Glacier Mass Balance

12 Key excerpts on "Glacier Mass Balance"

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  eBook - ePub

  Glaciers and Environmental Change

  • Atle Nesje, Svein Olat Dahl(Authors)
  • 2016(Publication Date)
  • Routledge

    (Publisher)

  hypsometry (distribution of glacier area over its altitudinal range). To overcome this problem, a balance ratio method was developed by Furbish and Andrews (1984). This approach takes account of both glacier hypsometry and the shape of the mass balance curve and is based on the fact that, for glaciers in equilibrium, the total annual accumulation above the ELA must balance the total annual ablation below the ELA. This can be expressed as the areas above and below the ELA multiplied by the average accumulation and ablation, respectively (for further details, see Furbish and Andrews, 1984; Benn and Evans, 1998: 84).

  4.8 Mass balance

  Glaciers and ice sheets are stores of water, exchanging mass with other components involved in the global hydrological system. Glaciers and ice sheets grow by snow and ice accumulation, and lose mass by different ablation processes. The difference between accumulation and ablation over a given time span is the mass balance, which can be either positive or negative. The mass balance reflects the climate of the region, together with glacier morphology and local topographic conditions. Mass balance measurements can therefore give information on the causes of retreat or advance of glaciers.

  One of the first systematic analyses of the annual mass budget of a glacier was made by Ahlmann (1927). Statistical relationships between mass balance and meteorological parameters have been investigated on several glaciers (Letréguilly, 1988; Pelto, 1988), and the physical relationships studied by Holmgren (1971), Kuhn (1979) and Braithwaite (1995), amongst others.

  Mass-balance variations can be associated with atmospheric circulation, linking them to atmospheric changes rather than single meteorological parameters. This approach was used by Hoinkes (1968) to show how glacier variations in Switzerland were related to cyclonic and anticyclonic conditions. Alt (1987) found that extreme mass balance years at the Queen Elisabeth Island ice caps, Canada, were related to the position of the Arctic front. In southwestern Canada, Yarnal (1984) found that two glaciers were sensitive to both large- and small-scale synoptic weather situations. Voloshina (1988) discussed why the position of the Siberian anticyclone forms an inverse relationship between the mass balance for glaciers in northern Scandinavia and in the northern Urals. The strength of the Aleutian low is important for the determination of the storm track and high mass balance in the Alaskan Range and the Cascades (Walters and Meier, 1989). McCabe and Fountain (1995) found that the winter balance of the South Cascade Glacier correlates to the pressure difference between the Gulf of Alaska and the west coast of Canada. Finally, Pohjola and Rogers (1997a,b) used atmospheric circulation and synoptic weather studies to explain variations in Glacier Mass Balance on Scandinavian glaciers. They also demonstrated that a high net balance on Storglaciären, the glacier with the longest mass-balance record in the world, is favoured by strong westerly maritime air flow which increases the winter accumulation. Holmlund and Schneider (1997) used a continentality index as a measure of the nature of climate, mass balance, and glacier-front response along a west–east transect in a region just north of the Arctic Circle in Scandinavia. These studies demonstrate the potential of the relationship between Glacier Mass Balance and synoptic weather studies. This is important when using glacier-front or ice-core records to reconstruct past atmospheric circulation.

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  eBook - ePub

  Glaciers

  • Peter Knight(Author)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  Letréguilly (1988) compared the mass balance of three glaciers at different distances from the coast in western Canada and found that winter precipitation was dominant in controlling annual variations in mass balance close to the coast but that summer ablation was dominant further inland. Variations in the mass balance of Peyto Glacier (550 km from the coast) were almost entirely related to summer temperature; variations at Sentinal Glacier (30 km) were mostly controlled by winter precipitation; and at Place Glacier (160 km) the two factors were closely matched in importance.

  Mass balance parameters can be measured in a variety of different ways. Indirect estimates based on proxy criteria such as those mentioned above provide one source of information. Field measurement of accumulation and ablation can be achieved using stakes, pits, cores and probes. Remote sensing of the glacier surface by satellite or aerial reconnaissance can identify positions of surface features such as the snowline at different times of year. Remote sensing can also measure changes in surface elevation and ice thickness through time. Meteorological and hydrological monitoring can reveal flux and storage of snow and ice. Energy budget models and precipitation data can be used to predict mass balance.

