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Under the force of the Andes

Map displays a comprehensive view of the topographic relief of South America, now subdivided into 35 units

Aconcágua, the highest mountain on the continent, in Argentina: mountain range is pushing against the rocky structures to the east

Jorge Díaz / WikicommonsAconcágua, the highest mountain on the continent, in Argentina: mountain range is pushing against the rocky structures to the eastJorge Díaz / Wikicommons

“Those are dunes!” was the surprised reaction of geographer Jurandyr Ross, a professor at the University of São Paulo, when he spotted the mounds of sandy soil sparsely populated with clusters of spiny plants, sheep and llamas adjacent to the tablelands known as mesetas in the Patagonian Desert of southwest Argentina, soon after Christmas of 2015. On a 16-day, 9,000-kilometer journey under an intense sun, Ross and other geographers dismissed any final doubts they had about the radar and satellite images they had used to prepare the relief map of South America on which he and his team had worked all year long. Published as part of an article in the August 2016 edition of the journal Revista Brasileira de Geografia, the new map replaces the earlier, rather simple one from the 1940s, and highlights 35 distinct units depicting the features of the three large continental blocks—the Andes Mountains to the west, the central great plains adjacent to the mountains, and the low-elevation plateaus—that make up nearly all of the Brazilian territory. These subdivisions, some of which are hundreds of square kilometers in size, offer a comprehensive view of the continent and reveal the connection between Brazil’s land relief and the Andes Mountains.

“Although the supporting structures of Brazil’s topography are very old, the present-day contours are the result of strong influences from Andean tectonic activity, which is geologically much more recent,” says Ross. The uplifting of the Andes range, which resulted from pressure exerted by tectonic plates on the ocean floor, caused the Amazon and other rivers in the Amazon Basin to change their direction of flow from west to east. In addition, according to Ross, the Serra do Mar and Mantiqueira mountain ranges along the coast, and the Paraíba Valley in the area around Taubaté, were formed as a result of compression and wrinkling as the Andes pushed against the rocky structure to the east.

“Today we’re living in a period of tectonic calm, but the reconfiguration of the land relief was once much more extreme, as a result of the Andes,” says geographer Silvio Rodrigues, a professor at the Federal University of Uberlândia, in the state of Minas Gerais. According to Rodrigues, the Andes still exert an influence on the continent because they lie on two active tectonic plates—the Nazca Plate and the South American Plate—which, through tectonic processes, generate energy that can extend out to the Atlantic coast. “Since Brazil’s topography is already quite well known, the most interesting aspect about this map is the analysis of the Andes and the Central Depression, between the Andes and Brazilian territory.”

Ross published a Brazilian topographic map in the book Geografia do Brasil (Edusp Publishers) in 1996, with a scale of 1:5 million (1 to 5 million; 1 centimeter on the map equals 50 kilometers). He followed this two years later with a 1:500,000 topographic map of the state of São Paulo (see Pesquisa FAPESP Issue nº 35). He then decided to do a synthesis of South American topography because he couldn’t find any updated maps to use in his classrooms. The only one he came across, when his work was already in progress, was the U.S. Geological Survey map from 1942. He used mainly radar images from NASA’s Shuttle Radar Topography Mission (SRTM) satellite. These were supplemented with Google Earth images, the geological map of South America produced by the Geological Survey of Brazil (CPRM)—a state-owned company under the Ministry of Mines and Energy, and academic research papers.

The new map, with a scale of 1:8 million, could be useful for environmental and economic planning purposes. “Relief, soils and climate influence human occupation and agribusiness,” says Ross. As a result, they create connections, such as the flat plains of Mato Grosso State associated with cultivation of soybeans and sugarcane, or the mountain valleys of Chile identified with fruit production. The types of relief, he observes, are expressions of both the Earth’s internal forces, such as movements of magma, and external forces such as erosion and inclement weather.

The map distinguishes the three basic continental blocks on the basis of differences in geological composition, soils and types of relief.

Mapa_246The block to the east includes low-elevation plateaus, with the principal Brazilian river basins depicted in blue on the map, the depressions shown in orange, and the coastal mountain ranges in red. It is the oldest part of the continent, dating back one billion years, and was formed in the geological era known as the Precambrian.

This block, along with present-day Africa and India, was part of the supercontinent Gondwana, which began to break up about 150 million years ago, in the Jurassic Period, which was also marked by the opening of the South Atlantic. The Amazon Craton, to the north and south of the Amazon River plains, forms the continent’s oldest rocky structures, about 2.5 billion years old. Shown in red are the hills and mountains that constitute the now heavily eroded vestiges of mountain ranges older than the Andes. “When they formed between 550 million and 1.5 billion years ago, they were as high as the Andes,” Ross points out.

To the west are the Andes Mountains, a block that is geologically more recent than the eastern portion. Ross pointed out the oldest and highest section, the Eastern Range, which is about 100 million years old and features elevations of 4,000 meters, in Bolivia and Argentina.

The youngest mountain
The predominant chain of mountains, stretching north-south across the continent at elevations of 1,500 to 2,600 meters, is the Western Range, which formed in two phases, one about 85 million years ago and the other 40 million years ago. The Coastal Range is even more recent, from the late Cenozoic, between 1.7 million and 23 million years old. Between the mountains there are valleys populated by cities such as Santiago, Chile, at an elevation of 800 meters, and the Atacama Desert, which Ross visited in November 2016 on another expedition, or field check trip, to verify image data. There, he was impressed by the white film of salt covering the arid red soil.

Stretching between the mountains and the low plateaus of Brazil is the South American Central Depression, formed by plains that contain stretches of floodplains, such as those of the Orinoco River in Venezuela, the Mamoré and Beni rivers in Bolivia and the Paraguay River in Brazil, Paraguay and Argentina. The average age of the surface of this area (shown in yellow on the map) varies from 10,000 to 3 million years old, and the highest elevation is 200 meters in the region between Paraguay and Bolivia. “This entire area—very low-lying with flat-topped hills, lightly sculpted valleys, plains, and wetlands called the Gran Chaco—was a large sea millions of years ago, before the Andes emerged,” Ross says.

Geographer Isabel Cristina Gouveia, a professor at São Paulo State University (Unesp) in Presidente Prudente, comments that two great Brazilian geographers of the past century—Aziz Ab’Saber and Fernando de Almeida—made significant contributions to knowledge about the national territory, even without the satellite images so easily obtained today. “Oddly enough,” she says, “even with high-resolution satellite images and resources from geographic information systems, there are still few studies that focus attention on geomorphological mapping as a method of analysis and systematization of knowledge about land relief.”

Scientific article
ROSS, J. L. S. Compartimentação do relevo da América do Sul. Revista Brasileira de Geografia. V. 61, No. 1, p. 21-58, 2016.