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

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

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

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

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

Seismic Ray Paths Animation

Locating Boundaries By Refraction

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

Internal Earth Structure from Seismology Crust: 0-60 km Mantle: 60-2900 km Core: 2900-6370 km

Seismic Moho and LVZ

Summary Crustal Properties

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

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

P wave Shadow Zone

S wave Shadow Zone

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.

D ‘’ Layer in Lower Mantle From: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28 , 509-37, 2000.

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.

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

Isostatic Adjustment: Erosion Animation

Isostatic Readjustment: Erosion

Post-glacial Crustal Rebound

Polar Glaciers Melting Animation From: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.html

Post-glacial Rebound Animation

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.

Gravitational Attraction

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.

Gravity Profile in Isostatic Equilibrium

Non-isostatically Compensated Profile

Negative Gravity Anomaly