A Brief History of Plate Tectonics

The proposal and subsequent confirmation that the earth's outermost layer is composed of fault-bounded lithospheric "plates" (averaging 6km thick in the ocean, and 30-60km+ on continents) which move with respect to each other over time is one of the greatest discoveries in the history of science. Plate tectonics, which is the study of these lithospheric plates and their interactions through time, provides a unified explanatory framework for a wealth of different geologic data. It would be only a slight overstatement to say that nothing in geology makes sense except in light of plate tectonics.

Amongst young-earth creationists, two views of the theory of plate tectonics are now common. In the first view, plate tectonics is just another "evolutionist" myth with no supporting evidence (Ham, 1992). In the second view, exemplified by the Runaway Subduction Theory of John Baumgartner, the historical reality of Phanerozoic plate movements (such as the rifting of Pangaea) is accepted, but is explained within the rather restrictive framework of a tectonic catastrophe coinciding in time with Noah's flood (~2500BCE), and lasting only "weeks or months" (J. Baumgartener, qtd. in Wieland et al., 1997). Before showing why both of these creationist views are untenable in light of the current evidence, a brief history of plate tectonics is in order.

The first hints that the continents had moved relative to each other over time were provided by the first reasonably accurate maps of the continents themselves. As early as 1596, the geographer Abraham Ortelius noted the closely symmetrical fit of the Americas to Africa and Europe. Ortelius supposed that at some point in the past, there had been a great "rupture" in which the American continents were "torn away from Europe and Africa" (qtd. in McPhee, p. 115).

The Scottish natural philosopher Thomas Dick made similar observations of the shapes of the two sides of the Atlantic, which he published in his 1838 Celestial Scenery; or, the Wonders of the Planetary System Displayed: Illustrating the Perfection of Deity and a Plurality of Worlds. Dick wrote that "a consideration of these circumstances renders it not altogether improbable that these continents were originally conjoined, and that, at some former physical revolution or catastrophe, they may have been rent assunder by some tremendous power . . ." (ibid p. 116).

Similar observations were made by Buffon, Francis Bacon, and Francois Placet. Today we know that the fit of the continents on either side of the Atlantic is actually much better than the early maps viewed by Ortelius and others would have suggested. For example, Edward Bullard (1965) showed that a very tight fit with almost no gaps or overlaps can be obtained by matching the continental slopes at a depth of 500 fathoms rather than the margins of exposed land. .

Antonio Snider-Pelligrini, in his 1858 Creation and its Mysteries Revealed, noted the similarity of fossil plants in coal beds of Europe and North America, thus introducing the fossil record as a line of supporting evidence for continental movement. Snider-Pelligrini also published some of the first maps showing what the prior supercontinent may have looked like.

Alfred Wegener's 1912 work Die entstehung der kontinente und ozeane, or The Origin of Continents and Oceans, is usually cited as the founding document of plate tectonics. As we've seen, however, the idea that continents have moved relative to each other had been proposed much earlier, which is not surprising, since the shape of the continental margins on either side of the Atlantic literally forced this conclusion. Eduard Suess had earlier proposed the existence of Gondwana, which was a megacontinent that included all modern southern continents (S. America, Africa/Arabia, India, Australia, and Antarctica). Wegener went one step further and proposed that all continents had once been joined in a supercontinent he named Pangaea.

Wegener supplemented the earlier observations with observations from stratigraphy and paleoclimatology. Since nothing was known of seafloor spreading in Wegener's day, his theory of continental drift proposed that the continents somehow "plowed through" the ocean crust. This rightly led to the rejection of Wegener's theory of "continental drift" by most North American geologists until after WWII, when the discovering of sea-floor spreading showed that continents ride on top of larger plates rather than through them.

When the Atlantic ocean began to open in the Jurassic/Cretaceous, many preexisting structural features were split into two (or more) pieces; these include large so-called crustal provinces, sedimentary basins and ore deposits, accretionary sutures and faults. Wegener and others showed how a reconstruction of the continents into a single land mass during the mesozoic brings these widely seperated features into proximity.

