The theory holds that the earth's crust is broken up into plates that float upon the much thicker layer of porous but very viscous material that makes up the earth's mantle.  Owing to slow convection within the mantle, the plates move at speeds ranging up to a few centimeters per year (or meters per century; or tens of km per million years).  Plates that lie above regions of upwelling in the mantle are moving apart and plates that lie above regions of downwelling in the mantle are being pushed together (Fig. 6-27). 

Earthquakes are observed to be to be concentrated along plate boundaries (Fig. 6-12).  Oceanic plates are thinner, but slightly denser than continental plates, so that when the two collide, the oceanic plate is pushed (or 'subducted') underneath the continental plate and disappears as it is incorporated into the mantle (Fig.  6-15a, 6-21).  Rocks in the subducted oceanic crust are subjected to increasingly higher temperatures and pressures as descend, causing physical and chemical transformations (referred to by geologists as 'metamorphosis': change of form) of certain rocks like carbonates (limestone).  Subducted ocean sediments also carry with them hydrated rocks (i.e., minerals with water molecules incorporated them).  As the temperature increases, the water molecules are released as steam within the mantle. 

One example of a region of subduction is the boundary of the Pacific and Juan de Fuca plates off the Washington coast, the site of mega-earthquakes every few hundred years.  Subduction zones correspond to trenches on the ocean floor. Volatile substances subducted into the mantle are expelled in volcanoes which are usually located nearby. 

Collisions between plate boundaries are often associated with volcanic activity (as in the Cascade range) and with the uplift of mountain ranges (Fig. 6-15).  The most dramatic of the earth's mountain ranges, the Himalayas, has been created by folding of the earth's crust following the collision of the Indian and Asian plates, which is still going on today.  The Rockies, Cascades and Sierra ranges have been created in a similar manner, by the collision of the Pacific and North American plates. 

Oceanic plates are continually being recycled.  The Pacific plate is being subducted along much of its boundaries.  Meanwhile, along the mid-Atlantic ridge, new oceanic crust is being formed as minerals upwelling in the mantle rise to the surface and cool (Fig. 6-13a).  As this newly formed crust diverges away from the mid-Atlantic ridge, the Atlantic plate widens, taking up the space lost by the Pacific plate as portions of it are subducted.  As the Atlantic widens and the Pacific shrinks, the continents may be viewed as drifting away from the Atlantic on trajectories that will eventually (in another 100-200 million years) cause them to converge over what is now the mid-Pacific.  A similar congregation of the continental plates occurred about 200 million years ago when the continental plates were clustered around what is now Africa.  The giant land mass that is believed to have existed at that time is called Pangea (all earth).

On the Paleomap project web site you can view a set of maps and animations of formation and break-up of supercontinents:

• Animation of the break-up of Pangea (200 million years ago to present): http://www.scotese.com/pangeanim.htm
• Animation of the formation of Pangea (540 million years ago to 240 million years ago): http://www.scotese.com/pzanim.htm
• Animation of the break-up of Rodinia and formation of Pannotia (750 million years ago to 540 million years ago): http://www.scotese.com/pcanima.htm
• What will happen over the next 250 million years? Check out a projection of future plate motions: http://www.scotese.com/futanima.htm

 Last Updated:
10/30/2001