1. Ch 21: Earth’s interior Probing Earth’s interior
Seismic waves and Earth’s interior
Discovering Earth’s major boundaries
Geodynamo
Earth’s internal heat engine

2. Ch 21: Earth’s interior - Study guide
·   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)?

3. earthquake happens (slip on a fault)
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

4. earthquake happens (slip on a fault)
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:
earlier lecture
locate earthquakes
determine what deep Earth is made of
today

5. 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

6. 1) Probing Earth’s interior
The nature of seismic waves
P waves always faster than S-waves
“secondary”
“primary”
wave paths are “bent” when crossing from one material into another

7. 1) Probing Earth’s interior
The nature of seismic waves
wave paths are “bent” when crossing from one material into another

8. reflections
refractions q3 q4 q2 q

9. 1) Probing Earth’s interior
The nature of seismic waves
wave paths are “bend” when going deeper in Earth
higher pressure = higher wave speed

10. 2) Seismic waves & Earth’s Interior Compositional layers
5 Physical/mechanical layers
crust km thick
mantle down to 2900 km depth
core km depth
layer
depth
lithosphere stiff/strong, km
asthenosphere soft/weak, km

11. crust mantle core boundaries between layers
3) Discovering Earth’s major boundaries
crust
mantle
core
boundaries
between
layers

12. 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

13. 3) Discovering Earth’s major boundaries
The “Moho”
Boundary between the crust and mantle
Discovered in 1909 by Andrija Mohorovicic

14. 3) Discovering Earth’s major boundaries
The Mantle
upper
mantle
400
Over 82% of Earth’s volume,
mainly peridotite (minerals
Olivine and pyroxene)
660
lower
mantle
mantle
upper mantle km
lower mantle km
D” region km D”
400 & 660 km depth
“phase transitions”
Minerals suddenly compress to
a more compact form (phase change)
See Fig. 21.7

15. Insert: Isostacy and crustal uplift/subsidence
“isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16

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

17. Isostatic rebound, adjustment:
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

18. crust mantle core 3) Discovering Earth’s major boundaries
about Mars sized
Nickel-iron alloy
Outer, liquid, spinning
Inner, solid
4 mio times atm. pressure at center

19. 3) Discovering Earth’s major boundaries
The core-mantle boundary
Boundary between the mantle and core
mantle
core
Core-mantle
boundary
Discovered in 1914 by Beno Gutenberg
…how?

20. P-wave shadow zone
Fig. 21.2

21. Core was discovered …from a “shadow zone”
S-wave shadow zone
Core was discovered …from a “shadow zone”

22. 3) Discovering Earth’s major boundaries
The inner core
Boundary between the outer liquid and solid inner core
mantle
core
Inner core
Discovered in 1936 by Inge Lehman
…how?

23. 3) Discovering Earth’s major boundaries
The inner core
She discovered reflections (‘echoes’) of seismic waves
‘Lehman
Discontinuity’

24. 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

25. 4) Earth’s magnetic field- Geodynamo
Geographic and magnetic poles do not coincide!
Fig

26. 4) Earth’s magnetic field
See time-line of magnetic field reversals (paleomagnetic time-scale), Fig Remember that magnetic reversals recorded in seafloor basalts were a major confirmation of seafloor spreading (Fig. Story 2.11).

27. 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
Ways to transfer heat:
1) conduction - molecular activity
2) convection - movement (circulation) of material
TSP 17.13

28. 5) Earth’s internal heat engine
Fig Convective flow in the mantle

29. 5) Earth’s internal heat engine
The geotherm: Increase of temperature with depth

30. Ch 21: Earth’s interior
What is the approximate distance from the surface to the center of the Earth?
A km
B km
C km
D. 24,000 km

31. Ch 21: Earth’s interior
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. a P wave
B. an S wave
C. a surface wave
D. all of the above

32. Ch 21: Earth’s interior
The two kinks in the seismic wave path are examples of seismic __________.
A. isostasy
B. reflection
C. refraction
D. tomography

33. Ch 21: Earth’s interior
Continental crust beneath mountains can be up to ___ kilometers thick
A. 10
B. 40
C. 70
D. 100

34. Which of the following statements is false?
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)

35. Ch 21: Earth’s interior
Which of the following regions in the Earth consists primarily of olivine and pyroxene?
A. the crust
B. the upper mantle
C. the lower mantle
D. the inner core

36. 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.

37. Ch 21: Earth’s interior What drives plate tectonics? A. erosion
What drives plate tectonics?
A. erosion
B. solar energy
C. thermal conduction
D. thermal convection