Volcanic Ash Trapped Ancient Forests as the Andes Mountains Were Born

An enormous volcanic eruption 22 million years ago froze a moment in Earth history, preserving the Andes in their youth. The eruption, which originated from the Lauca Caldera in northern Chile, blanketed the landscape in ignimbrite, a rock left behind after a pyroclastic flow. That flow swept over the ground like a broiling tsunami, carrying rock, volcanic gas and ash.

Now, a new study published Sept. 11 in the journal Science Advances has reconstructed what lay beneath that blanket. The researchers found rolling foothills rather than craggy peaks, and they measured how slowly the mountains rose.

What the Ignimbrite Revealed

The Andean eruption preserved a landscape in a moment frozen by volcanic deposits. The ignimbrite at Lauca Caldera is the same type of material that buried the Roman town of Pompeii after the eruption of Mount Vesuvius in A.D. 79. That eruption preserved buildings and people where they fell.

Byron Adams, a geomorphologist at University College London, described the significance in a statement. “Pompeii shows how volcanic eruptions can freeze a moment in human history,” he said. “This study shows that much larger eruptions can also freeze moments in Earth history, burying whole landscapes beneath volcanic deposits and preserving clues to how mountains were being built before the eruption.”

The Shape of the Blanket

Adams and his colleagues used the shape of the volcanic blanket to infer what was hidden beneath it. They also drew on what they know about how rivers shape mountains. Together, those tools let them estimate the slope of the original volcanic flow.

The original slope of the flow was about 1.5 degrees. From that, the researchers calculated the erosion of the landscape to figure out how fast the rocks in the area were rising. The answer was quite slow.

How the Andes Were Built

The Andes formed as the oceanic Nazca Plate pushed under the continental crust of South America. The researchers used the formation of certain minerals within rocks to determine when a particular rock rose through the crust and approached the surface. Those minerals mark the moment a rock began moving upward.

There is debate over how the Andes grew:

• Slowly and steadily over 40 or 50 million years
• Extremely slowly and then popped up more recently, in the last six to 10 million years

Adams said the new findings support the slow but steady hypothesis. “Our findings, which cover a large part of the middle of that history, support the slow but steady hypothesis,” he said.

The Method’s Reach

The same approach could be applied to other mountain ranges worldwide. The researchers noted that the technique depends on having a large-volume ignimbrite deposit that preserves a landscape before the eruption.

The paper, titled “Landscapes buried beneath large-volume ignimbrites reveal preeruptive uplift rates,” was authored by Adams, B. A., Cooper, F. J., Walsh, C., and Cashman, K. V. It appeared in Science Advances, volume 12, issue 37, with the identifier eaee5374.

The study adds to a growing body of research on mountain building. It also shows how eruptions can preserve landscapes across timescales far beyond human history.

What We Make of It

The Andean eruption produced a record that scientists can read. The ignimbrite froze a moment in Earth history, and the researchers used that moment to measure the rate of mountain building.

The slow rate of uplift — 0.16 mile per million years — is a striking figure. Over tens of millions of years, that slow rise produced the formidable range we see today.

The study supports the idea that the Andes grew steadily rather than in a sudden burst. That matters for understanding how mountains form across the planet.

The buried landscape offers a snapshot of a moment that otherwise would have vanished entirely. The researchers cannot dig down to see the buried hills. But they can see them in their minds, using the shape of the volcanic blanket and the processes that shaped it. That is a remarkable achievement.

Andes · geology · mountain building · Science Advances · volcanic eruption