From Snow to Firn to Glacier ice

Snow

How do we build a glacier? We start with a snowflake. Snow, over time, is compressed into firn, and then into glacier ice.

Snow falls in cold regions, such as mountain tops or in polar regions. In glaciology, snow refers to material that has not changed since it fell1.

Snow is very light and fluffy, and has a very low density. If the snow is wetter, it will have an increased density. Snowflakes have a hexagonal structure, and fallen snow has a significant amount of air in it.

Snow flakes by Wilson Bentley. Bentley was a bachelor farmer whose hobby was photographing snow flakes. ; Image ID: wea02087, Historic NWS Collection ; Location: Jericho, Vermont ; Photo Date: 1902 Winter. From Wikimedia Commons

Firn

Firn is usually defined as snow that is at least one year old and has therefore survived one melt season, without being transformed to glacier ice.

Firn is transformed gradually to glacier ice as density increases with depth, as older snow is buried by newer snow falling on top of it. Year after year, successive accumulation layers are built up. In the accumulation zone of a glacier, density therefore increases with depth; the rate depends on the local climate and rate of accumulation1. Firn transforms to glacier ice at a density of 830 kg m-3.

New snow (immediately after falling, calm conditions50-70
Damp new snow100-200
Settled snow200-300
Wind-packed snow350-400
Firn400-830
Very wet snow and firn700-800
Glacier ice830-923
Typical densities (kg m-3). From Cuffey and Paterson, 2010.
A scientist collecting snow and ice samples from the wall of a snow pit. Fresh snow can be seen at the surface and en:glacier ice at the bottom of the pit wall. The snow layers are composed of progressively denser en:firn. Taku Glacier, Juneau Icefield, en:Tongass National Forest, en:Alaska. From Wikimedia Commons

Firn transforms to glacier ice in 3-5 years in the temperate Upper Seward Glacier in the St Elias Mountains near the Alaska-Yukon border. Firn becomes ice at a depth of about 13 m1. At sites like this with rapid snow accumulation, the depth of a firn layer, and the load on it, increases rapidly with depth.

However, in cold, dry East Antarctica, firn becomes ice at a depth of 64 m at Byrd and 95 m at Vostok. 280 years are needed at Byrd, and 2500 at Vostok. Low temperatures slow the transformation. Temperatures at Vostok, the coldest region of Earth, are 30°C lower than Byrd, which explains the slower increase in density. In addition, slow accumulation gives slow burial, and a small load each year; the increase in density takes much longer.

Typically, the transformation of firn to ice takes 100-300 years, and a depth of 50 – 80 m1.

Glacier ice

Firn becomes glacier ice when the interconnecting air or water-filled passageways between the grains are sealed off (“pore closure”)1. Air is isolated in separate bubbles. This occurs at a density of 830 kg m-3. The air space between particles is reduced, bonds form between them, and the particles grow larger. This is a process known as sintering. Increasing pressure compresses the bubbles, placing the enclosed air under pressure and increasing the density of the ice2.

Fresh snowflakes, which have a complex shape, have a large surface area. Over time and under pressure, the surface area is reduced, the surface is smoothed, and the total surface area is reduced. Fresh, complex snowflakes are transformed into rounded particles.

Formation of glacier ice. Luis Maria Benitez, Wikimedia Commons

The transformation of firn to ice is much faster where there is melting and refreezing2.  Meltwater can percolate downwards, infilling porespaces, and the displaced air escapes upwards. If the snow is under 0°C, the water will freeze, producing areas of compact ice.  This will produce high density ice much more rapidly than in colder regions without melting.

The density of pure glacier ice is usually taken as 917 kg m-3. This strictly is only true at 0°C and in the upper layers of ice sheets and mountain glaciers; the density may be greater at the mid-depth ranges in polar ice sheets, where there are no bubbles and temperatures are -20°C to -40°C1.

Below 4 km of ice, such as at the centre of the East Antarctic Ice Sheet, the pressure would increase the density to 921 kg m-3.

Bubbles

Bubbles are common in glacier ice. Bubbles can contain liquid water or atmospheric gases, making them very useful for ice core research. The air in the bubble largely reflects the atmospheric concentrations when the ice formed1. In polar environments, bubbles in the ice occupy about 10% of the volume when firn turns to ice.

img_8369
Glacier ice with many bubbles exposed on the ice shelf. It is melting and thinning rapidly.
img_7878
Close up of white bubble-rich ice. Note the sharp junction between the coarse-clear ice and bubble-rich ice.

With greater depth in polar ice sheets, bubbles shrink as the overlying ice increases. The gas pressure within the bubbles therefore increases, and at certain depths, the gas attains a dissociation pressure. The bubbles begin to disappear as the gas molecules form clathrate hydrates1.  This process takes thousands of years.

Debris

Glacier ice contains various impurities in tiny amounts. By most scales, glacier ice is a very pure solid-earth material, because the processes leading to snowfall on a glacier – evaporation, condensation, precipitation – act as a natural distillation system1.

However, glaciers can contain impurities. The dirtiest glaciers are mountain glaciers, where debris can fall directly onto the ice surface. On ice sheets and glaciers, dust and other debris may blow onto the ice surface.

cimg0063-2
Iceberg laden with debris from a glacier, Antarctic Peninsula

Debris on the ice surface can affect the absorption of energy at the ice surface, and lead to increased or decreased melting.

unnamed-glacier-4
Supraglacial debris on Unnamed Glacier, James Ross Island, Antarctic Peninsula

Layers in the ice

Glaciers are composed of sedimentary layers in their accumulation zones, formed of annual layers of snowfall. These layers are initially parallel to the glacier surface. This is the primary stratification in structural glaciology.

In temperate and subpolar settings, the annual sedimentary layers consist of alternating thick layers of bubble-rich ice, which originated as winter snow, and thin layers of clear ice, which are the refrozen meltwater from the summer melt season.

img_7875
Glacier ice exposed in an ice-cored moraine. Note the foliation with coarse clear ice and white bubble-rich ice.
Primary stratification on a glacier on James Ross Island, Antarctic Peninsula.

Debris horizons may form, when summer melting concentrates debris (such as rockfall and wind-blown dust) on the ice surface.

