Ancient volcanic blanket preserves a buried Andean Pompeii from 22 million years ago

Some 21.9 million years ago, in what is now northern Chile, a catastrophic eruption at the Lauca Caldera sent searing mixtures of volcanic ash, rock fragments and gas surging across an entire stretch of the Andean landscape. In a single geological instant, the moving currents of hot material engulfed everything in their path and settled into a vast sheet of volcanic rock, sealing the terrain beneath it like a lid. Researchers led by University College London now report that this enormous deposit has acted as a time capsule, preserving the shape of a landscape that would otherwise have been worn away by erosion and crumpled by tectonic deformation long before anyone could study it. The comparison that keeps recurring among the scientists is Pompeii, the Roman town famously entombed by Vesuvius in 79 AD, except that here the buried entity is not a town but a whole sector of mountain terrain, frozen at a moment in Earth history tens of millions of years before humans existed.

The scale of the event is difficult to overstate. A caldera is a large volcanic depression that forms during major eruptions, when the rapid evacuation of a magma reservoir causes the ground above it to collapse inward. The Lauca Caldera eruption expelled what the researchers describe as a mountain’s worth of rock, a type of volcanic deposit known as ignimbrite, produced when pyroclastic flows sweep outward at high speed and weld together into solid rock as they cool. The resulting blanket covered an area six times larger than Chile’s capital, Santiago, or nearly three times the size of Greater London, and in places it reached a kilometre in thickness. That immense volume means the deposit’s very shape carries information: whatever terrain lies beneath it must once have fitted snugly under the volcanic sheet, and that geometric constraint is the key to the new study, published in Science Advances.

Because the buried surface cannot be excavated, the team had to infer it indirectly. Their approach was to simulate hundreds of possible ancient landscapes using models of how rivers sculpt mountain ranges over geological time. River incision is one of the dominant forces that carve relief into mountain belts, and the mathematical relationships governing how channel steepness responds to rock uplift, erosion and drainage area are well established in geomorphology. By generating a wide family of plausible pre-eruption topographies, each shaped by realistic fluvial processes, and then testing which of them could plausibly have been covered by the observed volcanic deposit, the researchers turned the ignimbrite sheet into a template. Only certain classes of buried terrain could satisfy the constraint that the overlying deposit, with its measured thickness and extent, could drape across it without impossible gaps or overlaps.

The outcome of that modelling exercise was strikingly decisive. The simulations showed that only relatively low-relief landscapes, comparable in character to gentle mountain foothills, could plausibly have been buried beneath the volcanic blanket. If the pre-eruption terrain had contained deep valleys and sharp ridges, the kilometre-thick deposit would have had to fill enormous voids or drape across slopes in ways that the observed geometry does not allow. The subdued surface implied by the deposit therefore became a geological measurement in its own right, a snapshot of how much vertical relief the Andes in this region had accumulated by the early Miocene, roughly 22 million years ago. From that snapshot, the team could work backwards to the tectonic process that produced it.

Their inference is that this part of the Andes was being built slowly and steadily. The rocks were being pushed upward at no more than a quarter of a kilometre every million years, a rate the authors quantify as 0.26 kilometres per million years. Expressed in human terms, that is equivalent to about 2.5 centimetres, roughly an inch, of uplift every hundred years. The logic is one of exclusion: had the rocks been rising substantially faster, the landscape would have developed steeper slopes and deeper valley relief, and such rugged terrain could not have fitted beneath the immense volcanic deposits. The shape of the ignimbrite blanket thus places a firm ceiling on how quickly the mountains could have been rising in the millions of years before the eruption, a constraint that no single mineral-based dating technique can provide on its own.

The finding carries significant weight in a long-running debate about how the Andes, the longest continental mountain chain on Earth, came to exist. One school of thought holds that the range grew slowly and steadily over tens of millions of years, driven by the sustained compression of the South American plate as oceanic crust slides beneath it. Another proposes that the mountains rose extremely slowly for a long period and then popped up more recently, within the last six to ten million years. The new results, which cover a large portion of the middle of that history, support the slow but steady hypothesis. Lead author Dr Byron Adams, of UCL Earth Sciences, notes that the method offers a way to look back at Earth history over far longer intervals than conventional approaches, which rely on chemical clocks in minerals that are limited to specific moments in time. In this case, he explains, the inferred subdued landscape would itself have taken millions of years to produce.

