Introduction
Continental Drift is the movement of the Earth's continents relative to each other by appearing to drift across the ocean bed. The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. Continental drift was first proposed in 1908 by American geologist Frank B. Taylor. However, Taylor's paper was mostly ignored and soon forgotten.
Then a German meteorologist, Alfred Wegener, began working on a theory of continental drift. The concept was independently and more fully developed by a German Geophysicist, Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism (though this was supplied later by Holmes) and others because of prior theoretical commitments. The idea of continental drift has been subsumed by the theory of plate tectonics, which explains how the continents move.
Like Taylor also Wegener could not explain the forces necessary to move the continents trough the crust. Wegener imagined the continents like gigantic ice floes swimming on and surrounded by the much denser oceanic crust. He proposed gravitational pull, tidal and centrifugal forces, but the English geophysicist Harold Jeffreys demonstrated that these forces are much too weak or if strong enough, had to stop earth’s rotation.
History
Abraham Ortelius (1596), Theodor Christoph Lilienthal (1756), Alexander von Humboldt (1801 and 1845), Antonio Snider-Pellegrini (1858), and others had noted earlier that the shapes of continents on opposite sides of the Atlantic Ocean (most notably, Africa and South America) seem to fit together.
W. J. Kious described Ortelius' thoughts in this way:
Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods" and went on to say:
"The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."
Writing in 1889, Alfred Russel Wallace remarks "It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had again and again changed places with each other." He quotes Charles Lyell as saying "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages" and claims that the first to throw doubt on this was James D. Dana in 1849.
In his Manual of Geology, 1863, Dana says "The continents and oceans had their general outline or form defined in earliest time. This has been proved with respect to North America from the position and distribution of the first beds of the Silurian - those of the Potsdam epoch - and this will probably prove to the case in Primordial time with the other continents also".
Dana was enormously influential in America - his Manual of Mineralogy is still in print in revised form - and the theory became known as Permanence theory. This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872-76, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore in what is now known as the continental shelf. This suggested that the oceans were a permanent feature of the earth's surface, and did not change places with the continents.
Furthermore, Alfred Wegener thought the continents had once been part of a single large land mass about 250 million years ago, which he called as Pangaea. Initially, he asserted, the original land mass had broken into two parts, two supercontinents, which he called Gondwanaland and Laurasia. Over millions of years, he suggested, Gondwanaland had broken apart into South America, Africa, India, Australia, and Antarctica, while Laurasia separated into North America and Eurasia.
Evidence
There are 4 evidences which support the theory of Continental Drift :
a.. Apparent fit of the continents
Eastern coast of South America and Western coast of Africa appeared to fit together. Similarly, North America and Eurasia and etc. appear to fit together. Evidence that South America and Africa might once have been joined to each other came from the research of the German geographer, Alexander von Humboldt. Von Humboldt travelled throughout South America, Africa, and other parts of the world, collecting plant and animal specimens and studying geography and geology. He observed many similarities between South America and Africa in addition to the apparent fit of continental coastlines.
Even, Alfred Wegener also supported this evidence. He thought it can’t be just a coincidence. But, he knew that he needed a strong evidence. So, he went to the library and got fascinated with the fossils, rocks and mountains.
b.. Fossil correlation
According to Alfred Wegener, fossils of animals and plants found on two or more continents were same. Like fossils of freshwater reptiles Mesosaurus were found in Eastern coast of South America and also in Western coast of Africa. Since, Mesosaurus could survive in shallow freshwater but not in salty water. Also they could not fly.
Since, water of ocean was salty and also it was impossible for Mesosaurus to swim in salty water for long. As fossils of Mesosaurus were found on two separate continents so, Wegener assumed that South America and Africa were once connected together and due to some circumstances they got separated and hence some Mesosaurus remained in South America and some in Africa. Similarly, fossils of triassic land reptiles Cynognathus were found on South America and Africa. Similarly, fossils of triassic land reptiles Lystrosaurus were found on Africa, India and Antarctica. Similarly, fossils of fern Glossopteris were found on South America, Africa, India, Antarctica and Australia. Similarly, fossils of the land vertebrate, Kannemeyrid, were found in Africa, North and South America, and Asia.
Von Humboldt and other naturalists also noticed many similarities among fossils of plants and animals on either side of the Atlantic. Before long, similarities across other oceanic gaps were observed. Plant and animal fossils found in India, for example, are often remarkably similar to those found in Australia. Living animals in widely separated lands are similar. For example India and Madagascar have similar mammals, which are quite different from those in Africa, even though it is now near to Madagascar.
