Pangaea

I

Introduction

Pangaea, name given the single supercontinent that existed on Earth during the late Paleozoic and early Mesozoic eras (about 300 million to 200 million years ago). Pangaea was made up of two connected continental masses: Gondwanaland to the south and Laurasia to the north. The modern continents are the result of the breakup of Pangaea, followed by the breakup of Gondwanaland and Laurasia. The processes that formed Pangaea and later broke it apart are known as plate tectonics, sometimes called continental drift.

The name Pangaea (Greek for “all land”) was coined in 1912 by Alfred Wegener, the German meteorologist who published the first scientifically argued theory of continental drift. Utilizing geological and fossil evidence, Wegener postulated the existence of Pangaea as a supercontinent that had existed throughout the early history of Earth. According to Wegener, Pangaea broke up to form the present continents beginning during the Cretaceous Period in the late Mesozoic Era and continuing through the following Cenozoic Era.

II

Confirmation of Continental Drift

Wegener’s ideas were largely rejected during his lifetime because he could provide no credible mechanism for continental movement. Developments in the Earth sciences during the past 50 years, however, have generally confirmed his basic ideas. The discovery of sea-floor spreading in particular provided a mechanism for continental drift as part of a theory of plate tectonics. Geologically active mid-ocean ridges create new areas of ocean floor as magma wells up and spreads outward. At ocean trenches called subduction zones the old ocean floor sinks back into the crust and is destroyed. Continents are carried on plates that move as the ocean floor spreads and sinks.

What scientists know about the movement of continents is best documented up to about 200 million years ago, the age of the oldest oceanic crust. Oceanic crust provides a better record because it is subject to less disturbance and distortion than continental crust. Even so, geological and paleontological studies of continental rocks have enabled researchers to extend the picture back to about 500 million years ago, near the end of the Cambrian Period (542 to 488 million years ago). The history of Pangaea as it formed, remained together, and then broke up can be outlined in relatively good detail.

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III

Continents Before Pangaea

Around 500 million years ago, the giant continent of Gondwanaland straddled the equator in the eastern hemisphere, with its southern portions near the South Pole. Gondwanaland comprised what were to become the continents of South America, Africa, Australia, and Antarctica, as well as the Indian subcontinent.

To the west of Gondwanaland were three large continental plates: the North American plate, including what is now Greenland and Scotland; the North European plate, or Baltica, comprising parts of the British Isles, Scandinavia, central and northern Europe, and European Russia; and, finally, the Scandinavian plate. The North American plate lay across the equator with Baltica to the southeast and the Scandinavian plate to the east. The gradual convergence of these three plates, plus a number of minor plates and microplates, over the next 200 million years would lead to the creation of Laurasia, the continental mass that subsequently broke up to form North America, Europe, and Asia, excluding India. At this time, marine life was evolving rapidly. The first plants are thought to have begun colonizing the land during the subsequent Ordovician Period (488 to 444 million years ago).

During the Ordovician and subsequent Silurian Period (444 to 416 million years ago), Gondwanaland began moving south and west across the South Pole. Then, about 400 million years ago, during the Devonian Period (416 to 359 million years ago), the western part of Gondwanaland—now the northern parts of South America and Africa—began moving northward toward the equator.

During this same period, Baltica converged on the North American plate to form a larger continental mass called Euramerica, or Laurussia, initiating one of the many periods of mountain-building (orogeny) that characterized the development of Laurasia. Because it was straddling the South Pole, glaciation affected parts of Gondwanaland throughout this period, and the climate in other parts ranged from subpolar to tropical at its northernmost extremities. North America, and subsequently Laurussia, by contrast, had a largely tropical to warm climate, and significant areas were underwater.

IV

Formation of Pangaea

About 280 to 260 million years ago, during the Permian Period (299 to 251 million years ago), northern Gondwanaland finally met up with southern Laurussia. At about the same time, the Siberian plate collided with northern Laurussia, initiating the mountain-building that created the Ural Mountains, completing the development of Laurasia, and creating one large land mass, Pangaea.

The overall shape of Pangaea was similar to a letter “V” lying on its side with the apex to the west. The northern and southern arms of the V, Laurasia and Gondwanaland respectively, were hinged on the Gulf of Mexico and stretched in a broad arc from pole to pole across one face of the globe. They were separated, in the east, by the Tethys Sea, while the global ocean known as Panthalassa (Greek for “all sea”) surrounded the land mass as a whole.

The creation of such a large land mass had profound effects on both the climate and atmospheric circulation. Strongly differentiated climatic belts emerged. During the early existence of Pangaea, climatic conditions led to extensive glaciation in southern Gondwanaland. The deflection north of equatorial currents led to the development of warm, moist conditions in northern Laurasia, farther north than such a climate might be expected. At the same time hot, dry conditions prevailed over much of Laurussia and northern Gondwanaland. Climatic differentiation was reflected in the development of strong regional differences in early Pangaean plants and animals, with distinct forms associated with Gondwanaland and with Laurasia. Later in the Permian, and throughout the following Triassic Period (251 to 200 million years ago), climates became milder as Pangaea moved northward.

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