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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.
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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; 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. • 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. • 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
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
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
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-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
