Continental Drift


Plate Tectonics Global Processes Outline

Continental Drift

The theory of continental drift was proposed by Alfred Wegener in 1910. The theory suggests that the continents on the surface of the Earth are not fixed in position relative to one another but wander across the ocean floor.

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Early in the Earth's history, all the continental plates had joined to form a large super-continent known as Pangea.

By the Mesozoic Era, Pangea had begun to break up. The break-up led to many changes in climate, fauna and flora.

The modern world is still in motion. North America drifts closer to Asia, and the movement creates the volcanoes and earthquakes particularly in the countries of the Pacific Rim.

There is considerable evidence for the the theory including:

The Changing Earth

The Earth's crust has been in motion since its formation 4.6 billion years ago. Broken into a patchwork of plates and floating on currents in the fluid viscoplastic upper mantle beneath, the plates continuously collide and pull apart. The continental crust is significantly less dense than either oceanic crust or the upper mantle rocks.

Continents Adrift

All continents are moving. The floor beneath your feet, even though it feels stable and motionless, rests upon a landmass that is in continuous motion. This movement is continental drift.

The plates that form the Earth's crust are no more than 50 kilometres thick. It has long been suspected that the plates are in motion, but the mechanism that drives them remained a mystery for many years. Today, we know the mechanism is plate tectonics.

The Jigsaw Puzzle

The east coast of South America and the west coast of Africa look as though they would fit together like the pieces of a jigsaw puzzle. With a bit of rearranging, most of the continents can be put together too. This was one of the first clues to continental drift, but other more direct now evidence supports the theory.

How it works

Although the evidence for continental drift became overwhelming, no mechanism was proposed until the early 1960's when Harry Hess and Robert Dietz described a process of ocean floor spreading supported by the symmetric banding of the magnetic signature of the sea floor parallel to ocean ridges.

Currents form within any liquid when it is heated, just as they do in a pot of boiling soup. Similar currents form with the Earth's thick, dense mantle. The source of heat is the decay of radiogenic istopes. Arthur Holmes was the first to propse this mechanism in 1945.

As the mantle is heated, it rises, creating massive, slow convection currents within the Earth. The heated rock spreads i laterally at the base of the solid lithosphere, dragging the frgmented crust with it. Volcanoes and earthquakes occur as a direct consquence of plate movements.


Plate Tectonics Global Processes Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9