Almost 22 million years ago the eruption of the Lauca volcanic caldera buried a part of the Chilean Andes, covering an area of at least 1,260 km3 with ignimbrite (pyroclastic material produced by an explosive eruption). Now a study published on Science Advances analyzed the mountain of pyroclastic material that covers an area almost as large as Molise and in some places up to a kilometer thick, to investigate the Andean orogeny and reveal new details on the history of this important mountain range.
Chilean Pompeii
The study was possible because the ignimbrite behaved a bit like the ash of Vesuvius with Pompeii, preserving the landscape below for 21.9 million years: «Instead of covering a city, a mixture of volcanic ash, rock fragments and gas poured over an entire landscape, submerging the ground below», explains Byron Adams, coordinator of the research.
Since the ignimbrite was able to completely cover the landscape, creating a fairly flat surface, the reliefs below must not have been very steep. Then using computer models to simulate hundreds of different possible landscapes and linking them to river erosion and crustal uplift, the authors estimated that, before the eruption, the western Andes were rising slowly, growing at about 0.26 mm per year.
Climate and mountain ranges
For years, geologists have wondered whether the Andes rose rapidly in the last million years or gradually over a longer period: understanding the Andean orogeny more deeply is important because the mountain ranges greatly influence the climate, modifying the circulation of the atmosphere and the distribution of precipitation and, potentially, contributing to the formation of arid areas such as the Atacama Desert in Chile.
Therefore, knowing that 21.9 million years ago the western Andes were still relatively low makes us understand that their influence on the climate of the time was very different from that of today. Frances Cooper, another author of the research, concludes by stating that “the same method could be applied to volcanic deposits in other parts of the world, helping to reconstruct landscapes buried for millions of years.”
