The Andes formed, and continue to form, because the floor of the Pacific Ocean is diving beneath the South American continent in a process called subduction. As the dense oceanic Nazca Plate slides beneath the lighter South American Plate, the enormous compressive forces crumple, thicken, and lift the western edge of the continent. But describing the Andes as a product of subduction is a bit like describing a novel as a product of typing. The real story involves ancient continental collisions, shifting slab angles, volcanic flare-ups, and feedback loops with climate and rivers that stretch back hundreds of millions of years.
Rocks Older Than the Mountains
The Andes sit on a foundation far older than the mountains themselves. Scattered along the chain from Colombia to Patagonia are exposures of basement rock that date to the Mesoproterozoic era, roughly 1,200 to 1,000 million years ago. These ancient blocks participated in the assembly of Rodinia, a supercontinent that existed long before any recognizable Andean topography. When Rodinia broke apart, some of those crustal fragments ended up on what would become North America, while others stayed attached to the Gondwanan side, eventually becoming part of South America’s western margin. Terranes like Arequipa and Pampia collided with the Amazon craton and remained stuck, experiencing episodes of stretching and rifting throughout the following hundreds of millions of years.1Journal of South American Earth Sciences. The Grenville-age basement of the Andes These old crustal blocks matter because they provided the raw material and the structural weaknesses that later subduction would exploit. The Andes did not rise from a blank slate; they inherited a patchwork of ancient scars.
When Did the Andes Start Rising?
One of the most important things to understand about Andean mountain building is that it did not happen all at once. The uplift was highly diachronous, meaning different parts of the range rose at very different times. In the northern Andes, the Western and Central Cordilleras of Ecuador began rising in the Late Cretaceous, around 70 million years ago, while the northwestern corner of the continent was still submerged beneath shallow seas.2Earth-Science Reviews. Andean mountain building since the Late Cretaceous: A paleoelevation reconstruction
In the central Andes, uplift migrated from west to east over tens of millions of years. The Western Cordillera rose during the Late Cretaceous and Paleocene, the western Puna plateau followed during the Paleocene, the eastern Puna came up in the early to mid-Miocene, and the Altiplano and Eastern Cordillera reached their current elevations only during the mid- to late Miocene, roughly 10 to 5 million years ago.2Earth-Science Reviews. Andean mountain building since the Late Cretaceous: A paleoelevation reconstruction So when someone asks “how old are the Andes,” the honest answer depends on which part you are pointing at. The initial subduction-driven deformation is ancient, but some of the most dramatic elevation gains happened in what geologists consider the recent past.
How Subduction Builds Mountains
Subduction does not just push rock upward like a hydraulic press. The main mechanism for building the Andes is crustal shortening: the continental crust gets compressed and folded into stacked sheets of rock that pile on top of one another, thickening the crust and raising the surface. The central Andes consist largely of Neoproterozoic through Cenozoic sedimentary rocks that have been shortened and thickened above the South American craton as it underthrusts from the east.3Earth and Planetary Science Letters. Differences between the central Andean and Himalayan orogenic wedges: A matter of climate
The scale of this shortening is staggering. In southern Bolivia, the Eastern Cordillera alone accommodated about 120 kilometers of shortening, meaning rocks that were once spread across 120 extra kilometers of horizontal distance got compressed and stacked into a zone of thrust sheets two to ten kilometers thick, now elevated roughly six kilometers above the adjacent lowlands.4Geosphere. Shortening and structural architecture of the Andean fold-thrust belt of southern Bolivia (21°S): Implications for kinematic development and crustal thickening of the central Andes That shortening accounts for about 37% of the total shortening across the entire Andean fold-thrust belt at that latitude. The result is a crust that reaches about 70 kilometers thick beneath the high Andes, thinning to around 35 kilometers in the lower ranges to the east.5Oxford Academic. Tomographic imaging of the Pampean flat slab: evidence of subduction erosion and volatile migration
Why the Slab’s Angle Matters
Not all subduction looks the same. The angle at which the oceanic plate dives beneath the continent varies along the length of the Andes, and those variations profoundly shape the geology at the surface. In some segments, the slab descends steeply, creating the classic arrangement of a deep trench offshore, an active volcanic arc, and a high plateau behind it. In other segments, the slab flattens out and slides almost horizontally beneath the continent for hundreds of kilometers before diving deeper.
Where the slab flattens, volcanism tends to shut off because the mantle wedge above the slab gets squeezed out, removing the source of melt. But the deformation migrates inland. In the Pampean flat-slab segment of Argentina, slab flattening began around 5 million years ago in the late Miocene, pushing deformation eastward from the high Andes into the Precordillera fold-and-thrust belt and eventually into the basement-cored uplifts of the Sierras Pampeanas, far from the trench.5Oxford Academic. Tomographic imaging of the Pampean flat slab: evidence of subduction erosion and volatile migration This is why you find active faulting and mountain building hundreds of kilometers east of where you would expect it based on a simple subduction model.
