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Three Worlds to Explore

Look Up? (Astronomy). Three Worlds to Explore. Look at Surface? (Geology). Look Down? (Geophysics). The Earth’s Interior. Deep wells and boreholes (4 to 12 km). Astronomical Observations. The Earth: What’s it made of?. Gravity. Seismology. N. S. Magnetism. Heat Flow.

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Three Worlds to Explore

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  1. Look Up? (Astronomy) Three Worlds to Explore Look at Surface? (Geology) Look Down? (Geophysics)

  2. The Earth’s Interior Deep wells and boreholes (4 to 12 km)

  3. Astronomical Observations The Earth: What’s it made of? Gravity Seismology N S Magnetism Heat Flow

  4. Density of Water 1 gm/cm3 Crust 2.6-3.1 gm/cm3 Mantle 3.3-5.7 gm/cm3 Outer Core 10-12 gm/cm3 Inner Core 13-14 gm/cm3

  5. P and S Wave Paths

  6. Relatively simple experiment to estimate some basic properties of the Earth’s interior: • Mass = M • Radius = R • Average Density = D • Rough Estimate of Variation of Density in Interior

  7. Newton’s Law of Gravitation r F F m M Force of attraction (F) is proportional to the masses, and is inversely proportional to the square of the distances between the masses.

  8. Newton’s Law of Gravitation r F F m M

  9. Henry Cavendish (1731-1810) determined the universal constant of gravitation G in 1798. torsion fiber massive lead spheres

  10. F m R M

  11. F m R ? Measure the force M Can measure in lab Use a known mass If we could measure R, we could determine M.

  12. How can we measure R? Greek Scientist:Eratosthenes (276-194 BCE)

  13. Eratosthenes (276-194 BCE) Observed the angles of the noonday Sun in two Egyptian cities that were roughly north and south of each other. Syene (presently Aswan) and Alexandria

  14. Eratosthenes (276-194 BCE) The angles differed by 7 degrees (or 1/50 of a complete circle). Circumference of the Earth must be 50 times the distance between the cities.

  15. Eratosthenes (276-194 BCE) The cities are 788 km apart. Circumference of the Earth must be 50 x 788 km = 39,400 km R = 39,400/2p = 6,271 km (modern value = 6,371 km)

  16. Astronomical Observations The Earth: What’s it made of? Gravity Seismology N S Magnetism Heat Flow

  17. F m Eratosthenes R Measure the force M Can measure in lab Use a known mass M = 6x1027 gm = 6,000,000,000,000,000,000,000,000,000 gm

  18. R M Average Density = 5.5 gm/cm3

  19. Average density of the Earth = 5.5 gm/cm3 Average density of crustal rocks = 2.7 gm/cm3 Dense Material? Density must increase with depth.

  20. Density must increase with depth. But, how can we obtain a more detailed picture of the variation of density with depth? Dense Material? Astronomical Observations

  21. Earth’s axis of rotation points towards different stars at different times. Gravitational forces from the Sun and the Moon cause the Earth to twist and turn in its orbit around the Sun.

  22. “Precession” of the Earth causes the North Pole to point to different parts of the sky during a 26,000 year cycle.

  23. “Precession” of the Earth is similar to the motion of a spinning top.

  24. From precession of the Earth it is possible to measure the moment of inertia of the Earth. Moment of inertia is a measure of how hard it is to twist an object.

  25. Moment of inertia is a measure of how hard it is to twist an object. The more that the mass is concentrated towards the center of an object, the easier it is to twist the object. Lower moment of inertia Higher moment of inertia

  26. Density of Water 1 gm/cm3 Crust 2.6-3.1 gm/cm3 Mantle 3.5-5.7 gm/cm3 Outer Core 10-12 gm/cm3 Inner Core 13-14 gm/cm3

  27. Astronomical Observations The Earth: What’s it made of? Gravity Seismology N S Magnetism Heat Flow

  28. Earthquake in Japan Magnitude 8.0 September 25, 2003 19:50 UTC

  29. Izmit Turkey Seismogram

  30. P and S Wave Paths

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