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 Siccar Point, Scotland “No vestige of a beginning, no prospect of an end.”

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.

Seismic Waves Seismic = vibrations from earthquakes. P-wave: - fastest - travel completely through Earth. S-wave: - slow - no liquids

Inner Layers Asthenosphere = Soft, plastic-like layer. Allows plates to move. Mantle = mostly solid rock = 80% of Earth’s volume. Outer Core = liquid iron/nickel - convection creates magnetic field. Inner Core = solid iron.

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

Plate Tectonics Alfred Wegener (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.

Mountain belts earlier than 260 ma

Pangaea

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

Magnetic Field Liquid outer core creates magnetic field Protects us from solar wind. Magnetic field reverses every 10k to 1,000,000 years.

Seafloor Spreading

Plate Tectonics Explains: 1.Mountains (Orogeny) 2.Earthquakes 3.Volcanoes 4.Ocean Trenches 5.All geological events.

Plates Lithosphere is broken into pieces (plates). New lithosphere is constantly: -being made (by volcanoes and ridges) -being lost (by subduction)

Plate Boundaries (Edges) Where plates meet other plates, three things can happen: 1.Diverge – move apart 2.Converge – come together 3.Transform – move past each other

eo/animations/ch2.htm