1. How Do We Know What It Is Like Inside The Earth? Pre-GPT scientific thought on the Earth before 1950 Department of Geological Sciences San Diego State University

2. What direct evidence tells you about the inside of the Earth?

3. How Symmetrical Is the Earth?

4. How Symmetrical Is the Earth? Very, a the rotation about its axis is fairly smooth Any reasonable Earth model proposed must be symmetrical Symmetry would be imposed by initial spinning while molten

5. The Interior Temperature of the Earth Is the inside of the Earth hot or cold? Is the inside of the Earth hot or cold? What evidence is there for this? What evidence is there for this?

6. Evidence that the Earth is hot inside: Volcanoes Hot Geysers Hot sulfurous gasses from vents

7. Deep Mines Evidence that the Earth is hot inside: The temperature in all deeper mines increases with depth: About 30 o C per 1000 m The limit to the depth of a deep mine is when the temperature is to hot for stand This image is in a South African Gold mine 3km or 2 miles deep in the Earth. The temperature is 49 C or 120 F Deepest mine is 3,585 m or 11,762 ft

8. Earth Radius~ 6,400 km. Deepest Mine 3.4 km Deepest mines only extend a tiny fraction into the average thickness the crust, never mind the rest of the Earth

9. What About Magnetism? Is the Earth’s magnetic? Is the Earth’s magnetic? What sort of properties does it have? What sort of properties does it have? Could you detect the Earth’s magnetic field Could you detect the Earth’s magnetic field

10. The Earth’s Magnetic Field: by compass Age of Exploration, 1400-1600, also discovered the compass and Earth’s magnetic field.

11. Magnetic Dip: the angle that magnetic lines leave or enter the Earth

12. Aurora Borealis and Aurora Australis, northern and southern lights

13. Most frequent is near 67 N and S latitude Maximum N. Pole Coverage Maximum S. Pole Coverage Earth’s magnetic field must be deep inside, And uniform and symmetrical to cause symmetrical compass variations and the Auras Borealis and Australis

14. What can gravity tell us about the Earth? Sir Isaac Newton Published his Philosophiae Naturalis Principia 1686 which included the definitive statement on gravitation I am So proud

15. Basic Gravity: Any two bodies in the universe attracted each other with a force that is GREATER if either is more MASSIVE GREATER if they are CLOSER Force, equal and opposite Force, equal and opposite Hey, this Took a lot of work

16. Gravity: The Formula G D m1m1 m2m2 Memorize this for the quiz

17. Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earth * Use torsion balance to measure the force between 4 iron balls * Measure the size and mass of the iron balls * Calculate “constant” in gravity formula Earth G Radius Ball mass Earth mass Ball weight Earth Mass = 6 x 10 24 kg Earth Average Density = 5.54 g/cm 3

18. Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earth: What does this mean? Earth Average Density = 5.54 g/cm 3 You should see this on my Facebook Henry Cavendish ~ 1789

19. Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earth: What does this mean? Earth Average Density = 5.54 g/cm 3 Surface rocks as Basalt, Granite ~ 3 g/cm 3 Deeper Rocks must be much denser than 5.54 g/cm 3 to compensate for lighter surface rocks Water = 1 g/cm3

20. What is the material at the center of the Earth? Iron -Nickel The Earth is thought to be similar to Meteorites, about 5% of the meteorites are Iron-Nickel metal. These are believed to have been the center of former bodies in space where the Iron-Nickel, a dense material, settled to the center when the body was molten. A later collision broke apart the body into many small parts. Iron-Nickel meteorite found in Oregon

21. Gravity is significantly affected by masses close to the Earth’s surface – even only a kilometer or so under ground Earth’s Radius 6,500 km

22. Ore bodies near the surface will cause gravity to be more and less – a way of searching for them

23. The British Great Arc Meridian Survey of India 1803-50

24. The British discovered that their plumb bobs were not pointing toward the center of the Earth What could be an explanation?

25. Plum bobs, gravity and floating Airy and Pratt proposed different explanations for the nature of the crust. Airy hypothesized that there was a uniform density crust to the Earth and it varied in thickness – he was more right. George Airy John Pratt

26. Isostasy and Floating Analogy: The crust floats on the mantel. A plate floats on a portion of mantle. Figure 2-5

27. Crust, Mantle, Core, Plates all - float with lighter on top, heavier below: Just as objects do in water Principle of Isostacy: “A body that is less dense than a surrounding fluid will displace a volume of the fluid equal to that body’s mass”

28. What Can Sound Tell Us? Hint: Earthquakes are mostly sound energy Earthquakes are used to probe the interior of the Earth, somewhat as a bat uses sound not only to detect the presence of objects but about the objects’ size and nature.

29. (Garrison, 1993) Primary (P) waves: Primary (P) waves: - compressional - analogy is sound waves Secondary (S) waves: Secondary (S) waves: - transverse (analogy is ocean waves) - ~1/2 velocity of P waves - do not pass through liquid By the way, surface waves (a third type slower than above two) cause the big disasters By the way, surface waves (a third type slower than above two) cause the big disasters

30. Prediction for Homogeneous Earth Structure (Garrison-3.7Top) (Garrison, 1993) Observation from Global Seismological Networks

31. (Pink Floyd, 1973)

32. .

33. Connect the dots: Then you get curves of temperature, density, P and S speed, properties and molecules Crust

34. The End