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

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

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

4. Crust 2.6-3.1 gm/cm 3 Mantle 3.3-5.7 gm/cm 3 Outer Core 10-12 gm/cm 3 Inner Core 13-14 gm/cm 3 Density of Water 1 gm/cm 3

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 M F r m F 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 M F r m F

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

10. M F m R

11. M F m R Measure the force Use a known mass Can measure in lab ? 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 = 39,400/2  = 6,271 km (modern value = 6,371 km)

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

17. M F m R Measure the force Use a known mass Can measure in lab M = 6x10 27 gm = 6,000,000,000,000,000,000,000,000,000 gm Eratosthenes

18. M R Average Density = 5.5 gm/cm 3

19. Average density of the Earth = 5.5 gm/cm 3 Average density of crustal rocks = 2.7 gm/cm 3 Density must increase with depth. Dense Material?

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. 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. Crust 2.6-3.1 gm/cm 3 Mantle 3.5-5.7 gm/cm 3 Outer Core 10-12 gm/cm 3 Inner Core 13-14 gm/cm 3 Density of Water 1 gm/cm 3

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

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

30. Izmit Turkey Seismogram

31. P and S Wave Paths