Download MYSTERIES OF PLANET EARTH

Chapter Eleven Geophysical
Properties of
Planet Earth
HOMOGENOUS EARTH
Exploring the Interior of the Earth
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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
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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:
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– 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
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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.
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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: 2.73.0 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; Pwave velocity from Moho to Asthenosphere: 88.3 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 400km, 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
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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
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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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
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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.
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Isostacy- equilibrium between lithospheric
segments and the asthenosphere beneath
them
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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