CR
Cryospheric Sciences

Glacial geomorphology

Image of the Week – Looking to the past for answers

Figure 1 – The lateral moraines of the Khumbu Glacier, Nepal (A+B). Taken from the true right of the glacier and the confluence with Changri Nup/Shar. A shows the original photo; B shows the annotation highlighting different moraines. Numbers assigned based on distance from glacier tongue. Dots represent where rock samples were collected from moraine crests. Yellow circle highlights walkers for scale. [Credit: Martin Kirkbride (photo), mapping and sample collection completed by Jo Hornsey]

We’re only just really starting to comprehend the state and fate of Himalayan glaciers due to a scarcity of research along the monumental mountain range. Climbers and scientists have been observing these lofty glaciers since the 1900s. However, is that looking back far enough? Glacier moraines, featuring in this Image of the Week, can reveal change extending back thousands of years.


You may look at Figure 1 and think ‘what is that?! It’s a mess!’ and you would be right to do so. The only glacier ice visible is where ice cliffs break the debris-covered surface of Khumbu Glacier (Figure 2), which begins in the Western Cwm. If you let your eyes adjust to the medley of rocks and many shades of brown, you can start to pick out lines and shapes. Some are highlighted by the sunlight whilst others take a more discerning eye. If your eyesight is very good, you can spot the people on a path in the lower right area (highlighted by the yellow circle), which give a sense of scale to this landscape. These huge mounds of rock and debris (called moraines), though appearing messy and interwoven, are vital pieces of evidence which show how much the glacier is shrinking; extending thousands of years back beyond the satellite record or human observations.

But why the mess?

The young Himalayan fold mountains produce huge amounts of debris due to the extreme weather and ongoing orogeny. The summer monsoon also provides significant amounts of intense precipitation, which erodes slopes and sediment, causing a highly mobile landscape, and a continual cacophonous supply of rocks and sediment to the Khumbu Glacier (Figure 2); creating a surface blanket known as debris cover. This debris alters how the glacier would normally melt and results in the surface of the glacier lowering through time, rather than the terminus retreating as so often seen on ‘clean’ ice glaciers. Though data collection is improving constantly, access to the Khumbu (nearly a week’s trek with several days of altitude acclimatisation) limits the range of monitoring techniques available and reduced oxygen controls your ability to collect data. Whilst there is observational data, such as satellite imagery and observations from explorers/scientists during the 1900s, it is limited in temporal and spatial resolution. This is where my research on glacier moraines comes in.

A longer time frame

I have spent the last year and ¾ (I am a PhD student; I am counting every second!) mapping the landforms which these great bodies of ice leave behind. This mainly consists of mapping lateral moraines (Figures 1 and2) as these represent the height of the glacier surface at the time it built the moraines and can be used to reconstruct a patterns of glacier evolution. These landforms have differing patterns (size, shape, preservation etc) between glacial valleys, sometimes even within the same valley, telling us that there are local and regional differences in glacier behaviour. To uncover when this all happened, I did what every person thinks I do when I tell them my PhD is based in geography; I went to collect rock samples from the glacier moraines (Figures 1 and 3).

Figure 2 – Mapped moraines. The moraines are identified by the different coloured lines (higher numbers represent older moraines). The dots represent the areas where rocks were sampled for dating. The contemporary glacier outlines were taken from the Randolf Glacier Inventory GLIMS data set. The Digital Elevation Model was taken from the High Mountain Asia 8m resolution data set (Shean, 2017). [Credit: Mapping of moraines and sample collection completed by Jo Hornsey].

Rocks can tell the time?

Well, no; they can’t. But using a technique known as Exposure Dating, we can assign an age to a rock surface if we’re sure that surface has been in that position on that landform since it was put there by the glacier. This means, if we choose rocks on the crests of the moraines (similar to the one I am stood on in Figure 3), we can interpret the age we get from that rock as the time that the glacier surface was there building that landform. If we do it for the mapped moraines of the Khumbu Glacier (Figure 2), then we can start to build a timeline of glacier recession. Thanks to studies in the 1980’s  and to a couple done in the early 2000’s (e.g. Richards, et al., 2000; Finkel, et al., 2003), we’re pretty confident that the most glacier proximal landforms were built around 500 years ago during a hemispheric wide cooling event. This event was significant enough to build the towering lateral moraines which are significantly larger than those mounds you can see bordering them in Figure 1. Importantly, this event occurred before the western industrial revolution. Therefore, by looking at moraine building events before this, we can recreate how glaciers were before humanity’s dependence on fossil fuels developed, i.e. a time where glaciers were able to reach a point of stability and build landforms. By including smaller, distal moraines as well as the mammoth slopes of those most proximal to the glacier, we can construct a chronology of the Khumbu Glacier’s behaviour over the Late Holocene (2500 years ago to present in the Himalaya), into the last point of stability, and onto the behaviour we see today.

So, then what?

