Giant Volcanic Blanket Reveals a Buried Andean 'Pompeii'

A vast sheet of volcanic rock in the Andes has preserved clues to a buried landscape that existed 22 million years ago, according to a study led by University College London (UCL) researchers and published in the journal Science Advances. The study describes how a giant volcanic eruption acted like Vesuvius, but across an entire landscape, burying the terrain beneath it and preserving information that would otherwise have been lost to erosion.

The researchers examined the vast expanse of volcanic rock, called ignimbrite, that erupted from the Lauca Caldera in northern Chile 21.9 million years ago. The deposit covered an area six times as large as Chile's capital, Santiago, and reached 1 kilometer deep in places. The 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 it. Only relatively low-relief landscapes, akin to gentle mountain foothills, could have been covered this way.

From this, the researchers inferred that this part of the Andes was being built slowly, with rocks pushed upward by no more than 0.26 kilometer every million years, equivalent to about 2.5 centimeters every 100 years. That rate is slow compared with many rapidly uplifting mountain belts, including parts of the Himalayas, where uplift and erosion can occur at rates of several millimeters to centimeters per year. The finding supports the idea that the Andes grew slowly and steadily over tens of millions of years, rather than rising more recently, in the last few million years.

Lead author Dr. Byron Adams of UCL Earth Sciences said the buried landscape was preserved a little like the city of Pompeii, but on a vastly larger scale, with the researchers using the shape of the volcanic blanket to infer what lies hidden beneath it. Co-author Dr. Frances Cooper, also of UCL Earth Sciences, said the same approach could be applied to volcanic deposits elsewhere in the world to reconstruct landscapes buried for millions of years.