A volcanic blanket has helped to estimate the rate of growth of the Andes 22 million years ago

Geologists have used a thick layer of volcanic rock in northern Chile as a record of the ancient topography. They estimate that, prior to a major eruption 21.9 million years ago, the rocks on the western slope of the Central Andes were rising at a rate of less than 0.26 km per million years.

The result was obtained not through direct observation of the buried surface, but through modelling. The study by Byron A. Adams and his colleagues has been published in the peer-reviewed journal *Science Advances*.

Details

The authors studied the Cardones ignimbrite – solidified deposits of hot flows of ash, debris and gas associated with the Lauka Caldera. This cover formed in the early Miocene, spread over a vast area and is up to a kilometre thick in places.

The surface of large ignimbrite sheets is usually gentle. The researchers modelled hundreds of possible versions of the relief that might have existed prior to the eruption, taking into account the action of rivers and the relationship between the steepness of river channels, the slope of the surface and the rate at which the rocks rose. Only relatively gentle foothills without excessively deep valleys could have been present beneath the observed layer.

From this, the team deduced the upper limit for the rate of rock uplift — less than 0.26 km per million years, or approximately 2.5 cm per century. According to the model, a faster uplift rate would have created a steep and highly dissected landscape, incompatible with the geometry of the preserved ignimbrite. This estimate is consistent with independent thermochronological and geothermobarometric data.

Why this is important

The study supports the hypothesis of a gradual rise in this part of the Andes over tens of millions of years, rather than just a sharp rise in the last 6–10 million years. The method may complement the ‘mineral clock’: the shape of the volcanic cover preserves information about the relief over a long period prior to the eruption.

However, the conclusion applies to the Cardones ignimbrite and the western slope of the Central Andes, and not automatically to the entire mountain system. The rate of rock uplift is also not directly equivalent to changes in surface elevation: this is influenced by erosion and other processes.

There is also a methodological limitation: the buried landscape is inaccessible for direct measurement, so its shape was reconstructed using a river erosion model. Furthermore, large ignimbrites themselves tend to preserve a gentler relief. The result places a limit on the permissible uplift rate and supports one version of the history of the Andes, but does not settle the debate once and for all.

Source

Research article: Byron A. Adams, Frances J. Cooper, Clementine Walsh, Katharine V. Cashman, ‘Landscapes buried beneath large-volume ignimbrites reveal pre-eruptive uplift rates’, Science Advances, 11 September 2026. DOI: 10.1126/sciadv.aee5374.

Official press release from University College London: Giant volcanic blanket reveals hidden ‘Pompeii’ of Andean landscapes. Original news story: Phys.org.