These shifting slab geometries, along with changes in how fast South America moves toward the trench, create alternating tectonic regimes along the chain. Different latitudes have experienced phases of compression, extension, and relative quiet at different times, which is why the Andes look so different at 23°S (the wide, high Altiplano-Puna plateau) compared to 43°S (a narrow, lower chain with active volcanism).6Tectonics. Tectonic Regimes of the Central and Southern Andes: Responses to Variations in Plate Coupling During Subduction
Volcanism and the Ignimbrite Flare-Up
The Andes are one of the most volcanically active mountain chains on Earth. Where the subducting slab descends steeply enough, water and other fluids released from the slab lower the melting point of the overlying mantle, generating magma that rises to feed the volcanic arc.7Geochemistry, Geophysics, Geosystems. Spatially and Geochemically Anomalous Arc Magmatism: Insights From the Andean Arc This is why the Andes host dozens of active stratovolcanoes, some reaching well above 6,000 meters.
But the most dramatic volcanic episode in the Andes was not a single eruption. Between about 10 and 1 million years ago, the Altiplano-Puna Volcanic Complex in the Central Andes experienced what geologists call an “ignimbrite flare-up,” a period of colossal explosive eruptions that produced at least 10,000 cubic kilometers of dacitic magma. These eruptions came in four pulses, at roughly 10, 8, 6, and 4 million years ago, with the eruptions growing more intense and concentrated over time before dropping off sharply after 4 million years ago.8Journal of Volcanology and Geothermal Research. Episodic construction of batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up One product of this period was the Vilama caldera, among the very large-volume volcanic structures that formed during the flare-up.9Journal of Volcanology and Geothermal Research. Geology of the Vilama caldera: A new interpretation of a large-scale explosive event in the Central Andean plateau during the Upper Miocene
These eruptions were not just surface spectacles. They are understood to be the surface expression of massive batholith formation happening at depth: enormous bodies of solidified magma building out the crust from below. Volcanism, in other words, has been both a builder and a shaper of the Andes.
The Pulsed Growth of the Altiplano
The Altiplano-Puna plateau, the broad high-elevation surface in Bolivia and adjacent countries that sits at roughly 3,700 to 4,000 meters, did not rise at a steady rate. Its growth was pulsed, driven by several distinct geodynamic processes operating in sequence and sometimes in combination.10Annual Review of Earth and Planetary Sciences. Tectonic Evolution of the Central Andean Plateau and Implications for the Growth of Plateaus
Crustal shortening and thickening account for a large share of the plateau’s elevation gain. But shortening alone cannot explain all of the observed uplift. At several points in the plateau’s history, dense material at the base of the thickened crust appears to have detached and sunk into the mantle, a process called delamination or lithospheric removal. When that heavy root drops away, the remaining lighter crust bobs upward. Evidence for punctuated episodes of this kind of removal has been identified in the Eastern Cordillera and Altiplano during the latest Oligocene to early Miocene and again in the late Miocene to early Pliocene.10Annual Review of Earth and Planetary Sciences. Tectonic Evolution of the Central Andean Plateau and Implications for the Growth of Plateaus Deep crustal flow, where hot, ductile lower crust moves laterally to even out thickness differences, added yet another mechanism of plateau modification during the late Miocene and early Pliocene.11Geological Society of America Memoirs. Shallowing and steepening subduction zones, continental lithospheric loss, magmatism, and crustal flow under the Central Andean Altiplano-Puna Plateau
The combination of these processes, crustal shortening, delamination, crustal melting, and deep flow, makes the Altiplano one of the most instructive examples of how a high plateau forms over time. It was not a single event but a layered history spanning tens of millions of years.
How the Andes Shaped Climate and Climate Shaped the Andes
Mountain building and climate are not independent processes. As the Andes rose, they began intercepting moisture-laden air masses from the east, wringing out precipitation on the eastern slopes and starving the western side. The Atacama Desert, one of the driest places on Earth, owes much of its extreme aridity to this rain shadow. Isotopic records from the Atacama suggest that the rain shadow intensified once the Andes reached mean elevations above about two kilometers, fundamentally changing the regional climate.12Earth and Planetary Science Letters. Evidence for the development of the Andean rain shadow from a Neogene isotopic record in the Atacama Desert, Chile
But the feedback runs in both directions. Crustal thickening elevates topography, which enhances precipitation on the windward side, which increases erosion, and that erosion may in turn influence where deformation localizes.13PubMed Central. Coordination between deformation, precipitation, and erosion during orogenic growth In other words, rain can help steer where mountains grow tallest by removing material from certain zones and leaving others to pile up. This coupling between tectonics and climate is one of the more active areas of research in Andean geology, and it helps explain some of the differences between the wet eastern flanks and the dry western side of the range.