  From the point of view of glaciology, the key elements of mass balance are accumulation of snow, the transformation of snow into glacier ice, and ablation, or release of material from the glacier. These processes control all other aspects of the mass balance and hence the general status of the glacier, and will be considered in the following sections.

  3.3 Accumulation

  The term 'accumulation' is generally taken to include all the processes by which snow or ice is added to a glacier. The main processes of accumulation include: direct precipitation of snow; freezing of liquid water; transport and deposition of snow by wind; deposition of snow or ice by avalanche; precipitation of rime or hoar; and freezing of water to the base of an ice shelf or floating ice tongue.

  Direct precipitation of snow accounts for the overwhelming bulk of accumulation in most glaciers. According to LaChapelle (1992) , snow consists of ice crystals in the atmosphere which grow large enough to fall and reach the ground. A snow crystal is a single ice particle which has a common orientation of the orderly array of molecules which make up its structure. A snow grain is a mechanically separate particle of snow which may or may not be a single crystal. Recent classifications of snow types has been provided by IAHS (1990) and LaChapelle (1992) . About 5% of global precipitation falls in the form of snow (Martinec, 1976 ). Accumulation is controlled largely by the frequency and intensity of snow precipitation events, which in turn depend partly on air temperature, as moisture transport is limited by the saturation vapour pressure of cold air. There is a general geographical correlation between low temperatures and low accumulation rates (e.g. Fortuin and Oerlemans, 1990

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  The Physics of Glaciers

  • W. S. B. Paterson(Author)
  • 2017(Publication Date)
  • Pergamon

    (Publisher)

  4 The Mass Balance of a Glacier Wavering between the profit and the loss. T. S. Eliot, Ash Wednesday INTRODUCTION Mass balance studies are concerned with changes in the mass of a glacier and the distribution of these changes in space and time. More particularly, to measure the change in mass in a given year. Such studies form an important link in the chain of events connecting advances and retreats of glaciers with changes in climate. Climatic fluctuations cause variations in the amount of snow that collects on the glacier and in the amount of snow and ice lost by melting. Such changes in mass initiate a complex series of changes in the flow of the glacier that ultimately results in a change in the position of the terminus. The present chapter deals only with measurement of changes in mass. Correlation of these changes with meteorologi-cal data, and the response of the glacier to the changes, will be discussed separ-ately. Measurements of this kind may have considerable practical importance. In several countries glacier-fed streams supply much of the water used by hydroelectric plants. Such streams have a distinctive pattern of run-off. A glacier acts as a natural reservoir that stores water during the winter and releases it in summer. Especially large quantities may be released in warm summers when water from other sources is in short supply. Mass balance measurements determine how much water can be stored and released in this way, and what variations can be expected from year to year. In this subject in the past, different authors have used the same terms with differ-ent meanings. Much confusion has resulted. We shall therefore start with some def-initions. Methods of measurement will then be described and some typical results given. Finally we shall discuss the special problem of determining the mass balance of the Antarctic Ice Sheet. 42 DEFINITIONS THE MASS BALANCE OF A GLACIER 43 The definitions given here are those now in general use (Anonymous, 1969).

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  The Physics of Glaciers

  • W. S. B. Paterson(Author)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  3 Mass Balance Wavering between the profit and the loss. T. S. Eliot, Ash Wednesday INTRODUCTION Mass balance or mass budget studies are concerned with changes in the mass of a glacier and the distribution of these changes in space and time; more particularly, with measuring the change in mass in a given year. Such studies form an important link in the chain of events connecting advances and retreats of glaciers with changes in climate. Climatic fluctuations cause variations in the amount of snow that collects on a glacier and in the amount of snow and ice lost by melting. These changes in mass initiate a complex series of changes in the flow of the glacier that ultimately results in a change in the position of the terminus. This chapter deals only with measurements of changes in mass. Correlation of these changes with meteorological data is discussed in Chapter 4 and the response of the glacier to the changes in Chapter 13. Measurements of this kind have practical application. In several coun-tries glacier-fed streams supply much of the water used by hydroelectric plants. Such streams have a distinctive pattern of run-off. A glacier acts as a natural reservoir that stores water during winter and releases it in summer. Especially large quantities are released in warm summers when water from other sources is in short supply. Mass balance measurements determine how much water can be stored and released in this way, and what variations can be expected from year to year. I start with some definitions, then describe methods of measurement and data analysis, and present some results. Finally I discuss the special 26 MASS BALANCE 27 problems in determining the mass balances of the Greenland and Antarctic ice sheets and their contribution to changes in sea level. DEFINITIONS The definitions given here are those in general use (Anonymous, 1969). Ideal definitions should be applicable to glaciers of all sizes and types.