In cold polar regions, annual layering forms instead by seasonal variation of snow metamorphism and wind deposition1.

This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration, Wikimedia Commons.

Blue glacier ice

Glacier ice is blue because the longer visible wavelengths are absorbed. The more energetic, blue, wavelengths are scattered back2.  The effect is greatest with deep, basal ice, which is bubble free and has large crystals. The blue colour tends therefore to be most intense in the calls of calved icebergs or fresh fractures.

Rough, weathered ice and fresh snow will appear white because preferential absorption does not occur.

prince-gustav-channel-iceberg
This iceberg is formed from basal glacier ice. It is blue an has basal dirt. Differential melting forms holes all over its surface.

Further reading

References

1.           Cuffey, K. M. & Paterson, W. S. B. The Physics of Glaciers, 4th edition. (Academic Press, 2010).

2.           Benn, D. I. & Evans, D. J. A. Glaciers & Glaciation. (Hodder Education, 2010).

Glacial ArcGIS Stories

There are many ArcGIS Story Maps around. Some are better than others; some take too long to load or are not well thought through. But some are excellent.

Glaciation: past, present and future

This great ESRI Storymap introduces glaciers in the present day and at the Last Glacial Maximum. It uses shapefiles from resources like GLIMS or the Randolph Glacier Inventory, and the Global LGM shapefiles from Ehlers and Gibbard. It’s well made and up to date. It also uses the BRITICE map to introduce glacial landforms across the British Isles and the LGM and Younger Dryas in Britain.

Glacier lake hazards in Alaska

This ArcGIS Story by the Alaska Climate Science Centre is better than most. It’s all about glacier hydrology and glacier lakes in Alaska. The videos and pictures are well made, and interspersed with explanatory figures.

Alaska Climate Science Centre ArcGIS Story Map

Disappearing Glaciers

A nice, well illustrated, introduction to glacier recession and mapping glacier change over recent decades.

Disappearing Glaciers Story Map

The recession of Glacier National Park Glaciers

This is a great introduction to using imagery to track and map glacier recession from 1966 to the present day.

Recession of Glacier National Park Glaciers

Glacial Landforms Story Map

This ESRI Story Map introduces a host of glacial landforms. It was a little slow to run for me, though.

Glacial Landforms Story Map

An Introduction to Sea Ice

A lovely storymap that introduces and illustrates sea ice, with illustrations of how it changes with the seasons at both poles.

ESRI StoryMap: An Introduction to Sea Ice

Mapping Mount Everest

This storymap, by Alex Tait from the National Geographic Society, tells us all about Mount Everest and how we map it, with some beautiful graphics.

Mapping Mount Everest

Glacial Landforms of Snowdonia

A straightforward storymap that highlights the glacial landforms in Snowdonia.

Snowdonia Story Map

Connecticut’s landscape is the story of glaciers

Learn about the glacial landforms of Connecticut.

Connecticut’s glacial landscape

Further reading

Eskers

This page was contributed by Dr Frances Butcher from Sheffield University.

What is an esker?

Eskers are ridges made of sands and gravels, deposited by glacial meltwater flowing through tunnels within and underneath glaciers, or through meltwater channels on top of glaciers. Over time, the channel or tunnel gets filled up with sediments. As the ice retreats, the sediments are left behind as a ridge in the landscape.

Esker in the foreland of Hørbyebreen, Svalbard. Panel A looks down the esker from its headward end, which is marked with an arrow in panel B. Panel B shows another view of the esker from its side, and the glacier that formed it in the distance. Image credit: Jakub Ondruch. Based on Storrar et al.1
A cross-sectional exposure through a branch of the Kinnity esker adjacent to Knockbarron Wood, Co. Offaly, Ireland (53.1114°N, 7.7435°W). Image credit: Frances Butcher

Eskers are important, because they can tell us about how ice sheets and glaciers behaved. They can tell us about meltwater, and help us reconstruct the former ice surface, and the orientation of the glacier’s snout.

What do eskers look like?

Eskers are usually metres to tens of metres high, and tens to hundreds of metres wide e.g., 2,3. In cross-section, their shape can be sharp-crested (triangular), round-crested (semi-circular), flat-topped (trapezoid), or multi-crested (having two or more crests).

Changes in cross-sectional shape along an esker in Finland. The colours show elevation
as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland

Eskers can range in length from hundreds of metres to hundreds of kilometres. The individual esker ridges that formed beneath the huge, continental-scale ice sheet that covered North America, for example, can extend up to ~100 km in length. Groups of aligned ridges can form fragmented esker chains up to ~300 km long4. Similarly long eskers in Scandinavia were formed by the Eurasian Ice Sheet.

Why are eskers important?

Eskers that formed in subglacial tunnels are valuable tools for understanding the nature and evolution glaciers and ice sheets. They record the paths of basal meltwater drainage near to the ice margin.

The weight of the overlying ice means that the subglacial meltwater is under high pressure. It can therefore flow uphill! This means that, on a local scale, eskers commonly go uphill and climb up local topography.

An esker climbing over hills (including drumlins recording an earlier right-to-left ice flow direction) in Finland. The colours show elevation as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland.

The path taken by the pressurised meltwater in subglacial channels is controlled mostly by the slope of the ice surface, rather than the slope of the bed. Eskers therefore tend to be oriented parallel to ice flow, and transverse to the ice terminus. As a result, the path of an esker section can be used to reconstruct the slope of the ice surface, and the orientation of the ice terminus at the time of its formation.

An esker in Finland, which terminates in a major ice-marginal moraine deposited by the Eurasian Ice Sheet. The ice terminus (white dashed line) was parallel to the moraine, and perpendicular to the esker. Ice flow was from bottom right to top left. The ice sheet terminated in a large ice-dammed lake when the moraine formed5. Where the esker entered this lake, it deposited a sediment fan, which forms part of the moraine. The colours show elevation as measured from aerial Light Detection and Ranging (LiDAR) at 2m horizontal resolution. The underlying shaded-relief map shows the shape of the land surface as generated from the elevation values, and is artificially illuminated from the top right of the image. Data credit: National Land Survey of Finland.

