Since it s early beginnings some 4.6 billion years ago, the earth has been constantly changing its face. Oceans have become deserts and what was once mountainous terrain has found itself submerged in salt water oceans. Underwater volcanoes create new surfaces daily which one day may host a new species of life. It is this metamorphic nature of the earth that interests geologists and paleontologists the world over. What was the appearance of the primitive earth? What changes must have taken place that resulted in the earth we know today? These are some of the questions that scientists have been addressing for years. There has been much speculation about the structural origins of the earth. The most thought-provoking being the idea that at one stage in the earth s history all the continents were joined together to form a single mass of land.
Francis Bacon first hypothesized the idea in the 1620 s, focusing on the parallel shores of South America and Africa. But it wasn t until 1910 when Alfred Wegener scientifically considered the matter of a supercontinent which he called Pangea (Stokes, 1973). Since this time, scientists have argued for and against Wegener s explanation of Pangea. Today skeptics still argue the idea even in the face of overwhelming scientific data. Fossil, geologic and paleomagnetic data clearly validate Wegener s hypothesis- proving the existence of the super continent.
Wegener first envisioned that the landmass of Pangea split apart and assumed the current continental positions. The driving force behind this magnificent change was what Wegener called continental drift. As geological evidence to support the idea of continental drift he used the good fit of South America and Africa (Stokes, 1973), which suggests that the two continents used to be joined or were in close proximity to one another. Opponents to continental drift believed that there was no mechanical mechanism capable of moving landmasses as large as continents. The idea of huge bodies of rock plowing through the earth s crust was ridiculous. It wasn t until 1960 when Harry H. Hess hypothesized that cracks in the sea floor open along the crest of mid-ocean ridges where new sea floor forms and spreads out laterally on either side of the crest. Robert S. Dietz took Hess s ideas further, naming the process sea floor spreading (which occurs at spreading zones ) and coupling it with the idea that old sea floor is absorbed beneath zones of deep ocean trenches and young mountains (Briggs, 1987). Dietz s idea of plates being absorbed into the asthenosphere at subduction zones (areas of converging plates) answered many questions. Since the earth maintains a constant size, it only seems logical that spreading of the crust would be balanced by absorption of old crust at plate margins.
Sea floor spreading has its own driving force called convection. Convection occurs in the mantle of the earth and the process involves heating of the mantle from below by the core and loss of heat to the crust in the upper mantle. The hotter slushy molten rock of the lower mantle expands and rises towards the crust and is replaced by the colder mantle contents near the surface. The result is the circular motion known as convection. It is this circular motion that drives plate movement in either a diverging or converging manner.
In 1963 and 1973 studies, J.Tuzo Wilson gave further credence to Dietz s sea floor spreading by observing that as one moves away from mid-ocean ridges the ages of oceanic islands increases. These and other discoveries led to the modern view of plate tectonics- the idea that proposes that the earth s crust is divided into a mosaic of constantly moving plates that contain the continents (Briggs, 1987).
The break up Pangea first began in the late Triassic, between North Africa and North America. The resulting rift was thought of as a complete separation of Pangea into Northern and Southern parts termed Laurasia and Gondwanaland respectively. But distribution of tetrapod fossils indicates a north-south connection that existed until the middle or late Jurassic (Briggs, 1987). Other rift formations soon followed with separation of South America and Africa occurring at their southern tips during the early Jurassic. India separating from Madagascar in the early Cretaceous, and the formation of continental connections such as the Bering Land Bridge in the late Cretaceous are a few examples of continental change (Briggs, 1987). The end of the Miocene marked a near completion of continental spreading to current present day positions (Stokes, 1973). Undoubtedly the process of plate movement is slow, just 5cm/ year or 1 km/ 20,000 years on average.
Early arguments for the existence of Pangea were based primarily on the good fit model that results when the continents are pieced together, but scientists and skeptics alike needed more proof. Over time, researchers began to piece together scientific data in order to unlock the secrets of Pangea. Proponents of Pangea used three sources of evidence: similar fossil and rock strata found on different continents and paleomagnetic data. Although not directly linked to the Pangea hypothesis, paleomagnetic data does support the theory of sea floor spreading which is one of the critical mechanisms of plate (continent) movement. In 1963, Fred Vine and Drummond Matthews proposed that as lava erupts at spreading zones and travels laterally away from the zone, it cools. As it cools, the magnetic minerals contained within the lava align with the magnetic field of the earth. Magnetic readings of the sea floor in these regions gave rise to positive and negative magnetic anomalies corresponding to stronger and weaker magnetic fields respectively (refer to figure). Vine and Matthews concluded that the difference in magnetic field strengths was a result of the reversal of the earth s magnetic field. Stronger readings were a result of the magnetic field as we know it today and weaker fields were due to a reversal of this field. Therefore, if an eruption occurred during a period of pole reversal, the magnetic data would be preserved as a pair of parallel bands in the lava on either side of the spreading zone. This is precisely what scientists observed (web site).
Further support for Pangea comes from cross-sectional studies of rock strata. Researchers have identified similar layers of earth in regions of Antarctica, Australia, South America, Africa and India. These core samples display identical banding consisting of layers of basalt, sandstone/shale, coal and glacial till (refer to figure). It seems unlikely that identical rock sequences could have formed independently on separate masses of land and at the same time. It is reasonable to conclude that these rock patterns developed on a single mass of land that existed early in the history of the earth (internet).
Of all the evidence suggesting the break up of Pangea into the continents of today, the most convincing is the number of plant and animal fossils found in areas of Africa, South America, India and Antarctica. The end of the Triassic was marked with substantial extinctions among terrestrial vertebrates (Behrensmeyer et al.). Fossils of some of these species have been found on different continents. Wegener proposed that species dispersed when the continents were connected and were then carried to their present positions as the continents drifted. Reassembling the continents into Pangea explains the distribution of Glossopteris (a type of fern). Glossopteris fossils have been found in South America, Africa, India and Australia, but when these regions are pieced together, the distribution can be accounted for over a much smaller area (web). The same reasoning can be applied to fossils of Mesosaurus (a small freshwater Early Permian reptile) found in South Africa and southern Brazil, and to Lystosaurus (a small mammal like reptile) found in regions of Antarctica and Africa (web).