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  The Future of the World's Climate

  • Ann Henderson-Sellers, Kendal McGuffie(Authors)
  • 2011(Publication Date)
  • Elsevier Science

    (Publisher)

  –2 at Pine Island Glacier, entails the melting of 44 m of ice per year because conduction of heat upwards into the glacier is unlikely to be significant (Cogley, 2005). More recently, Rignot et al. (2010) have reported rates of frontal melting at outlet glaciers in Greenland that are highly variable, but are comparable with rates of calving. The significance of this rapid basal and frontal melting for the force balance of the ice extends far upglacier, because it ‘pulls’ grounded ice across the grounding line.

  A concern with energy fluxes at the bottom of the glacier, hundreds or thousands of metres below the surface, may seem out of place in a book about the climate. But, whether the basal ice is grounded or afloat, lack of understanding of these basal energy exchanges is the fundamental reason for our inability to describe, let alone to predict, how the ice sheets and other tidewater glaciers respond to climatic forcing.

  8.3. Glacier Mass Balance

  8.3.1. Terms in the Mass-Balance Equation

  The mass balance, Δ M, is the change in the mass of the glacier, or part of the glacier, over a stated span of time. It is the sum of accumulation, C (all gains of mass), and ablation, A (all losses of mass, treated as negative quantities):

  (8.3)

  However, the relevant processes can be distinguished more clearly if it is written as:

  (8.4)

  where, in the terminology of Cogley et al. (2011), B is the climatic-basal mass balance and Af is frontal ablation. The climatic-basal balance is the sum over the extent of the glacier of the surface mass balance, the internal mass balance, and the basal mass balance. Frontal ablation is the sum of losses at the glacier margin by calving, subaerial melting, and sublimation and subaqueous melting.

  The surface balance is more often measured than the other balance components, some of which it is not practical to measure while some are zero on many glaciers. The so-called glaciological method of in situ measurement of the surface mass balance relies on stakes and snow pits, and yields no information about the internal and basal balances. In geodetic measurements, the balance is estimated by repeated mapping. The change of glacier volume is obtained as the difference of glacier surface elevation, which is multiplied by an assumed average density to obtain the change of mass. Again, the measurement is ambiguous as to the internal and basal balances, and in particular as to internal accumulation, which can produce a decrease of volume due solely to the increase of density following from refreezing. Gravimetric observations with the GRACE satellites (e.g., Arendt et al., 2008) measure mass change directly, but have spatial resolution of the order of 200 to 300 km and do not resolve the components of the climatic-basal balance, or indeed of the total balance.

