Chapter Eleven - Geophysical Properties of Planet Earth
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Transcript of Chapter Eleven - Geophysical Properties of Planet Earth
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Chapter Eleven -Chapter Eleven -Geophysical Geophysical Properties of Properties of Planet EarthPlanet Earth
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HOMOGENOUS EARTHHOMOGENOUS EARTH
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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
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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
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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
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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;
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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
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Ultramafic mantle minerals collapsingUltramafic mantle minerals collapsing
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Earth’s Mantle-Core BoundaryEarth’s Mantle-Core Boundary
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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
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Velocity Change between layersVelocity Change between layers
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Three major components of the EarthThree major components of the Earth
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Seismic WaveSeismic Wave
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The low-velocity zoneThe low-velocity zone
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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
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Shear Waves – Shadow zoneShear Waves – Shadow zone
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Shear Waves – contd.Shear Waves – contd.
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P-Waves GlobeP-Waves Globe
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P-waves Cut-awayP-waves Cut-away
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The Behavior of P- and S-wavesThe Behavior of P- and S-waves
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Solid Inner CoreSolid Inner Core
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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
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Gravitation attraction of the earthGravitation attraction of the earth
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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
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Negative Gravity anomalyNegative Gravity anomaly
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Positive Gravity anomalyPositive Gravity anomaly
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Positive gravity anomaly over ore depositPositive gravity anomaly over ore deposit
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Principle of isostacy - icebergsPrinciple of isostacy - icebergs
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Principle of isostacy - mountainsPrinciple of isostacy - mountains
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Isostatic adjustmentsIsostatic adjustments
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Magnetic field of a bar magnetMagnetic field of a bar magnet
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Prevailing Magnetic FieldPrevailing Magnetic Field
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Electrically conductive fieldElectrically conductive field
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Magnetic field polarity within magnetiteMagnetic field polarity within magnetite
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Terrestrial record of magnetic reversalTerrestrial record of magnetic reversal