Eskers produced by the last North American and Eurasian Ice Sheets probably record the final retreat of those ice sheets as climate warming increased the rate of meltwater production towards the end of the Pleistocene. Therefore, by studying eskers, we can better understand how glaciers and ice sheets respond to climate warming.

These palaeoglaciological insights are essential for predicting the responses of the contemporary Antarctic and Greenland Ice Sheets to human-induced climate change, and their potential contributions to sea level rise.

Where do eskers form?

Eskers are abundant across the land that was once covered by the former North American (Laurentide) Ice Sheet6, the Eurasian Ice Sheet6, and the British-Irish7 Ice Sheet. You can explore British Ice Sheet eskers using Britice map.

A map of large eskers in Canada, which were deposited by the former Laurentide Ice Sheet. Redrawn by Butcher 2019(8) from Storrar et al. 2013(6).

Subglacial eskers that formed in subglacial meltwater channels (termed R-channels, which are incised upwards into the basal ice) are the most common among those preserved on palaeo-ice-sheet beds. Good examples of more recently formed eskers are seen, for example, at Breiðamerkurjökull in Iceland2, and Høybyebreen in Svalbard8.

In the embedded Google Map below, look for the raised ridges of the eskers that formed in front of Breiðamerkurjökull. The zig-zagging eskers are largely in the direction of flow, whereas the moraines are parallel to the ice margin.

Eskers on paleo-ice-sheet beds are more abundant in areas of crystalline bedrock with thin coverings of surficial sediment than in areas of thick deformable sediment e.g., 9,4. This is because meltwater flowing at the bed is more likely to incise upwards into the ice to form an R-channel where the bed is hard; where the bed is deformable, meltwater is more likely to incise downwards10.

How long does it take to form an esker?

The timescales over which eskers form is a key topic of ongoing debate. Long eskers extending hundreds of kilometres over paleo-ice-sheet beds are not thought to have formed ‘synchronously’ i.e. at a single moment in time in continuous conduits extending deep into ice sheet interiors. Rather, their formation is thought to have been ‘time-transgressive’, with eskers ‘growing’ at their headward ends as their parent glaciers and associated meltwater conduits retreat across the landscapee.g., 11,12.

Under this mechanism, meltwater conduits need not extend more than a few tens of kilometres into the ice interior, beyond which the weight of the overlying ice would make it hard to form stable drainage conduits. Long esker systems may therefore take hundreds to thousands of years to form12

Shorter eskers (hundreds of metres to tens of kilometres in length) could form synchronously, possibly over periods of days-to-weeks, during high-magnitude drainage events such as glacial outburst floods13,14.

About the Author

Dr Frances Butcher is a planetary scientist researching glaciers on Earth and Mars. She completed her PhD entitled ‘Wet-Based Glaciation on Mars’ at the Open University (UK) in 2019. She is currently a member of the European Research Council (ERC) funded PALGLAC project at The University of Sheffield (UK), using glacial landforms on Earth to reconstruct the dynamics of the former Scandinavian Ice Sheet. Frances has been involved in preparations for the ESA-Roscosmos ExoMars (‘Rosalind Franklin’) Rover mission, which launches to Mars in 2022.

You can follow Frances on Twitter @fegbutcher

You can also follow the PALGLAC project @palglac

Dr Frances Butcher

References

1.         Storrar, R. D. et al. Equifinality and preservation potential of complex eskers. Boreas 49, 211–231 (2020).

2.         Storrar, R. D., Evans, D. J. A., Stokes, C. R. & Ewertowski, M. Controls on the location, morphology and evolution of complex esker systems at decadal timescales, Breiðamerkurjökull, southeast Iceland. Earth Surf. Process. Landf. 40, 1421–1438 (2015).

3.         Perkins, A. J., Brennand, T. A. & Burke, M. J. Towards a morphogenetic classification of eskers: Implications for modelling ice sheet hydrology. Quat. Sci. Rev. 134, 19–38 (2016).

4.         Storrar, R. D., Stokes, C. R. & Evans, D. J. A. Morphometry and pattern of a large sample (>20,000) of Canadian eskers and implications for subglacial drainage beneath ice sheets. Quat. Sci. Rev. 105, 1–25 (2014).

5.         Stroeven, A. P. et al. Deglaciation of Fennoscandia. Quat. Sci. Rev. 147, 91–121 (2016).

6.         Storrar, R. D., Stokes, C. R. & Evans, D. J. A. A map of large Canadian eskers from Landsat satellite imagery. J. Maps 9, 456–473 (2013).

7.         Clark, C. D. et al. BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet. Boreas 47, 11-e8 (2017).

8.         Butcher, F. E. G. Wet-Based Glaciation on Mars. (The Open University, 2019).

9.         Rattas, M. Spatial Distribution and Morphological Aspects of Eskers and Bedrock Valleys in North Estonia: Implications for the Reconstruction of a Subglacial Drainage System Under the Late Weichselian Baltic Ice Stream. Geol. Soc. Finl. Spec. Pap. 46, 63–68 (2006).

10.       Clark, P. U. & Walder, J. S. Subglacial Drainage, Eskers, and Deforming Beds Beneath the Laurentide and Eurasian Ice Sheets. Geol. Soc. Am. Bull. 106, 304–314 (1994).

11.       Hooke, R. L. & Fastook, J. Thermal conditions at the bed of the Laurentide ice sheet in Maine during deglaciation: implications for esker formation. J. Glaciol. 53, 646–658 (2007).

12.       Storrar, R. D., Stokes, C. R. & Evans, D. J. A. Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet. Geology 42, 239–242 (2014).

13.       Burke, M. J., Woodward, J., Russell, A. J., Fleisher, P. J. & Bailey, P. K. The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska and Skeiðarárjökull, Iceland. Geol. Soc. Am. Bull. 122, 1637–1645 (2010).

14.       Burke, M. J., Brennand, T. A. & Perkins, A. J. Transient subglacial hydrology of a thin ice sheet: insights from the Chasm esker, British Columbia, Canada. Quat. Sci. Rev. 58, 30–55 (2012).

The Younger Dryas Glacial Map

Welcome to the Younger Dryas Glacial Map! Here, you can explore the glaciation of the UK during the Younger Dryas glaciation. In the UK, this period is also called the “Loch Lomond Stadial”.

