seismic waves slow down.
(see p. 231)
9.
Still deeper in the upper mantle, below the low-velocity zone, is the Transitional Zone. Several
small jumps in velocity occur within this zone may be due to phase changes or changes in
mineral structure. (see p. 232)
10. There are no abrupt breaks in the seismic velocities through the lower mantle. This suggests
that the lower mantle is homogeneous.
11. Shear waves (S-waves) cannot travel through a liquid. S-waves passing through the earth
that intercept the outer core are blocked. This indicates that the outer core is molten, and the
size of the seismic shadow cast indicates the size of the core. This is known as the S-wave
Shadow Zone. (see p.234)
12. The depth to the inner/outer core boundary is determined by the travel times of P-waves that
travel through the outer core, are reflected off the inner/outer core boundary, and return. (see
p.235)
Use evidence from the following graph that
shows P- and S-wave velocities at various depths in the earth.
14
P-wave
12
10
Velocity
(km/sec)
8
S-wave
6
4
2
0
2 000
4 000
6 000
Depth (km)
a) Give one piece of evidence from the graph that the earth’s interior has a layered structure.
Graph shows sharp changes in wave velocities. S-waves disappear at
depth; P- and S-waves change velocities at the same depth.
 ← 1 mark


b) Give two pieces of evidence from the graph that could be used to determine the
upper and lower boundaries of the earth’s outer core.
Evidence for upper boundary: S-waves disappear or P-waves decrease in velocity. ← 1 mark
Evidence for lower boundary: P-waves increase in velocity.
← 1 mark
c) Give one piece of evidence from the graph that the density of material increases
with depth.
The wave velocities increase between boundaries as the density increases. ← 1 mark