Amazon Basin


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Amazon Basin

 

or Amazonia, in South America; the largest lowland on earth, with an area of more than 5 million sq. km.

The basin extends from the Andes to the Atlantic Ocean, between the Guiana and Brazilian plateaus in the basin of the Amazon, the largest river in the world in volume. It is situated in the geosyncline of the South American craton, which is mainly filled with Paleozoic marine and Mesozoic-Cenozoic continental deposits. The climate is hot (mean monthly temperatures 24–28° C) and moist (precipitation 1,500–3,000 mm and more per year).

Western Amazonia is a very broad (up to 1,600 km), level, and low-lying plain. The climate is equatorial and constantly moist. The rivers flow slowly in weakly incised broad valleys; these rivers are meandering and turbid (Rio Branco). The flat surfaces of the stream divides—the so-called terra firme (“hard earth”)—are covered by rain forests (selva, or hylea) on podzolic lateritic soils. In the river valleys, flood plains (várzea) and intermittently drowned and low delta (igapó) landscapes are distinguished; the latter are regularly submerged for several months. The várzea vegetation is similar to the eté and includes an especially large number of palms; rubber trees, or Hevea; kapok, or sumaúma (Ceiba); and cacao. The igapó vegetation is very sparse; imbaubal Cecropia is characteristic. Animals adapted to an arboreal life—including New World monkeys, sloths, and anteaters—are typical of the Western Amazon hylea. The land animals include giant armadillos, tapirs, and peccaries. There are also many birds, reptiles, amphibians, fishes, insects, and ants.

Eastern Amazonia (east of the mouth of the Rio Negro and Tapajós) is considerably narrower (up to 350 km). Marine Paleozoic and crystalline Precambrian rocks outcrop along the margins while the central portion has undergone recent subsidence, as a result of which the surface is intensely dissected, with buttes up to 350 m high; the rivers are straighter and deeper, with estuary-like mouths, many rapids, and transparent water that is darkened by decomposing vegetation (the Rio Negro). The climate is sub-equatorial. From June to September trade winds blow southeastward from the Brazilian plateau and cause droughts. This causes a considerable number of barren spots and savanna patches on red soils in the deciduous forests. Only bands of várzea and igapó on alluvial soils remain evergreen. Animals typical of open spaces make their appearance: these include brockets, anteaters, small armadillos, many rodents, and termites.

The Amazon lowland is very sparsely populated. Rivers are the basic paths of communication; along them are small settlements and two large cities—Manaus at the mouth of the Rio Negro, and Belém at the mouth of the Para River; a highway has been built between Belém and Brasilia. Manganese ore, discovered in 1945 (Serra do Navio in Amapa), nearby iron deposits, and oil fields in the lower Madeira valley (at Nova Olinda) are being developed.

REFERENCE

Lukashova, E. N. Iuzhnaia Amerika. Moscow, 1958.

E. N. LUKASHOVA

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