Debris-covered glacier landsystems

This article was written by Katie Miles from Aberystwyth University.

Debris-covered glaciers are valley glaciers that have a layer of rocks and sediment on top of the ice surface. While this can range from a thin smattering of rocks to a thick layer several meters deep (even on the same glacier surface), we typically define a debris-covered glacier as one with a mostly continuous layer of supraglacial debris over the ablation area, usually increasing in thickness towards the terminus1,2.

Debris-covered glacier landsystems are therefore generally similar to those of clean-ice valley glaciers, but with a few key differences and unique features.

The debris-covered Chola Glacier, Nepal. The width of the image is ~1.5 km (across the glacier terminus and glacier-dammed lake).

Where are debris-covered glaciers found?

There is no geographical limitation on debris-covered glaciers3 – they are found everywhere you might find clean-ice glaciers. There are even counterparts on Mars4! The most important control on whether a glacier surface is clean or debris-covered is the supply of debris from the surrounding topography. This means that you can have different glacier surfaces even in neighbouring valleys (e.g. Glaciers Noir and Blanc, French Alps).

The debris-covered Khumbu Glacier, Nepal. The glacier is clean in the upper ablation area (mid-left of the image), with the debris cover appearing and increasing in thickness towards the terminus (ice flow is towards the bottom right of the image).

Developing debris covers

Debris can be supplied to the ablation area in a number of ways, for example:

  • Rockfalls or avalanches from surrounding hillslopes, directly onto the ablation area;
  • Slumping from lateral moraines (usually spatially limited5);
  • Melt-out of englacial debris6.

Debris can be trapped in the ice through rockfalls in the accumulation area (becoming englacial when it is buried with the annual snowfall), falling into crevasses, or being entrained at the bed and later elevated to an englacial position by differential glacier motion. The debris is transported with the ice to the ablation area, and as the glacier surface melts, the debris melts out with it and adds to the amount of debris in the surface layer.

Folded englacial debris layers, approximately parallel with the ice surface, exposed in an ice cliff on Khumbu Glacier, Nepal.

Dynamics of debris-covered glaciers

The surface melt rates of debris-covered glaciers are dependent on the supraglacial debris layer thickness and thus vary spatially. Where debris layers are thick, particularly at the terminus, the ice surface is insulated, and heat has to be transferred through the debris layer before it reaches the ice surface.

This means that debris-covered glaciers are slower to respond to shifts in climate than clean-ice glaciers, and may persist for longer in the current warming climate – making them increasingly important and valuable water resources7,8.

Melt rates are greatest when the debris layer is only a few millimetres to centimetres thick9, so mass loss tends not to occur by terminus fluctuations, as for clean-ice glaciers. Instead, mass loss occurs primarily through surface lowering (thinning), with greatest rates often in the middle of the ablation area10. This difference produces some unique surface features that are not typically found on clean-ice valley glaciers.

Looking across the surface of Khumbu Glacier, Nepal. The different melt rates according to the local debris thickness results in a “hummocky topography” punctuated by ice cliffs and supraglacial ponds.

Unique features of debris-covered glaciers

Ice cliffs are steep faces of bare, exposed ice. These might be the only way you know you’re walking on a debris-covered glacier’s ablation area!

Ice cliffs on Khumbu Glacier, Nepal. Where melt rates are high, the clean-ice faces can become covered in a thin layer of sediment by meltwater transport.

Supraglacial ponds are pools of water, ranging in diameter from centimetres to kilometres. While not typically found on clean-ice valley glaciers, they are increasingly common on low-gradient areas of ice sheets and shelves. Ponds store meltwater and delay its transport downstream11.

A supraglacial pond on Khumbu Glacier, Nepal. The pond surface is partly frozen.

The bare ice faces and warm water of these features both act to further enhance glacier melt rates, resulting in a positive feedback cycle. Ponds and cliffs are commonly found together and referred to as “hotspots” of debris-covered glacier ablation.

