Rock glaciers are relatively small lenses of ice or frozen sediment that are covered by large amounts of seasonally frozen rock debris. The upper debris layer is known as the ‘active layer’, whilst the core of a rock glacier may comprise pure ice or large volumes of more fine-grained sediment.
Rock debris is typically sourced from avalanches above the ice surface, as well as other mass wasting processes [1,2]. The headwall above a rock glacier is a prime source of debris as it is exposed to sub-aerial weathering processes and is typically steep and unstable, often receding by several millimetres per year 
Internal deformation of the ice or sediment is predominantly how they flow downslope, unlike other glaciers which may also slide along their bed. The flow velocity of a rock glacier is therefore very slow – between about 1 cm and 100 cm per year .
Rock glaciers are periglacial features. That is, they exist on the very fringes of glaciated landscapes, where permafrost (rather than ice) typically dominates and erosion of recently exposed rock surfaces is prevalent.
Protalus Rock Glacier
Protalus rock glaciers (or protalus lobes) are also periglacial features with very similar properties to other rock glaciers as described above. The main difference that sets them apart from other rock glaciers is that they are even more saturated with rock debris and sediment.
This sediment is often referred to as ‘talus’, which typically undergoes mass wasting on a cliff side, forming a slope at the base. Internal deformation remains the primary driver for protalus rock glacier movement downslope. They will also have the same distinct lobate structure of a rock glacier.
Genesis and Evolution
If you think the description of a rock glacier sounds like that of a debris-covered valley glacier, you are not wrong. Some theorise that a rock glacier is a ‘transitional’ stage between a debris-covered glacier and an inactive till moraine .
However, this theory does not necessarily explain the genesis of rock glaciers that are primarily made up of permafrost, including protalus lobes. In this case, the avalanching of rock debris is thought to be a crucial process in preserving ice-rich permafrost .
Importance of Rock Glaciers
Understanding these processes of rock glacier genesis and evolution is crucial for predicting the future response of rock glaciers to climate change.
Hydrological resources from glacier ice help sustain 1.9 billion people, but are under threat from climate change . The quicker the ice melts, the more rapidly this life-sustaining water disappears.
Rock glaciers are more resistant to the warming climate as the rock ‘active layer’ reduces the ablation of ice , as explained above. Given the current global volume of rock glacier ice, this means that 68-102 trillion litres of water is potentially less vulnerable to rapidly warming global temperatures .
In summary, rock glaciers are therefore not only a unique periglacial landform, but have global importance in water resource management. Their size and sluggish movement should not distract from the fact that they are constantly evolving, yet how and why is still up for debate.
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About the Author
Hi I am Alex Clark, I am a PhD student currently working on a project that combines high-resolution geomorphological mapping with new chronological evidence to reconstruct palaeo-ice dynamics in NE Ireland during the Late Glacial (~18,000-15,000 years ago). I am primarily interested in past ice-climate interactions, digital mapping (GIS), and tephrochronology, but enjoy exploring other aspects of geography and the environmental sciences in my research.