Somewhere beneath the Andes, a whole world has been waiting—frozen in time by a volcanic blast so huge it buried an entire landscape the way Vesuvius buried Pompeii. Now, UCL scientists have found a way to read that hidden terrain without ever digging down to it.
Picture an immense sheet of rock that erupted from a place called the Lauca Caldera in northern Chile 21.9 million years ago. It spread far enough to cover an area six times the size of Santiago, Chile's capital, and in places the rock lay a full kilometer deep. It's a type of rock called ignimbrite—layers of volcanic ash, rock fragments and gas that blasted across the land and swallowed it whole.
But why should anyone care about rock buried so long ago? Because the eruption acted like a giant photograph. It preserved the shape of the land beneath it, and that shape holds secrets about how the Andes mountains were really built.
The researchers, publishing in the journal Science Advances, ran hundreds of computer models of possible ancient landscapes. They used what we know about how rivers carve mountain ranges to test which terrains could possibly fit beneath the volcanic blanket. The answer was surprising: only gentle, low-relief landscapes—like soft foothills—could have been hidden there. This part of the Andes was not a dramatic, jagged range when the blast struck.
From that, the team worked out that the mountains here rose slowly—no more than 0.26 kilometer every million years. That's about 2.5 centimeters, or an inch, per century. If the rocks had been shoved upward any faster, the slopes would have grown too steep to tuck beneath the enormous volcanic deposits.
This supports a big idea long debated by geologists: that the Andes grew slowly and steadily over tens of millions of years, rather than popping up suddenly in just the last few million years. For contrast, parts of the Himalayas thrust upward at several millimeters to centimeters every year—hundreds of times faster.
Lead author Dr. Byron Adams of UCL Earth Sciences explains it beautifully. "This landscape was buried by a giant volcanic eruption—a little like Pompeii, but on a vastly larger scale," he said. Instead of covering a town, the hot cloud engulfed an entire countryside. And because we can't physically dig down to see what's hidden, the team used the shape of the volcanic blanket itself as a key.
The approach is a new tool for reading Earth's history. Older methods rely on chemical "clocks" in the rock that only capture specific moments. This new method can measure mountain-building across millions of years at once. The team's estimate also matched earlier findings about minerals in the rock that cool slowly as they rise through the crust.
Co-author Dr. Frances Cooper points out a bigger payoff: the Andes shape the climate of an entire hemisphere, so understanding how they formed helps us grasp long-term climate change. And the same trick could be applied to volcanic deposits elsewhere on Earth—unlocking buried landscapes we never knew existed.