Pangaea

Pangaea, (Greek, “all land”), supercontinent that existed during the late Palaeozoic and early Mesozoic Eras (about 300 million to 200 million years ago), and which comprised two large continental masses: Gondwana to the south and Laurasia to the north. The term was originally devised by Alfred Wegener, the German meteorologist who in 1915 published the first scientifically argued theory of continental drift. Utilizing a variety of evidence, in particular geological and fossil data, Wegener postulated the existence of a primeval supercontinent, which he named Pangaea. This, he believed, had existed throughout the early history of the Earth, and broke up to form the present continents during the late Mesozoic and subsequent Cenozoic Era, especially during the Cretaceous Period.

Developments in the Earth sciences during the past 50 years have generally confirmed Wegener’s ideas, which had largely been rejected during his lifetime because he could provide no credible mechanism for continental movement. In particular, studies of the deep oceans—especially of ocean-floor creation, spreading, and destruction associated with mid-ocean ridges (see Oceanic Ridge) and ocean trenches, and of magnetic striping—have been able to provide not only a mechanism for continental drift as an integral part of plate tectonics theory, but also an accurate picture of that movement over geological time. Magnetic striping is the record of past changes in the orientation of the Earth’s magnetic pole, reflected in alternating bands of normally and reversely polarized ocean floor that extend out from, and parallel with, each side of mid-oceanic ridges. It occurs because magnetic minerals inside magma welling up from the ridges are aligned to the Earth’s magnetic orientation at the time; this alignment is “locked in” as the magma cools to create a new ocean floor. Magnetic striping can be used as part of palaeomagnetism (the study of the Earth’s magnetic field through time) to help determine the previous latitude of oceans and continental masses.

Our understanding of the movement of the continents, or rather of the lithospheric plates of which they form part, is most accurate up to about 200 million years ago. This is the age of the oldest oceanic crust, which is subject to less disturbance and distortion than continental crust. Even so, palaeomagnetic studies of continental rocks, combined with developments in geology and palaeontology, have enabled researchers to extend the picture back to about 500 million years ago, near the end of the Cambrian Period. Most are agreed that at this time a large continental mass, Gondwana, was to be found straddling the equator in the Eastern hemisphere, with its southern portions near the South Pole. It comprised what was to become the continents we now call South America, Africa, Australia, and Antarctica, as well as the Indian subcontinent. To the west of Gondwana 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 south-east 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 (495 to 443 million years ago).

During the Ordovician and subsequent Silurian Period, Gondwana began moving south and west across the South Pole. Then, about 400 million years ago, during the Devonian Period, the western part of Gondwana—now the northern parts of South America and Africa—began moving northwards towards the equator. During this same period, Baltica converged on the North American plate to form a larger continental mass called Eurasia, 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 Gondwana throughout this period, and the climate in other parts ranged from sub-polar to tropical at its northernmost extremities. North America, and subsequently Laurussia, by contrast, had a largely tropical to warm climate, and significant areas were under water. About 280 to 260 million years ago, during the Permian Period, northern Gondwana 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 Gondwana 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 ocean known as Panthalassa 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 favoured an extensive glaciation in southern Gondwana. In northern Laurasia, the deflection north of equatorial currents led to the development of warm, moist conditions in areas farther north than might have otherwise been expected. At the same time hot, dry conditions prevailed over much of Laurussia and northern Gondwana. Climatic differentiation was reflected in the development of provinciality in early Pangaean flora and fauna, with distinct forms associated with Gondwana and with Laurasia. Later in the Permian, and throughout the subsequent Triassic Period, climates ameliorated as Pangaea moved northward.

Although plate tectonics and palaeomagnetism have proved the existence of a supercontinent such as Wegener postulated, it was not as long-lived as he had presumed. Pangaea was the product of a long and geologically energetic period, but it lasted little more than 100 million years. Approximately 240 million years ago, during the Triassic Period, Pangaea began to split apart, along the line of what is now the Southern Atlantic and Gulf of Mexico. The break-up was due to the effects of plate movement which caused Pangaea to rotate as well as move north. However, different parts were rotating at different speeds and in different directions. The result of this was not only to separate Gondwana and Laurasia, but also, eventually, to cause these two continents to break up, mainly during the Cretaceous period (142 to 65 million years ago). South America and Antarctica parted from the west and south-east sides of Africa respectively. The Atlantic Ocean opened up as North America hinged away from Eurasia and, about 50 million years ago, India started to move across the Tethys Sea, to collide with the southern side of Asia about 20 million years ago, creating the Himalaya mountains in the process.

Many researchers believe that Pangaea was not unique in Earth’s history. They postulate the existence of an earlier supercontinent, Rodinia, that existed in the southern hemisphere and which split in two about 750 million years ago to create the North American, Baltica, and Siberian plates, and numerous minor and microplates. The larger land mass left behind was a forerunner of Gondwana. Some researchers also predict that a new supercontinent, to be located in the Northern hemisphere, is in the process of formation through contemporary plate movements.