A vast sheet of volcanic rock in the Andes has revealed clues about a landscape that existed 22 million years ago, according to a new study led by researchers from University College London (UCL). Published in the journal Science Advances, the study shows that a massive volcanic eruption, similar in scale to the one that buried Pompeii, covered an entire region in the northern Chilean Andes, preserving details of the ancient terrain that would otherwise have been erased by erosion or geological changes. The eruption came from the Lauca Caldera, a large volcanic depression formed during major eruptions, and released a type of volcanic rock called ignimbrite. This rock covered an area six times the size of Santiago, Chile’s capital, and in some places reached a depth of 1 kilometer (0.6 miles). To understand what the landscape looked like before the eruption, the researchers used computer models to simulate hundreds of possible ancient landscapes, testing which ones could have fit beneath the volcanic deposit. They found that only landscapes with low relief—similar to gentle foothills—could have been buried by the eruption. This suggests that the region of the Andes where the eruption occurred was rising very slowly, with rocks being pushed upward at a rate of no more than 2.5 centimeters (about an inch) every 100 years. If the uplift had been faster, the landscape would have had steeper slopes, which wouldn’t have fit under the massive volcanic deposits. This finding supports the idea that the Andes built up gradually over tens of millions of years, rather than rising rapidly in the last few million years. The slow uplift rate contrasts with other mountain ranges, like the Himalayas, where rocks can rise and erode at much faster rates—several millimeters to centimeters per year. Lead author Dr. Byron Adams of UCL Earth Sciences compared the volcanic event to the eruption of Vesuvius, but on a much larger scale. Instead of burying a single town, the eruption engulfed an entire landscape with a mixture of volcanic ash, rock fragments, and gas. The study also aligns with earlier research that examined minerals in the rocks, which recorded changes in temperature over the past 50 million years. These changes reflect the cooling of the rocks as they moved upward through Earth’s crust. Dr. Adams noted that this method offers a new way to estimate how quickly mountains form over long periods, unlike traditional methods that rely on chemical “clocks” limited to specific times. The researchers estimate it would have taken millions of years to create the gentle landscape that existed before the eruption. The study helps resolve a long-standing debate about the timing of Andes mountain formation. Some scientists believe the Andes rose slowly and steadily over 40 to 50 million years, while others think they rose slowly and then rapidly in the last 6 to 10 million years. This research provides support for the slow and steady growth model. Co-author Dr. Frances Cooper emphasized the importance of understanding the Andes’ development, as the mountains have a major influence on regional and global climate. The study’s methods could also be applied to other volcanic deposits around the world, helping to reconstruct landscapes that have been buried for millions of years.