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

http://www.glg.ed.ac.uk/courses/field/siccarpt/

Siccar Point, Scotland

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

Earth Structure

Earth starts as molten ball of rock.

Elements begin to differentiate (layers based on density).

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 = earthquakes

P-wave: (pump)- fastest- travel completely through Earth.

S-wave: (snake)- slow- no liquids- cause most damage

Outer Layers

Lithosphere (crust + hard upper mantle)

- outermost layer of Earth- includes continents and ocean floor

Asthenosphere = plates move on this. Convection occurs here.

Inner LayersMantle

- mostly solid rock- 80% of Earth’s volume.

Outer Core

- liquid iron/nickel- rotation creates magnetic field.

Inner Core = solid iron.

Crust

Continental Crust: – Less dense– Old

Oceanic Crust: – Denser– Young (why?)

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.

Mountain belts earlier than 260 ma

Pangaea

Magnetic FieldOuter core creates magnetic field

Protects us from solar wind (radiation).

Magnetic field reverses every 10k to 1,000,000 years.

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.

Seafloor Spreading