An ice cliff adjacent to a (frozen) supraglacial pond on Khumbu Glacier, Nepal.

As the ponds expand, they can merge, storing more water and contributing even more to glacier ablation. In some cases, this can result in one large proglacial lake12 – either at the terminus (dammed by the terminal moraine) or further upglacier where melt rates were highest and the most ponds produced.

Such a lake is then dammed by the “dead” glacier ice below, which has been cut off from the main active glacier as the lake has expanded. This ice-cored moraine can be eroded over time by the warmer lake water, potentially making the dam unstable.

The terminus and calving front of Imja Glacier into its proglacial lake, Imja Tsho. The width of the glacier front is ~0.75 km and has retreated rapidly since the formation of the proglacial lake, which measured ~2.7 km in length in 2018, filling the basin between the lateral and terminal moraines. Some recently calved icebergs are floating just in front of the glacier terminus.

Unstable proglacial lake dams, whether moraine- or ice-dammed, can occasionally produce a risk of a Glacial Lake Outburst Flood (GLOF). GLOFs can present considerable hazards to communities living downstream, though mitigation strategies are increasingly being implemented.

For example, damage from the 2016 Lhotse Glacier GLOF, Nepal, was minimal due to the construction of gabions around the downvalley village of Chukhung13. However, many proglacial lakes are stable because water leaves through an outlet channel, and in some regions of the world (such as the Karakoram), the number of these lakes is actually decreasing14.

About the Author

This article was contributed by Katie Miles from Aberystwyth University. Katie is a PhD candidate and lecturer at Aberystwyth University, specialising in glaciers and glacial hydrology. She’s interested in what happens beneath the ice surface, and her PhD research investigated this for Khumbu Glacier, a high-elevation debris-covered glacier in Nepal. You can follow Katie on Twitter @Katie_Miles_851. She has had two field seasons on Khumbu during 2017 and 2018 with the EverDrill research project.

Katie Miles

References

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2.        Miles, K. E. et al. Hydrology of debris-covered glaciers in High Mountain Asia. Earth-Science Rev. 207, 103212 (2020).

3.        Scherler, D., Wulf, H. & Gorelick, N. Global Assessment of Supraglacial Debris-Cover Extents. Geophys. Res. Lett. 45, 11,798-11,805 (2018).

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5.        van Woerkom, T., Steiner, J. F., Kraaijenbrink, P. D. A., Miles, E. S. & Immerzeel, W. W. Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya. Earth Surf. Dyn. 7, 411–427 (2019).

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8.        Immerzeel, W. W. et al. Importance and vulnerability of the world’s water towers. Nature 577, 364–369 (2020).

9.        Østrem, G. Ice Melting under a Thin Layer of Moraine, and the Existence of Ice Cores in Moraine Ridges. Geogr. Ann. 41, 228–230 (1959).

10.      Watson, C. S. & King, O. Everest’s thinning glaciers: implications for tourism and mountaineering. Geol. Today 34, 18–25 (2018).

11.      Irvine-Fynn, T. D. L. et al. Supraglacial Ponds Regulate Runoff From Himalayan Debris-Covered Glaciers. Geophys. Res. Lett. 44, 11,894-11,904 (2017).

12.      Benn, D. I. et al. Response of debris-covered glaciers in the Mount Everest region to recent warming, and implications for outburst flood hazards. Earth-Science Rev. 114, 156–174 (2012).

13.      Rounce, D. R., Byers, A. C., Byers, E. A. & McKinney, D. C. Brief Communications: Observations of a glacier outburst flood from Lhotse Glacier, Everest area, Nepal. Cryosph. 11, 443–449 (2017).

14.      Gardelle, J., Arnaud, Y. & Berthier, E. Contrasted evolution of glacial lakes along the Hindu Kush Himalaya mountain range between 1990 and 2009. Glob. Planet. Change 75, 47–55 (2011).