Geography of South America - Topography and Geology

Topography and Geology

The geographical structure of South America is deceptively simple for a continent-sized landmass. The continent's topography is often likened to a huge bowl owing to its flat interior almost ringed by high mountains. With the exception of narrow coastal plains on the Pacific and Atlantic Oceans, there are three main topographic features: the Andes, a central lowland, and the extensive Brazilian and Guiana Highlands in the east.

The Andes are a Cenozoic mountain range formed (and still forming) by the continuing convergence of the American and Pacific tectonic plates. In their northern and central reaches the Andes are quite wide and contain extensive plateau such as the Altiplano and a number of major valleys such as the Rio Magdalena. These contain three of the world's highest capitals: Bogotá, Quito and highest of all, La Paz, Bolivia. The southern Andes have been eroded by the Patagonian Ice Sheet and are much lower and narrower. There are a number of large glaciers in the northern part, but from latitude 19°S to 28°S the climate is so arid that no permanent ice can form even on the highest peaks. Permafrost, however, is widespread in this section of the Altiplano and continuous above 5,600 metres (18,373 ft).

The climate of the coastal belt west of the Andes shows violent contrasts, including two of the world's wettest regions in the Colombian Chocó and southern Chile, and the world's driest desert, the Atacama. This dry area is cooled by the Humboldt Current and upwelling, giving rise to the largest fisheries in the world. There are two small transition zones between the perhumid and perarid regions: around Guayaquil with summer rain, and the Mediterranean climate region of central Chile. Both these regions have highly erratic rainfall strongly influenced by El Niño events, which bring major floods. In contrast, the high plateaux of the Andes are drier than normal during El Niño episodes.

The very fertile soils from the erosion of the Andes formed the basis for the continent's only pre-Columbian state civilizations: those of the Inca Empire and its predecessors (Chavín, Nazca, Mochica, etc.). The area is still a major agricultural region. The Altiplano contains many rare minerals such as copper, tin, mercury ore. The Atacama is mined for its nitrates. Peru east of the Andes is regarded as the most important biodiversity hotspot in the world with its unique forests that form the western edge of the world's largest rainforest, the Amazon Rainforest.

East of the Andes is a large lowland drained by a small number of rivers, including the two largest in the world by drainage area—the Amazon River and the more southerly Paraná River. The other major river of this central lowland is the Orinoco River, which has a natural channel linking it with the Amazon. Most of this central lowland is sparsely populated because the soils are heavily leached, but in the south is the very fertile pampas of Argentina—one of the world's major food-producing regions where wheat and beef cattle are pre-eminent. The natural vegetation of the northern lowlands are either savanna in the northern llanos and southern campos, or tropical rainforest throughout most of the Amazon basin. Efforts to develop agriculture, outside of fertile floodplains of rivers descending from the Andes, have been largely failures because of the soils. Cattle have long been raised in the llanos of northern Colombia and Venezuela, but petroleum is now the dominant industry in the northern lowlands, making Venezuela the richest country in the continent.

The eastern highlands are much older than the Andes, being pre-Cambrian in origin, but are still rugged in places, especially in the wet tepuis of Venezuela, Guyana and Roraima. The Amazon River has cut a large valley through a former highland, and to the east is a relatively low plateau comprising the Nordeste and Southeast regions of Brazil. In the north of this region is the arid sertão, a poor region consistently affected by extremely erratic rainfall, and the humid zona da mata, once home of the unique Atlantic Rainforest with many species not found in the Amazon, and now a centre for sugarcane. Further south, the main land use is coffee, while São Paulo is the economic heart of the continent with its industry.

South of about Santa Catarina, the highlands fade out to low plains in Uruguay.

East of the Andes in Argentina, there are a number of rugged, generally dry lslands, the highest of which is the Sierra de Cordoba near the city of that name. Argentine Patagonia is a Paleozoic plateau now heavily dissected by rivers flowing from the Andes.