Only gentle terrain fits beneath Chile's 21.9-million-year-old ash sheet

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 ^[7]. 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 ^[4]. Only relatively low-relief ones, akin to gentle mountain foothills, came through ^[5].

"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 ^[10].

The rate that follows from those surviving landscapes is 0.26 km per million years, which works out to 0.26 mm a year ^[6][1]. 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 ^[9]. Taking "several millimetres" as 3 mm and "centimetres" as 1 cm, the Lauca figure is roughly 12 to 38 times slower ^[2].

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 ^[7]; every slower history fits equally well. And it describes rock uplift in one sector at one moment, 21.9 million years ago ^[2][6].

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 ^[13]. 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" ^[12].

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" ^[15]. The eruption sits about 12 million years before the earliest date proposed for that late rise ^[3].

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 ^[19]. 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].