CONTINENTAL DRIFT: The Geological Jigsaw

By: Staff Reporter
The jigsaw fit, one of the strongest evidences of Continental Drift Theory was propagated by A Wegener in 1912. Although remarked upon way back in 1620, with several scientists believing that the present day continents were the fragmented pieces of preexisting larger landmasses (supercontinents), it was only in the 1920s that the Theory gained prominence.
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Our earth is about 4.6 billion years old and undergoes a continual cycle of change – the shifting patterns of land and sea, the creation of new crustal rocks, the destruction and recycling of old rocks, etc. are elements of that change. A glance at the map of the world would clearly show that if the continents were cut out like pieces of jigsaw, many could be roughly fitted together. This was first remarked upon as long ago as 1620, but it was only in 1912 that the Austrian meteorologist Alfred Wegener noting the similarities in ancient fossil remains between the rocks of western Africa and eastern South America argued that it could not be mere coincidence, and that it could mean that the two continents were once joined.

Article 4 Fig 1

About 300 million years ago (mya), claimed Wegener, the continents formed a single mass (Fig.1), called Pangaea (Greek for ‘all the Earth’). Consequently, about 280 mya, the Pangaea rifted or split due to tidal and pole fleeing forces and its pieces begun moving away from each other ever, plowing through the oceanic crust. In 1929, the last edition of his book saw the addition of convective currents as the main driving force of the drift.

Article 4 Fig 2

Although Wegener was not the first to suggest that the continents had once been connected – he was the first to present extensive evidence from several fields. The idea was not taken seriously at first, but in the later 1950s and early 1960s, new techniques in geochemistry and geophysics as well as the traditional methods of palaeontology (the study of fossils) and stratigraphy (the study of rock layers), revealed that Wegener had been right, and the continents had in fact drifted. A further new understanding was that Earth’s lithosphere is segmented and that the individual pieces or plates are in constant motion. The term plate was in fact first used by J Tuzo Wilson in 1965 to denote a dozen or so large and small, rigid slabs of solid rock that remain in relative motion to one another as they ride atop hotter, more mobile material. There were also other evidences such as placer deposits and migratory habits of animals, amongst others, to support Wegener’ claim (Fig. 2).

The first split between the northern half, now named Laurasia (Europe, North America and Asia) and the southern half, named Gondwanaland (South America, Africa, India, Antarctica, Australia and New Zealand), began 300-280 mya. The gap between the two continents was sufficiently large to form the first circumglobal sea, allowing ocean currents to travel around the world, resulting in a warm equitable climate everywhere. The seas that formed between the splitting continents, were warm and rich in nutrients and marine life, laying down all the mineral oil we now mine.

Where the continents collided, high mountain ridges were formed such as in Europe (colliding with Africa): The Pyrenees, the Alps and more. An interesting case is that of the Indian subcontinent which travelled all the way north to collide with Asia forming the Himalayan mountains. Australia broke away from Antarctica at a later stage, pulling New Zealand with it.

Interestingly, scientists today point out that continents may have been drifting for much longer. New evidence with respect to tectonic plate movements suggests that the continents may always have been drifting apart, then moving in together again. In a cycle of 500-600 my, the continents may have been dancing to and fro for over 1500 my. As scientists gather more data, this puzzle may be pieced together more accurately.

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