An introduction to Glacier Mass Balance

Glacier mass balance | Measuring mass balance | Mass balance gradients | Mass balance through time | Further readingReferences | Comments |

Glacier mass balance

The mass balance of a glacier is a concept critical to all theories of glacier flow and behaviour. It is simple enough, really: mass balance is simply the gain and loss of ice from the glacier system1. A glacier is the product of how much mass it receives and how much it loses by melting.

Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.
Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.

Mass balance can be thought of as the ‘health of a glacier’; glaciers losing more mass than they receive will be in negative mass balance and so will recede. Glaciers gaining more mass than they lose will be in positive mass balance and will advance. Glaciers gaining and losing approximately the same amount of snow and ice are thought of as ‘in equilibrium’, and will neither advance nor recede.

For clarification: when we talk about glaciers advancing, receding or being in equilibrium, we are talking about the position of their snout. Glaciers will continually flow under the force of gravity; ice is continually being moved from the upper reaches to the lower reaches, where it melts.

Accumulation zone

The glacier system receives snow and ice through processes of accumulation. Surface accumulation processes include snow and ice from direct precipitation, avalanches and windblown snow. There may be minor inputs from hoar frost. The snow and ice is then transferred downslope as the glacier flows.

Unnamed Glacier, Ulu Peninsula, James Ross Island. Small valley glacier.
Unnamed Glacier, Ulu Peninsula, James Ross Island. The accumulation zone for this glacier extends from the plateau downwards.

Precipitation falling as rain is usually considered to be lost to the system. Internal accumulation may include rain and meltwater percolating through the snowpack and then refreezing. Basal accumulation may include freezing on of liquid water at the base of the glacier or ice sheet2.

The figure below summarises the inputs and outputs from a glacier system; the inputs are the processes of accumulation (including precipitation (snow, hail and rain) and other sources of accumulation such as wind-blown snow and avalanching.

The Glacier as a System. Inputs are largely from precipitation, and also from wind-blown snow and avalanches. The glacier loses mass (ablates) mainly by the processes of calving and surface and subaqueous melt. In this simplified figure, processes of internal and basal accumulation are ommitted. See Cogley et al. 2011 for more information.

Ablation zone

The Glaciers as a System figure above summarises the key processes of ablation for a glacier.

Glaciers lose mass through processes of ablation. Surface ablation processes include surface melt, surface meltwater runoff, sublimation, avalanching and windblown snow. Glaciers on steep slopes may also dry calve, dropping large chunks of ice onto unwary tourists below. Glaciers terminating in the sea or a lake will calve photogenic icebergs.

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

Other processes of ablation include subaqueous melting, and melting within the ice and at the ice bed, which flows towards the terminus2.

Equilibrium line altitude

Accumulation usually occurs over the entire glacier, but may change with altitude. Warmer air temperatures at lower elevations may also result in more precipitation falling as rain. The zone where there is net accumulation (where there is more mass gained than lost) is the accumulation zone. The part of the glacier that has more ablation than accumulation is the ablation zone. Where ablation is equal to accumulation is the Equilibrium line altitude.

Equilibrium line altitudes in a hypothetical glacier
Equilibrium line altitudes in a hypothetical glacier

The snowline at the end of the summer season is often used to demarcate the equilibrium line on satellite images of glaciers. Above the snowline, where there is more accumulation than ablation, snow remains all year around and the glacier is a bright white colour. Below the snowline, there is more ablation than accumulation, so there is no snow left at the end of the summer, and the duller, grey-blue coloured glacier ice is visible.

The figure below shows an outlet glacier of the North Patagonian Icefield. The bright white parts in the upper reaches of the glacier are in the accumulation zone; the darker more blue areas on the glacier tongues are in the ablation zone. The Equilibrium Line Altitude here is approximately equal to the snow line.

Medial moraines on the North Patagonian Icefield (Landsat image). Each medial moraine separates out an individual flow unit.

Accumulation Area Ratio

The Accumulation Area Ratio (AAR) is the ratio of the area of the accumulation zone to the area of the glacier. It is a value between 0 and 1 (Cogley et al., 2011).

