Only gentle terrain fits beneath Chile's 21.9-million-year-old ash sheet
A UCL-led team simulated hundreds of ancient landscapes to see which ones could have been buried by the Lauca Caldera's ignimbrite in northern Chile. Only low-relief terrain could. That bounds local uplift near an inch a century.
The Scientist · Science desk
What happened
- A UCL-led study in Science Advances treats a giant volcanic deposit as a cast of the landscape it buried, preserving information erosion and deformation would otherwise have destroyed.
- The ignimbrite from northern Chile's Lauca Caldera covers an area six times the size of Santiago and reaches a kilometre deep in places.
- Those surviving landscapes imply rock in this part of the Andes was pushed upward by no more than 0.26 kilometre per million years, about an inch every 100 years.
- The authors say that rate supports the view that the Andes grew slowly and steadily over tens of millions of years.
Why it matters
- What's now possible capability Chemical clocks in minerals fix single moments of cooling; Adams said this method can estimate rock uplift over a much longer period, giving an average rate across an interval.
- What it limits constraint The result closes off the fast end of the Andean debate for this sector and leaves every slower history admissible.
- What it sets up precedent Any large volcanic sheet with a mapped shape becomes a candidate archive of the terrain under it, an approach Cooper said could be applied to deposits elsewhere in the world.
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The constraint is geometric. Ignimbrite that thick can only bury terrain shallow enough to be covered, and the authors argue that faster uplift in this sector would have produced steeper slopes than the deposit could have hidden [12]. So the team worked the problem backwards. They generated hundreds of candidate pre-eruption landscapes using models of how rivers shape mountain ranges, then tested which of those surfaces the deposit could plausibly have buried [3]. Only relatively low-relief ones, akin to gentle mountain foothills, came through [11].
"We cannot dig down to see the buried landscape, but we can use the shape of the volcanic blanket and what we know about how rivers shape mountains to infer what is hidden beneath it," lead author Byron Adams of UCL Earth Sciences said [6].
The rate that follows from those surviving landscapes is 0.26 km per million years, which works out to 0.26 mm a year [4][17]. The comparison the authors themselves draw is with rapidly uplifting parts of active mountain belts, including parts of the Himalayas, where uplift and erosion can run at several millimetres to centimetres a year [13]. Taking "several millimetres" as 3 mm and "centimetres" as 1 cm, the Lauca figure is roughly 12 to 38 times slower [20].
That figure is a ceiling. Uplift faster than 0.26 km per million years is excluded because the relief it would have carved does not fit under the deposit [12]; every slower history fits equally well. And it describes rock uplift in one sector at one moment, 21.9 million years ago [2][4].
The authors note their rock-uplift estimate agrees with earlier work on minerals that recorded temperature changes over the past 50 million years, cooling being the signature of rock travelling upward through the crust [15]. Two methods, one sampled from rock and one inferred from a landscape nobody can sample, land in the same place. Adams said current methods "look at chemical 'clocks' in the rock, but these are limited to specific moments in time. Our method can estimate rock uplift over a much longer period" [14].
On the underlying dispute, Adams said: "There is debate over whether the Andes grew slowly and steadily over 40 or 50 million years or whether they rose extremely slowly and then popped up more recently, in the last six to 10 million years. Our findings, which cover a large part of the middle of that history, support the slow but steady hypothesis" [7]. The eruption sits about 12 million years before the earliest date proposed for that late rise [18].
The published account of the work does not spell out which model parameters were varied across the hundreds of simulations, or the criterion by which a candidate landscape was accepted or rejected [10]. Co-author Frances Cooper, also at UCL Earth Sciences, said: "The Andes have a major influence on regional and global climate, so reconstructing their history is important for understanding long-term climate change" [16].
What to watch
- A thermochronology dataset from the same Lauca sector returning a faster rock-uplift rate would break the agreement the paper leans on.
- A test of the same inversion on a buried surface that has since been drilled or re-exposed, where the inferred relief can be checked against rock.
- Published rates for other central Andean sectors between 22 and 10 million years ago that sit above 0.26 km per million years.
