1. Chapter 12: Earth’s Interior

2. Propagation of Seismic Waves
P waves travel through both liquids & solids
S waves travel only through solids
P waves travel faster than S waves in all materials
Velocity of seismic waves depends on density (ρ) & elasticity of material
Velocity of waves increases with depth b/c pressure (P) makes rocks more elastic
Refraction of waves occurs when they pass from one material to another

3. P waves P waves Compressional First to arrive (primary)
Travel through both liquids & solids

4. S waves S waves Shear waves Second to arrive
Travel through solids only

5. Propagation of Waves Through Solid of Increasing P
Increasing velocity with depth
Through Uniform Solid
Constant velocity with depth

6. Wave Propagation Through Earth
Because Earth has differentiated, waves travel at different speeds through the different layers

7. Layers Defined by Composition
Crust:
3-70 km thick outer skin
Mantle:
km solid silicate-rich shell
Core:
Fe-rich sphere with 3486 km radius

8. Layers Defined by Physical Properties (Seismic Waves)
Lithosphere:
3-250 km thick crust & uppermost mantle
Cold and rigid
Asthenosphere:
km lower upper mantle
Warm and soft
Mesosphere: (lower mantle)
km lower mantle
More rigid than asthenosphere due to increased P
Inner Core:
2270 km thick
Liquid b/c s-waves don’t penetrate
Convection produces Earth’s magnetic field
Outer Core:
Fe-Ni metallic sphere with radius of 3486 km
Solid because of increased P

9. Earth’s Layers

10. Earth’s Major Boundaries
The Mohorovičič discontinuity (Moho)
Divides Crust from Mantle
Seismic velocities higher in Mantle so deeper waves arrive earlier
Core-Mantle Boundary
P-wave shadow zone 105°-140°
Due to high refraction of waves
S-waves do not penetrate at all
Inner-Outer Core
P-waves reflected off surface of solid inner Core
P-waves faster in inner Core than in outer Core

11. The Moho

12. Core-Mantle Boundary

13. Core-Mantle Boundary

14. Discovery of Inner Core

15. The Crust Averages <20 km thick
Deep oceanic crust compositionally different from continents
Continental Crust
ρ ~ 2.7 g/cm3
Older (up to 4 Ga)
Felsic (enriched in K, Na and Si)
Oceanic Crust
ρ ~ 3.0 g/cm3
Younger (< 180 Ma)
Mafic (enriched in Mg, Fe)

16. The Mantle >82% (by volume)
Knowledge from xenoliths and experiments
410 km discontinuity
Abrupt increase in wave velocities
Phase change from olivine to spinel
660 km discontinuity
Divides Asthenosphere and Mesosphere
Spinel changes into perovskite
D’’ layer
Deepest 200 km
Sharp decrease in P-waves velocities
Implies partially molten
Hot spot plumes may originated from this layer

17. The Mantle
Phase change

18. Core-Mantle Boundary
Origin of Hot Spot Plumes ?

19. The Core Larger than Mars 1/6 of volume and 1/3 of mass
Temperature > 6700°C
Average density g/cm3
Knowledge from Meteorites
Probably began all liquid
Solidifying with cooling of Earth

20. Earth’s Magnetic Field
Inner core rotates about 1° faster per year
From west to east (same as surface)
Axis of rotation offset ~10°

21. Earth’s Internal Heat Engine
Geothermal gradient:
20°-30° per km
Rate of increase much lower in Mantle & Core
Heat Sources:
Radioactive decay
Heat of crystallization (core)
Accretional heat
Crust: conduction
Mantle: convection

22. Mantle Convection