Chapter Eleven - Geophysical Properties of Planet Earth

Chapter Eleven -Chapter Eleven -Geophysical Geophysical Properties of Properties of Planet EarthPlanet Earth

HOMOGENOUS EARTHHOMOGENOUS EARTH

Exploring the Interior of the EarthExploring 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 InteriorEarth’s Interior

Inaccessibility of Earth’s InteriorDeepest hole drilled ~ 13 kmGeneral 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 LayersEarth’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.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.Earth’s layers-contd.Crust-Mantle Boundary: Moho discontinuityMantle: 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 collapsingUltramafic mantle minerals collapsing

Earth’s Mantle-Core BoundaryEarth’s Mantle-Core Boundary

Earth’s layers-contd.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 layersVelocity Change between layers

Three major components of the EarthThree major components of the Earth

Seismic WaveSeismic Wave

The low-velocity zoneThe low-velocity zone

Shadow ZonesShadow ZonesShadow 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 occursP-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 zoneShear Waves – Shadow zone

Shear Waves – contd.Shear Waves – contd.

P-Waves GlobeP-Waves Globe

P-waves Cut-awayP-waves Cut-away

The Behavior of P- and S-wavesThe Behavior of P- and S-waves

Solid Inner CoreSolid Inner Core

GravityGravity 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 earthGravitation attraction of the earth

Gravitation-contd.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 anomalyNegative Gravity anomaly

Positive Gravity anomalyPositive Gravity anomaly

Positive gravity anomaly over ore depositPositive gravity anomaly over ore deposit

Principle of isostacy - icebergsPrinciple of isostacy - icebergs

Principle of isostacy - mountainsPrinciple of isostacy - mountains

Isostatic adjustmentsIsostatic adjustments

Magnetic field of a bar magnetMagnetic field of a bar magnet

Prevailing Magnetic FieldPrevailing Magnetic Field

Electrically conductive fieldElectrically conductive field

Magnetic field polarity within magnetiteMagnetic field polarity within magnetite

Terrestrial record of magnetic reversalTerrestrial record of magnetic reversal