The “Balanced Budget AAR” is the AAR of a glacier with a mass balance equal to 0 (i.e., it is in balance with climate, and is neither growing or shrinking). The balanced-budget AAR of non-calving glaciers is usually between 0.5 and 0.6 on average, although the range of variation is substantial, and depends on climatic and topographic factors.

On many glaciers, the AAR correlates well with the climatic mass balance and the ELA. The correlation is inverse; a lower ELA implies a higher AAR.

Some glaciological parameters of a mountain glacier shown in, A, cross-sectional view and, B, plan view. The accumulation area plus ablation area represents the total glacier area. The positive net mass balance (bn) of the accumulation area and the negative net mass balance of the ablation area are separated by the equilibrium line altitude (ELA) where net mass balance is zero (bn=0). The accumulation area ratio (AAR) is determined by dividing the accumulation area by the total area of the glaciers. The length of a glacier is measured from the terminus along its mid-line (dashed line) to its uppermost margin. Modified from Andrews (1975, p. 33, fig. 3-1A). Source: USGS Professional Paper 1386

So what is Glacier Mass Balance?

So, glacier mass balance is the quantitative expression of a glacier’s volumetric changes through time.In the figure below, Panel A shows how temperature varies with altitude. It is colder at the top than it is at the bottom of the glacier. This is crucial, as surface air temperature strongly controls melting and accumulation (as in, how much precipitation falls as snow or ice).

Mass balance (b) is the product of accumulation (c) plus ablation (a). Mass balance (b) = c + a Mass balance is usually given in metres water equivalent (m w.e.). It varies over time and space; accumulation is greater in the higher reaches of the glacier, and ablation is greater in the lower, warmer reaches of the glacier (Panel B in the figure).

Mass balance also varies throughout the year; glaciers typically get more accumulation in the winter and more ablation in the summer (Panel C in the figure). Glacier mass balance therefore usually can therefore be expressed as a mass balance gradient curve, showing how c + a varies attitudinally across the glacier (Panel D in the figure). The balance gradient is the rate of change of net balance with altitude3. A glacier’s net mass balance is a single figure that describes volumetric change across the entire glacier across the full balance year.

Principles of glacier mass balance
Principles of glacier mass balance

Measuring Mass Balance

Glacier mass balance is normally measured by staking out a glacier. A grid of ‘ablation stakes’ are laid out across a glacier and are accurately measured. They can be made of wood, plastic, or even bamboo like you’d use in your garden. These stakes provide point measurements at the glacier surface, providing rates of accumulation and ablation.

Jonathan Carrivick prepares to stake out Glacier IJR45 on James Ross Island.
Jonathan Carrivick prepares to stake out Glacier IJR45 on James Ross Island.

These methods are time consuming, logistically challenging and arduous; the stakes will need to be visited several times through the balance year. Accumulations and ablation are generally measured by reference to stakes inserted to a known depth into the glacier, and fixed by freezing and packing in3. The location is fixed with GPS.

Automatic weather stations on the glacier surface are key to understanding energy fluxes on the glacier. Probing, snowpits and crevasse stratigraphy are also used to measure mass balance on glaciers, ideally supplemented with stakes.

Remote sensing of glacier mass balance is obviously a good alternative, as it allows many glaciers to be assessed using desk-based studies. It is a cheap and simple alternative to arduous fieldwork, but ground truthing of mass balance measurements will always be necessary. Researchers from Aberystwyth University use satellite measurements to track changes in the mass balance of the Greenland Ice Sheet.

Mass balance gradients

The mass balance gradient of a glacier is a key control in factors such as the glacier’s response time.

A glacier’s mass balance gradient is critically determined by the climatic regime in which it sits; temperate glaciers at relatively low latitudes, such as Fox Glacier in New Zealand, may be sustained by very high precipitation. They will therefore have a greater mass balance gradient (more accumulation, more ablation). These wet, maritime glaciers may have a shorter response time and higher climate sensitivity than cold, polar glaciers that receive little accumulation but also have correspondingly low ablation. These cold, dry glaciers may respond more slowly to climate change.

