1. Seeing Inside the Earth
INTERLUDE D
ESSENTIALS OF
GEOLOGY
3rd Edition
Stephen Marshak
Seeing Inside the Earth

2. The Earth’s surface hides a hot interior.
Fig. D.CO
The Earth’s surface hides a hot interior.

3. The 19th century image of the Earth’s interior.
Fig. D.1
Oceanic crust
Continental crust
Mantle
Core
Crust
A simple 3-layer Earth model.
Continental and oceanic crust are not the same.
The 19th century image of the Earth’s interior.

4. The propagation of earthquake waves — 1.
Fig. D.2a,b
An earthquake sends out waves in all directions.
Seismic waves travel at different velocities in different rock types.
Wave front
Seismic ray
Sandstone
Peridotite
The propagation of earthquake waves — 1.

5. The propagation of earthquake waves — 2.
Fig. D.2c,d
Both P-waves and S-waves can travel through a solid, but only P-waves can travel through a liquid.
P-waves travel faster in solid iron allow than in liquid, such as molten iron alloy.
S-wave
P-wave
Solid
Liquid
Solid iron alloy
Molten iron alloy
The propagation of earthquake waves — 2.

6. Refraction and reflection of waves.
Fig. D.3a,b
Air
Water
Incoming light
Faster
Slower
Reflected
Refracted
A ray of light reflects and refracts when it crosses the boundary between different materials.
A ray entering slower material bends away from the boundary,
but a ray entering faster material bends toward the boundary.
Refraction and reflection of waves.

7. Fig. D.4a,b
A nearby seismograph detects crustal seismic waves first.
A distant seismograph detects mantle seismic waves first, because waves travel faster in mantle rock.
Crust
Mantle
Moho
Discovery of the Moho.

8. The velocity of P-waves in the mantle changes with depth.
Fig. D.5
P-wave velocity (km/s)
Depth (km)
1000
200
400
600
800 5 7 9 11
LVZ
Upper
Mantle
Transition
zone
Lower
mantle
Lithosphere
The velocity of P-waves in the mantle changes with depth.

9. The velocity of seismic waves changes with depth in the mantle .
Fig. D.6a,b,c
In a stack of discrete layers, rays bend at each boundary.
If velocity increases gradually with depth, rays curve smoothly.
The velocity of seismic waves increases with depth in the mantle, so rays curve.
Seismic ray
Mantle
Focus
The velocity of seismic waves changes with depth in the mantle .

10. P-waves do not arrive in the P-wave shadow zone.
Fig. D.7a
P-waves do not arrive in the P-wave shadow zone.
P-wave shadow zone
103°
143°
Focus
Shadow zones and the discovery of the Earth's core — 1.

11. S-waves do not arrive in the S-wave shadow zone.
Fig. D.7b
S-waves do not arrive in the S-wave shadow zone.
S-wave shadow zone
103°
Focus
Shadow zones and the discovery of the Earth's core — 2.

12. Seismic waves reflect off the inner core-outer core boundary.
Fig. D.7c
Source
Seismic waves reflect off the inner core-outer core boundary.
Seismograph
Mantle
Outer
core
Inner
Incident
ray
Reflected ray
Shadow zones and the discovery of the Earth's core — 3.

13. Note the error! The outer core is liquid (the liquidus lies to the left of the geotherm, so it melts).
The inner core is solid (the geotherm is not enough to melt it because of high pressure).
Fig. D.7d
A graph of the geotherm and melting curve for the Earth.
Depth (km)
6000
Temperature (°C)
1000
2000
3000
4000
5000
Geotherm
Mantle
Melting curve
Solid outer core
Liquid inner core
Liquid and solid inside the Earth.

14. Inner core-outer core boundary
Fig. D.8
Depth (km)
Velocity (km/s)
400
2900
4155
6371
670
P-wave
S-wave
Core-mantle boundary
Inner core-outer core boundary 4 8 12
The velocity-versus-depth profile of the Earth.

15. Tomographic image of the whole Earth.
Fig. D.9a
Tomographic image of the whole Earth.
Inner core
Outer core
Faster
Slower
Mantle
Tomographic images of the Earth's interior — 1.

16. Tomographic images of the Earth's interior — 2.
Fig. D.9b,c
Lower Mantle
North America
Convecting cell
Mantle plume
Plate graveyard
Pacific Ocean
660 km
Close-up tomographic image of the mantle.
Modern view of the Earth's complex and dynamic interior.
Tomographic images of the Earth's interior — 2.

17. Seismic-reflection profiling.
Fig. D.10a,b
Trucks thump the ground to generate a seismic signal.
A seismic-reflection profile. Color stripes are subsurface layers.
Ground surface
1000 ft
100 ft
Seismic-reflection profiling.

18. The Earth's geoid is distorted by highs and lows in the gravity field.
Fig. BXD.1.1
The Earth's geoid is distorted by highs and lows in the gravity field.
Geoid high
Geoid low
An exaggerated representation of the geoid.

19. Interlude D: Figure Credits
BXD.1: ESA; D.10a: John Q. Thompson, courtesy of Dawson Geophysical
Company; D.10b: John Q. Thompson, courtesy of Dawson Geophysical Company.