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; oceanicdensity 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.
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
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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; 2km 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
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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
 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
 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
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Chapter Summary
P- & S-waves velocity in various earth’s layers
Direction of propagation of P- and S-waves
Moho discontinuity
Evidences for the presence of Ni and Fe in earth’s core
Shadow zones for S- and P-waves
Magnetism – Electricity & movement of electrons
Speed of rotation of earth’s core
Isostacy; gravity anomaly
Principle behind Magenetic stratigraphy