Mass balance gradients of some typical glaciers.

In the figure above, temperate glaciers with greater mass balance gradients are represented by the shallower lines; more mass is transferred from the top to the bottom of the glacier. Cold, polar-type glaciers with smaller mass balance gradients are represented by the steeper lines.

Mass balance through time

The Cumulative mass balance is the mass of the glacier at a stated time, relative to its mass at some earlier time. Some glaciers have mass balance measurements going back decades, which means that scientists can analyse how mass balance is changing over time.

These measurements give us detailed information about climate change, as glaciers are sensitive ‘barometers’ to our changing world. Usually, the net mass balance over the balance year is plotted on a graph. There are several projects monitoring glaciers all over the world, and these analyses show that glacier mass balance is generally decreasing (becoming more negative) over time.

30 year glacier mass balance for 30 reference glaciers in the Alps.
30 year glacier mass balance for 30 reference glaciers in the Alps. From the World Glacier Monitoring Service and Alpine Glacier Mass Balance.

In Europe, European Environment Agency has records of many glaciers, and makes their cumulative mass balance measurements publically available. The Glaciers (CLIM 007) analysis shows that the vast majority of European glaciers are receding, with the rate of recession accelerating since the 1980s.

Cumulative specific net mass balance of European glaciers (mm water equivalent) from 1946 to 2010
Cumulative specific net mass balance of European glaciers (mm water equivalent) from 1946 to 2010. From the Glaciers (CLIM 007) assessment.

The North American region shows a similar trend, with a generally declining mass balance each year.

North American glacier mass balance. Image courtesy of Mauri Pelto
North American glacier mass balance. Image courtesy of Mauri Pelto

Further afield, the IPCC AR4 shows cumulative specific net mass balance of glacierised regions worldwide. The differing behaviours of different regions shows the variable strength of climate change.

Cumulative mean specific mass balances (a) and cumulative total mass balances (b) of glaciers and ice caps, calculated for large regions (IPCC AR4)
Cumulative mean specific mass balances (a) and cumulative total mass balances (b) of glaciers and ice caps, calculated for large regions (IPCC AR4)

Further reading

More information on glacier accumulation and ablation

How glaciers flow:

Also of interest:

Wider reading:

References


1.            Benn, D.I. &Evans, D.J.A. Glaciers & Glaciation. London: Hodder Education. 802 (2010).

2.            Cogley, J.G., Hock, R., Rasmussen, B., Arendt, A., Bauder, A., Braithwaite, R.J., Jansson, P., Kaser, G., Moller, M., Nicholson, L., & Zemp, M. Glossary of Glacier Mass Balance and related terms. Paris: IHP-VII Technical Documents in Hydrology No. 86, IACS Contribution No. 2, UNESCO-IHP. 124 (2011).

3.            Hubbard, B. &Glasser, N.F. Field Techniques in Glaciology and Geomorphology. Wiley. 412 (2005).

Glacier response time

What is glacier response time? | The role of glacier size | The role of mass balance gradients | The role of glacier slope and hypsometry | Calculating glacier response times | Summary | Further reading | References | Comments |

What is glacier response time?

Glacier response time is the length of time taken for a glacier to adjust its geometry to a new steady state after a change in glacier mass balance1, caused by a changing climate2. Mountain glaciers worldwide are currently thinning and receding, but their behaviour in the future is highly variable, as their recession is controlled by many parameters. Understanding glacier response time is important for understanding how quickly particular glaciers will change in length and volume under a given climatic scenario; if we are to be able to estimate future sea level rise from shrinking glaciers, we need to know how quickly they can change2. Will glaciers shrink in response to short term climate fluctuations, or are longer and more sustained climatic changes required before a glacier changes its geometry?

Mountain glacier mass balance since 1970, excluding the Greenland and Antarctic ice sheets. From the Global Warming Art Project.

Mountain glacier mass balance since 1970, excluding the Greenland and Antarctic ice sheets. Mountain glaciers worldwide currently show significant thinning and recession. From the Global Warming Art Project.

