The Atacama Desert, located on the Pacific coast of South America, is the driest non-polar desert on Earth. It stretches over a 1,600-kilometer strip of land in northern Chile. The region’s extreme aridity is a consequence of a unique interplay of geographic, oceanic, and atmospheric phenomena. These combined forces create an environment where some locations receive less than a millimeter of rainfall annually and have gone decades without measurable precipitation. Understanding the formation of this landscape requires examining the physical barrier, the cold ocean flow, and the large-scale pressure systems that dominate the area.
The Geological Prerequisite: Uplift of the Andes Mountains
The primary physical structure contributing to the desert’s dryness is the massive elevation of the Andes Mountains to the east. This geological feature resulted from the continuous convergence of the oceanic Nazca Plate and the continental South American Plate. The denser oceanic lithosphere of the Nazca Plate is forced down and under the lighter South American Plate in a process called subduction, which began approximately 170 million years ago.
The friction generated by subduction causes the leading edge of the South American Plate to fold and shorten, steadily building the mountain chain. The ongoing movement of the Nazca Plate sustains the uplift of the Andes. This continuous mountain building created a high-elevation barrier that physically separates the desert from eastern moisture sources.
The Oceanic Driver: Influence of the Humboldt Current
The second major mechanism driving the Atacama’s coastal aridity is the Humboldt Current, also known as the Peru Current. This cold, north-flowing current originates from the Antarctic Circumpolar Current, transporting frigid water along the western coast of South America. Its flow is enhanced by coastal upwelling, where strong southerly winds force deep, cold water to the surface.
The presence of this cold water profoundly affects the atmosphere above it. As air passes over the cold ocean, it is cooled from below, stabilizing the lower atmosphere. This stability creates a persistent temperature inversion layer, trapping cool, moist air beneath a warmer, drier layer higher up.
The inversion layer effectively suppresses the vertical movement of air required for cloud development and rainfall. While the cold water causes fog, known locally as garúa, to form along the coast, this moisture does not convert into significant precipitation. This mechanism ensures the air mass reaching the coast cannot rise high enough to condense into rain clouds, contributing directly to the coastal desert conditions.
Atmospheric Dynamics: The South Pacific Anticyclone and Rain Shadow
The large-scale atmospheric component contributing to the desert’s dryness is the permanent South Pacific Anticyclone, a massive high-pressure system positioned over the southeastern Pacific Ocean. This system generates persistent atmospheric subsidence, meaning air constantly sinks down toward the surface over the region. As air descends, it warms and compresses, which actively suppresses cloud formation and prevents the development of large storm systems.
The presence of this anticyclone keeps the skies clear for most of the year. The circulation patterns of this high-pressure cell also induce the southerly winds that drive the cold coastal upwelling of the Humboldt Current, linking the oceanic and atmospheric drivers. This combination of sinking air and stable coastal conditions maintains a state of perpetual atmospheric drought.
Simultaneously, the towering Andes Mountains create a dramatic rain shadow effect that blocks moisture from the South American interior. Moisture originating from the Atlantic Ocean is carried west until it encounters the eastern slopes of the Andes. The air mass is forced to rise, cooling and releasing its moisture as precipitation on the eastern side of the range.
By the time the air descends the western slopes toward the Atacama, it is significantly dried out. This process, in conjunction with the high Chilean Coast Range acting as a second barrier closer to the Pacific, creates a double rain shadow effect, sealing off the central desert from virtually all sources of moisture.
The Timing of Hyper-Aridity and Unique Desert Characteristics
Scientific evidence suggests that while the Atacama has experienced arid to semi-arid conditions for 150 million years, the extreme hyper-aridity began much later. The transition to a perpetually dry state is closely linked to the final major uplift phases of the Andes and the full establishment of the cold Humboldt Current system. Geological data indicates that hyper-arid conditions have been prevalent in the inner core of the desert for at least the last 8 to 15 million years.
This ancient and sustained dryness has resulted in unique geological and biological conditions. The extremely low erosion rates have preserved the landscape’s topography. The lack of water has also led to the accumulation of vast, commercially relevant nitrate deposits that formed over millions of years. Furthermore, the extreme dryness has resulted in many high-altitude mountain peaks remaining completely free of glaciers.