At this time (12,900 to 11,700 years ago), there was a period of abrupt cooling. Glaciers began to grow again in much of upland Britain. There was a large ice field, running the length of the Western Highlands in Scotland. This icefield was surrounded by numerous smaller icefields, ice caps, valley glaciers and cirque or niche glaciers. Glaciers also grew in Snowdonia, the Brecon Beacons, the Lake District, and on the Hebridean Islands.

Glaciation of Britain during the Loch Lomond (Younger Dryas) Stadial. From Bickerdike et al., 2018

Younger Dryas Glacial Map

This glacial readvance left behind a very distinctive geomorphological imprint on the UK. You can explore these data using our Younger Dryas Glacial Map! This is an ArcGIS Online Map that shows the geomorphological evidence for glaciation and the reconstructed glaciers and ice caps.

The Younger Dryas Glacial Map includes all the geomorphological information relating to the Younger Dryas glaciation of the UK. The data were compiled in a series of papers by Bickerdike et al. (2016, 2018a, 2018b). Here, we have hosted these shapefiles on ArcGIS Online and made them publically available. Click the image below to launch the Younger Dryas Glacial Map.

Click the image to launch the Younger Dryas Glacial Map

How to use the map

Launch the Younger Dryas Glacial Map by clicking here.

Read the information on the Spash Screen and then say OK. You will see a map of the UK (as the figure above).

Zoom in and out

You can zoom in on a selected area by pressing Shift and drawing a box with your mouse (SHIFT and DRAG). Click the Home icon on the left hand tool bar to return to the default extent. You can also zoom by scrolling with your mouse, or by using the + and – buttons.

Tools and toolbars

There are a number of icons in the left hand corner. You can hover over them in the webmap for an explanation.

Icons in the Younger Dryas Glacial Map.

On the vertical bar, the + and – buttons allow you to zoom in and out, and the House returns you to the full map extent. The circle shows you where you are. The square box makes the map full-screen; press escape to return. The left and right arrows take you to the past or next extent.

The five horizontal buttons are, in turn, Measure, Basemap, Legend, Layer, and Information (same as the Splash screen). You can turn layers on and off using Layer List, and fiddle with their transparency and other settings.

You can view the Metadata by clicking on the three little dots to the right of each layer in the Layer List.

Change the Basemap

If you wish, you can change the Basemap to a digital terrain model, satellite imagery, or some other kind of basemap. Change it to “Light Grey Canvas” to speed up drawing time.

Try zooming in, and then changing the Basemap to “Satellite Imagery” to see high-resolution satellite imagery of your field of view.

Choose “Terrain with Labels” to see a digital terrain model.

Popup information

Try zooming in over some features on an area of the map (press shift and draw a square with the mouse). Clicking a feature will select it, and bring up a popup. The Popup has the attribute information (including the reference of the original authors who first mapped the feature), and a description and photograph of a typical example.

Popup for Hummocky Moraine, with the satellite imagery basemap

Attribute tables

Zoom to a new area and once the map has loaded, click the little black tab at the base of the screen. This will bring up the Attribute Table for all the layers. By default, they are filtered to the map extent. You can therefore view the attribute data held for each layer in the current map view.

Try zooming over a small area and viewing the attribute data for each type of landform.

Attribute table for Moraine Ridges in the current view.

Select Tool

The Select Tool is in the top right corner. You can use this tool to select different landforms, and then view them in the attribute table. You may wish to turn Basemap to “Light Grey Canvas” if drawing time is slow.

Tick the check box in the Select Tool drop-down menu for the layer that you want to select. Then draw a box with your mouse to select some features.

You can view the selected features in the Attribute Table, by clicking on the three little dots on the right of the Select drop-down menu.

Using the Select tool

Geomorphological Data in the Younger Dryas Map

The glacier outlines were reconstructed in Bickerdike et al (2018a) using a number of key landforms: moraines, meltwater channels, drift limits, and trimlines. From these data, Bickerdike et al. (2018a) reconstructed the ice limits and the extent of ice-dammed lakes. The geomorphological evidence has been organised into a series of shapefiles, each containing a different landform type.

The database contains over 95,000 individual features, which are organised into thematic layers and each attributed to its original citation. The evidence includes moraines, drift and boulder limits, drift benches, periglacial trimlines, meltwater channels, eskers, striations and roches moutonneés, protalus ramparts and ice-dammed lakes.

Younger Dryas Glacial Geomorphology in Scotland (from Bickerdike et al., 2018a).

You can view the entire geomorphological database in this PDF map (Bickerdike et al., 2016).

Moraines

There are three moraine shapefiles in the Younger Dryas Glacial Map. Moraine ridges are the most abundant; these are the moraine ridges that were deposited at the terminus of glaciers during the Younger Dryas glaciation of Britain.

Some researchers represent the shape and distribution of individual moraine mounds as polygons, whilst others record only ridge crests as line features or just general areas of ‘hummocky moraine’. These different styles of mapping cannot be reconciled within a single layer in ArcMap, and so are split into separate layers accordingly (‘Moraines (detail)’, ‘Moraine_Ridges’ and ‘Moraine_Hummocky_Area’).

Generally, in areas where mapping of individual features overlapped areas of general ‘hummocky moraine’, only the detailed features were digitised to prevent the database becoming too cluttered.

Younger Dryas moraines in Coire Ardair, Scotland

Drift Limits

Drift limits denote the extent of glacier till or sediment that was directly deposited by the glacier. Drift limits can be used alongside moraines to demarcate the maximum extent of the ice limits.

Younger Dryas drift limits. From Bickerdike et al. 2018a

Trimlines

Trimlines form at the ice surface in valleys; the area below the trimline was covered in glacier ice and wasa subjected to glacial erosion. The area above the trimline was exposed above the ice surface, and subjected to frost-shattering and periglacial processes. This leads to a distinct difference in appearance in bedrock above and below the trimline. This can help to reconstruct the ice surface.

Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Photo credit: J. Martin.

Meltwater channels

Meltwater channels were formed by the passage of water and can form under the glacier (subglacial), in front of the glacier (proglacial) or on the valley side, between the glacier and any valley sides (ice-marginal).

