South America is a geologically dynamic continent, defined by the towering Andes mountain range along its western edge. This immense cordillera is a direct result of powerful tectonic forces, making the region a major component of the Pacific Ring of Fire. The mountains host a significant concentration of volcanoes, which have shaped the landscape and pose a natural hazard. Quantifying the total number of these volcanic structures is complicated because the definition of a single volcano varies greatly among geologists.
Defining and Counting South American Volcanoes
Determining a definitive number for South America’s volcanoes is challenging because volcanologists use different classification criteria. The most conservative and widely used count focuses on Holocene volcanoes, defined as those showing evidence of activity within the last 10,000 years. By this measure, the continent hosts approximately 150 to 200 Holocene-active volcanic centers, which are considered potentially capable of future eruption.
Including all recognizable volcanic cones and structures, such as those active in the Pleistocene epoch, raises the total figure to well over 500 structures. The conservative Holocene count is the most scientifically relevant figure for assessing current hazard risk. This disparity highlights the difficulty in providing a simple numerical answer.
The Primary Volcanic Zone: The Andean Arc
The vast majority of South America’s volcanic structures are concentrated within the Andean Volcanic Belt (AVB), which runs for approximately 7,000 to 8,000 kilometers along the western margin of the continent. This extensive arc is segmented into four distinct zones separated by long stretches where no volcanism occurs. These volcanic gaps are caused by localized shifts in the underlying tectonic process.
The Four Volcanic Zones
The Northern Volcanic Zone (NVZ) spans Colombia and Ecuador, containing numerous prominent peaks, including approximately 55 volcanoes in Ecuador alone. South of a major volcanic gap in Peru, the Central Volcanic Zone (CVZ) extends through Peru, Bolivia, and northern Chile and Argentina. This segment is characterized by some of the world’s highest volcanic peaks, rising above the high-altitude Altiplano plateau.
The Southern Volcanic Zone (SVZ) begins around the latitude of Santiago in Chile and continues south, a region densely populated with active stratovolcanoes. Finally, the Austral Volcanic Zone (AVZ) represents the southernmost segment, featuring sparse volcanism in the remote, ice-covered terrain of Patagonia. Chile and Argentina host the highest concentration of volcanic edifices overall.
The Tectonic Engine: Subduction and Formation
The existence of the Andean Volcanic Belt is driven by subduction, a process where one tectonic plate slides beneath another. Here, the oceanic Nazca Plate and the Antarctic Plate are descending beneath the continental South American Plate. This geological collision occurs along the Peru–Chile Trench and generates the heat and pressure necessary for volcanism.
Magma Generation
As the oceanic plate sinks deeper, increasing temperatures and pressures cause minerals within the rock to release water and other volatile compounds. This water rises into the overlying mantle rock, lowering its melting point and causing it to partially melt. The resulting molten rock, or magma, is buoyant and begins to ascend through the continental crust.
The magma eventually pools in reservoirs beneath the surface, feeding the volcanoes that form the Andean chain. Where the subducting plate angle is steep, magma rises easily, resulting in active volcanic zones. Conversely, areas where the plate subducts at a shallow angle, known as flat-slab segments, inhibit the formation and ascent of magma, explaining the volcanic gaps observed along the Andes.
Current State of Activity and Monitoring
The volcanoes of the Andean Arc exhibit a wide range of activity, from continuously erupting mountains to those dormant for centuries. Many are classified as historically active, meaning they have erupted since 1450 CE. Due to the proximity of major cities and populations to the Andes, constant observation is a priority for regional safety.
Monitoring efforts are carried out by governmental and scientific bodies, such as Chile’s National Geology and Mining Service (SERNAGEOMIN) and the Peruvian Geophysical Institute. These agencies use sophisticated systems to track subtle changes that might precede an eruption. Common monitoring techniques include:
- Deployment of seismometers to detect earth tremors.
- GPS sensors to measure ground deformation as magma moves beneath the surface.
- Instruments to analyze gas emissions.
Several volcanoes are under intense scrutiny. Cotopaxi in Ecuador is dangerous due to its size and potential to generate massive mudflows (lahars) that threaten surrounding valleys. Nevado del Ruiz in Colombia is closely watched following its devastating 1985 eruption, which was caused by lahars from melted snow and ice. Villarrica in Chile is known for its persistent active lava lake.