Clarity's read
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Claim ledger
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- [1]
A new study led by UCL (University College London) researchers, published in Science Advances, shows that a giant volcanic eruption buried the terrain beneath it and preserved information that would otherwise have been lost to erosion and deformation.
- [2]
The volcanic rock studied erupted from the Lauca Caldera in northern Chile 21.9 million years ago; a caldera is a large volcanic depression formed during major eruptions.
- [3]
The research team simulated hundreds of possible ancient landscapes, using models of how rivers shape mountain ranges, to test which ones could plausibly have been buried beneath the volcanic deposit.
- [4]
The team inferred that this part of the Andes was being built slowly, with rocks pushed upward by no more than 0.26 kilometre every million years, equivalent to 2.5 cm, or about an inch, every 100 years.
- [5]
The finding lends support to the idea that the Andes grew slowly and steadily over tens of millions of years rather than popping up more recently, in the last few million years.
- [6]
Lead author Dr Byron Adams, based at UCL Earth Sciences, said: "We cannot dig down to see the buried landscape, but we can use the shape of the volcanic blanket and what we know about how rivers shape mountains to infer what is hidden beneath it."
- [7]
Adams said: "There is debate over whether the Andes grew slowly and steadily over 40 or 50 million years or whether they rose extremely slowly and then popped up more recently, in the last six to 10 million years. Our findings, which cover a large part of the middle of that history, support the slow but steady hypothesis."
- [8]
Cooper said the same approach could be applied to volcanic deposits elsewhere in the world.
- [9]
The ignimbrite covered an area six times as large as Chile's capital, Santiago (or nearly three times as large as Greater London), reaching 1 kilometre (0.6 miles) deep in places.
- [10]
The published account describes the simulations as testing which landscapes could plausibly have been buried, without specifying which model parameters were varied or the criteria used to accept or reject a candidate landscape.
- [11]
They found that only relatively low-relief landscapes, akin to gentle mountain foothills, could have been buried beneath the deposit.
- [12]
If rocks had been pushed upward faster in this region, the landscape would probably have developed steeper slopes that could not have fit beneath the immense volcanic deposits.
- [13]
This rate is slow compared with many rapidly uplifting parts of active mountain belts, including parts of the Himalayas, where uplift and erosion can occur at rates of several millimetres to centimetres per year.
- [14]
Adams said: "Current methods to estimate the speed at which mountains build look at chemical 'clocks' in the rock, but these are limited to specific moments in time. Our method can estimate rock uplift over a much longer period."
- [15]
The paper's authors noted their estimate of rock uplift was consistent with earlier research looking at minerals in the rock that recorded specific changes in temperature over the last 50 million years; these temperature changes represent the rock cooling as it travels upward through Earth's crust.
- [16]
Co-author Dr Frances Cooper, also based at UCL Earth Sciences, said: "The Andes have a major influence on regional and global climate, so reconstructing their history is important for understanding long-term climate change."
- [17]
An uplift rate of 0.26 kilometre per million years is 0.26 millimetre per year.
- [18]
The 21.9-million-year-old eruption predates the earliest date in the proposed six-to-10-million-year late-rise window by about 12 million years.
- [19]
Adams said: "In this case, we estimate it would have taken millions of years to produce the subdued landscape we infer prior to the eruption."
- [20]
Reading the source's Himalayan comparison range as 3 mm/yr at the low end and 1 cm/yr at the high end, those rates are about 12 to 38 times the inferred Lauca-area rate of 0.26 mm/yr.
Sources
3 independent publishers whose own reporting we read for this story.
- discovermagazine.comDevastating Volcanic Eruption Engulfed Part of Chile 21.9 Million Years Ago, Burying Evidence of Andes Uplift
1 article · September 11, 2026
- livescience.comSouth American 'Pompeii': Volcanic eruption 22 million years ago preserved a snapshot of the Andes forming
1 article · September 14, 2026
- phys.orgGiant volcanic blanket reveals hidden 'Pompeii' of Andean landscapes
1 article · September 11, 2026
Topics and entities
Topics
- Rock uplift rates
- Landscape evolution modelling
- Tectonic geomorphology