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The Natural Environment

Geological History

The oldest and most stable structural element of the continent is the shield area of the Brazilian and Guiana highlands of the east and northeast. It comprises a Precambrian (before 570 million years ago) complex of igneous and metamorphic rocks. In most places the shield is overlaid by sedimentary rocks, mostly of Paleozoic age (570 million to 225 million years ago), although some areas of younger basalts occur, notably in southern Brazil. Fossils found in the Brazilian Highlands offer evidence of continental drift, indicating that in the Permian period the continent was linked to Gondwanaland, a great landmass incorporating Africa and Asia.

The complex that underlies the Patagonian Plateau is largely mantled by sediments deposited in the Mesozoic Era (225 million to 65 million years ago) and Tertiary Period (65 million to 1.6 million years ago) and by basalts of recent formation.

Material eroded from the old shield areas contributed to the thick deposits of sediments in the surrounding seas. These sedimentary formations were uplifted repeatedly in the Mesozoic Era to form the coast ranges of Chile and southern Peru and the higher and more extensive Andes. This mountain-building process, which continued through the Tertiary Period, was accompanied by intrusions of magma (molten rock) and by the formation of volcanoes. Volcanic and seismic activity continues all up and down the continent's western rim. The glaciers of the southernmost Andes are remnants of the great ages of glaciation of the Quaternary Period (beginning 2.5 million years ago). The erosion of the highlands continues to contribute sediments to surrounding lowlands.

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