Ice-marginal meltwater channels can extend up onto plateaus, where they form extensive networks. Ice-marginal meltwater channels often form where the ice is thin and cold-based, and frozen to the substrate. Subglacial drainage was inhibited, for at least some of the melt season.

Meltwater channels can therefore help to determine the location of the ice margin, and also determine the nature and style of glaciation.

From Bickerdike et al., 2018a. Meltwater channels in the Monadhleith mountains, Scotland.

Activities

Using the instructions above, allow the students to explore, interact with, and get used to the map. Encourage them to use all the functions and investigate the glacial landforms themselves.

Does anyone live near to any glacial landforms? Has anyone been to anywhere on holiday that might have been glaciated during the Younger Dryas glaciation?

Encourage the students to use the popups to work out which was the biggest icefield, and which were second and third largest. How big were they? Use the Measure tool to measure the length of the icefields.

Zoom in to an area with detailed moraine ridges, like the terminal moraines in the Lake District. Turn off the icefields by unchecking “Younger_Dryas_Extent” in the Layer List. Turn on the satellite imagery and see if the students can view the landforms for themselves.

Ask the students to think about the different kinds of glacial landysystems they can find in the Younger Dryas of Britain. For example, is there evidence for a cirque landsystem? What about a glaciated valley?

What are the processes active in these different landsystems? What does this tell you about the style of glaciation?

References

Bickerdike, H. L., Evans, D. J. A., Ó Cofaigh, C., & Stokes, C. R. (2016). The glacial geomorphology of the Loch Lomond Stadial in Britain: a map and geographic information system resource of published evidence. Journal of Maps, 12(5), 1178–1186. https://doi.org/10.1080/17445647.2016.1145149

Down the PDF map from Bickerdike et al. 2016.

Bickerdike, H. L., Evans, D. J. A., Stokes, C. R., & Ó Cofaigh, C. (2018a). The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review. Journal of Quaternary Science, 33(1), 1–54. https://doi.org/doi:10.1002/jqs.3010

Bickerdike, H. L., Ó Cofaigh, C., Evans, D. J. A., & Stokes, C. R. (2018b). Glacial landsystems, retreat dynamics and controls on Loch Lomond Stadial (Younger Dryas) glaciation in Britain. Boreas, 47(1), 202–224. https://doi.org/10.1111/bor.12259

Introduction to Glaciofluvial Landforms

“Fluvioglacial” means erosion or deposition caused by flowing meltwter, from melting glaciers, ice sheets and ice caps. Glacial meltwater is usually very rich in sediment, which increases its erosive power.

Fluvioglacial landforms include sandar (also known as outwash plains; they are braided, sediment-rich streams that drain away downslope away from a glacier), kames and kettles, meltwater channels, and eskers.

Glaciofluvial outwash plain

Glaciofluvial systems are characterised by strong changes in flow magnitude and frequency. Flow magnitudes can fluctuate strongly on a daily basis, as melt increases and decreases over day and night. It also fluctuates seasonally, in the summer (ablation) and winter (accumulation) seasons.

Glacier meltwater can flow supraglacially (on top of the glacier ice), englacially (within the glacier ice), subglacially (below the glacier ice) and proglacially (in front of, and away from, glacier ice). Surface meltwater can reach the bed by draining through the bases of crevasses and moulins.

Fig. 1
A schematic illustration of a land-terminating section of the Greenland ice sheet, highlighting the main meltwater pathways and stores in the hydrological system. Critical areas of uncertainty regarding the future evolution of the hydrological system are numbered as discussed in the text: (1) darkening of the ice sheet, (2) surface firn densification processes, (3) surface to bed connections at higher elevations, (4) cryo-hydrologic warming, (5) rates of channelisation at the ice bed interface, (6) subglacial sediments and till deformation, and (7) basal melt rates. All of these processes are also relevant to tidewater glacier systems. From Nienow et al., 2017.

These pages outline some of the key glaciofluvial landforms associated with the passage of glacial meltwater. For more information, see Glacier Hydrology.

Glaciofluvial landforms

Glaciofluvial landforms are landforms created by the action of glacier meltwater. They can be erosional, or depositional landforms, and can form underneath, on top of, in front of, and around the edges of former glaciers.

Introduction to glacial landforms

Glaciers are one of the most powerful forces shaping our local landscape. As glaciers flow downhill from mountains to the lowlands, they erode, transport, and deposit materials, forming a great array of glacial landforms. They can erode mountains, and change their morphology. Large glaciers and ice sheets can deposit great swathes of sands and gravels, forming swarms of hills called drumlins. Ice sheets deposit great thicknesses of glacial tills, and glaciers and ice sheets form moraines at their terminus. These pages will explain these concepts in more detail.

Erosional glacial landforms

In their upper reaches, glaciers can erode bedrock by quarrying, pucking, abrasion and polish. Rocks and debris embedded in the ice scratches the rock below.

Polished and striated bedrock in Utah. Photo credit: Bethan Davies

The photograph below shows ice-scoured, smoothed bedrock in Greenland. The passage of the ice, with lots of debris embedded in it, has scratched and abraded the rocks, making them smooth. Over time, roche moutonnees develop, which are smooth on one side but have a blunted downstream face.

Bethan Davies standing on ice-scoured bedrock in Greenland. She is pointing in the direction of former ice flow.
Polished and striated bedrock in Utah. Note the plucked face on the down-stream end. Photo credit: Bethan Davies

This erosion creates deep hollows in mountain sides, called cirques. Multiple cirques on a mountain may cause a pyramidal peak as they form back-to-back. Cirques are one of the most visual and characteristic glacial landforms of glaciated mountains.

Classic glacial cirque basin. Cwm Clyd in the Glyderau mountains of Snowdonia. Image from GoogleEarth.

Larger glaciers can excavate a glacial trough, which has a parabolic, or U-shape.

The parabolic glaciated valley of Glen Coe, Scotland. Photo credit: Bethan Davies

Trimlines on the valley side mark out the former ice surface. The area below the trimline was smoothed by the passage of glacier ice. In the photo below, the trimline was formed during the “Little Ice Age”, when the glacier reached the moraines visible in the bottom of the photograph. The valley side above the trimline was not glaciated at this time, and so is more vegetated and weathered.

Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Photo credit: J. Martin.

Depositional glacial landforms

Lower down, in the ablation zone, deposition becomes more important and shear stress lessens. This can lead to the deposition of vast thicknesses of unsorted sediments called tills.

Two tills
Two tills rest on top of Magnesian Limestone bedrock at Whitburn Bay, overlain by deformed glaciofluvial sands (sands deposited by a proglacial river). Note the large, faceted boulders at the boundary between the two tills.

Around the margins of the glacier, lateral moraines may develop in the ablation zone, and terminal moraines may form at the end of the glacier. In front of the ice margin, there may be small scale streamlined ridges called flutes.

The ridges in the forefield of this glacier are moraines. They are made up of debris carried by the glacier, and deposited in ridges at its terminus.
Push moraines and flutes in a recently deglaciated glacier forefield.
Esmeralda Moraine. A subaerial terminal push moraine with symmetrical sides. Photo credit: Bethan Davies

Under larger ice masses such as ice sheets, drumlins may form. These are elongated hills made up of glacial sediments (sands, gravels, boulders, unsorted muds) that form in the direction of ice flow.

Drumlin at Holwick, Teesdale. Photo credit: Bethan Davies

Under faster-flowing ice streams, mega-scale glacial lineations may form. These landforms are important for telling us about directions and dynamics of ice flow under former ice sheets.

Belgica Trough Mega Scale Glacial Lineations, Antarctic Peninsula.

Glaciofluvial landforms

Glaciers are wet. Temperate glaciers, which are those in more moderate climates and that have meltwater at their base, produce huge volumes of water each melt season. This results in a characteristic suite of glaciofluvial landforms.

Meltwater stream on Mendenhall Glacier, Alaska. From: Gillfoto, Wikimedia Commons

All this water produces a whole suite of glacial landforms. These include eskers (ridges of sediments that form underneath a glacier), kame terraces, sandar (braided gravel-rich outwash streams), and meltwater channels.

A sinuous esker ridge, and several smaller eskers, mapped from satellite imagery in the Lago Cochrane-Pueyrredón valley. Copyright: J. Bendle.

Meltwater may cut meltwater channels underneath and around the margins of the glacier. Ice-marginal meltwater channels usually form around sub-polar glaciers where water cannot get underneath the glacier, which is frozen to its bed. They therefore form in a lateral position, between the glacier and the valley flank, or around the snout. These meltwater channels can therefore mark out the position of the former ice margin.

Lateral meltwater channel in Lunedale, Pennines. Image credit: Bethan Davies.

The glaciofluvial rivers that drain away from glaciers are typically very laden with sediment. If the valley floor is quite low-angle, as is common in glacial valleys, then the river tends to form a braided pattern, with bars of gravelley sand forming. These rivers are very active and their form changes regularly. These features are called sandars.

Glaciofluvial outwash from Nef Glacier, Patagonia

Glaciolacustrine landforms

Many temperate glaciers terminate in glacial lakes, which results again in a characteristic suite of glacial landforms. Lakes may form in front of glaciers, occupying the glacial overdeepening, and may be dammed by moraines, by the ice itself, or by bedrock.

An ice-dammed lake on the northern margin of the Russell Glacier, in western Greenland.

Moraine-dammed and ice-dammed lakes may be susceptible to hazardous Glacial Lake Outburst Floods.

Rapid growth of glacial lakes in the Bhutan-Himalaya in response to retreating glacier termini. Photo: NASA/USGS, Wikimedia Commons.

Sediments in glacial lakes may be varved, with winter and summer layers being laid down each year.

Varves in a glacial lake. Photo credit: Jacob Bendle.

The location of former glacial lakes may be marked out by shorelines, raised deltas, beaches, and grounding line fans or morainal banks.

Raised delta in Patagonia. The high flat delta top formed when the lake was higher, due to ice damming the outflow. The lower delta is forming in the lake today.

Glacial landsystems

The types of glacial landforms that are generated are particular to glacier flow, basal processes, the substrate (soft and deformable? Hard crystalline bedrock?), the basal driving stress and thermal regime, and the ice temperature.

There are diagnostic landforms associated with wet-based sheet flow, ice streams, and surging ice. These diagnostic suites of landforms are called glacial landsystems.

Ice streamsSurging ice Sheet flow Cold-based ice
Mega scale glacial lineations (MSGLs)Looped medial morainesMarginal / subglacial / glaciofluvial domainsMay be very little modification of previous landforms
Progressive elongation of landforms down-iceThrusted end morainesPush, dump, squeeze morainesSmall glaciotectonic structures
Trough mouth fansConcertina eskersSubglacial till, flutes, drumlins, overridden morainesSome deposits with a coarse, sandy to boulder-gravel texture.
Till, glaciotectonised sedimentsTill, glaciotectonite, complex till stratigraphiesRoche moutonnees, striated and polished bedrockLittle evidence of fluvial reworking, but aeolian reworking may be common.
Drumlins, meltwater channels, terminal moraines, grounding linesCrevasse-squeeze ridges; flutingsTill, glaciotectonite 
“Sticky spots” (bedrock bumps/cold-based ice/dry bed)Hummocky moraineSandur, eskers, kame terraces,  proglacial lakes, braided channels, pitted outwash 

In these pages, you can learn more about glacier erosional and depositional landforms. There are case studies to illustrate the key points.

Once you have a grounding in the different kinds of glacial landform, take a look at the Glacial Landsystems pages, where the different suites of landforms that make up characteristic glacial landscapes are highlighted.

You can learn about the techniques that researchers use to understand these landforms, including geomorphological mapping and chronostratigraphy (dating glacial landforms).

There are sections of the website here on the characteristic landforms associated with the last Antarctic Ice Sheet, British-Irish Ice Sheet and the Patagonian Ice Sheet, and even glaciers on Mars.

Meltwater channels

What are meltwater channels?

Each year, glaciers melt. Meltwater channels are erosional features, cut into rock and sediment by flowing water beneath or close to ice-sheet margins1,2. They can cut sizeable troughs, meaning that they are very visual indicators of the location of the former ice margin.

