REVIEW: Seismic Waves give us information on: 1) Plate boundary locations 2) Types of plate boundaries 3) Internal Structure of the Earth 4) How Plates can move – Brittle Lithosphere “floating” on hot plastic Asthenosphere.

We can use gravity to examine the Earth’s Interior

ISOSTACY

Topography of the Sea-floor Development of the fathometer (echo –sounding) for World War II

Harry Hess and Sea-floor Spreading Hess had no real evidence that this was occurring. He called is own theory “Geopoetry” Evidence would be provided by new data from The Earth’s Magnetic Field

The Earth has a magnetic field – a Geodynamo

A B

The Earth’s Magnetic Field has reversed itself many times during the geologic past.

Polar Wandering? Or……… Continental Drift?

So, Earth’s Magnetism Explains: Apparent polar wandering Magnetic reversals recorded in rocks Seafloor spreading

Final Proof of Sea-floor spreading came from dating the Ocean floor

MAP OF PLATE BOUNDARIES

1) Divergent Boundaries