The response time of a glacier is largely a function of its mean thickness and terminus ablation3 (Table 1), and of its hypsometry and mass balance gradient4,5 (the change in accumulation and ablation with elevation; a glacier with a steeper mass balance gradient receives more accumulation and has more ablation than a dry glacier with little accumulation and ablation). High mass balance gradients are indicative of high flux through the ELA. High gradients are usually found in mid-latitude maritime regions, where the maritime environment represents a major heat and moisture source6. Glacier size also affects glacier response time7.

Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.

Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.

Table 1. Estimated glacier response times as a function of thickness and terminus ablation. From Cuffey and Patterson, 2010.

  Thickness (m) Terminus ablation (m per year) Response time (years)
Glaciers in temperate maritime climate 150-300 5-10 15-60
Glacier in high-polar climate 150-300 0.5-1 150-600
Ice caps in Arctic Canada 500-1000 1-2 250-1000
Greenland Ice Sheet 3000 1-2 1500-3000

The role of glacier size

Small glaciers contribute significantly to present observed sea level rise (Gardner et al., 2013)

Small glaciers contribute significantly to present observed sea level rise (Gardner et al., 2013)

Smaller glaciers have shorter response times, and this is one of the reasons why small glaciers currently contribute significantly to sea level rise10,11. The high sensitivity of small ice masses to climate change is a function of their small system scale, and their proximity (compared with larger ice sheets) to the melting point9. However, it is difficult to apply a uniform response time widely across multiple glaciers. There is no straightforward relationship between glacier size and change in  ice volume under any given climate scenario12.

Short, steep glaciers in maritime environments (with a correspondingly steep mass balance gradient) reach equilibrium following a change in mass balance forcing after only a few years, larger valley glaciers in around a century, and continental ice caps with gentle slopes much longer4. The smallest cirque glaciers will reflect annual changes in mass balance, almost without delay8. Franz Josef Glacier, in New Zealand, is 11 km long and 35 km2 in area, has a steep surface gradient and a steep mass balance gradient, and has a response time of 21-24 years9. The Greenland Ice sheet, on the other hand, is estimated to have a response time of 1500-3000 years5.

The implication of this is that, given the same climatic forcing, glaciers of different lengths and thicknesses will respond in different ways, with variable numbers of advances and retreats. These should generally correlate regionally. Moraines occurring at only a few glaciers at a specific time are interpreted as reflecting shorter term climatic oscillations and a glacier with a short response time4. Larger, flatter glaciers tend to smooth the climatic signal, with a delay of several decades.

The role of mass balance gradients

Mass balance gradients of hypothetical glaciers

Mass balance gradients of hypothetical glaciers

A key factor in controlling glacier response to climatic perturbations is the mass balance gradient, the change in net balance with altitude, which is largely governed by the temperature lapse rate2,6. There is a linear relationship between mass balance gradient and mass balance sensitivity. Glaciers with a steeper mass balance gradient flow faster. The mass balance gradient of a glacier is affected by glacier size, hypsometry (the variation of glacier area with altitude13) and glacier slope.

Wet, maritime glaciers have a steeper mass balance gradient than dry continental glaciers. Warm, wet glaciers with a steep mass balance gradient and high sensitivity have a small response time, and cold and dry glaciers with low mass balance gradients and sensitivity have typically a longer response time. Temperature-sensitive glaciers may show the most rapid response to climate change at present, but may not be the most important contributors to global sea level rise over longer time periods2.

The role of glacier slope and hypsometry

Glacier hypsometry controls the mass balance elevation distribution over a glacier13. Glacier hypsometry is determined by valley shape, topographic relief and ice volume distribution. The altitudinal distribution of a glacier controls its sensitivity to a rise in the Equilibrium Line Altitude (ELA); glaciers with a large, relatively flat accumulation area will be more sensitive to a small increase in ELA than glaciers with a steeper accumulation area.

