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Ch 21: Earth’s interior
1. Probing Earth’s interior
2. Seismic waves and Earth’s interior
3. Discovering Earth’s major boundaries
4. Geodynamo
5. Earth’s internal heat engine
Ch 21: Earth’s interior - Study guide
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Know Earth’s major layers (and depths) based on
compositional and mechanical differences.
Properties of P-and S waves, and which ones move through
solid, which through liquid (molten) rock?
Difference between refraction and reflection
What is the Moho and how was it discovered?
How was the core-mantle boundary discovered?
Variations of S-and P wave velocities with depth. How can
you explain the changes in velocity inside the
asthenosphere? What does it tell you about the composition
of the mantle?
Why does earth have a magnetic field?
What is the source of heat inside earth and how it is
transferred through earth (conduction and convection)?
1) Probing Earth’s interior
 earthquake happens (slip on a fault)
 seismic waves travel away from earthquake
 they carry info about material they travel through
1) Probing Earth’s interior
 earthquake happens (slip on a fault)
 seismic waves travel away from earthquake
 they carry info about material they travel through
 earthquakes make P-waves & S-waves
 seismologists use them to:
 locate earthquakes
earlier lecture
 determine what deep Earth is made of
today
1) Probing Earth’s interior
 The nature of seismic waves
Seismic wave speeds:
 depend on material properties
 are faster in more rigid materials
 increase with increasing depth (higher pressure)
P waves:
 compressional waves: are fastest
 vibrate material back/forth in direction wave travels
S waves:
 shear waves: slower than P-waves
 vibrate material side-to-side from direction wave travels
Don’t pass through liquids
1) Probing Earth’s interior
 The nature of seismic waves
 P waves always faster than S-waves
“primary”
“secondary”
 wave paths are “bent” when crossing from one
material into another
1) Probing Earth’s interior
 The nature of seismic waves
 wave paths are “bent” when crossing from one
material into another
reflections
refractions
q4 q3
q2 q2
1) Probing Earth’s interior
 The nature of seismic waves
 wave paths are “bend” when going deeper in Earth
 higher pressure = higher wave speed
2) Seismic waves & Earth’s Interior
 Compositional layers
crust
3-70 km thick
mantle down to 2900 km depth
core
2900-6370 km depth
 5 Physical/mechanical layers
layer
lithosphere
stiff/strong,
asthenosphere soft/weak,
depth
0-100 km
100-660 km
3) Discovering Earth’s major boundaries
 crust
 mantle
 core
boundaries
between
layers
3) Discovering Earth’s major boundaries
 The Crust
Thickness: ~ 30 km (continents, 70km under mountains)
3-15 km (oceanic)
Composition:
Continents: felsic (granite)
and mafic (gabbro) rocks
Oceani: Basalt, Gabbro
3) Discovering Earth’s major boundaries
 The “Moho”
Boundary between
the crust and mantle
Discovered in 1909 by
Andrija Mohorovicic
3) Discovering Earth’s major boundaries
 The Mantle
Over 82% of Earth’s volume,
mainly peridotite (minerals
Olivine and pyroxene) mantle
upper mantle 0 - 660 km
lower mantle 660-2900 km
D” region 2600-2900 km
upper
mantle
lower
mantle
D”
400 & 660 km depth
“phase transitions”
Minerals suddenly compress to
a more compact form (phase change)
See Fig. 21.7
Insert: Isostacy and crustal uplift/subsidence
“isostacy” = balance between gravitational force
and buoyancy force, see Figure Story 16.16
Insert: Isostacy and crustal uplift/subsidence
Less dense crust floats on top of the denser
and deformable rocks of the mantle
“isostacy” = balance between gravitational force
and buoyancy force, see Figure Story 16.16
Insert: Isostacy and crustal uplift/subsidence
Isostatic rebound, adjustment:
Readjustment of the isostatic equilibrium
after the ice-shield is removed, as happens still in Scandinavia
and Canada.
See 21.1: Isostacy and postglacial uplift
3) Discovering Earth’s major boundaries
 crust
 mantle
 core
about Mars sized
Nickel-iron alloy
Outer, liquid, spinning
Inner, solid
4 mio times atm. pressure at center
3) Discovering Earth’s major boundaries
 The core-mantle boundary
Boundary between
the mantle and core
mantle
Discovered in 1914 by
Beno Gutenberg
…how?
core
Core-mantle
boundary
P-wave shadow zone
Fig. 21.2
S-wave shadow zone
Core was discovered …from a “shadow zone”
3) Discovering Earth’s major boundaries
 The inner core
Boundary between
the outer liquid and
solid inner core
Discovered in 1936 by
Inge Lehman
…how?
mantle
core
Inner core
3) Discovering Earth’s major boundaries
 The inner core
She discovered reflections (‘echoes’) of seismic
waves
‘Lehman
Discontinuity’
4) Earth’s magnetic field- Geodynamo
 The Core
Core
behaves like a
dynamo and
thus sustains
Earth’s magnetic
field.
Polarity
of Earth’s magnetic
field reverses about
every million years
4) Earth’s magnetic field- Geodynamo
Fig. 21.11
Geographic and magnetic poles do not coincide!
4) Earth’s magnetic field
See time-line of magnetic field reversals (paleomagnetic timescale), Fig. 21.15 . Remember that magnetic reversals recorded in
seafloor basalts were a major confirmation of seafloor spreading (Fig.
Story 2.11).
5) Earth’s internal heat engine
 3 reasons for internal heat:
1) radioactive decay of uranium, thorium, potassium
2) heat released as inner core crystallized
3) from colliding particles during Earth formation
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Ways to transfer heat:
1) conduction - molecular activity
2) convection - movement (circulation) of material
TSP 17.13
5) Earth’s internal heat engine
Fig. 17.14 Convective flow in the mantle
5) Earth’s internal heat engine
The geotherm: Increase of temperature with depth
Ch 21: Earth’s interior
What is the approximate distance from the surface to the center of the Earth?
A.
B.
C.
D.
700 km
2900 km
6400 km
24,000 km
Ch 21: Earth’s interior
earthquake
*
seismograph
4. The two kinks in the seismic wave path are examples of seismic __________.
A. isostasy
B. reflection
C. refraction
D. tomography
Answer = C (page 485)
What type of seismic wave is depicted by the ray path in the diagram?
A.
B.
C.
D.
a P wave
an S wave
a surface wave
all of the above
Ch 21: Earth’s interior
earthquake
*
seismograph
The two kinks in the seismic wave path are examples of seismic
__________.
A. isostasy
B. reflection
C. refraction
D. tomography
Ch 21: Earth’s interior
Continental crust beneath mountains can be up to ___ kilometers thick
A.
B.
C.
D.
10
40
70
100
Ch 21: Earth’s interior
Which of the following statements is false?
A. P waves travel slower in the crust than in the mantle.
B. The crust is denser than the mantle.
C. The crust-mantle boundary is called the Mohorovicic
discontinuity.
D. The oceanic crust consists of basalt and gabbro.
Answer = B (page 487)
Ch 21: Earth’s interior
Which of the following regions in the Earth consists primarily of
olivine and pyroxene?
A.
B.
C.
D.
the crust
the upper mantle
the lower mantle
the inner core
Ch 21: Earth’s interior
Which of the following statements about the Earth’s core is true?
A. The inner core and the outer core are both liquid.
B. The inner core and the outer core are both solid.
C. The inner core is liquid and the outer core is solid.
D. The inner core is solid and the outer core is liquid.
Ch 21: Earth’s interior
What drives plate tectonics?
A.
B.
C.
D.
erosion
solar energy
thermal conduction
thermal convection