I’m glad you asked. Once we have a chronology for the glacier’s behaviour, we can start to compare it to the modern-day behaviour. This can be done using direct comparisons between glacier extent and thickness or using glacier models. Models can be useful as they are able to recreate the behaviour between the moraine building phases. I will be applying my chronology to a dynamic glacier model known as iSOSIA, one that was adapted to be able to simulate the development of debris covered glaciers. The chronology will act as parameters within the model (so that it knows the moraines must be built by a certain time), and the model can then recreate the glaciers behaviour whilst it was building these striking landmarks. We can use the model to improve our understanding of how these glaciers have become debris covered, what they would be like if they weren’t, and what might happen to them as climate change continues.

What is this all for?

Having travelled to the Khumbu valley and spent days staggering around on the debris-covered glacier trying in vain to catch my breath, the sense of our impact on the world hit home quite dramatically. Whilst the Khumbu valley is a particularly busy valley, you’re still days away from any form of infrastructure. The moment you travel off the well-worn path, it’s the most incredibly peaceful landscape. Not because it’s silent; the glacier is constantly making noise as it slowly flows down the valley and debris shifts around the surface. It is because it is entirely natural. When there are no helicopters to be heard, in that moment, you could be the only person alive. In the face of an ever-expanding world, I believe it is important to protect and preserve these natural spaces, and those dependent on them.

Humans will not last forever, or even for a long time in the grand scheme of things, but if we’re not careful, our impact might. I don’t think future populations of any species deserve that.

Figure 3 – Looking back up the valley whilst I contemplate a rock. Pumori summit can be seen to the left of the photograph, and Nuptse summit can be seen on the right. Existential questions on a postcard please. [Credit: Martin Kirkbride]

Further Reading

Edited by Scott Watson


Jo Hornsey is a PhD student at the Department of Geography in Sheffield, UK. She is researching the changing extent of Himalayan glaciers over the last 2500 years with specific focus on the Little Ice Age event ~500 years ago; dating the patterns of glacier retreat in the Khumbu Valley; using this information to improve accuracy of the iSOSIA model; and applying IPCC Climate Scenarios to analyse future glacier behaviour in the Khumbu Valley. In her downtime she can be found walking, climbing, running, acting, playing Dungeons and Dragons, and invariably trying to show you pictures of her cats.
Twitter – @joshornsey
Email – jhornsey1@sheffield.ac.uk

Image of the Week – When “Ice, Ice Baby” puts rocks “Under Pressure”

Image 1: Composite image of the Aiguille Verte, the heavily-fractured headwall of the Glacier d’Argentière near Chamonix, France [Credit: D. Dennis].

Bowie and Queen said it first, and Vanilla Ice brought it back. But now, I’ve set out to quantify it: Pressure. Rocks in glacial landscapes can experience many different kinds of pressure (forces), from sources like regional tectonics or even the weight of the glacier itself. Our hypothesis is that smaller-scale pressures, caused by the formation of ice in small bedrock cracks (frost-weathering), have a large effect on the sculpting of landscapes in cold regions. This post will share how we evaluate these processes and their dependence on temperature, as well as discussing the broader effects for glacier and glacial landscape evolution.


Walking through the valley in the shadow of glaciers

Growing up just outside Glacier National Park, USA, at nearly the exact edge of the former Laurentide Ice Sheet, I became familiar with the romantic lore of how we understand glacial landscapes (Images 2, 3). Observing these glacial landscapes later throughout my formal Earth science education, I came to understand mountains as passive resistors to the relentless efficiency of glacier advance, erosion, and retreat—offering evidence of past glaciations but nonetheless devoid of agency in the rise and fall of icy stadials.

My current PhD research, however, investigates a slightly-modified premise: that glaciers and their landscapes respond in concert with climate, and that dividing the dynamics governing the ice and the rock may not be as straightforward as once thought. My work is a sub-project of the Climate Sensitivity of Glacial Landscape Dynamics (COLD) project, funded by the European Research Council (ERC) and lead by Dirk Scherler at the Deutches GeoForschungsZentrum (GFZ) in Potsdam, Germany.

Image 2: The author on holiday in Glacier National Park, Montana, circa 2001, demonstrating an early aptitude for glacial geomorphology and cosmogenic nuclide geochemistry. His affinity for popular German footwear at a young age foreshadowed his eventual move to Germany to study glaciology and geomorphology [Credit: D. Dennis].

Image 3: This image of Chamonix Valley and the M. Blanc massif conceptually outlines how average annual temperature may change with elevation in steep hillslopes. The highest peaks in the massif tower up to nearly 4000 m over Chamonix Valley, which sits at appx.1000m. This corresponds to a nearly ~20 °C difference in annual average temperature. [Image adapted from Google Earth].

