1. Chapter 17 Earth’s Interior and Geophysical Properties
Photo credit: NASA Apollo 17

2. Evidence from Seismic Waves
Earth’s Internal Structure
The Crust
The Mantle
The Core

3. Seismic Reflection Controlled by: Snell’s Law Elastic properties
of the layers

4. Seismic Refraction Ray paths are “bent” across velocity or seismic
discontinuities.
Bend away from
boundary normal
if V2 > V1
Bend toward
if V1 > V2 V1 V2

5. Ray Paths and Snell’s LawV1 V2
In this example, V2 > V1
Modified from: Brown & Mussett, 1981

6. Seismic Ray Paths Animation

7. Locating Boundaries By Refraction

8. Radial Distribution of Ray Paths
Rays curve
upward because
of generally
increasing
velocity
with depth
Vp and Vs

9. Internal Earth Structure from Seismology
Crust: 0-60 km
Mantle: km
Core: km

10. Seismic Moho and LVZ

11. Summary Crustal Properties

12. Mantle Tomography
Blue = Fast (cold)
Red = Slow (hot)

13. Deep Earth Velocity Structure
NB that velocity is measured from earthquake waves, but density must
Be inferred from inversion of the velocity combined with elastic modulii

14. P wave Shadow Zone

15. S wave Shadow Zone

16. Pallisite Meteorites - Rocks from a CMB?
Olivine
Non-seismological
evidence of
core - mantle
exchange!
Fe-Ni
Metal Alloy
Pallisite meteorites are classified as stoney-irons and are thought
to be derived from the CMB region of an asteroidal parent body.

17. D ‘’ Layer in Lower Mantle
From: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28 , , 2000.

18. Isostasy and Gravity
Isostasy is the gravitational equilibrium of bodies of different density.
Isostatically compensated uplift zones are stabilized by low density roots.
Isostatic re-adjustment can be caused by many different processes.

19. Isostasy Low density High density
Compensation depth is arbitrary and selected for convenience.
Total column mass (or gravitational acceleration) must be equal.

20. Isostatic Adjustment: Erosion Animation

21. Isostatic Readjustment: Erosion

22. Post-glacial Crustal Rebound

23. Polar Glaciers Melting Animation
From:

24. Post-glacial Rebound Animation

25. Isostatic Uplift due to Crustal Thickening
NB that uplift would depend on the amount and density of
accumulated magma accreted to the base of the crust.

26. Gravitational Attraction

27. Gravity Meter’s Response to Density Variations
high
gravity
low
High density
Low density
Local gravity high can be related to a high density body within the crust,
while a gravity low can be related to a lower than average density.

28. Gravity Profile in Isostatic Equilibrium

29. Non-isostatically Compensated Profile

30. Negative Gravity Anomaly