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  Physical Geology

  The Science of Earth

  • Charles Fletcher(Author)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  Alpine glaciers are recognized as sensitive indicators of climate, such that changes to their size over long periods point to a warming or cooling climate. More specifically, researchers track a glacier’s mass balance to discover glacier trends. The mass balance is the difference between accumulation and loss of snow on a glacier over a period of time (usually one year). When wastage exceeds LO 15.5 Describe the response of various ice environ- ments to global warming. Ice environments around the world—including glaciers, ice sheets, permafrost, sea ice, and periglacial environments— are melting because of global warming. –1400 –1200 –1000 –800 –600 –400 –200 0 Mean annual mass balance (mm w.e.) –22000 –17000 –12000 –7000 –2000 3000 1980 1985 1990 1995 2000 2005 2010 2015 Cumulative mean annual mass balance (mm w.e.) All glaciers 40 reference glaciers FIGURE 15.21 The World Glacier Monitoring Service collects data on global alpine glacier change over time. Its data show that the annual global glacier mean mass balance (in millimeters water equivalent) has been decreasing since 1980. The same data, plotted as cumula- tive mass balance, show a total reduction in thickness of over 12 meters for all glaciers globally (blue dashed line) and for 40 intensely researched reference glaciers (red line). 476 CHAPTER 15 Glaciers and Paleoclimatology snow accumulation, a glacier loses mass; it thins, and its leading edge (terminus) retreats. You can explore more about the concept of mass balance in the Critical Thinking Exercise, “Mass Balance of a Glacier.” Statistics attest that worldwide, glaciers are losing more mass than they are gaining (Figure 15.22), with 2016 marking the twenty-sixth consecutive year of negative mass balance for glaciers around the world.

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  Geomorphology

  The Mechanics and Chemistry of Landscapes

  • Robert S. Anderson, Suzanne P. Anderson(Authors)
  • 2010(Publication Date)
  • Cambridge University Press

    (Publisher)

  Note the many-hundred meter lowering of the ELA in the LGM, and the corresponding greater extent of the glacial coverage of the topography (after Skinner et al ., 1999 , with permission from John Wiley & Sons, Inc.). Mass balance 217 glacier. This is simply the spatial integral of the pro-duct of the local mass balance with the hypsometry (area vs. elevation) of the valley: B ¼ ð z max 0 b ð z Þ W ð z Þ d z ð 8 : 2 Þ This exercise is carried out annually on numerous glaciers worldwide. See Figure 8.6 for an example from the Nigardsbreen, Norway. The Norwegians are interested in the health of their glaciers because they control fresh water supplies, but also because a significant portion of their electrical power comes from subglacially tapped hydropower sources. It is a common misconception that a consider-able amount of melting takes place at the base of a glacier, because after all the Earth is hot. Note the scales on the mass balance profiles. In places, many meters can be lost by melting associated with solar radiation. Recall that the heat flux through the Earth’s crust is about 41 mW/m 2 (defined as one heat flow unit, HFU), a trivial flux when contrasted with the high heat fluxes powered in one or another way by the sun (about 1000 W/m 2 ). The upward heat flux from the Earth is sufficient to melt about 5 cm of ice per year. As far as the mass balance of a glacier is concerned, then, there is little melt at the base. If nothing else were happening but the local mass gain or loss from the ice surface, a new lens of snow would accumulate, which would be tapered off by melt to a tip at the ELA (or snowline) each year. Each successive wedge would thicken the entire wedge of snow above the snowline, and would increase the slope everywhere. But something else must happen, because we find glaciers poking their snouts well below the ELA, below the snowline. How does this happen? Ice is in motion. This is an essential ingre-dient in the definition of a glacier.

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  Modern and Past Glacial Environments

  Revised Student Edition

  • John Menzies(Author)
  • 2002(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Thus a glacier’s accumula-tion area has a b n > 0, and the ablation area a value of b n < 0, while at the equilibrium line b n = 0. When and if b n = 0, for the total ice mass, a steady state is reached. In reality, this state is rarely, if ever, attained because of the changing spatial geometry of an ice mass over time and the impact of past variations in b w and b s . This latter problem increases when mass balance calculations are made for large ice sheets. Values obtained of total accumulation for the Antarc-tic and Greenland Ice Sheets are, within a reasonable margin of error, accurate, but total ablation values remain elusive. Whether these major ice sheets are stable or whether they are gaining or losing mass at present remains unknown. 3.4.4. Mass Balance Gradients The calculation of mass balance can be made by considering an energy balance equation that is altitude dependent. This approach utilizing the entire balance for a year (over a time step-wise function of 30 min) involves the atmospheric temperature, snowfall and atmospheric transmissivity for solar radiation related to altitude allowing a balance gradient to be calcu-lated (see Menzies, 1995, eq. 4.4, p. 109). The response of mass balance gradients of differing glacier types to variations in climate and thus mass balance parameters is complex. The variations may consist of one or a combination of the following effects: (1) change in accumulation to the ice mass at the surface or base; (2) change in the energy flux from the atmosphere to the surface of the ice mass; (3) change in the geothermal heat flux; and (4) change in the length of the ablation season.