Response time of glaciers. From Haeberli, 1995

Response time of glaciers. From Haeberli, 1995

Steeper glaciers therefore typically have shorter response times4,5,8. In the New Zealand Southern Alps, smaller, steeper mountain glaciers have recently had minor readvances following short term climatic oscillations, while larger, low-gradient valley glaciers have continued to recede, as they have done for the last century14.  This is because, in general, if the glacier surface gradient is small, changes in mass balance occur slowly with distance15, whilst with steep glaciers, changes in mass balance occur rapidly with distance.

Calculating glacier response times

Mathematical estimates

In reality, response time actually refers to the time taken for a glacier to complete most of its adjustment to a change in mass balance5. This is because glaciers continue to adjust at an ever decreasing rate for a very long time after the change. Response time therefore typically refers to the time taken for a glacier to complete all but a factor 1/e (or 37%) of its net change1,5,9. Glacier response time can be estimated for a particular glacier from the simple equation,

 equation1

Where H is the thickness of the glacier and bt is the scale of ablation at the terminus of the glacier15,16. This calculates response time taken for the volume of the glacier to reach steady state following a change in mass balance. It will only give order-of-magnitude estimates2. This equation predicts that response time will increase linearly with glacier thickness, and that larger glaciers will have longer response times15. However, thicker glaciers are also longer, and longer glaciers push their snouts further into the ablation zone. The mass balance therefore gets increasingly negative at the snout as the glacier gets longer.

Numerical models

Glacier response times are usually calculated using glacier models9,17,18. Numerical flowline models take into account glacier geometry and mass balance when calculating glacier response time and climate sensitivity. Oerlemans (1997) defines glacier response time as the time taken for the glacier volume to go from one steady state (V1) to another (V2) under a given climatic forcing (C1 to C2)18. Response time for glacier volume can therefore be written as:

equation2

Oerlemans (1997) used a step change of 0.5K and -0.5K to force a change in Franz Josef Glacier18, and observed how long it took to reach equilibrium for both length and volume. More recently, Anderson et al. (2008) used a numerical model to impose a step-change in mass balance on Franz Josef Glacier, and observed how long it took for the glacier to complete two-thirds of its response9. This approach has also been used to compare the response time of glaciers of different sizes using different models1.

Summary

Glacier response time is an important factor to take into account when analysing glacier response to climate change. Not all glaciers will respond in a uniform way to a change in environmental conditions, as their response times are governed by ice thickness, ablation at the terminus, mass balance gradients, hypsometry and ice surface slope.

Further reading

References


1.            Leysinger Vieli, G.J.M.C. & Gudmundsson, G.H. On estimating length fluctuations of glaciers caused by changes in climatic forcing. Journal of Geophysical Research: Earth Surface 109, F01007 (2004).

2.            Raper, S.C.B. & Braithwaite, R.J. Glacier volume response time and its links to climate and topography based on a conceptual model of glacier hypsometry. The Cryosphere 3, 183-194 (2009).

3.            Jóhannesson, T., Raymond, C. & Waddington, E. Time-scale for adjustment of glaciers to changes in mass balance. Journal of Glaciology 35, 355-369 (1989).

4.            Kirkbride, M.P. & Winkler, S. Correlation of Late Quaternary moraines: impact of climate variability, glacier response, and chronological resolution. Quaternary Science Reviews 46, 1-29 (2012).

5.            Cuffey, K.M. & Paterson, W.S.B. The Physics of Glaciers, 4th edition, 704 (Academic Press, 2010).

6.            Rea, B.R., Evans, D.J.A., 2007. Quantifying climate and glacier mass balance in north Norway during the Younger Dryas. Palaeogeography, Palaeoclimatology, Palaeoecology 246, 307-330.

7.            Harrison, W.D., Elsberg, D.H., Echelmeyer, K.A. & Krimmel, R.M. On the characterization of glacier response by a single time-scale. Journal of Glaciology 47, 659-664 (2001).

8.            Haeberli, W. Glacier fluctuations and climate change detection. Geogr. Fis. Dinam. Quat 18, 191-199 (1995).

9.            Anderson, B., Lawson, W. & Owens, I. Response of Franz Josef Glacier Ka Roimata o Hine Hukatere to climate change. Global and Planetary Change 63, 23-30 (2008).

