To discover how Plate Tectonics works we must study the Earth using several fundamental physical principles & properties:

1) Seismic Waves2) Gravity3) Isostacy4) Magnetism

Seismic Waves give us information on:1) Plate boundary locations2) Types of plate boundaries3) Internal Structure of the Earth4) How Plates can move – Brittle Lithosphere

“floating” on hot plastic Asthenosphere.

To discover how Plate Tectonics works we must study the Earth using several fundamental physical principles & properties:

1) Seismic Waves2) Gravity3) Isostacy4) Magnetism

Gravity: One of the 4 Fundamental forces in the universe1) Gravity is a force of attraction between any two objects that have mass.2) The strength of the force is proportional to the mass of

the objects and inversely proportional to the distance between them.

3) Differences in the force of gravity can be measured by how a weight stretches a spring at different locations – this is how a scale works.

We can use gravity to examine the Earth’s Interior

Why a positive Gravity Anomaly in the upper Mid-West?Why a negative anomaly across the Rocky Mts.?

Ancient Rift ZoneOur Continent began to pull apart over a billion years ago but stopped.

Dense basaltic magma flowed upward to surface, hence the positive gravity anomaly.

Why a negative anomaly across the Rocky Mts.?

We need to understand property #3 - Isostacy

1) Seismic Waves2) Gravity3) Isostacy4) Magnetism

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 fromThe Earth’s Magnetic Field

1) Seismic Waves2) Gravity3) Isostacy4) Magnetism

The Geodynamo

Polar Wandering?

Or……. Continental Drift?