Plate Tectonics and

Continental Drift

Continental Drift

• Alfred Wegener believed that the continents were once connected.

• This large continent was called Pangea.

• In 1915, he proposed his theory of Continental Drift.

Evidence For Continental Drift

• Matching fossils were found along the boundaries of continents that have very different climates.

• Glacial deposits found in South America, Africa, India, and Australia.

Sea Floor Spreading Theory• With sonar, scientists determined that the

ocean floor was composed of mountains and deep valleys.

• Harry Hess authored the Sea Floor Spreading Theory to explain that the ocean floor is constantly spreading along the Mid-Atlantic Ridge.

Mid-Atlantic Ridge

• New sea floor is created by oozing magma in the center of the MAR.

• Old sea floor is forced down into the mantle in the trenches near the boundaries of the oceans and continents.

• Rocks near the MAR were much younger than rocks that had been forced outward from the MAR.

Plate Tectonic Theory

• A new theory based on the Sea Floor Spreading Theory and The Continental Drift theory were combined to form the Plate Tectonic Theory.

• Plate Tectonic Theory: States that the earth’s crust and upper mantle are broken into sections called plates. The plates move around on the mantle.