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THE EARTHS INTERIOR • Seismic Wave Review 1. p-waves and s-waves 2. velocity = Distance / time

THE EARTHS INTERIOR • Seismic Wave Review 1. p-waves and s-waves 2. velocity = Distance / time 3. seismic waves travel at different speeds through different materials. Geologic Discontinuities

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THE EARTHS INTERIOR • Seismic Wave Review 1. p-waves and s-waves 2. velocity = Distance / time

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  1. THE EARTHS INTERIOR • • Seismic Wave Review • 1. p-waves and s-waves • 2. velocity = Distance / time • 3. seismic waves travel at different speeds through different materials

  2. Geologic Discontinuities • seismic waves reflect and refract at discontinuities or boundaries between geologic materials (ie. rocks) • 2. velocity relationships

  3. Geologic Discontinuities 3. Paths of Seismic Waves i. homogeneous composition = straight ii. non-homogeneous composition = curved

  4. Composition and Structure of the Earths Interior 1. Crust: Oceanic and Continental Crust i. Continental Crust (1) felsic (granitic): 6 km / sec (2) 35 km thick ii. Oceanic Crust (1) mafic (basalt): 7 km / sec (2) 5-10 km thick (3) denser than continental crust

  5. Composition and Structure of the Earths Interior 2. Isostasy i. continental crust floats on top of oceanic curst

  6. 3. Mantle • i. Andrija Mohorovi i (1909 • (1) discovered a discontinuity at a depth of about 30 km • (2) Mohorovičić Discontinuity (Moho) • (a) p-waves below travel at 8km / sec • (b) p-waves above travel at 6.5km / sec • (3) the Moho separates the crust from the mantle

  7. 3. Mantle • ii. low-velocity zone • (1) 100 - 250 km deep • (2) p- and s-wave velocities decrease • (3) corresponds to the asthenosphere • (a) a layer in which the rocks are close to there melting point and are less elastic • (b) magmas are produced here(?) • (c) discontinuous (?)

  8. 4. Core • i. 1906 R.D. Oldham • (1) discovered that seismic waves arrived later than expected at seismic stations more than 130 degrees from an earthquake focus • (2) he postulated the existence of a core that transmits seismic waves at a slower rate than shallower depths • only very weak p-waves between 103-143 degrees from an earthquake focus: P-wave shadow zone • ii. Inge Lehman (1936) • (1) postulated that the Earth has a solid inner core • (2) explains the existence of weak p-waves in the shadow zone

  9. 4. Core • iii. Harold Jeffreys (1926) • (1) discovered that s-waves were no just simply slowed by the core but were blocked by it • (2) large and complete s-wave shadow at locations greater than 103 degrees

  10. Internal Heat and Heat Flow • 1. heat is due to meteor impact and radioactive decay • 2. earth cools by conduction and convection • i. conduction • (1) hot particles transfer heat to cool particles • (2) very inefficient and cannot explain the solid mantle (must be another mechanism) • ii. convection • (1) heated liquid expands and rises since it is less dense than cool material around it

  11. Magnetic Field • 1. William Gilbert (1600) realized the earth is a big magnet • 2. supports theory that the outer core is liquid

  12. Magnetic Field • 3. paleomagnetism • i. the earths magnetic field is recorded in cooling rocks that contain material that will align to a magnetic field - thermoremanent magnetism

  13. Magnetic Field • 3. paleomagnetism • iii. since rocks that are 3.5 billion years old record a magnetism, the earth had a magnetic field at that time - fluid core • iv. magnetic stratigraphy • (1) about ½ of the rocks on earth contain a magnetic field opposite to today's field • (2) patterns have been recognized and we can use it as a stratigraphy (a sequence) to tell approximate age

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