Fossil plants in India, South Africa, Australia, Antarctica and South America are similar to each other. This so-called Glossopteris flora is quite different from plants found in other parts of the world at the same time. The same applies to fossil animals.
c.. Rock & Mountain correlation
Von Humboldt noticed that the mountain ranges near Buenos Aires, Argentina, are similar to mountain ranges in South Africa. Other mountain ranges in Brazil extend to near the seashore and stop. Similar mountain ranges begin at the corresponding seashore in Ghana in Africa. All of these mountain ranges appear to have the same age and to be formed of the same kinds of rock. The rock strata in these and other mountain ranges would match perfectly if the coastlines of the two continents were lined up. Von Humboldt also observed similar patterns among mountain ranges in Europe and North America.
Even, Alfred Wegener also noticed the similar patterns among mountain ranges in Northern Europe and North Eastern United States. He found that these mountain ranges are made of same kind of rock and the same age of rock. By 1912 Wegener had developed a theory suggesting that continental rocks were stronger and lighter than seafloor rocks. He also suggested that the seafloor rocks were like very thick tar. He concluded that the stronger continents were able to drift around on the weaker seafloor rocks.
d.. Paleo-climate data
Wegener also pointed out that ancient climatic zones seemed to have lain in different places from the present zones. According to him, present day Tropical Rain Forest of South America and Africa have glacial striation(scratches in bed-rock). But, how it could be possible that Tropical Rain Forest have glacial striation unless it was not always a Tropical Rain Forest earlier. According to Wegener, it could be possible only if the continents were not always in their one Equatorial regions like they are now. They were once down near the South pole where it could have been cold enough to have glaciers and they have since drifted apart. He pointed out that where great ice sheets have melted in recent geological times in Scandinavia and North America, the land is rising as fast as a centimeter a year. This vertical uplift requires horizontal inflow of matter below and implies that flow and motion do take place within the earth.
But Wegener was not much satisfied with this, so, he researched more. During his research he came across a sedimentary rock, also called Bituminous coal. He then, found a very interesting fact that coal is found in several parts of world like Tropical Plains, Antarctica, Northern Europe, Northern Asia, Southern tip of Africa, Australia and in Northern United States. Alfred knew that coal can be found in Tropical Climatic regions. But, these are not tropical climate, so, according to him, there might be a tropical climate in these regions in past.
Plate Tectonics
The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates, are dotted with many active volcanoes that form the so-called Ring of Fire. The Ring provides excellent examples of plate-boundary volcanoes, including Mount St. Helens. However, some active volcanoes are not associated with plate boundaries, and many of these so-called intra-plate volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an intra-plate volcanic chain, developed by the northwest-moving Pacific plate passing over an inferred “hot spot” that initiates the magma-generation and volcano-formation process.
Tectonic Activity :
At first, other scientists did not accept Wegener’s theory of continental drift. But scientists now know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. Over time, tectonic activity changes the Earth’s surface, rearranging and reshaping its landmasses.
Today, scientists believe that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago. The continents are still moving today. Underwater exploration has revealed seafloor spreading. Seafloor spreading is the process of new crust forming between two plates that are moving apart. Along a network of mountain ranges in the ocean, molten rock rises from within the Earth and adds new seafloor to the edges of the old. As the seafloor grows wider, the continents on opposite sides of the ridges move away from each other. North America and Europe, for example, are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. If you could visit the planet in the future, you might find part of California separated from North America, becoming an island in the Pacific Ocean. Africa will eventually split in two along the Great Rift Valley. It is even possible that another supercontinent may form someday.
Types of Plate Movement
Divergence, Convergence, and Lateral Slipping
At the boundaries of the plates, various deformations occur as the plates interact; they separate from one another (seafloor spreading), collide (forming mountain ranges), slip past one another (subduction zones, in which plates undergo destruction and re-melting), and slip laterally. Each of these types of movement produces unique geological consequences. The Himalayas are formed as two continental plates collide.
As one plate is forced under the other, friction causes enormous amounts of heat that builds up until a volcano forms and erupts. Earthquakes are often the result of sudden movement of two adjacent plates. The plates "lock up" until enough force is generated to break them apart, causing the quake. One of the world's most famous earthquake zones, the San Andreas Fault, lies at the boundary of the Pacific and the North American plates.
Divergent Plate Movement: Seafloor Spreading
Seafloor spreading is the movement of two oceanic plates away from each other, which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a mid-ocean ridge. The zone where the oceanic plates are moving away from each other is called a zone of divergence. Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960's.
Convergent Plate Movement:
When two plates collide, some crust is destroyed in the impact and the plates become smaller. The results differ, depending upon what types of plates are involved. Oceanic Plate and Continental Plate - When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction. Two Oceanic Plates - When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, forming volcanoes in the vicinity. Two Continental Plates - When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards.
Lateral Slipping Plate Movement:
When two plates move sideways against each other, there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.
