Gondwanaland and Laurasia

Earth scientists theorize that the present-day continents were produced by dividing a supercontinent called Pangaea into two gigantic landmasses, Gondwanaland and Laurasia, which continued to fragment. The continents moved into their present locations in accord with a phenomenon known as continental drift, which scientists believe is still taking place.

Formation of the Continents

Austrian geologist Eduard Suess first postulated the existence of an ancient supercontinent in 1885. He called this great continent Gondwanaland, after a region in India inhabited by the Gond peopl, an Indigenous group listed as a Scheduled Tribe. Whereas Suess proposed that much of Gondwanaland had sunk, in 1929 German geophysicist Alfred Lothar Wegener suggested that a supercontinent that he called Pangaea had broken up and that the individual continents had drifted to their present positions. In 1937, South African geologist Alexander Du Toit envisioned two primordial continents: Laurasia in the north and Gondwanaland in the south; subsequently, both parts broke again to form the present continents. English geologists Edward Bullard, J. E. Everett, and A. G. Smith's computerized assembly of the ancient continents in 1965 seems to verify Du Toit's continental arrangement.

The standard explanation of the formation of the continents begins 300 million years ago, at the end of the Carboniferous and start of the Permian geological periods, with the formation of Pangaea, a V-shaped supercontinent. The northern extension of Pangaea, called Laurasia, straddled the equator and included North America, Europe, and Asia; the southern arm of Pangaea, called Gondwanaland and comprising South America, Africa, India, Australia, and Antarctica, was positioned so that its northern parts were in the tropical latitudes and its southern parts were beneath the polar ice cap. Although the Tethys Sea lay between Laurasia and Gondwanaland, they were linked from northwest Africa to North America and southern Europe.

The total area of Pangaea, when measured down to the 1,000-fathom isobath, seems to have been 200,000 square kilometers, or roughly 40 percent of the earth's surface. When the future continents were still part of Pangaea, they were to the south and east of their present locations. If New York had existed then, it would have been on the equator and at longitude 10 degrees east instead of 74 degrees west. Spain would have been near its present longitude, but it, too, would have been on the equator. Japan would have been in the Arctic, and India and Australia would have bordered the Antarctic.

This loose grouping of partly linked continents remained constant for the next 150 million years, although a few changes were slowly occurring. During this period, Gondwanaland gradually drifted northward. In addition, Gondwanaland, which had formed a huge basin-like area, was slowly being eroded, and the debris was deposited into the basin itself or into the long troughs that surrounded Pangaea. Toward the end of this period, this enormous basin was transformed into a collection of separate basins.

This scene was violently disrupted approximately 160 million years ago when gigantic floods of basaltic lava spread out on all southern continents except for South America, where lava flows did not occur until 40 million years later. As a result of the lava flow, the cracks from which the lava was released sank, forming basins. When the water in these basins evaporated, it left salt deposits bordering most of the southern continents.

The predecessors of the present deep oceans were formed between India and Somaliland as the areas sank even lower. Similar seas spread along the coast of India some 10 million years later, separating it from Australia. The seas reached southwest Africa roughly 120 million years ago, the Congo 110 million years ago, and Nigeria 105 million years ago. While the seas were spreading, the Benue Trough, which cuts across the bulge of Africa from Nigeria through Algeria, was slowly being filled. Instead of driving a great distance down the trough, however, the seas moved westward, merging with marginal seas that had been moving eastward between the bulge of Africa and northern Brazil. Thus, when the seas finally met 92 million years ago, Africa became separated from South America. The Indian Ocean opened approximately 160 million years ago, and the South Atlantic Ocean opened roughly 120 million years ago, but neither ocean began to widen until about 100 million years ago.

Breakup of Gondwanaland

Between 100 and 80 million years ago, Gondwanaland began to break up. South America rotated away from Africa before it drifted westward to its present position. At approximately the same time, India rotated away from Africa, moved northward, and collided with Asia. The Himalayan Mountains are a direct result of this collision. Before finally separating into their present positions during the past 50 to 60 million years, Australia and Antarctica drifted away from Africa. After rotating slightly, Africa moved northward to encroach upon Europe.

Laurasia had been lying lower than Gondwanaland at the beginning of this 300-million-year period, except the Appalachian-Caledonian mountains, the Russian Urals, and the eastern Siberian mountains, which all gradually wore down during the next 50 million years. During the next 150 million years, the seas that covered most of North America slowly receded to the south and west. Shallow seas also covered Europe west of the Urals. These seas, which spread northward from the Tethys, shifted their location periodically during the next 200 million years. A large portion of northeast Asia was covered with marine sedimentary troughs. Some of these troughs cut China off from the rest of Asia at the beginning of this period. Shallow seas spread out from these troughs over parts of China and western Siberia. The first strong geological activity occurred in Siberia 200 million years ago, when flood basalts up to 2.5 kilometers thick spread out over 500,000 square kilometers.

