Chapter Eleven - Geophysical Properties of Planet Earth

HOMOGENOUS EARTH

Exploring the Interior of the Earth Geophysics- the study of the foundational properties of the Earth’s interior. Geophysicists- identify thickness, density, composition, structure and physical state of the layers of the Earth’s interior Knowledge of Earth’s interior comes mainly from seismological station that records seismic body waves. Analysis of waves arrival time recorded by seismographs Seismic tomography- using same principles similar to CAT Scans to generate 3-D images of the Earth’s interior P-wave travels through solid and liquid while S-wave travels only through solids Refraction and reflection occur at contacts between different layers

Earth’s Interior Inaccessibility of Earth’s Interior Deepest hole drilled ~ 13 km General observations about Wave Propagation: P waves compress mail material through which they travel; Medium returns to original volume; Travel through sold (Elastic) faster than Liquid or Gas (inelastic) S waves travel as shear waves; admitted by elasticity of solids; omitted by inelasticity of liquid or gas; seismic wave velocity increases with depth

Earth’s Layers Earth is divided into continental and oceanic crust between different composition thickness & structure Seismic discontinuity- MOHO- boundary between crust and mantle Transitional zone within the mantle (slowing) Crust- silica rich igneous/metamorphic rocks- continental 20-70 km (12.5-45 mi) ~2.7-3g/cm3; oceanic- density 3.0 gm.cm3 Mantle- upper 3.3 g/cm3 up to 400 km and more. Lower P velocity at boundary between mantle and core 700-2900 km (440-1800 mi) Asthenosphere-region (100-350 km (62-217 mi)) where P & S slow down Seismic discontinuity- mantle core

Earth’s layers-contd. Crust composed of Silicate-rich igneous rocks Sampled directly by drilling Studied extensively by seismic analysis P-waves: ~6 km/s in continental crust; ~ 7 km/s in oceanic crust Continental Crust: Thickness varies between 20-70 km; P-wave velocity varies between 6-7 km/s; density: 2.7-3.0 g cm-3 Oceanic Crust: Studied by Deep-Sea Drilling; seismic analysis; 200-m deposit marine sed; 2-km layer of pillow sediment;6-km layer of Gabbro; aver. Den. ~ 3 g cm-3;

Earth’s layers-contd. Crust-Mantle Boundary: Moho discontinuity Mantle: Density varies – 3.3-5.5 g cm-3; composed of elastic/plastic solids; Changes in P- & S-wave velocities reveal mantle layers; P-wave velocity from Moho to Asthenosphere: 8-8.3 km/s; P-wave velocity in Asthenosphere: < 8 km/s; Asthenosphere is partially molten because of unique temperature and pressure combination Transition Zone: Below Asthenosphere; At 400-km, Mg olivine compresses to form spinel; At 700-km, spinel and other minerals change to metallic oxides

Ultramafic mantle minerals collapsing

Earth’s Mantle-Core Boundary

Earth’s layers-contd. Lower Mantle: 700-2,900 km deep; P-wave velocity from Asthenosphere to base of mantle: 8.3-13.6 km/s; composed of dense Mg silicates and oxides Mantle-Core Boundary: P-wave velocity slows from 13.6 to 8.1 km/s; S-waves cease; Outer core: Liquid Iron-Nickel mix, density 10-13 g cm-3 CORE: 1/6TH Earth’s volume, 1/3RD Of the Earth’s Mass; Pressure >3 Million atmosph.; Temp.~4,700ºC; Composition: IRON-NICKEL, Consistent with Seismic data, meteorite data, and mathematical model

Velocity Change between layers

Three major components of the Earth

Seismic Wave

The low-velocity zone

Shadow Zones Shadow zones- S-Shadow zone- P-Shadow zones- segments of the earth opposite an Earthquake’s focus where no direct S & P waves can be received S-Shadow zone- produced because shearing S-wave cannot travel through liquid, hence S-Shadow zone occurs P-Shadow zones- are produced as P-waves are refracted when they enter a zone of lower rigidity both zones help to confirm that earth’s outer core is liquid

Shear Waves – Shadow zone

Shear Waves – contd.

P-Waves Globe

P-waves Cut-away

The Behavior of P- and S-waves

Solid Inner Core

Gravity Force of attraction that an object (A) exerts on another object (B), i.e. Force of gravity is proportional to mass of A x mass of B distance 2 Gravimeter- measures variation in Earth’s gravity. Gravity depends on the altitude of the land, latitude, and distance from the Earth’s center of gravity. Gravity anomalies- difference between actual gravimetric measurement to the expected theoretical values- positive attraction will be lower than expected and negative attraction higher than expected

Gravitation attraction of the earth

Gravitation-contd. Isostacy- equilibrium between lithospheric segments and the asthenosphere beneath them Magnetism- force associated with moving charged particles that enables certain substances to attract or repel similar materials- magnetic reversal, paleomagnetism

Negative Gravity anomaly

Positive Gravity anomaly

Positive gravity anomaly over ore deposit

Principle of isostacy - icebergs

Principle of isostacy - mountains

Isostatic adjustments

Magnetic field of a bar magnet

Prevailing Magnetic Field

Electrically conductive field

Magnetic field polarity within magnetite

Terrestrial record of magnetic reversal