The Atlantic margins of both Africa and South America contain flood basalts and associated feeder dikes which appear to have been generated by the rifting process. Both flood basalts, the Parana in South America and the smaller Etendeka basalt province in Africa, have been dated to 130-135 my (Milner et al., 1992; Turner et al., 1994). Similar rift basalts on the northern Atlantic margin, such as the 300m thick diabase Palisades Sill of the Newark group, which yields ages of 193 million years, show that the Atlantic opened slowly and progressively along a north-south trend, not all at once in a globally synchronous catastrophe. This is also suggested by the fact that the oldest oceanic crust along the north Atlantic margins date to the early Jurassic, whereas the oldest oceanic crust along the southern Atlantic margins dates to the Cretaceous.

Wegener, following the work of Juan Keidel and others, introduced paleoclimatological evidence for past supercontinents. He showed how the otherwise mysterious distribution of Carboniferous period glacial deposits in Africa, South America, Antarctica, India, Madagascar, Arabia and Australia would make sense if all these landmasses were joined together during the Paleozoic as Gondwana (later, with his father in law, Wegener went on to write one of the first books on paleoclimatology). Striated pavements created by glacier flow were used to infer the direction of ice flow away from centers of accumulation. In the case of the Carboniferous pavements, they seem to show, in many cases, ice moving onto continents from centers of accumulation in the sea. This pattern makes no sense given the present day distribution of the these landmasses.

Wegener showed that when these landmasses are reassembled into their proposed predrift positions, these paleozoic glacial deposits fit together like a puzzle, and show ice flowing in an outwardly radial pattern, and are limited to a much more narrow latitudinal distribution, in the same manner as modern polar glaciers. "This hypothesis has now been confirmed; from their lithology, many of the boulders in South American tills have been traced to a source that is now in Africa" (Plummer and McGeary, p. 422). Paleomagnetic data later showed that the center of ice flow deduced from bedrock striations coincided almost exactly with the south pole during the Carboniferous.

Note that this map is a projection. If applied to a globe, the glacial deposits form a partial ring around the south pole. Paleomagnetic evidence confirms that Gondwana was located over the south pole during much of the paleozoic (Crowell, 1978). The youngest tillites (Lower Permian) are found in Australia and the earliest (Lower Carboniferous) in southern South America. Click here to view a reconstruction of the Gondwana APW path.

Cyclothems in Laurentia

During the late Carboniferous period Laurussia and Siberia collided to form Laurasia; meanwhile Gondwana continued to move north. Pangaea formed as a result of the collision of the Gondwana and Laurasia supercontinents.The resulting compression formed some of the largest mountains of all time.

At the same time that Gondwana was being glaciated near the south pole, Laurussia was partially straddling the equator. As the Carboniferous glaciers waxed and waned, the resulting glacioeustatic changes in sea level (150 to 200 m.), combined in some cases with local tectonic subsidence, caused the shore to repeatedly transgress and regress over an enormous area where the Permo-Mississippian coals are now found. Each time the glaciers receded, sea level rose again, burying coastal swamps and forests of Lycopod trees and seed ferns under shale and limestone. Each time the glaciers began to accumulate again, sea level dropped, and the forests returned to the coastal areas.

The resultingly cyclic sedimentary deposits, known as cyclothems, are dustributed across the Laurasian landmasses, including North America, northwest Europe, and western Russia. Each cyclothem is the record of a single transgression/regression cycle. In Illinois, up to 60 individual cyclothems are stacked on top of each other (Condie, p. 301). In each cyclothem (as originally defined), a coal bed rests atop a paleosol underclay, and is covered by a transgressive sedimentary sequence of shale grading upward into marine limestone, followed by a regressive sedimentary sequence and topped by an unconformity. These cyclotherms are a major source of coal.

