What is a debris-covered glacier, and why do I care about them?
When people think of a glacier they most likely picture a spectacular glittering white and blue mass of ice wending its way down a mountain valley, but in many mountain ranges, the lowest reaches of big glaciers are covered by a continuous layer of sand, gravel and rock. These glaciers are known as debris-covered glaciers, and they are particularly common in the mountains of high Asia (Kääb et al., 2012).
The debris-covered Ngozumpa glacier in the Khumbu Himal is the longest glacier in Nepal. The dark peak in the middle background is Mt Everest, known locally as Sagarmatha, and in Tibet as Chomolungma. Map and photo are from the BBC (http://www.bbc.com/news/science-environment-16317090) and GlacierWorks (http://more.glacierworks.org/glacier/ngozumpa-glacier/) respectively.
This debris cover arguably makes the glaciers look less impressive than cleaner glaciers – I’ve certainly heard some disappointment voiced from tourists who climb up the glacier moraine from Gokyo to get their first glimpse of the mighty Ngozumpa Glacier (see photo), and wonder why they are looking out onto what looks like a choppy grey sea of rocks. More importantly though, the debris cover changes the way the glacier behaves in response to climate conditions, with implications for predicting local meltwater supply, how glaciers will change in the future and what these glaciers are contributing to ongoing sea level rise. This matters because the glaciers of high Asia constitute the largest concentration of glacier ice outside the polar regions and yet remain relatively poorly understood (Bolch et al., 2012).
How does the surface cover of rock debris affect the glacier?
Field and laboratory studies have shown that a thin debris cover causes ice to melt faster than it normally would, while beyond a certain thickness of debris cover the ice melt rate decreases with increasing debris thickness. This can be readily understood if we consider that melting ice at the surface of a glacier is dependent on the amount of energy supplied to the ice surface. The dark surface of the debris cover means that a higher proportion of incoming solar radiation is absorbed at the surface of the debris than at the surface of bright exposed glacier ice. When the debris cover is thin, this extra energy is efficiently transferred to the ice beneath, causing accelerated melt. When the debris cover becomes thicker, however, the energy absorbed at the surface is used to heat the rock material and much of this heat is re-released to the atmosphere at night, so that much less of the energy delivered to the surface reaches the underlying ice, and consequently melt rates are reduced compared to exposed ice.
The relationship of debris cover thickness and melt rate can be readily observed in its extreme form on glacier surfaces where thin is found melted into hollows called cryoconite holes (left: Cluster of cryoconite holes on Vadret da Morteratsch, Grisons, Switzerland. Photo J. Alean) and thick blocks of rock form glacier tables (right: Glacier table on Vadret Pers, Grisons, Switzerland. Photo August 2000, J. Alean). Photos from Glaciers Online (http://www.swisseduc.ch/glaciers/)
As the glacier is like a conveyer belt for the rock debris it carries, the debris is concentrated, and becomes thicker towards the end of the glacier, which effectively protects the glacier terminus. This means that debris-covered glaciers can survive for longer at lower altitudes than neighbouring clean ice glaciers. This can be easily seen in the landscape, where the clean ice glaciers are only found high above the debris-covered glacier tongues.
A number of scientifically interesting consequences arise as a result of how the debris cover protects the glacier terminus, which have been nicely summarized in the context of eastern Himalayan glaciers (Benn et al., 2013).
- Firstly, the terminus position of a debris-covered glacier under a given climate is not at the same elevation as a clean ice glacier would be. This is significant if one wants to use former glacier terminus positions (marked by moraine deposits) as a stand-in record for climate conditions.
- Secondly, in addition to slowing melt near the terminus, the general trend in debris thickness means that thin debris at the upper end of the debris cover increases the melt there, and the glacier long profile becomes concave over time. This in turn causes the ice flow to slow down and the glacier terminus can become stagnant.
- Thirdly, the surface lowering to a concave profile, in conjunction with the large confining marginal moraines, makes it difficult for meltwater to drain off the glacier surface, and instead small meltwater ponds form in hollows in the rough terrain.
At the moment, the behaviour of debris-covered glaciers is only just beginning to be taken into account in regional models of runoff generation and glacier change, and much remains to be learned about these glaciers. For example, the description I gave above of how a continuous debris cover alters surface melt rate is not the whole story, as the debris cover is usually not continuous, but instead debris slides off steeper slopes and exposes the underlying ice, which can then melt very fast as this exposed ice is very much out of its comfort zone, with the surrounding debris re-emitting plenty heat, and depositing dust onto the ice, both of which increase the melt of exposed ice. Building on pioneering work (Sakai et al., 2000) these processes are now being studied intensively by scientists trying to figure out how important they are to the glacier as a whole.