An explosive volcanic eruption in northern Chile nearly 22 million years ago buried an ancient Andean landscape under more than 1,260 cubic kilometers of hot ash, pumice, and gas, creating a geological time capsule that reveals how slowly the mountains rose, according to a study published in Science Advances.
How the Cardones Ignimbrite Preserved Northern Chile’s Ancient Topography
The catastrophic event, which researchers compared to a massive scale version of the Pompeii disaster, emplaced the enormous Cardones ignimbrite during the early Miocene about 21.9 million years ago, according to Byron Adams and Frances Cooper of UCL Earth Sciences. As fast-moving pyroclastic density currents swept across the terrain, the resulting deposits exceeded a kilometer in thickness in certain areas and spread over a region several times larger than a major metropolitan center. Rather than obliterating the geological record, the thick volcanic blanket froze valleys, ridges, and river networks in place.
Because the upper surface of the Cardones ignimbrite originally sloped at a gentle angle of about 1.5 degrees with an uncertainty of roughly 0.3 degrees, it established a physical limit on the terrain underneath, according to the study’s geometric models. For the volcanic flows to completely bury the preexisting topography without major peaks projecting through the deposit, the underlying land could not have possessed steep relief.
Did you know? The Cardones eruption produced roughly 300 cubic miles of volcanic material. Its surface originally rose less than 8 feet over the length of a football field.
Landscape Evolution Modeling Reveals Slow Rock Uplift Rates
To determine what lay beneath the volcanic cover, Adams, Cooper, and their colleagues Clementine Walsh and Katharine Cashman constructed two-dimensional computer models simulating river networks cutting into rising terrain over long periods, according to findings reported by Earth.com. The research team generated 560 synthetic landscapes by varying rock uplift rates from 0.1 to 2 kilometers per million years while adjusting how easily rivers could erode the bedrock.
When the team restricted erosion efficiency to values matching the region’s actual climate and bedrock, compatible landscapes required average rock uplift rates below approximately 0.26 kilometers per million years, or roughly 2.6 centimeters every century, according to the study. The slow Chilean uplift rate demonstrates that the Andes in this region grew gradually over millions of years rather than surging upward rapidly late in their history.
Implications for the Long-Term History of the Central Andes
The speed of Andean mountain building has long sparked debate among geologists, with competing theories proposing either a steady 40-to-50-million-year rise or a sudden acceleration during the last six to 10 million years, according to Adams. The buried Cardones landscape provides an independent physical constraint that supports the slow and steady hypothesis for the middle of that timeline.
Traditional techniques, such as thermochronology tracking mineral cooling histories as rocks move upward, often rely on chemical clocks and complex thermal models. In contrast, the volcanic-landscape approach uses physical geometry, asking what topography could have existed beneath a dated geological reference plane. The models also indicate that developing such subdued terrain required at least 14 million years to reach steady-state relief.
Frequently Asked Questions
How old is the Cardones ignimbrite deposit?
The Cardones ignimbrite formed approximately 21.9 million years ago during the early Miocene epoch, according to research published in Science Advances.
What was the estimated uplift rate of the Andes based on the study?
Researchers calculated that the average pre-eruption rock uplift rate remained below approximately 0.26 kilometers per million years, or about 2.6 centimeters per century.
Where is the Cardones ignimbrite located?
The enormous volcanic deposit is located in northern Chile, associated with the Lauca Caldera and deeply incised by the Lluta River.
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