Pangea

Pangea (Greek for “all earth”) was a superconti-nent that gradually formed during the early Paleozoic era (beginning approximately 450 million years ago) and broke apart beginning in the Middle Jurassic era (approximately 180 million years ago). At its most complete state in the Early Jurassic, Pangea consisted of nearly all of the world's continental crust sutured together into a single giant landmass. This landmass was C-shaped, centered on the equator, and surrounded by a single giant ocean, termed Panthalassa. Pangea enclosed an eastward-facing body of water called the Tethys Sea, of which the modern Mediterranean is a much-reduced remnant, and was home to vast subtropical deserts. The existence of a single giant continent had profound effects on life, and many important evolutionary events occurred on Pangea, including the radiation of the dinosaurs and the early evolution of birds and mammals.

During the early history of geology it was thought that Earth was a static, stable planet whose surface remained largely unchanged through time. However, beginning in the early 1900s, [Page 965]German meteorologist Alfred Wegener accumulated substantial evidence showing that the continents had not always occupied their present positions. Wegener discovered that South America, Africa, India, Australia, and Antarctica shared a suite of unique Mesozoic fossils, including a tropical plant flora characterized by the fern Glossopteris and a reptile fauna that included the tusked, piglike Lystrosaurus. Because modern animals do not range across all continents, owing to the existence of oceans, mountain ranges, and other barriers, Wegener hypothesized that these landmasses must have been linked during the Mesozoic era (225–65 million years ago), and have since moved to their present, widely divergent positions. Additionally, Wegener chronicled closely matching rock units shared by Africa and South America, as well as evidence of former equatorial climate belts and glaciations shared between now divergent continents. Taken together, these facts suggested to Wegener that all of the continents had once been joined together into a supercontinent, which he named Pangea. Initially Wegener's ideas were controversial, as he could provide no plausible mechanism for continental motion. However, further research in the years after World War II roundly supported Wegener's observations and firmly established the existence of a Paleozoic-Mesozoic supercontinent. Today, many geologists believe that Earth is characterized by a supercontinent cycle, in which these giant landmasses form and disintegrate roughly every 500 million years.

The formation of Pangea was gradual and can be traced to the breakup of a previous superconti-nent, Pannotia, approximately 750 million years ago. Pannotia split into three large landmasses, including a large southern platform called Gondwana and the more northern Proto-Laurasia, which subsequently split into several smaller land-masses. Two of these smaller landmasses, termed Laurentia and Baltica, collided in the Late Ordovician (about 450 million years ago) and were joined several million years later by Avalonia, a slice of crust comprising present-day New England, Nova Scotia, and Great Britain. Meanwhile, Gondwana was fragmenting into many small landmasses that periodically collided with the Laurentia-Baltica-Avalonia landmass, known as Euramerica. These smaller collisions occurred throughout the Devonian, Mississippian, Pennsylvanian, Permian, and Triassic periods (about 420–200 million years ago), until nearly all of the world's continental crust was completely sutured together during the Early Jurassic.

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