Rivers of rock
High in the Aragonese Pyrenees, above 2,200 meters, the area contains features that appear to be slow-moving rivers of rock. These formations, known as rock glaciers, are tongue-shaped lobes of angular boulders and debris that creep downslope at a few centimeters to a meter per year. They are piles of stone; concealed within is a core of ice, a remnant of permafrost from colder climatic periods like the Little Ice Age or even the much older Younger Dryas period, which ended around 11,700 years ago.
The Pyrenees host dozens of these landforms, particularly in the highest massifs such as Posets, Maladeta, and Infiernos. These rock glaciers are the southernmost examples of mountain permafrost in Europe, making them highly sensitive indicators of climate change. The thick layer of rocky debris is an insulating layer, protecting the ice core from the summer sun and slowing its melt. This insulation allows permafrost to persist in a region where surface temperatures are now often too high to support it. Geophysical surveys using techniques like electrical resistivity and ground-penetrating radar confirm the presence of these frozen masses deep within the rock glaciers' structure.
Drilling for ancient climate data
Scientists from organizations like the Pyrenean Institute of Ecology (IPE-CSIC) are studying these unique formations to understand past climates and predict future changes. By drilling deep into the rock glaciers, they extract ice cores that contain a frozen record of environmental history. Trapped within the ice are bubbles of ancient air, pollen, and other organic matter that show what the atmosphere and landscape were like thousands of years ago. Some of the ice discovered in Pyrenean caves and rock glaciers dates back over 6,000 years.
This research is urgent. The Pyrenees have experienced a temperature increase of about 1.3°C since 1949, a rate faster than the global average. This warming is causing the ancient ice to melt at an accelerating pace. The degradation of this permafrost means the loss of a climate record but also increases the risk of natural hazards. As the ice that binds the rock debris together thaws, it can destabilize steep mountain slopes, potentially leading to more frequent rockfalls and landslides. Projects like PERMAPYRENEES are currently working to monitor the thermal state of the permafrost and assess the risks associated with its thaw.