Another line of evidence invoked by Du Toit, Wegener and others to support the existence of supercontinents in the past involves the distribution of fossils. For instance, it was pointed out that late paleozoic-middle mesozoic deposits on the continents of Gondwana preserved similar fossils, such as the extinct seed-fern Glossopteris and the mammal-like reptile Lystrosaurus. Other examples include Cynognathus and the freshwater reptile Mesosaurus. Fossil mesosaurs occur only in South African and South American freshwater deposits. Du Toit pointed out that "We find . . . the Irati shales [of Brazil] are identically lithologically and palæontologically with the carbonaceous . . . 'White Band' of the Dwyka [in Africa], each containing the reptile Mesosaurus . . . not known in other parts of the world." Because it is unlikely that the mesosaurs could have traversed thousands of kilometers of saline open ocean, their geographic distribution corroborates other evidence that the continents of the Southern Hemisphere formerly were joined together. In addition, the basins in which these Mesosaurs occur, the Parana basin of south South America and the Cape-Karoo basin of southern Africa, also possess very similar stratigraphic columns and tectonic subsidence histories (see Tankard et al., 1995, p. 33), again corroborating their previous geographic continuity.

In a 1998 article appearing in Nature (1998; 392, 171 - 173. Evidence for a late Triassic multiple impact event on Earth), Spray et al. prevent evidence of a Triassic period multiple impact event, similar to the 1994 Shoemaker-Levy 9 multiple-impact event on Jupiter. Though the respective craters from this impact are currently widely distributed, each is dated to ~214mya. Their abstract reads:

As we said above, one of the main reasons for the rejection of Wegener's continental drift theory was his assumption that continents must have moved through stationary oceanic crust to reach their current positions. In 1962, Harry Hess proposed an entirely different mechanism. According to Hess' Sea-floor Spreading Hypothesis, continents do not plow through ocean crust; instead, they are simply riding atop larger plates. For instance, the North American continent represents only a portion of a larger plate which includes the eastern half of the Atlantic ocean.

On this hypothesis, new ocean crust is continually being created at spreading ridges, such as the Pacific Rise, the Indian Ridge, and the Mid-Atlantic Ridge system. These spreading ridges rise about 2.5km above the ocean floor and are characterized by high heat flow and shallow, tensional earthquakes. The generation of new oceanic crust at spreading ridges is compensated elsewhere by destruction of oceanic crust at subduction zones/trenches, such as the Aleutian trench. Trenches are characterized by low heat flow and deep, compressional earthquakes.

As plates diverge from a spreading ridge, sheets of magma rise between them, forming another diabase dike in what is known as the sheeted dike complex. Typically, the sheeted dike complex is overlain by pillow basalts and underlain by gabbroic igneous rock. Crystal size increases with depth. The pillow basalts, for instance, contain only very fine crystals, indicating that they cooled quickly, as we would expect, since pillow basalts are extruded subaqeously. Crystal size is larger in the diabase dikes, and larger still in the underlying gabbro. As we will discuss below, large slivers of such oceanic crust have been accreted onto the side of continents in the past, and are often found in collisional foldbelts between continental cratons.

Although GPS and laser technologies were not yet developed in 1962, Vine and Matthews found a way to test the sea-floor spreading hypothesis using paleomagnetic data. They reasoned that if sea-floor spreading occurs at midocean ridges, then the same sequence of geomagnetic reversals found in terrestrial lava flows should be found, in a bilateral pattern, on either side of the mid-ocean ridges. Magnetic mapping of the oceanic crust revealed precisely this pattern, although in some cases spreading is faster on one side of a ridge than the other. You can see a few examples here. Today, of course, the seafloor spreading hypothesis can be directly tested with precise geodetic measurements. The results indicate that spreading is in fact occuring at these ridges.

The general proposition of Wegener and his predecessors, that the continents have moved relative to each other, has now been established beyond any doubt. Also established is the proposition that these processes continue to occur today, though a few creationists still deny this. The argument can now focus on the much more contentious issue of how quickly these processes have operated in the past.