1. Siccar Point, Scotland
The unconformable contact of Old Red Sandstone on vertical Silurian rocks noted by James Hutton, who in 1788 first recognized the significance of its meaning in relationship to necessity for an old age for the earth
“No vestige of a beginning, no prospect of an end.”

2. Earth Structure
Earth starts as molten ball of rock. Elements begin to differentiate (layers based on density).

3. Earth Structure
How do we know its structure? -- deepest drill only 7 miles deep. -- infer from earthquakes (seismic activity). We use P-waves and S-waves.

4. Seismic Waves Seismic = earthquakes P-wave: (pump) - fastest
- travel completely through Earth.
S-wave: (snake)
- slow
- no liquids
- cause most damage

6. Outer Layers
Lithosphere (crust + hard upper mantle) - outermost layer of Earth - includes continents and ocean floor Asthenosphere = plates move on this. Convection occurs here.

8. Inner Layers Mantle - mostly solid rock - 80% of Earth’s volume.
Outer Core
- liquid iron/nickel
- rotation creates magnetic field.
Inner Core = solid iron.

10. Crust Continental Crust: Oceanic Crust: Less dense Old Denser
Young (why?)

12. Plate Tectonics Alfred Wegener - 1915 (Continental Drift) Evidence:
-- Continents fit like a jigsaw puzzle.
-- Same geology on each side of ocean. ex. Appalachian mountains, coal beds
-- fossils of identical land animals on each side of ocean.

14. Mountain belts earlier than 260 ma

15. Pangaea

16. Magnetic Field Outer core creates magnetic field
Protects us from solar wind (radiation).
Magnetic field reverses every 10k to 1,000,000 years.

18. Proof Not until the 1960’s. Seafloor spreading hypothesis:
If the seafloor is spreading, then magnetic reversals will match on each side of the spreading ridge.

21. Seafloor Spreading