Continental Drift Theory (the Jigsaw Theory), Evidences and Causes of Continental Drift

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Continental driftis defined as the movement of the Earth’scontinentsrelative to each other, appearing to “drift” across the ocean bed. Continental drift theory was initially proposed byAbraham Orteliusin 1596 and was later developed fully by a geophysicist and meteorologist,Alfred Wegenerin 1912. This theory explains that the earths continent move gradually over the surface of the planet on a substratum of magma; this movement is what Wegener termed the continental drift and it is also called the Jigsaw theory.

History of Continental Drift

The history of earths land surfaces since the end of Permian period the last period of the Pangaea (pangea) shows that all the continents of the world were joined together to form one single continent. Pangaea is surrounded by a single universal ocean Panthalassa. At about 300 million years ago, at the beginning of the Mesozoic era, pangea began to split into fragments. The event that has occurred since that time is indeed extraordinarily complex and very incompletely known. However, it is believed that the outcomes of Pangaea are the present continents and seas in which we live and are surrounded by. It has also been established that land, sea, and the present position of the continents have not always been the same. Sometimes in the Mesozoic era, all the world land constituted two great continents LAURASIA and GONDWANALAND. Laurasia was made up of what are now North America and Asia; while Gondwanaland was made up of what are now Antarctica, Australia, South America, Africa and India.

LAURASIA and GONDWANALAND, the two great continents that make up the world land in the Mesozoic era
LAURASIA and GONDWANALAND, the two great continents that make up the world land in the Mesozoic era

 

During the breakup of the continents, Australia was one of the first continents to breakup and drift apart, since then, it has never had land connection with any other area. It is thought that this may be the reason why Australia has a different flora and fauna (plant and animals) from other parts of the world today because animals and plants have not been able to reach Australia, because of the immensity of the ocean surrounding Australia. However, the other continents have stayed together for much longer period of time. Later, India drifted northward and joined Asia; Africa then drifted northward and joined with Europe. The Atlantic Ocean began to separate South America from Africa, when joined together, South America and Africa fit perfectly together like a jigsaw puzzle in outline as well as their rock type and geological structure. At the same time, North America broke up from Europe and finally joined South America. It is thought that this movements help to explain the similarities which are found inflora and fauna of North America and Europe as well and the striking differences between Flora and fauna of North America and those of South America. Also read factorsaffecting theglobal distribution of plants and animals. Wegners assumptions are summarized below using these four evidences.

Wagener 4 evidences of continental drift theory

  1. Similar rocks found in South America and Africa- rock similarities occur on two different continents and same mountain ranges as well.
  2. Similar glacial deposit and ancient climates- same glaciation marks on different continents, fossils indicate tropical climates were now cold.
  3. Fossils records- there were fossils found in Argentina that are identical to that Africa.
  4. Continent seem to fit together like puzzle pieces- The eastcoastof South America and the west coast of Africa seem to fit together like pieces of ajigsaw puzzle.
The map shows how the Earth continents were constantly drifting away
The map shows how the Earth continents were constantly drifting away

 

A map of the continents inspired Wegener to assumed the connections between the Earths continents, this he explained using some evidence such as the Earth’s geologic history, climatological evidence and of plant and animal fossils on the matching coastlines of South America and Africa, which are now separated by the Atlantic Ocean. Wegener, who was trained as a meteorologist, was fascinated by the interlocking fit of Africa and South America’s coastlines. Wegener then outlined an impressive amount of evidences to support his observations that Earth’s continents were once joined by a single universal ocean called Pangaea before the breakup.

Pangea or Pangaea- a supercontinent that existed during the late Paleozoic and early Mesozoic eras before the breakup of the continents
Pangea or Pangaea- a supercontinent that existed during the late Paleozoic and early Mesozoic eras before the breakup of the continents

 

Wegener knew that fossil plants and animals such asMesosaurus a freshwater reptile like crocodile was only found in South America and Africa during the Permian period. He also matched up rocks on either side of the Atlantic Ocean like puzzle pieces. When the rockstrataof the margins of separate continents appeared very similar, he suggests that these rocks were formed in the same way, which means they were joined together initially. For instance, the Appalachian Mountains in United States and Caledonian Mountains in Scotland were seen to fit together. Furthermore, the Karroo strata in South Africa and Santa Catarina rocks in Brazil are similar in structure and in physical characteristics.

The rejection of Alfred Wageners theory

Wageners theorywasnotwidely accepteddue to some missing facts, many geophysicists and geologists based on the following reasons rejected His theory:

  • Wegener had no good models to explain why the continents moved apart, he believed that the rotation of the earth caused thecontinents to move apart,
  • He believed that the Earth’s spin was sufficient to cause continents to move, but geologists knew that rocks are too strong for this to be true.
  • Wegener thought that the continents were moving through the earth’s crust, like icebreakers plowing through sea ice. Geologists noted that plowing through oceanic crust would distort continents beyond recognition.

Furthermore, it had been shown that floating masses on a rotatinggeoidwould collect at the equator, and stay there. This would explain one, but only one, mountain building episode between any pair of continents; it failed to account for earlierorogenicepisodes. Secondly, masses floating freely in a fluid substratum, like icebergs in the ocean, should be inisostaticequilibrium (in which the forces of gravity and buoyancy are in balance). However, gravitational measurements showed that many areas are not in isostatic equilibrium. Lastly, there was the problem of why some parts of the Earth’s surface (crust) should have solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties. Moreover, decades later, scientists came to confirm some of Wegener’s ideas, such as the past existence of a supercontinent joining all the world’s landmasses as one.

What causes continental drift?

However, today many scientists believed that continental drift is caused by the movement of the tectonic plates (massiveslabs of rock) in which the continents rests on top. As of 1953, the idea of continental drift was displaced by theory ofplate tectonics, which explains how the continents move. Scottish Geologist, Arthur Holmes, in 1931 came up with some observations as to why the continents moved apart. He believed that heat trap in the Earth caused convection currents. There were areas where fluids trapped beneath Earth’s crust heat up, rise, spread laterally, than cool and drop back down.

Regardless of Wegener implausible evidence for continental drift, he died in 1930 at the age 50 of a likely heart attack while carrying out a research in Greenland, Wegener never lived to see how his theory gained wider acceptance.

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