The main framework of fractures that would eventually form the North Atlantic was already in place 300 million years ago. Actual separation, however, began farther south. Even though the northern continents were linked to Gondwanaland at the beginning of this period, North America separated from Africa more than 200 million years ago after the eruption of some volcanoes in eastern North America and Morocco. The proto-Central Atlantic was formed as a result of this rupture. Although Europe was still connected to North America after this movement occurred, a shallow sea formed a layer of sediments between 3 and 4 kilometers thick; traces of this sediment can still be found along the Atlantic continental shelf of North America. The Atlantic Ocean widened to one-quarter of its present width and connected to the Labrador Sea some 120 million years ago, separating Greenland and Canada. While the Atlantic continued to expand some 70 to 80 million years ago, the opening began between Europe and Greenland after the eruption of flood basalts.

Study of Continent Formation

Scientists have employed a wide variety of methods to study the formation of the continents. Glaciation has long been used to determine the original fracturing of the significant landmasses and estimate their separation rate. The close grouping of the glaciated areas provides some of the most convincing evidence for continental drift. Glacial scratches on rocks found in Gondwanaland point to ice movement from areas now submerged under the ocean. In addition, some materials deposited in the glacial drift, such as Brazilian diamonds, are completely foreign to the lands where they now repose and, therefore, provide proof for continental drift.

The study of terrestrial vertebrates also provides clues to the existence of Pangaea and Gondwanaland. After the land bridge theory fell into disrepute, the contiguity of the continents early in the earth's history seemed to be the only way to explain the fact that fauna is comparatively homogeneous throughout the world. The most convincing evidence for continental drift from terrestrial vertebrates is provided by an early Permian reptile called Mesosaurus, whose remains are found only in South America and South Africa. Since this reptile was equipped for swimming only in shallow fresh water, it probably could not have swum the distance between the two continents.

The evolution of plants also supplies evidence for the existence of a supercontinent. The presence of araucarian pines in South America, the Falklands, Australia, and the South Pacific has led scientists to infer former land connections. The distribution of the Glossopteris land plants in the Lower Gondwana Formations is larger than that of the glacial deposits. The cold climate stimulated the growth of this unique plant. These plants could not have developed independently of one another in widely separated continents but could have thrived in a supercontinent covered by enormous ice sheets.

Modern technology has provided scientists with more advanced methods of exploring the possibility of a supercontinent. Gathering paleomagnetic data is one such method. Magnetic particles in the seafloor rocks recorded the direction of the earth's magnetic field when the rock hardened. As the ocean floor formed, the direction of the field reversed itself from time to time. By paleomagnetically determining the pole positions for sedimentary and igneous rock in different continents, scientists have been able to determine the continents' relative positions for any time. Some of the data that they have acquired in this fashion support the existence of Pangaea.

Examination of Precambrian cratons also provides invaluable information regarding continental drift. Drift can be demonstrated by matching pieces of cratons across facing continents. For example, the cratons between Guyana and West Africa fit very well together and are well documented.

Computers have greatly facilitated the complicated process of reassembling the continents. In 1965, following a 500-fathom isobath, Bullard, Everett, and Smith used a computer to fit the continents together into their original configuration. Even though computerized models have brought an air of mathematical precision to reconstructing Pangaea, there is still considerable disagreement about specific features. For example, did Madagascar lie against east or southeast Africa when they were part of Pangaea?

Significance

The study of ancient landscapes is of great value to the economy. The same techniques geomorphologists devised to comprehend fossil landforms and deposits can also be brought to bear to locate ore deposits. Geomorphologists have also applied their methods to detect placers and to discover petroleum. Using structure contour maps, the search for an “oil pool” almost always involves geomorphology applied to problems of fluid entrapment.

Knowledge of ancient landscapes also aids scientists in predicting the occurrence of natural catastrophes. Scientists have mapped out a worldwide system of ocean trenches and ridges, probably formed when the oceans were formed, and the continents moved to their present locations. They have observed that many deep earthquakes occur beneath oceanic trenches and that volcanic activity is concentrated along ocean ridges.

Finally, scientists have used the theory of continental drift to predict what the earth's surface will look like millions of years from now. They predict that the California coast may tear away from the mainland and drift north toward Alaska, that Africa and South America may move even farther apart, and that Australia may collide with Asia.

Because the breakup of Gondwanaland and Laurasia had significant impacts on the global climate, its study is especially important as global climate issues become increasingly dire in the twenty-first century. Its breakup consistently affected biodiversity and changed the course of evolution for many species. Understanding the changes that occurred in ocean circulation, carbon dioxide levels, sea level changes, and temperature enables scientists to better understand these issues within the context of the modern Earth. 

Principal Terms

basalt: a hard, dense volcanic rock

Carboniferous period: the fifth of the six periods in the Paleozoic era; it preceded the Permian period

craton: the crystalline portion of a continent; it may have a sedimentary rock veneer

fathom: a unit of length equal to 1.8 meters, used principally in the measurement of marine depths

geomorphology: the study of the origin of landscapes

isobath: the contour lines of continental slopes

Paleozoic era: the era immediately before the Mesozoic era; it included the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods

placer: a deposit of sand or gravel containing eroded particles of valuable minerals

trough: a long, narrow depression, as between waves or ridges

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