Chapter 12: Earth’s Interior

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

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

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

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

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

Layers Defined by Composition Crust: 3-70 km thick outer skin Mantle: 70-2900 km solid silicate-rich shell Core: Fe-rich sphere with 3486 km radius

Layers Defined by Physical Properties (Seismic Waves) Lithosphere: 3-250 km thick crust & uppermost mantle Cold and rigid Asthenosphere: 250-660 km lower upper mantle Warm and soft Mesosphere: (lower mantle) 660-2900 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

Earth’s Layers

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

The Moho

Core-Mantle Boundary

Core-Mantle Boundary

Discovery of Inner Core

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)

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

The Mantle Phase change

Core-Mantle Boundary Origin of Hot Spot Plumes ?

The Core Larger than Mars 1/6 of volume and 1/3 of mass Temperature > 6700°C Average density 11-14 g/cm3 Knowledge from Meteorites Probably began all liquid Solidifying with cooling of Earth

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

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

Mantle Convection