Theory Of Continental Drift Alfred Wegener It is

Theory Of Continental Drift • Alfred Wegener • It is this German to whom we credit with the proposal of theory of Continental Drift.

• While pondering the similarities between the coastlines of South America and Africa, Wegener came up with an idea: • What if the continents were once all connected and just drifted over the years?

Wegener’s evidence for Continental drift • Ferns and Reptiles in many different continents • Glaciers that were then tropical forests • Same types of rocks on continents close to each other (yet separated by an ocean) • The coastlines of Africa and South America

The Flaw in Wegener’s Theory • He thought that each of the continents were a separate plate--they were just drifting on a neverchanging ocean. • Like styro-foam floating on a pool of water!!

Canadian Correction • J. Tuzo Wilson • the 1960’s Canadian scientist who resurrected Wegener’s theory after years of disbelief by the science community. • Today, we know that's false, thanks to the discovery of crustal plates. • The plates of the earth are not composed of just land; they're composed of ocean too.

Canadian Correction – cont’d • In some cases, the plates are just land, in others they're just ocean, and, in still other cases, they consist of land ocean. • They each have different boundaries and move in all different directions.

Plates of the earth p. 12

Continental Drift • refers to the movement of the more than 20 plates (9 major) due to convergent, divergent, and transform boundaries. • The continents drift at a rate of 2 inches a year. • Started 200 million years ago • Pangea (land) & Panthalasa (sea)

More about Continental Drift • our text p. 10 • watch it at: • http: //www. ucmp. berkeley. edu/geology/tectonics. html

Plate Tectonics • Tectonic plates move or float on top of the upper mantle. • However they do not float freely. • The plates are forced in specific directions by the flow of magma beneath.

Plate Tectonics – cont’d • Plates move with the flow of magma. • The magma closer to the core heats and then rises towards the surface as its density decreases. • Once the rising magma reaches the lithosphere it moves in opposite directions. • The magma forms convectional currents.

Plate Tectonics – Convection Currents

Divergent Boundaries • Tensional Forces occur where two tectonic plates are pushed apart. The tension is created as the plates move away from each other. • Ridge Zones sometimes occur where two plates move apart. The magma rises between the plates and forms a ridge. • Again caused by convectional currents in the magma

Tensional Forces – Ridge Zones This diagram above shows “Sea Floor Spreading”

Where’s the TENSION?

Convergent Boundaries • Compressional Forces occur where two tectonic plates come together. They compress against each other. • Subduction Zones sometimes occur where compressional forces result from two plates colliding and one plate slips under the other. • Again caused by convectional currents in the magma

Compressional Forces – Subduction

Where’s the Subduction / Compression?

More about Plate Tectonics • our text p. 10 - 12 • watch it at: • http: //www. pbs. org/wgbh/aso/tryit/tecto nics/intro. html