Theory Of Continental Drift

• Alfred Wegener

• It is this German man to whom we credit with the proposal of the 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?

Wegner’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 Wegner’s Theory

• He thought that each of the continents were a separate plate--they were just drifting on a never-changing ocean.

• Like styro-foam floating on a pool of water!!

Canadian Correction

• J. Tuzo Wilson

• the 1960’s Canadian scientist who resurrected Wegner’s theory after years of disbelief by the science community.

• Today, we know that 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 and 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/tectonics/intro.html