GEO21014 Glacial Processes and Hazards

what is a glacier?

• Any large mass of perennial ice that originates on land by the recrystallization of snow or other forms of solid precipitation and that shows evidence of past or present flow ― Meier

• A mass of temperate ice will start to exhibit flow when the vertical thickness of the ice body reaches c. 30 m

what is glaciology?

• the study of glaciers of all types

• the processes of ice formation and flow

glacial hazards

• Hazards over a large temporal and spatial scale 

  • E.g avalanches, outburst floods, sea level rise 

• Also a legacy of landscape changes pose hazards including moraine formation and steep valley walls

hazard assessment and mitigation

• Model and research key processes 

• Think of different ways to reduce and mitigate impacts 

sea level change past and present

• Since last glacial maximum sea level has risen 120 metres

• More recently more stable in holocene 

• But upticked now in last 20-30 years

• Greatest component is ocean warming - thermal expansion

• Greenland is main contributor - more recently become more dominant 

• Increasing in future due to warming - contributions may change 

glacial formation and function

• Glaciers transfer precipitation that has collected to form glacier ice 

• Compaction and build up of snow - if it doesn't melt in summer - the weight of snow densifies - loses air spaces - slowly transforms into ice - water also percolates into the gaps and refreezes 

• Snow → firn → glacial ice 

glacier locations

• Typically in polar regions but also can occur up to equator - because of altitude 

• High altitude or high latitude

types of glacier

• Mountain glaciers 

  • Typically land terminating 

  • Smaller and thinner 

  • Shorter residence times - 100 of years - sensitive to change - responds rapidly to climate change - retreating 

• Ice sheet - outlet- glaciers 

  • Ocean termination 

  • Larger, faster, thicker 

  • Long residence time for ice within the ice sheet 

Mountain glaciers 

• Discrete bodies of ice in mountain areas 

• Ice accumulation and flow determined by host/pre existing topography e.g river networks and fluvial valleys 

Outlet glaciers 

• Drain ice from ice sheet interiors 

• Rapid flow - carasses occur - track these to monitor velocity 

• On reaching the ocean they terminate 

  • Calving front - chunks of icebergs calve off into the ocean 

  • Calving front - warm conditions 

• Ice shelves 

  • Floating over sea 

  • Occur in colder conditions 

  • Can be thousands of km ling 

  • Fast flow 

measuring glaciers

• Length change - monitor retreating from front of glacier - also measure the loss of mass vertically 

  • Surface elevation change 

• Mass balance change - gains vs losses 

• Velocity change 

length change observations

• In general over last 100 years glaciers are retreating - but some fluctuations eg more precipitation in warmer climates 

satellite methods

• Retreat mapping 

  • Now global - many sources provide monthly/daily coverage 

• Feature tracking 

  • Melt events - changes in velocity?

• GRACE

why and how do glaciers form?

• when snow remains on the ground for a number of years

• temperatures during the summer are most crucial - as this determines whether the snow remains

• needs to be sufficiently cold during summer and sufficient snowfall during winter

• in polar and high altitude regions

• the weight of subsequent snowfall gradually compresses the underlying snow forcing the snow to recrystallize to simpler and more packable orms

• the new crystals grow larger and intervening air pockets smaller as the overlying weight increases

• eventually the air content is restricted to bubbles and glacier ice is formed

• after a few winters (depending on snowfall rate) firn - an intermediate state between snow and ice - is formed

• continued compression - further expansion of ice crystals - remaining air is compacted into bubbles

• the time taken for snow to turn to glacier ice depends on the snowfall accumulation rate and the presence of meltwater in the snowpack

• high accumulation + meltwater = the fastest

• presence of meltwater freezing and refreezing speeds up the process of densification

density differences between snow/firn/glacier ice

new snow = 50-70 kg/m3

firn = 400-830

glacier ice = 830-917

characteristics of glacial ice

blue colour

• air bubbles compressed out and more dense means it absorbs a small amount of longer wavelength red light leaving a bluish tint in the reflected light

• when glacier ice is white there are many tiny air bubbles still in the ice or tiny fractures and it reflects all wavelengths equally

climate archives

• analysing the chemistry of the air trapped in bubbles can tell us about the climate at the time that the ice was formed

• once the air bubbles became isolated from the atmosphere

• this idea has been used to great effect reconstructing past climate change from ice cores

what is glacier mass balance

• difference between gains and losses - positive or negative over a given time period

• two categories of processes

• accumulation - adds mass to the glacier

• ablation - takes away mass

• mass balance varies over time and space

ablation processes

melting and runoff

• surface melting most important - particularly for land terminating glaciers

• subaqueous melt at lake and ocean terminating glaciers - melting of ice underwater - sensitive to temperature and motion of water

• basal melt - when ice is in contact with substrate

• frictional heating due to differential motion at the ice bed

• geothermal heat from radioactive decay of elements in the earth’s core and mantle

• basal melt induced by rainfall runoff

meltwater that refreezes elsewhere on or within the glacier does not count as mass loss - just redistribution

highest melt rates = where steep mass balance gradients enable glaciers to extend into relatively warm low altitudes e.g Franz josef glacier in new zealand (23m/yr at the terminus)

sublimation

• transition from solid ice straight to water vapour; cold dry and windy climates where air humidity at the glacier surface is greater than in the free atmosphere above

solid ice discharge

• either directly in a lake or ocean (iceberg calving) or motion of ice across the grounding line (the point at which a glacier begins to float) into an ice tongue or shelf

