Volcanic Blanket Reveals Development of Central Andes

The geologic history of the Central Andes is typically a story of erasure, where wind and water strip away evidence of how mountains actually grow. However, a massive volcanic event 22 million years ago created a rare physical archive on the western slopes of the range. The eruption of the Lauca caldera released over 1,260 cubic kilometers of gas, ash, and debris, depositing a layer of ignimbrite up to one kilometer thick. This volcanic blanket acted as a sealant, freezing the underlying paleolandscape and protecting it from millions of years of erosion. A team led by Byron Adams from University College London utilized this layer to resolve a fundamental tension in geology: the conflict between tectonic uplift and erosional wear. The central premise is that the geometry of the ignimbrite surface dictates the maximum possible steepness of the land buried beneath it. Because the volcanic layer is nearly flat—with a slope of only 1.5 degrees—the pre-existing terrain could not have possessed significantly steeper gradients. The difficulty lies in the fact that a flat landscape is an ambiguous signal. It could indicate that the mountains rose slowly, or that they rose rapidly but were immediately ground down by aggressive river erosion. To disentangle these variables, the researchers employed computer models, testing hundreds of scenarios by varying the rates of uplift against the coefficients of rock decay.