10.            Gardner, A.S., Moholdt, G., Cogley, J.G., Wouters, B., Arendt, A.A., Wahr, J., Berthier, E., Hock, R., Pfeffer, W.T., Kaser, G., Ligtenberg, S.R.M., Bolch, T., Sharp, M.J., Hagen, J.O., van den Broeke, M.R. & Paul, F. A Reconciled Estimate of Glacier Contributions to Sea Level Rise: 2003 to 2009. Science 340, 852-857 (2013).

11.          Hock, R., de Woul, M., Radic, V. & Dyurgerov, M. Mountain glaciers and ice caps around Antarctica make a large sea-level rise contribution. Geophysical Research Letters 36, L07501 (2009).

12.          Oerlemans, J., Anderson, B., Hubbard, A., Huybrechts, P., Jóhannesson, T., Knap, W.H., Schmeits, M., Stroeven, A.P., van de Wal, R.S.W., Wallinga, J. & Zuo, Z. Modelling the response of glaciers to climate warming. Climate Dynamics 14, 267-274 (1998).

13.          Jiskoot, H., Curran, C.J., Tessler, D.L. & Shenton, L.R. Changes in Clemenceau Icefield and Chaba Group glaciers, Canada, related to hypsometry, tributary detachment, length-slope and area-aspect relations. Annals of Glaciology 50, 133-143 (2009).

14.          Winkler, S., Chinn, T., Gärtner-Roer, I., Nussbaumer, S.U., Zemp, M. & Zumbühl, H.J. An introduction to mountain glaciers as climate indicators with spatial and temporal diversity. Erdkunde, 97-118 (2010).

15.          Bahr, D.B., Pfeffer, W.T., Sassolas, C. & Meier, M.F. Response time of glaciers as a function of size and mass balance: 1. Theory. Journal of Geophysical Research: Solid Earth 103, 9777-9782 (1998).

16.          Jóhannesson, T., Raymond, C.F. & Waddington, E.D. A Simple Method for Determining the Response Time of Glaciers. in Glacier Fluctuations and Climatic Change, Vol. 6 (ed. Oerlemans, J.) 343-352 (Springer Netherlands, 1989).

17.          Klok, E.J. & Oerlemans, J. Deriving historical equilibrium-line altitudes from a glacier length record by linear inverse modelling. The Holocene 13, 343-351 (2003).

18.          Oerlemans, J. Climate Sensitivity of Franz Josef Glacier, New Zealand, as Revealed by Numerical Modeling. Arctic and Alpine Research 29, 233-239 (1997).

Deformation and sliding

Glacier mass balance | Glacier flow | Internal deformation | Basal sliding | Subglacial deformation | Different types of glacier flow | References | Comments |

Glacier mass balance

Components of mass balance of a glacier. From the USGS

Components of mass balance of a glacier. From the USGS

How do glaciers move? A glacier is a pile of ice, and as such, deforms under the force of gravity. Glaciers flow downslope because they accumulate mass (ice) in their upper portions (from precipitation and from wind-blown snow) and ablate (melt, sublimate and calve ice bergs) in their lower portions.

This means that a glacier in a steady state (equilibrium) will not change in steepness or size, because accumulation = ablation. The altitude with zero net accumulation or ablation on the glacier is the equilibrium line altitude. Changes in rates of accumulation or ablation will lead to glacier advance or recession; if the accumulation area of a glacier shrinks, for example, and the equilibrium line altitude rises, then the glacier will recede.

Glacier mass balance is the difference between accumulation and ablation. It is therefore controlled by both temperature and precipitation. If accumulation is greater than ablation, then the glacier has positive mass balance and will advance. If ablation is greater than accumulation, then the glacier has negative mass balance and will recede.

Note: see Common Misconceptions in Glaciology. Glaciers always flow downslope under the weight of their own gravity. A receding or shrinking glacier still flows (although it might flow very slowly!); it’s just that it’s melting faster than it’s acquiring snow in its upper reaches. As a result, the glacier will thin and the snout position will recede backwards.

Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.

Glacier mass balance and atmospheric circulation. By NASA. From Wikimedia Commons.

In theory, glaciers discharge ice from the accumulation area to the ablation area and maintain a steady-state profile. The velocity, the balance velocity, is controlled by glacier mass balance and glacier geometry (Jiskoot et al., 2011). Some glaciers have dynamic flow driven by other factors, for example, surging glaciers, tidewater glaciers, ice streams or ice-shelf tributary glaciers.

Glaciers flow through ice deformation and sliding

Glaciers always flow downslope, through the processes of deformation and sliding. Glacier flow, velocity and motion is controlled several factors (Jiskoot et al., 2011), including those listed below:

  • Ice geometry (thickness, steepness),
  • Ice properties (temperature, density),
  • Valley geometry,
  • Bedrock conditions (hard, soft, frozen or thawed bed),
  • Subglacial hydrology,
  • Terminal environment (land, sea, ice shelf, sea ice), and
  • Mass balance (rate of accumulation and ablation).

When glaciers flow downslope, gravitational driving stresses are resisted (resistive stress). The driving stress is controlled by gravitational acceleration, ice density and temperature, ice thickness and ice surface slope. Resistive stresses mainly operate at the glacier bed, and comprise basal drag or lateral drag against valley walls.

This driving stress means that glaciers move in one of three ways:

  1. Internal deformation (creep)
  2. Basal sliding
  3. Soft bed subglacial deformation.

All glaciers flow by creep, but only glaciers with water at their base (temperate or polythermal – see Glacial Processes) have basal sliding, and only glaciers that lie on soft deformable beds have soft sediment deformation.  If all three factors are present, you have the ingredients to contribute to fast ice flow (see Ice Streams).

Internal deformation

If the glacier flows just by internal deformation, then it is likely that rates of creep decrease with depth, with fastest ice movement at the surface and slowest (or no) ice movement at the base and at the valley sides, where resistive stresses are highest (Jiskoot et al., 2011). Ice deforms because it is plastic. If large stresses are applied it can crack in a brittle manner (forming crevasses or calving ice bergs).

The video below shows a huge calving event at Helheim Glacier, Greenland, in July 2010. It was made by the Swansea Glaciology Group.

Basal sliding

Glaciers can slide because ice melts under pressure, resulting in a film of water at the ice-bed interface. This can facilitate decoupling and enhance fast ice flow. If the glacier bed is rough, with many bumps and obstacles, this increases melting and ice flow. This process is known as regelation. If water pressures become high enough, cavities can form at the ice-bed interface, causing sliding with bed separation. This reduces basal friction and allows faster ice flow. Sliding velocity is controlled by basal shear stress and effective pressure, which is the difference between ice overburden pressure and water pressure (Jiskoot et al., 2011).

This video shows the glacier flowing above a cavity beneath the ice on Mont Blanc glacier.

Subglacial deformation

Subglacial till (see Glacial Processes) comprises unconsolidated, unsorted or poorly sorted sediments ranging from boulders to clay. In Norfolk, till sequences are over 20 metres thick. Fine sediments, such as clay and sand, are not cohesive and therefore deform readily when shear stress is applied to them if they have a high pore-water pressure (so, like basal sliding, subglacial deformation depends on high basal water pressures). If basal shear stress (the gravitational driving dress) is greater than the yield strength of the till, deformation occurs, resulting in some fantastic glaciotectonic sequences (see picture gallery below and papers by Davies et al., 2009,  2012a, 2012b).

Different types of glacier flow

Glaciers do not just flow in a steady state, however. We have cold-based glaciers, which have little flow velocity; polythermal glaciers, which are partly frozen to their bed; wet-based glaciers which have sheet flow (as is described in the above sections); ice streams, which have very rapid flow velocities; and surging glaciers, which have periods of rapid flow separated by quiescent periods of slow flow. The key differences in temperate glacier flow is summarised in the table below. Each of these different flow regimes results in a set of different and diagnostic glacial landforms.

Further Reading

Other sources:

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