The Scientific Method

1. The Scientific Method

2. Goal: Explain How Universe Works • Scientific method: the process where ideas are formed, tested, and refined • Step 1: • Questions sparked by observation or experiments

3. Strange Observations • Alfred Wegener • 1910 Pangaea

4. But what does it all mean? • Step 2: • Formulate hypothesis: tentative explanation built on strong supporting evidence • Continental drift? • Continents were once part of giant supercontinent, Pangaea (“all earth”) and have since drifted apart to their current locations

5. Wegener’s Hypothetical Earth • Late Carboniferous • 300 Ma • Middle Tertiary • 50 Ma • Early Quaternary • 10 ka

6. But… (the hardest part) • Step 3: • All hypotheses must be tested • Counter-evidence  you’re wrong, back to beginning • Support  confidence, becomes scientific theory  basis for future tests

7. Wegener couldn’t test it… • Mechanism for continental migration?! • His idea: continents floated like boats on ocean basin crust pulled by tides of Sun and Moon • Unfortunately, forces are much too weak • Open and shut? • Wegener was apparently wrong

8. The Building Blocks of Earth Science

9. Eratosthenes (~250 B.C) • Experiment • If the Sun is far enough away, its rays are parallel • A well in Syene, a pole in Alexandria 800 km apart at same time, same day • No shadow in Syene but shadow in Alexandria

10. Disproving the Flat Earth Theory • Earth must be curved • Calculated that earth was a sphere, 250,000 stadia in circumference at the equator • Not sure how big a stadion • Attic stadion (~185 m): 46,620 km (over by 16.3%) • Egyptian stadion (~157.5 m): 39,690 km (<1%!) • Actual circumference 40,075.16 km

11. Isaac Newton • Discovered how to measure the Earth’s mass by the strength of gravity • Density (D) = Mass (M) / Volume (V) • With Eratosthenes’ circumference… • Could estimate average Earth (D) = 5.5 g/cm3

12. What Was the Earth Made Of? • Major problem: • Surface rocks mostly low density = 2.7 g/cm3 • Some iron-rich rocks had density = 3.5 g/cm3 • So where was the dense material? • Something REALLY DENSE had to account for the high average density

13. The Dense Nougat Center • Emil Wiechert, German Physicist  Pressure increases with depth, makes sense that deep rocks, more compact, higher density  Iron-nickel meteorites densest material identified • Wiechert’s hypothesis: Earth must be layered and iron/nickel settled to core under gravity  Si-rich low-density “mantle” – sandstones, etc  Iron-rich “core”

14. How to Go from This to This?

15. Earthquakes Provide Answers • What are earthquakes or seismic waves? • Waves that travel through rock • What is a wave? • Movement of energy with little permanent movement of mass • Frequency = peaks/s • Velocity = frequency * wavelength

16. Body Waves(Waves that pass through the Earth) • P-wave = • compressional • S-wave • shear (secondary)

17. Surface Waves(Waves that occur at the Earth’s surface) • Love = side to side • Rayleigh = up and down

18. Notes About Earthquakes • All earthquakes involve all wave types • Body waves travel fastest • P-wave (5-6 km/sin crust), S-wave (3.5 km/s) • Seismic waves travel faster through denser, more rigid rocks • P-waves can travel through liquids, solids, gas • S-waves cannot travel through liquids, gases

19. How Do We Know?

20. The Seismograph • Recording earthquakes

21. What is an Epicenter? • Epicenter –location on earth’s surface • Focus – place where fault first moved • Recall P and S-wave times