The sluggish uplift rate implied by the study stands in sharp contrast to many rapidly deforming parts of active mountain belts elsewhere in the world. In portions of the Himalaya, for example, uplift and erosion can proceed at rates of several millimetres to centimetres per year, orders of magnitude faster than the Andean rate inferred here. The comparison underlines how differently mountain belts can behave depending on the details of plate convergence, crustal strength and climate-driven erosion. It also illustrates why a single global model of mountain building is inadequate: the Central Andes, at least in this region and during this interval, were accumulating elevation with almost glacial patience, inch by inch across geological time, rather than lurching upward in dramatic pulses.

The researchers note that their estimate of rock uplift is consistent with earlier independent evidence, particularly studies of minerals in the rock that recorded specific changes in temperature over the last 50 million years. As rocks travel upward through the Earth’s crust, they cool, and certain minerals preserve chemical signatures of the temperatures they have passed through. These thermochronological records act as a partial archive of uplift history, but each mineral system captures only a bounded window of time and temperature. The new volcanic-template approach complements that archive by covering a much longer continuous span, and the agreement between the two independent lines of evidence strengthens confidence in the central conclusion that the Andes in this region have grown gradually rather than abruptly.

Beyond resolving a regional debate, the study offers a genuinely new tool for reconstructing planetary history. Dr Frances Cooper, also of UCL Earth Sciences, who has worked in this part of the Andes for many years, emphasises that the mountains exert a major influence on regional and global climate, so reconstructing their history matters for understanding long-term climate change. The positions and heights of mountain ranges steer atmospheric circulation, control patterns of rainfall and even affect global carbon cycling through the weathering of rock. Because the same analytical approach could be applied to large volcanic deposits elsewhere in the world, wherever voluminous ignimbrites blanket older terrain, the method may allow scientists to resurrect landscapes that have been hidden for millions of years, converting some of the planet’s most violent eruptions into unexpectedly precise instruments for reading its deep past. The research was supported by AXA and the Royal Society, and builds on earlier work in the Central Andes supported by BHP.

The ignimbrite sheet at Lauca belongs to a broader family of explosive eruptions known as caldera-forming events, among the most voluminous volcanic phenomena on Earth. When such eruptions occur, pyroclastic density currents can travel tens of kilometres from the vent in a matter of hours, which is why the resulting deposits can blanket entire drainage networks rather than isolated sites. The preservation potential of these deposits is what makes them scientifically valuable: once welded into rock, an ignimbrite protects the buried surface from the fluvial erosion and tectonic shortening that would otherwise erase it, much as fine ash at Pompeii protected frescoes and streets from the elements.

Northern Chile’s Central Andes are particularly rich in such deposits, because the region hosted a major episode of ignimbrite volcanism during the early Miocene, when large silicic magma systems repeatedly erupted across the Altiplano-Puna plateau region. Each successive sheet offers a potential snapshot of the terrain that existed at the moment of burial, meaning stacked ignimbrites of different ages could in principle be read as a sequence of frames in a long film of landscape evolution. The Lauca deposit, at 21.9 million years old, captures an especially early chapter in the range’s growth.

The study also highlights how geomorphology and volcanology can be combined in unexpected ways. Rather than treating volcanic deposits purely as hazards or as records of eruption dynamics, the researchers exploited them as geometric archives, an approach that requires no direct access to the buried surface and could complement traditional field mapping in remote, high-altitude terrain.

Subject of Research: Using a large ignimbrite deposit from the Lauca Caldera eruption to reconstruct a buried Andean landscape and infer long-term rock uplift rates.

Article Title: Giant volcanic blanket reveals hidden ‘Pompeii’ of Andean landscapes

Article References: Giant volcanic blanket reveals hidden ‘Pompeii’ of Andean landscapes. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Andes, Lauca Caldera, ignimbrite, volcanic eruption, rock uplift, landscape evolution, UCL, Science Advances, geomorphology, mountain building, Chile, pyroclastic flows