chapter eleven - geophysical properties of planet earth
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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. - PowerPoint PPT PresentationTRANSCRIPT
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