Plate Tectonics Topics

Principles of Plate Tectonics (Part 3)

The Wilson Cycle

• The cycle of rifting, drifting, and colliding plates is called the Wilson Cycle.

• The cycle begins with the rifting of a large continent (a supercontinent) to form a young ocean. Over time, the ocean widens to form a "passive margin" style of geology. Eventually, the ocean begins to close and the style of geology changes to subduction and mountain-building. The cycle ends with the complete closing of the ocean, a continent-continent collision and the formation of a new supercontinent.


• First, continental rifting begins when magma from the asthenosphere rises toward the surface. Continents overlying the rising magma are stretched and faulted.

East African Rift
The East African Rift. Note that as the African continent stretches, and normal faulting creates a central rift valley that includes volcanoes. (Photograph courtesy of NASA)

The East African rift system is an example of present-day rifting.  The African continent lies over an area where a convection current in the asthenosphere is rising, causing Africa to be stretched apart.  This young rift basin has not pulled apart enough to allow sea water to enter.  Basalt rich volcanoes have pierced the valley floor.

Formation of Young Ocean

• The continental fragments separate, and new seafloor crust, made of basalt, forms a mid-ocean ridge in the rift.  Seafloor spreading begins along the young mid-ocean ridge. The now separated continents "drift" apart as the ocean widens.

Red Sea
The Arabian Peninsula, the Red Sea, and Africa. The Mediterranean Sea is in the lower left corner and the Nile River valley is in the lower right corener. (Photograph courtesy of NASA)

This satellite view (above photo, looking south from the Nile delta) shows the young ocean basin of the Red Sea.  The oldest seafloor rocks in the Red Sea are about five million years old, indicating that the Red Sea began to form at that time.