On the trail of Andean paleo-avalanches in Peru’s Cordillera Blanca

The tropical Andes are strongly affected by climate change, which is reflected in the retreat of the cryosphere and the degradation of permafrost conditions. This thermo-mechanical evolution of mountain massifs is leading to a significant increase in gravity hazard in the high mountains and adjacent valleys.

Historical records show that the valleys of the Peruvian Andes have suffered severe natural disasters over the last century, with tens of thousands of victims, notably in the Cordillera Blanca during the Huaraz (1941), Ranrahirca (1962) and Yunguay (1970) disasters. All these events are linked to the close proximity of urban centers to extremely high relief areas, whose massifs are in the process of transition as a result of climate change. In addition, regional seismicity contributes to the triggering of unstable processes such as snow avalanches, rock-ice avalanches and collapses, landslides and glacial lake outburst floods.

An ISTerre team (Swann Zerathe / IRD, Benjamin Lehmann et Julien Carcaillet / CNRS) has launched a project to study the spatio-temporal impact of these extreme avalanches in the Rio Santa Valley between the Cordilleras Noires and Blanches. This work is being carried out in collaboration with the following Peruvian institutes INGEMMET(Instituto Geológico, Minero y Metalúrgico) and INAIGEM (Instituto Nacional de Investigación en Glaciares y Ecosistemas de Montaña).

The Rio Santa Valley, home to towns with populations in the tens of thousands, including Huaraz (130,000 inhabitants), is one of the areas most exposed to gravity hazards, as it is dominated by glacial massifs at altitudes of over 6,000 metres (Huandoy - 6,395 m, Huascaran - 6,768 m and Huantsan - 6,369 m). In June 2023 and 2024, field campaigns were carried out to map paleo-events and collect samples for cosmogenic nuclide dating (GTC Platform, ISterre).

These campaigns identified numerous geomorphological witnesses (multi-decametric boulders, stratigraphic sections, lacustrine sequences, etc.) to events of greater magnitude/volume than historical avalanches. The existence of a landslide of almost a cubic kilometer (or hundreds of millions of m3) initiated on the piedmont and having modified the entire morphology of the valley has also been identified.
Last but not least, evidence of ancient disasters affecting populations was discovered. These included pre-Columbian ruins covered by avalanche deposits, and downstream, an avalanche deposit rich in ceramics and bone debris, suggesting a movement over several hundred meters depending on flow directions. These field investigations and discussions with S. Wagner (Archaeologist, Wisconsin-Madison University), have enabled us to identify a site of human occupation that has suffered a catastrophe at least similar to those observed in the last century, possibly 3,000 to 4,000 years old in view of the ceramics found (Chavín culture).

The continuation of this study will be carried out at ISTerre, within the framework of Ronald Concha’s thesis, which is financed within the framework of the France 2030 program by the Targeted Project IRIMONT of PEPR Risques (ANR-22-EXIR-0003) et le CNES.