Temperature as a control in glacial landscapes

Glaciers exist in locations with temperatures that are, for some portion of the year, below freezing, as this is a condition required for snow to persist through the melt season and to form ice. Temperature is therefore an important primary control on the stability of glaciers. These cold temperatures, however, impact mountain environments beyond just the formation/decline of glaciers, and several decades of recent research have shown that temperature is an important controlling factor on the type and magnitude of erosion (the act of dislodging and transporting rock) in cold landscapes.

Mountain glacier valleys are commonly characterized by steep head- and sidewalls which frame the glacier within (like in our Image of the Week). At our field sites in the French, Swiss, and Italian Alps, these rockwalls can tower up to 1500 m above the surface of the glaciers, corresponding to a temperature gradient of ~10 degrees (Image 3). Therefore, the rocks at different elevations are exposed to different temperature conditions, which could lead to differences in the rate of erosion.

Image 4: Permafrost degradation and frost-weathering in the steep hillslopes of the M. Blanc massif commonly lead to the deposition of debris on the glaciers at the base of the mountains. Shown here is Glacier d’Argentière (France) with patches of surface debris [Credit: D. Dennis].

Erosion in steep rockwall faces

Frost-weathering processes occur only at temperatures at or below zero, therefore requiring the same cold temperature conditions that form glaciers. At these temperatures, liquid water present in small cracks in the bedrock freezes. The pressure exerted on the rock by the ice as it freezes causes the rock to fracture, leading to large cracks in the bedrock (Image 5). Erosion occurs when the ice in the crack becomes large enough and its corresponding fracture wide enough that the rock can no longer remain attached and it falls from the rockwall surface.

Erosion can also occur when the ice in the crack melts and no longer “cements” the surface together. Because temperatures in glacial landscapes are commonly quite cold, much of the bedrock is considered permafrost (permanently-frozen ground), and remains frozen throughout the year. In the Alps, however, warmer temperatures over the past decades have caused the permafrost to thaw, melting the lenses of ice and causing larger and more frequent rockfalls.

Temperature conditions are therefore important for both the rate at which cracks form in rocks (and erode from the surface) in addition to permafrost stability and the size/frequency of rockfalls. As temperatures change in mountain regions due to global warming, this could lead to considerable changes in debris production.

Image 5: A cropped version of our Image of the Week, showing the base of the Aiguille Verte, headwall of Glacier d’Argentière. Large fractures in the bedrock are clearly visible. These may have grown from much smaller cracks that formed due to frost-weathering.

The hillslope/glacier surface connection

After material erodes from the surface of the headwall, it is often deposited onto the surface of the glacier (Image 3). As mentioned above, the deposition of material can occur both at a constant rate or sporadically (as in the case of permafrost-thaw rockfalls), depending on the controlling process. As such, determining the actual representative rate at which these headwalls erode is challenging.

Though this work can be complicated, we believe it to be important, as debris deposited on the surface of glaciers can insulate the ice from the effects of temperature (Image 4, Video 1). Though the global distribution of debris-covered glaciers is much smaller than debris-free glaciers, debris-covered glaciers make up a non-trivial fraction of the glaciers in populated mountain regions where they may be important fresh water sources, contribute to glacial hazards, or allow for the generation of hydropower. Understanding the supply of debris to these glaciers (via erosion), and how it may change, is therefore an important component of forecasting their evolution under warming climates.

Video 1: This drone footage from the Arolla Glacier, Switzerland, shows the steep relief which can develop as a result of differential melting. Debris thicker than 2-4 cm insulate ice, leading to topographic relief on the glacier surface as exposed ice melts and covered ice is protected. [Credit: D. Gök, GFZ]

Re-evaluating the dynamic glacial landscape

Though studies of frost-cracking and debris-covered glaciers individually are not necessarily brand new inventions, our methods for combining the two are rather novel. In doing so, we are linking the evolution of glacier with the evolution of the landscape itself, and investigating an interesting feedback loop induced by changes in climate. Should erosion rates increase with warmer temperatures, and the mountains therefore supply more debris to glacier surfaces, this could extend the “lifetime” of the glacier by insulating it; likewise, if erosion rates decrease, less debris supplied to already-covered glaciers could lead to less insulation and (comparatively) higher melt rates. This interplay demonstrates the complexity of Earth system processes, and the need to take these complexities into account when considering the effects of climate changes.

To summarize

Pressure, pushing down on rock,
Pushing laterally against rock, can cause them to fall.
Under (thick) debris, glacier melt will slow,
Despite higher temperatures,
And global warming.

Will it ever stop? I don’t know.
Turn up the temperatures, then no more (ice and) snow.
At the end of the day, frost-weathering needs ice,
When water can’t freeze, ice-cracking’s no dice.

Edited by David Docquier


Donovan Dennis is a PhD student at the Deutches GeoForschungsZentrum in Potsdam, Germany. He is interested in many aspects of glaciology and glacial geomorphology, and currently investigates the geomorphic feedbacks on glacial landscape erosion. He previously worked on post-deposition alteration of stable water isotope signals in snow and ice. He tweets as @donovan__dennis.

Contact Email: dennis@gfz-potsdam.de