In the southern Andes, glaciers have been a powerful erosional force. In Patagonia, the onset of alpine glaciation around 6 million years ago led to a two- to threefold increase in the average erosion rate of the source region, with the most deeply carved valleys experiencing a fifteen-fold increase in erosion.14PubMed Central. Transient glacial incision in the Patagonian Andes from ~6 Ma to present Glaciers have sculpted the dramatic fjords and U-shaped valleys that define the Patagonian landscape, showing how climate processes actively reshape the mountains even as tectonic forces continue to build them.
Reversing the Amazon
One of the most striking consequences of Andean uplift had nothing to do with the mountains themselves. Before the Andes reached their current heights, the drainage of northern South America looked nothing like it does today. During the Miocene, much of western Amazonia was occupied by a vast mega-wetland system known as the Pebas system. Rivers in this region flowed generally northward or westward, not eastward toward the Atlantic.
As the Andes rose and the continental surface tilted progressively to the east, this mega-wetland system gradually disappeared, and by the late Miocene the transcontinental Amazon River had established itself, connecting the Andes to the equatorial Atlantic margin.15Earth and Planetary Science Letters. Drainage reversal of the Amazon River due to the coupling of surface and lithospheric processes The formation of the modern Amazon was not driven by Andean surface uplift alone. Mantle convection beneath the continent helped tilt northern South America eastward, enabling the river’s current course.16Nature Geoscience. Miocene drainage reversal of the Amazon River driven by plate–mantle interaction The Andes, in effect, helped create the Amazon as we know it by rerouting an entire continent’s drainage and flooding the lowlands with a vast influx of Andean sediment.
An Engine for Biodiversity
The uplift of the Andes did not just reshape rivers and climate. It created an astonishing array of new habitats at every elevation, from lowland rainforest to alpine grasslands above 4,000 meters, and it isolated populations on opposite sides of the range. Phylogenetic studies have shown that this uplift played a major role in the extraordinary diversification of the Neotropical flora.17PubMed Central. Tracing the impact of the Andean uplift on Neotropical plant evolution The eastern flanks of the Andes, where moisture from the Amazon basin meets steep terrain, are among the most species-rich places on Earth for plants, birds, and amphibians.
The sediment carried down from the rising Andes also transformed the Amazon basin itself. A region-wide mosaic of different soil types developed on this Andean substrate, becoming extraordinarily rich in species, particularly in western Amazonia. These biodiversity patterns are not recent; they are rooted deep in pre-Quaternary history, meaning the biological richness we see today is fundamentally a product of tens of millions of years of mountain building.18PubMed. Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity The Andes are often called the species pump of South America for good reason: by simultaneously creating new high-altitude environments and reshaping the lowlands, the range drove evolution on a continental scale.
Humboldt and the Rock Named After the Andes
The scientific study of how the Andes formed has its own interesting history. Alexander von Humboldt’s expeditions through South America in the early 1800s produced the first geological cross-sections of the Andes that included lithology and stratigraphy. Humboldt recognized chains of volcanoes stretching hundreds of kilometers along the range, parallel to its axis, and linked their distribution to major earthquakes, earning him recognition as the first Andean volcanologist.19Springer Professional. Humboldt’s Interpretation of the Andean Geology Rock samples he collected during those travels eventually led to the definition of a new volcanic rock type: andesite, named after the Andes themselves. Andesite is the characteristic intermediate volcanic rock of subduction zones worldwide, so in a way, the Andes gave their name to the very rock type that best represents their volcanic heritage. Humboldt did not have the framework of plate tectonics to explain what he saw. That understanding would take another century and a half to develop. But his careful measurements and cross-sections laid the groundwork for everything that followed.
Active Deformation Today
The Andes are not a finished product. The Nazca Plate continues to slide beneath South America at several centimeters per year, and the forces that built the range are still operating. GPS measurements along the south-central Chilean margin reveal heterogeneous deformation patterns that track the boundaries between different megathrust earthquake segments, reflecting the ongoing tug-of-war between the locked subduction interface and the overriding plate.20Geochemistry, Geophysics, Geosystems. Active faulting and heterogeneous deformation across a megathrust segment boundary from GPS data, south central Chile (36–39°S) The 2010 magnitude-8.8 earthquake in Chile and the 1960 magnitude-9.5 event, the largest earthquake ever recorded, both occurred along this margin.
Active faulting in the Andes extends well beyond the coastal zone. Upper-plate faults throughout the forearc and within the high Andes themselves accommodate ongoing deformation, posing seismic hazards to cities like Santiago, Lima, Quito, and Bogotá. The volcanic arc remains active as well, with eruptions in the past few decades from volcanoes like Calbuco in Chile and Nevado del Ruiz in Colombia serving as reminders that the processes building the Andes continue in real time. Geologically speaking, the Andes are still young, still growing, and still very much alive.