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If you have ever hiked in the high peaks of Colorado, the Wasatch Range in Utah, or the Tetons in Wyoming, you have almost certainly seen a rock glacier, perhaps without even knowing it. These are slow-moving masses of rock debris and ice that flow downhill in the same way that traditional glaciers do. However, they are covered by a thick layer of rock and boulders that can easily be mistaken for stable ground. To the casual observer, they look like stationary piles of rubble rather than dynamic ice formations.
There are at least 1,500 active rock glaciers across the western United States, and they play a critical role in the region's water security. While the iconic white, icy glaciers that people typically picture have been shrinking and, in some cases, disappearing entirely, our new study shows that rock glaciers and their frozen water remain mostly stable despite rising global temperatures. This stability is not accidental. The thick debris mantle that covers the ice acts as a protective shade, keeping the ice colder underneath. This process is similar to how ski areas in summer cover their slopes with reflective blankets to prevent melting.
As a result of this natural insulation, rock glaciers continue to provide meltwater for streams during the summer months just as they have for centuries. Crucially, they are not disappearing at the alarming rate seen in their naked-ice counterparts. This makes them vital resources for mountain ecosystems that are facing increasing heat and drought.
We are scientists who study glaciers around the world. In our new paper, we examined how different types of glaciers are changing beneath the soaring peaks of the Teton Range in Wyoming. Our goal was to understand which ice sources will remain reliable in a warming climate.
The data from the Tetons revealed a stark contrast. Between 2014 and 2022, the white, icy glaciers in the region thinned by 2.75 feet per year (0.84 meters per year). This rate of loss is about seven times faster than the average rate observed in the previous half-century. In contrast, rock glaciers were close to stable. They lost only about 0.16 feet (0.05 meters) per year during the same 2014-2022 period. Furthermore, there was no significant change in their stability relative to the earlier period of 1967-2014.
Mountain glaciers operate on a cycle where they partially melt in the summer and rebuild their mass as snow falls in the winter. However, as temperatures rise, these glaciers are losing more ice each year than they gain. Climate models project that the vast majority of glaciers in temperate mountain ranges like the Tetons will melt away completely by the end of this century. This disappearance represents the loss of a critical source of water for mountain streams and lakes, which are essential for both wildlife and human use.
However, where rock glaciers are present, the picture is different. Their protected ice will continue to release meltwater into the streams below, buffering these waterways against warming temperatures and drying conditions. Because of this sustained water supply, streams fed by rock glaciers have emerged as potentially critical climate refugia. These are places that are likely to stay cooler while everything around them warms.
This cooling effect has profound implications for biodiversity. A wide array of species already live in the cold meltwater that emerges from rock glaciers. This ecosystem includes tiny aquatic insects like stoneflies and larger predators like bull trout that rely on eating them. As traditional glaciers fade, the ties between cold-water animals and rock glaciers will likely become even tighter. These streams become safe havens for species that cannot survive in warming waters.
A prime example is the meltwater stonefly (Lednia tumana). This aquatic insect was listed under the U.S. Endangered Species Act in 2019 due to climate-related habitat loss. It previously relied heavily on the fading glaciers of Glacier National Park. However, our research indicates that the stonefly can also be found downstream of rock glaciers. These stable ice sources are likely to give the stonefly a chance of survival as other glaciers disappear. The rock glaciers act as a biological safety net for these specialized species.
Our study showed that having a major ice source feeding a stream significantly limits the warming of that stream over time. We compared the temperature changes in different types of mountain waterways over a ten-year period.
We found that streams fed by rock glaciers warmed slowly, by about 1.1 degrees Fahrenheit (0.6 degrees Celsius) over the decade. In comparison, icy glaciers warmed by about 1 degree Fahrenheit (0.9 degrees Celsius). Streams that were fed primarily by seasonal snowpack, small patches of ice, and groundwater warmed much more rapidly, by 6.1 degrees Fahrenheit (3.4 degrees Celsius) over the same period. The vulnerability of snow-fed streams was highlighted in one instance where the small snowfields feeding one of our long-term study sites largely disappeared. Consequently, the stream below it stopped flowing entirely by late summer, demonstrating the fragility of non-glacial water sources.
It is important to clarify that rock glaciers will not replace the glaciers and snowfields that are disappearing. The scale of water storage in rock glaciers is significant, but it is not infinite. A recent study estimates that rock glaciers in the region hold the equivalent of 0.6 cubic miles (2.5 cubic kilometers) of water. This volume is about one-fifth the amount contained in mountain glaciers. While this is a substantial reserve, it cannot fully compensate for the total loss of the region's glacial ice.
Furthermore, climate projections show that even rock glaciers are not immune to the effects of a warming climate. Under current warming projections, many rock glaciers could become ice-free by the end of the century. Understanding how much ice is contained in these formations and how fast they are likely to melt is vital. This information helps natural resource and land managers plan for the landscapes they will be managing later this century.
Rock glaciers also offer unique opportunities for scientific research that extends beyond Earth. They serve as analogs for studying what appear to be debris-covered glaciers on Mars. Scientists have sought to better understand these rock glacier-like features on the Red Planet, as the environmental conditions share some similarities with high-altitude terrestrial environments. Researchers are using Earth-based rock glaciers to test technology, such as the use of drone-based radar systems to measure ice and debris thicknesses. The insights gained from these Earth-based tests can be applied to interpret data from Mars, potentially revealing how much water ice might be hidden beneath Martian surface debris.
So, the next time you are out in the mountains, staring off into the distance, look carefully for these large fields of rock that appear to be flowing down the mountainside. Pay close attention to that small trickle of meltwater emerging from the toe of the rock glacier. It is a sign of resilience in a changing world.
While meltwater from rock glaciers alone certainly will not make up for the glaciers that have been lost, it could help mitigate the most severe impacts where rock glaciers persist. They are not a complete solution to the climate crisis, but they are a critical component of our mountain water infrastructure. Recognizing their value is the first step toward protecting these hidden climate havens for future generations.