Further Reading

Prothero, Donald R., and Robert H. Dott. Evolution of the Earth. 6th ed. Boston, Mass.: McGraw-Hill, 2002.

Windley, Brian F. The Evolving Continents. 3rd ed. Chichester, England: John Wiley & Sons, 1995.

Pangaea Pangaea was the supercontinent that formed in the Late Paleozoic, lasting from about 300-200 million years ago, and included most of the planet's continental masses. The former existence of Pangaea, meaning all land, was first postulated by Alfred Wegener in 1924, when he added the Australian and Antarctic landmasses to an 1885 supercontinent reconstruction of Gondwana by Eduard Suess that included Africa, India, Madagascar, and South America. He used the fit of the shapes of the coastlines of the now dispersed continental fragments, together with features such as mineral belts, faunal and floral belts, mountain ranges, and paleoclimate zones that matched across his reconstructed Pan-gaean landmass to support the hypothesis that the continents were formerly together. Wegener proposed that the supercontinent first broke up into two large fragments including Laurasia in the north and Gondwana in the south, and then continued breaking up, leading to the present distribution of continents and oceans. The scientific community did not generally accept Wegener's ideas at first, but since the discovery of seafloor magnetic anomalies and the plate tectonic revolution, the general

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Map showing the distribution of landmasses in the supercontinent of Pangaea framework of his Pangaea model has proved to be generally valid.

The Pangaean supercontinent began amalgamating from different continental fragments with the collision of Gondwana and Laurentia and Baltica in the Middle Carboniferous, resulting in the Alleghenian, Mauritanide, and Variscan orogenies. Final assembly of Pangaea involved the collision of the South China and Cimmerian blocks to the Paleo-Tethyan margin, resulting in the early Yanshanian and Indonesinian orogenies in the Middle to Late Triassic.

The formation of Pangaea is associated with global climate change and rapid biological evolution. The numerous collisions caused an overall thickening of the continental crust that decreased continental land area and resulted in a lowering of sea level. The uplift and rapid erosion of many carbonate rocks that had been deposited on trailing or passive margins caused a decrease in the carbonate strontium 87/strontium 86 ratios in the ocean. During the final stages of the coalescence of Pangaea, drainage systems were largely internal, erosion rates were high, and the climate, with large parts of the supercontinent lying between 15° and 30° latitude, became arid, with widespread red-bed deposition. Soon however, the effects of the erosion and burial of large amounts of carbonate and the associated drawdown of atmospheric C02 caused climates to rapidly cool, resulting in high-latitude glaciations.

The main glaciations of Pan-gaea started in the Late Devonian and Early Carboniferous, began escalating in intensity by 333 million years ago, peaked in the Late Carboniferous by 292 million years ago, and ended in the Early Permian by 272 million years ago. These glaciations resulted in major global regressions as the continental ice sheets used much of the water on the planet. Wegener, and many geologists since, used the distribution of Pangaean glacial deposits as one of the main lines of reasoning to support the idea of continental drift. If the glacial deposits of similar age are plotted on a map of the present distribution of the continents, the ice flow patterns indicate that the oceans too must have been covered. However, the planet does not hold enough water to make ice sheets so large that they can cover the entire area required if the continents have not moved. If the glacial deposits are plotted on a map of Pangaea however, they cover a much smaller area, the ice flow directions are seen to radiate outward from depocenters, and the amount of water on Earth can accommodate the total volume of ice.

Pangaea began rifting and breaking apart about 230 million years ago, with numerous continental rifts, flood basalts, and mafic dikes intruding into the continental crust. Major breakup and seafloor spreading began about 175 million years ago in the central Atlantic, when North and South America broke away from Pangaea, 165 million years ago off Somalia, and 160 million years ago off the coast of northwest Australia. sea levels began to rise with breakup because of the increase in volume of the mid-ocean ridges that displaced seawater onto the continents, forming marine transgressions. Episodic transgressions and evaporation of seawater from restricted basins led to the deposition of thick salts in parts of the Atlantic, with some salt deposits reaching 1.2 miles (2 km) thick off the east coast of North America, Spain, and northwest Africa. Several rifts along these margins have up to 6 miles (10 kilometers) of nonmarine sandstones, shales, red beds, and volcanics associated with the breakup of Pangaea. Many are very fossiliferous, including plants, mud-cracks, and even dinosaur footprints attesting to the shallow water nature of these deposits.

Breakup of the supercontinent was also associated with a dramatic climate change and high sea levels. The increased volcanism at the oceanic ridges released many gases to the atmosphere, inducing global warming, leading to a global greenhouse, ideal for carbonate production on passive or trailing continental margins.

See also Paleozoic; supercontinent cycles.

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