The Atacama Desert is an expansive plateau located along the Pacific coast of South America, primarily within northern Chile. This vast, barren landscape is globally recognized as the driest non-polar desert on Earth. Some areas receive less than one millimeter of annual rainfall, creating an environment so hostile it has been used to simulate conditions on Mars. The desert’s formation results from a unique confluence of geographical and oceanic factors that effectively block all sources of moisture.
The Role of the Andes Mountains
One of the primary geographical features contributing to the Atacama’s dryness is the massive wall of the Andes Mountains to the east. These mountains stand directly in the path of moisture-laden air masses carried westward from the Amazon basin.
When this moist air encounters the height of the Andes, it is forced to rise sharply, a process known as orographic lifting. As the air rises, it cools, causing the water vapor to condense and precipitate as rain or snow on the eastern slopes. By the time the air descends on the western side, where the Atacama Desert is situated, it has been stripped of almost all its moisture. This phenomenon is known as the “rain shadow effect,” which leaves the land on the leeward side profoundly dry.
The Influence of the Cold Humboldt Current
The second mechanism responsible for the desert’s formation lies in the Pacific Ocean, where the cold Humboldt Current flows northward along the coast. This current brings frigid water to the surface through upwelling, chilling the air directly above it. This maintains coastal temperatures far cooler than expected for the subtropical latitude.
This chilling creates a stable atmospheric condition known as a temperature inversion, trapping a layer of cold, dense air beneath warmer air higher up. Since the cold air near the surface cannot rise, convective activity is suppressed, preventing the formation of rain-producing clouds. While this system inhibits rainfall, the cold water generates dense marine fog, known locally as camanchaca, which provides minimal moisture for specialized coastal ecosystems. This oceanic influence maintains the hyper-aridity of the coastal strip by preventing Pacific moisture from reaching the interior.
Geological History and Persistence
The extreme aridity of the Atacama is not a recent phenomenon. Geological evidence indicates this region has experienced arid to semi-arid conditions for an immensely long time, possibly since the late Jurassic period, approximately 150 million years ago. The core of the desert has been in a state of hyper-aridity for at least 10 to 15 million years, making it one of the oldest deserts on the planet.
This remarkable persistence is linked to the long-term stability of the tectonic and oceanic systems. The South American continent’s stable position ensures the Andes continue to act as an effective rain barrier. The ongoing subduction of the Nazca Plate beneath the South American Plate maintains the high elevation of the mountain range. Evidence of this sustained dry climate is found in extensive mineral deposits, such as nitrates and ancient salt lakes, or salares, which formed through the evaporation of water over deep time.