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  Remote Sensing of Glaciers

  Techniques for Topographic, Spatial and Thematic Mapping of Glaciers

  • Petri Pellikka, W. Gareth Rees, Petri Pellikka, W. Gareth Rees(Authors)
  • 2009(Publication Date)
  • CRC Press

    (Publisher)

  When melt water percolates through the seasonal snow and reaches the imperme-able glacier ice, it may refreeze there and form layers of superimposed ice. When the seasonal snow is wasted and the superimposed ice appears at the surface, it displays a different structure and has different optical properties to glacier ice transformed from snow. The transfer of latent energy of melting by percolation effectively distributes the summer energy surplus from the surface throughout the seasonal snow and gives the snow a mean annual temperature of 0 ◦ C even at elevations where the mean annual air temperature is below freezing. Temperate ice thus prevails in the Alps up to altitudes of about 3400 m, and cold ice may be found, preferentially on north slopes, above that altitude. Recent warming, however, is upsetting such empirical relations. High latitude glaciers are more often of the cold type (below freezing tempera-tures throughout) or polythermal (having both cold and temperate ice). In Arctic and Antarctic ice sheets and glaciers the build-up of superimposed ice by the refreezing of percolating melt water may be an important part of accumulation. 2.5 MASS BALANCE: DEFINITIONS AND KEY PARAMETERS At any location on a glacier, the resultant of accumulation and ablation is the specific mass balance b ( x , y ) which is expressed in kg m − 2 a − 1 , or after division by the den-sity of water as m of water equivalent (m w.e.) per year. Averaging over the entire glacier area yields the mean specific balance b in the same units. Multiplying b (m w.e. per year) by the total glacier area S gives the volume balance B (m 3 w.e. per year). The specific mass balance profile b ( h ) results from averaging specific balance b ( x , y ) over altitude intervals or elevation bands, (e.g. 2900–3000 m). An example is given in Figure 2.6. The equilibrium line altitude (ELA) is defined as that altitude where b ( h ) intersects the axis of b ( h ) = 0.

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  Glaciology and Glacial Geomorphology

  • Navodita Bhatnagar(Author)
  • 2019(Publication Date)
  • Delve Publishing

    (Publisher)

  a −1 over the same period is closer to the agreement with the geodetic mean mass balance of –0.37 m w.e. a −1 for the Himalayan Range over 1975–2015. However, these screened glaciological mass balances remain too sparse in time and space to obtain a robust regional average and an unambiguous temporal trend. Our analysis highlights the sensitivity of the regional average to the addition/subtraction of just a few glaciological measurements. Glaciological measurements should be retained for the understanding of physical processes, validation of remotely sensed measurements, calibration/validation of glacio-hydrological models and development of process-based models for future glacier changes. Our recommendation is that glaciological mass balances should not be used for the computation of regional mass balance (Sherpa and others, 2017) and for sea-level rise contribution from the Himalayan range. This recommendation is paired with suggested improvements in the benchmark glacier network. Mass-balance modeling is becoming widely used in the HK region with growing satellite and recent in situ meteorological data availability. A few studies estimated mass balances using different models such as the hydrological model for Siachen Glacier; temperature index model for Chhota Shigri, Langtang and Mera glaciers; albedo model for Chhota Shigri and regression (mass balance-meteorological parameters) model for Kangwure Glacier (Fig. 3d; Supplementary Table S9). Modeling of mass balance over the historic period of observations using both in situ and satellite measurements may finally give high spatial and temporal resolution, long-term continuity and geographic completeness , thus filling data gaps and addressing current inhomogeneities (Supplementary Tables S4 and S8). Based on in situ field measurements, Vincent and others (2013) showed that Chhota Shigri Glacier was near balanced conditions during the1990s.

