South America has two main deserts, the Atacama and
the Patagonian Deserts, both along its Pacific coastline, with the
Andes as their natural boundary in the east.
The Atacama Desert is the more northerly and lies mainly in Peru and
Chile, stretching from the Equator to about 20o South, some
3,000 km (1875 miles) in length and rarely more than 200 km (125 miles
wide). The Andes, with heights of up to 6,000 m (19,700 ft)
effectively block the passage of moist tropical air from the Amazon Basin,
shedding their moisture as rainfall when they rise over the Amazonian
Andes foothills.
Along the coast, the cold waters of the Humboldt Current that
originate in the Antarctic, flow north. The cold water and the air
above it causes a thermal inversion that dries the air, condensing any
remaining moisture into insubstantial short-lived mists, the Camanchaca.
The Atacama is often said to be the driest desert in the world,
enduring continuous draught for many years in succession. There are
no significant subterranean moisture reserves, and where they are found,
the water is heavily contaminated with boron that is toxic to most forms
of life. It is a mineral rich area, particularly in deposits of
sodium nitrate that together with guano gathered from the off-shore
seabird colony islands were a significant source of the world's
agricultural fertilizer industry up to the 1920s.
At some locations the Atacama is a low, arid coastal plateau, while
over a considerable proportion a range of low mountains rise as cliffs of
up to 450 m. height from the ocean, increasing in height to rocky crags
towering to 2,000 m. Behind these mountains is found a valley
corridor that reaches up to the Andean foothills. The coastal hills
prevent water flowing down from the Andes from reaching the Pacific, so
that their waters evaporate, leaving large salt pans surrounded by
alluvial fans from the mountains, consisting mainly of pebble and gravel,
with some sand desert areas with typical dune formations. Occasional
volcanic cones rise from this barren moonscape. The surface layer of soil
can best be described as volcanic in origin and often broken down into
small particles, 'sandy', in texture
Animal life is scarce. Infrequently the Andean Condor can
be seen high up in the sky on the lookout for carrion. In the cactus
areas, the cactus wren can be found while elsewhere the oven
bird, a South American endemic, builds its dome shaped mud nests on
stumps or posts.
The dominant animal group consists of lizards, including a number of
colourful pot-bellied species of Iguana. One of the species
can reach lengths of up to 2 m and although fierce and dragon-like in
appearance, is actually a quite timid scavenger.
