Rock Glaciers Move Mountains - Perhaps Right Under Your Skis
Shortcut URL: https://serc.carleton.edu/69112
A rock glacier is an geomorphic feature that includes ice and talus. An "active" rock glacier meets two important criteria: 1) it contains ice currently and 2) it is moving and deforming, traveling downhill faster than the regions around it. As a rock glacier loses ice and/or stops moving, it becomes "inactive". In some cases, a rock glacier has a consistent body of internal ice that is then covered by a mantle of rock talus from a nearby slope. The rock layer plays an important role in insulating the internal ice and preventing it from melting away during each summer. The ice in a rock glacier deforms and allows the rock glacier to flow. Rock glaciers can be distinguished from the rocky area around them by their lobate shape that is formed by their motion (Fig. 1). The layering of rock and ice are not necessarily consistent across rock glaciers, and there is still notable debate about the origin of the ice in rock glaciers: is it old ice that remains from a previous glacier or new ice formed by modern-day precipitation? The answer may not be the same everywhere.
Lone Peak Rock Glacier
Hundreds of rock glaciers exist in the Northern Rocky Mountains of the U.S., including in southwest Montana, where Lone Mountain is located. There is geomorphic evidence that ten distinct rock glaciers existed recently on Lone Mountain. Though most of them are inactive, there may be ice in more than one at the present day. The largest Lone Mountain rock glacier, known as the Lone Peak rock glacier (LPRG) (Fig. 1), is the most active today and once stretched about 1.5 kms in length (though it is not active today across this whole length). The LPRG was first studied in the early 1970s. At the time, the region had just been slated for development as a ski resort and it was important to document the characteristics of the land surface before the transition. More recently, researchers from Montana State University have probed the LPRG with ground-penetrating radar and seismic refraction surveys to look at the internal structure of the rock and ice, used digital elevation models to examine the surface shape, and excavated part of the talus mantle to get ice samples (Fig. 2 and Fig. 3).
The Lone Peak rock glacier has at least two morphologically distinct regions in regards to the surface expression of the rock glacier. The uppermost section is active today and has a transverse ridge morphology, which is commonly observed in roughly one third of the glaciers in the U.S. Northern Rockies, including southwest Montana. These transverse ridges look like waves, creating ridges and troughs that stretch across the glacier, perpendicular to the downslope flow direction (somewhat visible in Fig. 1 and 2). The lower segments of the rock glacier are not active today and, because the rocks in this area are no longer moved along by internal ice flow, lichen and grass have been able to grow in these segments.
The Montana State University researchers were able to describe some of the important characteristics of the LPRG (Fig. 2). Using ground-penetrating radar, they found that the rock glacier may be quite thick; they did not find a bedrock surface despite collecting data to depths of up to 100 m. As for the top of the rock glacier, their research revealed about 2-3 m of unfrozen, unsorted talus, and more in other parts of the rock glacier. Underneath this top mantle there is a frozen layer of ice and debris, with some sections of the ice containing less debris than other layers. The thickness of this internal ice remains a question for future research.Understanding the motion of the LPRG is particularly important because it is located in the middle of a popular ski resort. In fact, the base station for the Big Sky Resort's ski tram is built on top of the toe of the active section of the rock glacier (Fig. 4). Thanks in part to the early research on the rock glacier, engineers were aware of it when designing and constructing the ski tram. In building the structure, they had to drill down into the rock and ice to create the foundation. The structure actually moves along with the rock glacier, traveling ~20 cm each year. As a result, the resort has to plan for changes to their infrastructure, like lengthening the ski tram cable, due to the rock glacier motion.
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