Looking at a globe or a map of the world, it is easy to see a close resemblance between the coastlines of western Africa and eastern South America.  Noticing this similarity, it is tempting to mentally merge these two massive continents and fit them together as if they were interlocking pieces in a very big puzzle.  Conversely, it is also not too difficult to imagine that these two continents were once joined together, and subsequently drifted apart, creating a space in between that is now the Atlantic Ocean.  This idea of 'drifting continents' was first hypothesized by Alfred Wegener, a German meteorologist, in his book The Origin of Continents and Oceans, first published in 1915.  In this book, Wegener proposed that all of the world's continental land masses were once united in a single, massive continent that he named Pangaea (a Greek word, meaning 'all land'), and that they subsequently drifted apart.  To portray his idea, Wegener produced a series of maps, incremented in time, and showing a progression of continental positions which began with Pangaea and ended with the present day configuration of the continents.  Wegener used many sources of data to produce this series of maps, including geological, paleontological and climatological information.  His idea, however, was not well received.  The majority of the scientific community rejected Wegener's hypothesis.  They voiced their objections on the basis that there was no known force which could push such massive continents through the very dense rock that comprised the ocean basins.

 
             As time passed, however, evidence for Wegener's hypothesis mounted.  In 1937, a South African geologist named Alexander du Toit published a book titled Our Wandering Continents, in which he provided convincing paleoclimatological evidence of Wegener's hypothesis.  Du Toit noticed that glacial deposits in present day Africa formed at the same time when massive coal deposits formed in what are present day Northern Hemisphere localities (coal is generally found in areas where ancient forests once thrived, generally tropical forests).  To explain this distribution, du Toit argued that the areas of glacial deposits in Africa could have been located near the south pole, and at the same time the areas containing the coal deposits could have been located in tropical latitudes, and that they subsequently moved to their present day locations.  This scenario was precisely what Wegener had earlier proposed.  Though compelling, du Toit's evidence still wasn't quite enough to sway the tide of scientific thought.
 

             It wasn't until the 1960's that general acceptance of continental drift began to take hold.  In 1962, Harry H. Hess published his landmark synthesis titled 'History of Ocean Basins.'  In this paper, Hess combined observations from four disparate areas of geologic research:  (1)  Maps of the sea floor topography obtained from echo sounders, which showed large ridges along the mid-oceanic regions;  (2)  Maps of the variations in magnetic orientation of minerals in the ocean basins obtained from magnetometers, which showed geomagnetic reversals in patterns which were symmetric with respect to the mid ocean ridges;  (3)  Timing information about the reversals in the Earth's magnetic field obtained from continental rocks which had been radiometrically dated;  and (4)  Accurate maps of the locations of earthquakes obtained from a recently installed world-wide net of seismometers, showing that earthquakes were distributed in concentrated lines along well defined boundaries throughout the world.  Hess noted that the mid-ocean ridges closely correspond to the median lines of the ocean basins, and that these ridges are also areas of high heat flow and shallow earthquakes.  Hess and others reasoned that the oceanic crust in these areas is moving apart in opposite directions and at right angles to the ridges.  This reasoning was bolstered by the periodic reversals in remanent magnetic orientation of the minerals of the ocean floor.  These reversals appear as magnetic stripes remarkably parallel to the mid-ocean ridges, shown in Figure 1 on the following page.  The timing of these reversals, calibrated by radiometric dating, corresponded to rates of spreading consistent with the large displacement of the continents suggested by du Toit and Wegener.  These observations sparked a revolution in thinking about the Earth's structure.
 

             Perhaps the biggest breakthrough to come in the 1960's was the formulation of a mechanism by which the continents move about.  In his original idea, Wegener envisaged the continents as moving like shallow rafts through the stationary oceanic crust, not unlike an ice-breaking boat plowing through a layer of ice on the surface of a lake.  Wegener, however, could not provide a motive force behind such movement.  Further, seismic evidence suggested that the continents were deep structures, and it would be very difficult for such deep structures to plow through the strong oceanic crust.   When the idea of sea-floor spreading was proposed, it became apparent that the continents are not plowing through the oceanic crust as Wegener suggested.  Rather, the continents are passive, being pushed around by the spreading sea floor.  At some boundaries, the oceanic crust is pushed under the continent, creating subduction zones.  At other boundaries, the oceanic plate butts directly into the continental plate, resulting in large forces applied to the continental plate.  The motive force causing the spreading of the sea floor was suggested to be thermal convection in the layer of the Earth below the crust, called the mantle.  This process is described as somewhat like the rising currents one sees in a pan of water which is being heated from below.  As hot plumes of magma in the Earth's mantle buoyantly rise towards the surface, the forces push apart the crust above, and these areas are seen at the mid-ocean ridges.  New ocean floor is being produced at these ridges as the hot magma below pushes its way to the surface.  As this new ocean floor is created, the older segments are pushed aside, outward from the mid-ocean ridge and towards the bordering continents.  At subduction zones, where the dense oceanic crust meets the lighter continental crust, the oceanic crust is pushed underneath the continent and recycled into the mantle.  In other areas, where the oceanic crust abuts the continental plates, large forces result.  In the process, the continents are moved about the surface of the Earth.  Since the 1960's, massive amounts of data have been collected which lend credence to the idea of drifting continents.  With modern global positioning technology, it is even possible to directly detect continental movement, which is on the order of 2 to 10 centimeters per year in most areas.
 

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