Meltwater channels can therefore be used to work out the location of the ice margin, and help understand the pattern of glacial retreat3, especially in places where moraines are limited. They can also give glacial geologists insights into the thermal regimes of past glaciers. They formed commonly in Britain during the Younger Dryas glaciation.

Different kinds of meltwater channels

Subglacial meltwater channels

Surface meltwater can flow on the surface of the glacier, and perhaps penetrate to the bed of the glacier. The water here may form subglacial meltwater channels.

Supraglacial meltwater stream on Mendenhall Glacier, Alaska. From: Gillfoto, Wikimedia Commons

Meltwater can flow under the glacier ice, or around its margins. Where the water can reach the bed of the glacier, it may form a subglacial meltwater channel. These typically have an undulating long profile, and may descend down the slope at an angle. They may not have current streams in them, and can form complex systems that split and meet up again3.

Meltwater propagates to the glacier bed through crevasses and moulins

These subglacial meltwater channels may also be called Nye Channels. Again, these are erosional, subglacial features that are cut into bedrock and sediments, underneath the glacier.  These features can range from a few tens of metres to thousands of metres long1.

The largest subglacial channels are called Tunnel Valleys. Tunnel valleys can be up to 100 km in length.

Subglacial meltwater channels can form networks, similar to those that form on ground today. Flow is driven by pressure gradients as well as elevation, so these channels can flow uphill and therefore have undulating long profiles1, that go up and down. Channels do tend to avoid high points on the bed, so at a local scale, they will avoid bedrock highs, and will usually form at topographic low points or cols.

Subglacial meltwater channels are associated with temperate, wet-based glaciers that have ice at melting point at their bed2. The presence of subglacial meltwater channels can therefore tell investigators something about the thermal regime of the past glacier.

Ice-marginal / lateral meltwater channels

However, if the stream cannot reach the bed, perhaps because the glacier is frozen to its bed (polythermal or cold-based glaciers), the streams may flow to the sides of the glacier, and follow the glacier edge (the lateral margin), or the snout. Meltwater is forced to go around the glacier1.

In this case, it the water may cut ice-marginal meltwater channels. This is most common for cold-based subpolar or polythermal glaciers, where the meltwater cannot reach the base of the glacier as it’s frozen to its bed. Subglacial meltwater channels do not form here2.

Ice-marginal meltwater channels can track the position of the glacier snout. Once the glacier is gone, the ice-marginal meltwater channels are left behind, often forming nested sequences that mark out the former ice margin (e.g., see geomorphological map below).

The map below shows meltwater channels that formed on a high plateau in the Monadhliath Mountains in Scotland, during the Younger Dryas glaciation4,5. These meltwater channels formed in an ice-marginal position, and where moraines are absent, can be used to trace the ice margin (the snout of the glacier).

Meltwater channels in the Monadhliath Mountains. Most of the icefield here was reconstructed by Bickerdike et al. (2018) using meltwater channels. Image credit: 6

Ice-marginal channels can also lie in a lateral position, such as along a valley flank (e.g. see schematic cartoon below). An example of a lateral meltwater channel is shown below in the photograph of a meltwater channel from Lunedale, in the Pennines.

Formation of meltwater channels in the lateral and subglacial positions. The upper picture depicts a valley glacier, with lateral meltwater channels forming at the ice margin, and subglacial meltwater channels forming in the valley bottom. The lower picture gives a detailed view of the formation of lateral meltwater channels.

Instead of flowing downhill, lateral meltwater channels are parallel with contemporary contours, and can form a series of channels, parallel to each other3. They are perched on valley sides and may form networks. Lateral meltwater channels can terminate abruptly, or may end in downslope chutes.

Lateral meltwater channel in Lunedale, Pennines. Image credit: Bethan Davies.

Lateral meltwater channels may form in a marginal or sub-marginal position. Marginal meltwater channels form between the glacier and the valley side, but sub-marginal ones form just underneath the glacier, again in a lateral position3.

Lateral meltwater channels have helped to determine the ice surface of an ice stream in the Vale of Eden (Pennines, UK), during the last phases of the last glaciation7. In the figure below, yellow meltwater channels are the subglacial channels that formed underneath the glacier. Lateral meltwater channels formed at the ice margin against the valley side, with blue ones forming at the ice surface and the red meltwater channels being submarginal ones, formed just under the ice.

Ice-marginal, sub-marginal and subglacial meltwater channels in the Vale of Eden7.

Proglacial meltwater channels

In front of the glacier, water drains away downslope through proglacial meltwater channels. Meltwater channels may form a braided pattern, and cut through and dissect moraines. Here is an example of some proglacial meltwater channels that formed on the outwash deposits in front of the North Patagonian Icefield during the Last Glacial Maximum.

Proglacial meltwater channels cut into the surface of outwash deposits, and dissecting moraine ridges (right)

Proglacial meltwater channels form most commonly where there is abundant meltwater in temperate settings. The meltwater drains away downslope, away from the ice margin2.

Meltwater here usually contains a high volume of sediment, and they typically degrade into a network of shallow, sediment-floored channels separated by gravel bars. These collectively make up an outwash plain or sandar.

Glaciofluvial outwash

Summary

Meltwater channels vary greatly in terms of availability of meltwater, glacier thermal regime, and the local geology or availability of sediment. They can form subglacially, ice-marginally or laterally, or proglacially. They can be used to help understand the position of a former ice margin, and give insights into the glacier’s thermal regime.  

