This article was written by Camryn Kluetmeier.
Rock glaciers are part of a continuum, with clean-ice glaciers at one end, debris-covered glaciers in the middle, and rock glaciers at the far end. Like glaciers, rock glaciers flow downslope, but the speed at which they flow is variable on a timescale of days, seasons and decades. Some rock glaciers are speeding up, while others are slowing down in response to climate change. This can affect how stable they are, and can affect their potential as a water resource or increase their hazard potential.
How fast do rock glaciers flow?
Like ice glaciers, rock glaciers move downslope under the influence of gravity, but there are important differences from ice glaciers in how rock glaciers deform and what factors control their deformation.
Rock glaciers flow much slower than ice glaciers, with speeds reaching a maximum of around several meters per year [1, 2]. In contrast, some glaciers can move several meters a day. Slower rock glaciers may only move several millimeters per year [3].
Active and inactive rock glaciers
Unlike ice glaciers, rock glaciers can persist on the landscape as a pile of rocks long after all the internal ice that causes movement downslope has melted away. We therefore divide rock glaciers into active and inactive forms [4].
Active rock glaciers contain enough internal ice to move downslope and often have surface features that indicate motion. A series of ridges and furrows is a common feature that makes active rock glaciers look like a lava flow or a heaping ice cream serving.
Very steep frontal and lateral slopes are also a sign of an active rock glacier because internal ice is acting like glue, holding rocks together past the angle where they would normally tumble downslope (like in the above photograph).
Inactive or relict rock glaciers no longer move downslope and have very little or no internal ice. These features often have partial vegetation cover and subdued surface topography when compared with active rock glaciers.
How do rock glaciers move?
Rock glacier movement is primarily driven by a shearing force near the base of the feature [5,6]. An example of a shearing force is pushing a deck of cards one direction at the top and the opposite direction at the bottom, causing the cards to stretch out.
This has been studied by drilling boreholes into rock glaciers and observing horizons that move much faster than the surrounding ice-rock mixture [6].
In addition to a primary basal shear horizon, some rock glaciers have a secondary shear horizon at the base of the active layer. The active layer is the top layer of a rock glacier which freezes and thaws seasonally.
Rock glaciers also experience motion due to plastic deformation of internal ice, just like ice glaciers.
Speeding up and slowing down
How fast a rock glacier moves varies across many timescales. On an hourly to daily timescale, rock glacier speed up has been linked to heavy precipitation and spring snowmelt events [8].
Seasonally, rock glacier velocities are typically marked by a strong acceleration in late spring to summer followed by a deacceleration in the late autumn. This seasonal cycle is water-controlled, with more liquid water reaching the basal shear horizon in the spring and summer [3,9].
Within the past several decades, some rock glaciers in the European Alps have destabilized, speeding up by several orders of magnitude and then ceasing to move at all [10]. This is thought to be caused by warming permafrost and air temperatures due to climate change.
Summary
Rock glacier motion is highly variable and there is still a lot left to discover about how rock glaciers move and the mechanisms that govern them. It’s important to understand rock glacier activity to gain insights into how they may respond to climate change and assess their potential as a hazard and water resource.
About the Author
Camryn Kluetmeier earned a B.A. in environmental geology from Middlebury College in 2022, completing a thesis on inventorying active rock glaciers in Utah, USA using satellite remote sensing tools. She has delved into the world of glaciology as a student on the Juneau Icefield Research Program (2022) and a Summer Student Fellow at Woods Hole Oceanographic Institution (2021).
Currently, Camryn is working as a research specialist on the calibration and validation of the NASA Surface Water and Ocean Topography satellite based at the University of North Carolina at Chapel Hill (2022-2024). She plans to eventually pursue a Ph.D. linking glacial change to shifting water resources with climate change. You can follow her on twitter at @ckluetmeier.