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Chapter Eleven - Geophysical Properties of Planet Earth

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1 Chapter Eleven - Geophysical Properties of Planet Earth


3 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

4 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

5 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 km ( 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 km ( mi) Asthenosphere-region ( km ( mi)) where P & S slow down Seismic discontinuity- mantle core

6 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 km; P-wave velocity varies between 6-7 km/s; density: 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;

7 Earth’s layers-contd. Crust-Mantle Boundary: Moho discontinuity
Mantle: Density varies – g cm-3; composed of elastic/plastic solids; Changes in P- & S-wave velocities reveal mantle layers; P-wave velocity from Moho to Asthenosphere: 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

8 Ultramafic mantle minerals collapsing

9 Earth’s Mantle-Core Boundary

10 Earth’s layers-contd. Lower Mantle: 700-2,900 km deep; P-wave velocity from Asthenosphere to base of mantle: 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 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

11 Velocity Change between layers

12 Three major components of the Earth

13 Seismic Wave

14 The low-velocity zone

15 Shadow Zones Shadow zones- S-Shadow zone- P-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

16 Shear Waves – Shadow zone

17 Shear Waves – contd.

18 P-Waves Globe

19 P-waves Cut-away

20 The Behavior of P- and S-waves

21 Solid Inner Core

22 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

23 Gravitation attraction of the earth

24 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

25 Negative Gravity anomaly

26 Positive Gravity anomaly

27 Positive gravity anomaly over ore deposit

28 Principle of isostacy - icebergs

29 Principle of isostacy - mountains

30 Isostatic adjustments

31 Magnetic field of a bar magnet

32 Prevailing Magnetic Field

33 Electrically conductive field

34 Magnetic field polarity within magnetite

35 Terrestrial record of magnetic reversal

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