Further Reading

References


1. Benn, D. I. & Evans, D. J. A. Glaciers & Glaciation. (Hodder Education).
2. Atkins, C. Meltwater Channels BT – Encyclopedia of Snow, Ice and Glaciers. in (eds. Singh, V. P., Singh, P. & Haritashya, U. K.) 735–737 (Springer Netherlands, 2011). doi:10.1007/978-90-481-2642-2_351
3. Greenwood, S. L., Clark, C. D. & Hughes, A. L. C. Formalising an inversion methodology for reconstructing ice-sheet retreat patterns from meltwater channels: applications to the British ice sheet. J. Quat. Sci. 22, 637–645 (2007).
4. Boston, C. M., Lukas, S. & Carr, S. J. Overview of Younger Dryas glaciation in the Monadhliath Mountains. in The Quaternary of the Monadhliath Mountains and the Great Glen: Field Guide (eds. Boston, C. M., Lukas, S. & Merritt, J. W.) 41–58 (Quaternary Reseach Association, 2013).
5. Boston, C. M., Lukas, S. & Carr, S. J. A Younger Dryas plateau icefield in the Monadhliath, Scotland, and implications for regional palaeoclimate. Quat. Sci. Rev. 108, 139–162 (2015).
6. Bickerdike, H. L., Evans, D. J. A., Stokes, C. R. & Ó Cofaigh, C. The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review. J. Quat. Sci. 33, 1–54 (2018).
7. Davies, B. J. et al. Dynamic ice stream retreat in the central sector of the last British-Irish Ice Sheet. Quat. Sci. Rev. 225, 1–21 (2019).

Geography Southwest

GeographySouthwest.co.uk is a resource hub for students and teachers. It has a range of resources at primary, secondary (KS3, KS4, post-16) and University level.

It includes lesson plans and resources, powerpoint presentations, and more.

The aim of the website is to promote geography and geographical education, and is a collaborative project driven by enthusiastic geographers who have volunteered their time to create a wide-ranging and dynamic resource.

The site is free at point of access with no subscription charges.

The site’s creators are working with academics to increase the exposure of geographical resources, with co-hosting and authoring new resources.

Mass Balance teaching resources

This page highlights some of the excellent teaching resources available for exploring glacier mass balance.

For more ideas, see the Resources for Teachers page.

World Glacier Monitoring Service (WGMS)

The World Glacier Monitoring Service (WGMS) provides a host of resources for the reference glaciers monitored for mass balance.

World Glacier Monitoring Service

There are annual mass balance reports, and these are presented with some clear graphics showing cumulative glacier mass balance.

Annual mass balance of reference glaciers with more than 30 years of ongoing glaciological measurements. From the WGMS
Global_Cum_MB
Cumulative mass change of reference glaciers. Cumulative values relative to 1976 are given. From the WGMS.

WGMS Glacier Browser

The WGMS have produced a browser where you can view the mass balance records of different reference glaciers. The map is based on ArcGIS Online and allows students to explore reference glaciers worldwide.

WGMS Fluctuations of Glaciers Browser

The numbers in the circules highlight the number of types of measurement and glaciers in each area. As you zoom in, you can click through the individual glaciers and see a graph of mass balance observations, surges, and front variations over time.

This resource allows you to explore observations on glaciers worldwide, and examine the dataset easily to see if glaciers really are receding.

Raw mass balance data

These data are all available in the following publication:

WGMS (2020, updated, and earlier reports). Global Glacier Change Bulletin No. 3 (2016-2017). Zemp, M., Gärtner-Roer, I., Nussbaumer, S. U., Bannwart, J., Rastner, P., Paul, F., and Hoelzle, M. (eds.), ISC(WDS)/IUGG(IACS)/UNEP/UNESCO/WMO, World Glacier Monitoring Service, Zurich, Switzerland, 274 pp., publication based on database version: doi:10.5904/wgms-fog-2019-12.

The WGMS provides mass balance data, which could be used by students to plot as graphs or as data: https://wgms.ch/latest-glacier-mass-balance-data/. You can also download and explore the full database.

Case study: Bahia del Diablo, Vega Island, Antarctic Peninsula

An example of a student exercise could be to look at the Mass Balance Point data for a single year for a single glacier from the WGMS dataset, and plot elevation against balance for each point. Students could then plot a graph of elevation against mass balance and get the mass balance gradient through time. These are plotted in the WGMS Bulletin for comparison.

An example could be Glaciar Bahia del Diablo on Vega Island on the Antarctic Peninsula. This is a reference glacier that has been monitored since 2009.

James Ross Island and Vega Island, northern Antarctic Peninsula.

Glaciar del Diablo is on the northern side of the island, and is a land-terminating glacier.

Using the mass balance point data from the WGMS, students could attempt to plot the net balance for each point over certain years.

Net balance versus altitude from Glaciar Bahia del Diablo. WGMS 2020

OGGM Glacier Simulator

You can explore more about glacier mass balance using the OGGM Glacier Simulator.

This is an interactive web application that allows you to learn about how glaciers flow, shrink and grow, and what parameters influence their size.

The webpage has information and guided tutorials.

USGS Lemon Creek Glacier

The USGS has an excellent resource on the mass balance of Lemon Creek Glacier, a World Reference Glacier.

Lemon Creek Glacier, Alaska

This has resulted in a publication showing a reanalysis fo the USGS Benchmark Glaciers (O’Neel et al., 2019). Point data were collected at each glacier over many years. These point datasets allow glacier-wide mass balance to be calculated. The datasets are rather complicated, probably too much so for post-16 education, but the “Glacier-wide solutions” spreadsheets could be used to calculate glacier mass balance from annual winter and summer balances.

The data are available for download from the Alaska Science Centre.

Students could also use the dataset from the WGMS to plot glacier mass balance gradients over time.

References

O’Neel, S., McNeil, C., Sass, L., Florentine, C., Baker, E., Peitzsch, E., . . . Fagre, D. (2019). Reanalysis of the US Geological Survey Benchmark Glaciers: Long-term insight into climate forcing of glacier mass balance. Journal of Glaciology,65(253), 850-866. doi:10.1017/jog.2019.66

WGMS (2019): Fluctuations of Glaciers Database. World Glacier Monitoring Service, Zurich, Switzerland. DOI:10.5904/wgms-fog-2019-12. Online access: http://dx.doi.org/10.5904/wgms-fog-2019-12

WGMS 2020. Global Glacier Change Bulletin No. 3 (2016-2017). Zemp, M., Gärtner-Roer, I., Nussbaumer, S. U., Bannwart, J., Rastner, P., Paul, F., and Hoelzle, M. (eds.), ISC(WDS)/IUGG(IACS)/UNEP/UNESCO/WMO, World Glacier Monitoring Service, Zurich, Switzerland, 274 pp., publication based on database version: doi:10.5904/wgms-fog-2019-12. PDF (20 MB)