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  Principles of Glacier Mechanics

  • Roger LeB. Hooke(Author)
  • 2019(Publication Date)
  • Cambridge University Press

    (Publisher)

  Significantly, their contribution appears to have nearly doubled since 2001 (Table 3.2). ....................................................................................................................... SUMMARY In this chapter we have discussed snow accumulation and the transformation of snow to ice. We found that in polar environments, where little if any melting occurs, the physical and chemical stratigraphy in an annual layer of snow persists for many thousands of years and can be used to date the ice by counting annual layers. We then explored the change in mass of a glacier or ice sheet: the sum of changes due to meteorological effects, to dynamic thickening or thinning, and to terminus advance or retreat. We defined some terms used to discuss meteorological effects, particularly summer, winter, and net balance, and summarized satellite techniques for measuring mass balance. We then used a perturbation approach to study the influence of temperature, radiation, and precipitation on net balance. It turned out that the net balance of glaciers in continental environments was sensitive, primar- ily, to summer temperature, while that of glaciers in maritime areas was sensitive to both winter balance and summer temperature. Radiation balance, principally due to differences in cloud cover, could play a role in either environment. The lower budget gradient and consequent more sluggish behavior of polar glaciers compared with their temperate counterparts turned out to be largely related to the shorter melt season in polar environments. We also saw that dynamic thickening or thinning, and terminus advance or retreat are closely linked. If a rapid glacier advance is accompanied by a commensurate amount of thinning, the mass does not change. However, such an advance may result in rapid melting of the terminus and hence retreat.

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  Glacial Geology

  Ice Sheets and Landforms

  • Matthew M. Bennett, Neil F. Glasser(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  The mass balance – accumulation minus ablation over a year – of a glacier determines whether it will grow or decay over time. A positive balance, more accumulation than ablation, will result in glacier expansion. A negative mass balance, more ablation than accumulation, will result in glacier decay. Accumulation and ablation occur on different parts of the ice sheets. It is this spatial imbalance that drives glacier flow. The stronger this imbalance the faster the glacier will flow. Glaciers flow through three different mechanisms: (i) by internal deformation; (ii) by basal sliding; and (iii) by subglacial deformation. The temperature distribution at the base of a glacier, the basal thermal regime, is important in determining the operation of these processes. Basal thermal regime is controlled by the influx of geothermal heat from the Earth and by frictional heat released during glacier movement. Glaciers may be either warm-based or cold-based. In warm-based glaciers, meltwater is released at the base of the glacier and ice flow occurs by all three processes: internal deformation, basal sliding and subglacial deformation. Cold-based glaciers are frozen to their beds and therefore can flow only by internal deformation: this restricts their geomorphological impact. The basal thermal regime of a glacier may vary both in space and time and as a consequence the geomorphological potential of a glacier will also vary. The rate or velocity of glacier flow is controlled primarily by the mass balance gradient of the glacier: the steeper the gradient the faster the ice flow. Some glaciers maybe subject to periods of rapid flow known as surges, which result from an instability at the glacier bed. The growth and decay of glaciers is controlled by their mass balance budget, which is a function of climate.

  SUGGESTED READING

  Probably the most comprehensive and accessible treatments of the mechanics of glaciers are in books by Paterson (1994) and Hooke (2005). The book edited by Bamber and Payne (2004) contains a number of excellent contributions concerning measuring and monitoring changes in the Earth’s contemporary ice sheets and glaciers. Bamber et al. (2007) review observations of rapid climate change and how these might affect the Greenland and Antarctic Ice Sheets. Hubbard and Glasser (2005) outline methods for measuring Glacier Mass Balance. The importance of mass balance in determining glacier activity is considered by Andrews (1972). The application of basal shear stress in testing glacier reconstructions is covered in Pierce (1979), Thorp (1991) and Bennett and Boulton (1993). There are numerous empirical and theoretical papers on glacier flow, including Blake et al. (1994), Iverson et al. (1995), Willis (1995), Engelhardt and Kamb (1998) and Hubbard (2002). A whole issue of Journal of Geophysical Research, Volume 92, is devoted to fast-flowing glaciers (e.g., Clarke, 1987). Bennett (2003) reviews the behaviour of ice streams. Subglacial deformation is dealt with by Boulton and Hindmarsh (1987), Alley et al. (1986, 1987a,b) and Kamb (1991). Bamber et al. (2006) and Peters et al.

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