The Andes Mountains formed from the collision between the Nazca Plate and the South American Plate. This is a subduction boundary, where the dense oceanic crust of the Nazca Plate dives beneath the lighter continental crust of South America. The process has been building the Andes for tens of millions of years and continues today, making it one of the most tectonically active mountain ranges on Earth.
How Subduction Built the Andes
The Nazca Plate, which makes up a large portion of the southeastern Pacific Ocean floor, moves roughly eastward at 70 to 80 millimeters per year relative to South America. Where it meets the continent, it plunges downward into the Earth’s mantle. This descent creates the Peru-Chile Trench, a massive underwater gash running along South America’s western coast that reaches depths of more than 8,000 meters in its southern stretches.
As the Nazca Plate sinks, it drags water-rich ocean sediments down with it. At depth, this water lowers the melting point of surrounding rock, generating magma that rises to feed the volcanic chain along the Andes. The same downward pull compresses and crumples the edge of the South American Plate, folding and thrusting rock upward. This combination of volcanic buildup and tectonic compression is what created the towering peaks we see today, including Aconcagua at 6,959 meters, the highest point in the Western Hemisphere.
Tectonic vs. Volcanic Growth
Not all of the Andes’ height comes from the same process. Research comparing the contributions of tectonic compression and magmatic addition finds that the ratio is roughly 2:1. In other words, about two-thirds of the crustal thickening beneath the Andes comes from rock being squeezed, folded, and stacked by plate collision. The remaining third comes from molten rock rising from below and adding new material to the base of the crust. By the late Eocene (around 35 million years ago), this combination had already built the Andean crust to a minimum thickness of about 42 kilometers.
Major Phases of Mountain Building
The Andes didn’t rise in one smooth event. Geologists recognize several distinct pulses of mountain building, each named for regional characteristics. In central and northern Peru, three phases stand out. The Incaic phase during the Eocene (roughly 40 to 50 million years ago) raised the western mountain range and created major thrust-and-fold belts. The Quechua 1 phase in the early Miocene reactivated those structures and pushed deformation eastward into the high plateau region. The Quechua 3 phase in the late Miocene built the sub-Andean zone, the foothills on the range’s eastern flank.
These phases correspond to changes in how fast and at what angle the Nazca Plate was subducting. Faster convergence and shifts in the slab’s dip angle triggered new episodes of compression and uplift across different parts of the range.
The Altiplano-Puna Plateau
One of the most striking features the Andes produced is the Altiplano-Puna Plateau, a high-elevation flat zone stretching 1,800 kilometers through Bolivia, Peru, Argentina, and Chile. At 350 to 400 kilometers wide, it’s one of the largest plateaus on Earth that formed without a continent-on-continent collision (the more common way plateaus like the Tibetan Plateau form).
Uplift in the Altiplano region began around 25 million years ago, coinciding with an increase in the convergence rate between the Nazca and South American plates and a shallowing of the subduction angle. The Puna section, farther south, started rising 5 to 10 million years later. While horizontal shortening of thermally softened rock accounts for most of the plateau’s thickness, known surface shortening explains only 70 to 80 percent of the observed crustal thickening. The rest likely comes from magma additions from below, thinning of the dense lower lithosphere, and other deep processes.
Four Volcanic Zones Along the Chain
The Andean Volcanic Belt is the longest continental volcanic arc on Earth, but it isn’t continuous. It breaks into four distinct segments: the Northern Volcanic Zone (in Colombia and Ecuador), the Central Volcanic Zone (southern Peru, Bolivia, northern Chile, and Argentina), the Southern Volcanic Zone (central Chile and Argentina), and the Austral Volcanic Zone (southern Chile and Argentina).
Between these zones are gaps where no active volcanoes exist. These gaps correspond to regions where the Nazca Plate subducts at a very shallow angle, sliding nearly horizontally beneath the continent instead of diving steeply. A steep descent is what allows water to be released at the right depth to trigger magma generation. Where the plate is too flat, that process stalls and volcanism shuts off, even though the plates are still converging.
Earthquakes and Ongoing Activity
The subduction zone beneath the Andes generates earthquakes at a wide range of depths. Shallow earthquakes occur near the trench where the plates first make contact. Deeper earthquakes, some with focal depths greater than 500 kilometers, trace the path of the sinking Nazca Plate far beneath the continent. These deep events may partly reflect remnants of earlier subduction cycles, not just the current one.
GPS and geological data show the Andes are still rising. In the Central Andes, uplift rates over the past several million years average 0.2 to 0.3 millimeters per year, amounting to 2,300 to 3,400 meters of surface uplift since the late Miocene. In Colombia’s Eastern Cordillera, uplift was even faster between 2 and 5 million years ago, reaching rates of 0.6 to 3 millimeters per year. The Nazca Plate continues its relentless eastward push, meaning the Andes are still growing, still producing earthquakes, and still feeding magma to their volcanic peaks.