• can result in very rapid loss of several km3 of ice e.g helheim glacier in greenland

• calving from ice shelves does not count as this mass has already been lost from the glacier (i.e it is floating on the ocean)

dry calving

• serac collapse and avalanches

• possible in steep mountainous terrain

accumulation processes

snowfall

• usually the most important form of accumulation

deposition of other forms of ice

• depth hoar: temperature gradients between the insulated ground and cold snow surface lead to sublimation and deposition of uprising water and vapour in snowpack

• Freezing rain: supercooled raindrops freeze to the ice surface

• rime ice: water droplets within fog freeze to the ice surface

superimposed ice

• ice that forms from the re-freezing of water saturated snow

avalanches

• enable the existence of glaciers outside the climate envelope (where normally snowfall would not survive between winters)

wind blown snow

• can cause significant variations in local mass balance

• important for survival of small cirque glaciers

basal freeze on

• terrestrial or aqueous

• spatial variations in pressure mean basal water freezes onto overlying ice

• water remains liquid at lower temperatures when the pressure is high (pressure melting point)

highest accumulation rates = mountainous regions with frequent onshore winds and much orographic precipitation (topography induced)

lowest accumulation rates = very dry and cold conditions. e.g the interior of antarctic ice sheet

surface mass balance and ablation

• Many mass balance processes occur at the glacier surface and are collectively termed surface mass balance (SMB)

• Whether the ice surface gains or loses mass is partly dependent on the surface energy balance - the net energy produced by 

  • Solar radiation: mostly short wave radiation 

  • Long wave (infrared) radiation: emitted from the atmosphere and earth surface when heated 

  • Sensible heat: exchange with the atmosphere (like wind chill)

  • Latent heat: related to phase changes (water, vapour, ice)

  • Heat supplied by rain

• If the energy balance is positive after the glacier surface has been raised to the melting point, the surface will ablate, if it is negative, ice will accumulate 

• Note also the importance of surface debris which typically hinders ablation 

• Latent heat example: freezing of liquid water releases energy while melting of ice absorbs energy 

• Albedo of the glacier surface controls the ratio of incoming to outgoing solar radiation; higher albedo surfaces reflect more radiation 

determining surface mass balance - temperature and snowfall lapse rate

• Mean temperature lapse rate under normal conditions = -6.5 degrees per km altitude 

• Snowfall typically increases with elevation for two reasons 

  • Orographic precipitation (most air is forced to rise by topography)

  • Proportion if precipitation falling as snow increases with elevation due to cooler temperatures

• Snowfall may reduce at high elevations because cold air cannot hold sufficient moisture (blue dotted line)

glacier mass balance: typical variation with altitude

• Variations of temperature and precipitation with altitude result in typical ‘glacial zones’ 

• Dry snow zone 

  • theoretically no melt - but in greenland there isn’t anywhere like that now - but still in antarctica 

• Percolation zone 

  • Surface melt but refreezes in snow 

• Wet snow zone 

  • The whole snowpack can experience melt and refreezing (not just the surface)

• Superimposed ice zone

  • Meltwater refreezes to colder ice surface 

• Bare ice zone 

  • Each year’s accumulation of snow and ice is fully melted and ice from previous years can be removed 

nolin and payne (2007)

mass balance: visible features

Equilibrium line altitude (ELA) - a line joining locations where ablation is exactly balance by accumulation for a mass balance year 

• Equilibrium line should balance with snow line and firn line - if glacier is in steady state and equilibrium

mass balance gradient

• variation of mass balance with altitude

• concept important for how a glacier might respond to climate change - related to temperature and precipitation lapse rates

• Glaciers with a high mass balance gradient have greater ablation and accumulation - temperate maritime climates 

• Mass balance gradient is the average of accumulation and ablation gradients 

• Ablation gradient typically slightly steeper than accumulation, so inflection point in mass balance gradient at ELA

• Balance ratio = ablation gradient/accumulation gradient 

• Glaciers with high balance ratios (greater rate if change of ablation with elevation) have small ablation areas compared with the area of the glacier and vice versa

glacier mass balance: typical seasonal pattern

• Mass balance is usually seasonally governed, with distinct periods of net positive (winter) and negative (summer) mass balance (winter accumulation regime 

• A mass balance year extends between two successive mass balance minima 

• Most accumulation occurs in the winter, with only a small amount in the summer 

• Most ablation occurs in the summer with only a small amount in the winter 

• The sum of the winter (positive) and summer (negative) balances is the net balance 

how is mass balance quantified?

direct field measurements: representative point measurements distributed across a glacier

• Ablation 

  • Repeated measurement of the length of stakes drilled into the ice surface 

• Accumulation 

  • Short-term accumulation: can also be measured by ablation stakes

  • Seasonal accumulation: measured by reference to upper surface of the previous season’s firn

  • Multi-year accumulation: measured by digging a snow pit, or extracting a shallow ice core and identifying key seasonal variations in snow structure and density

• Net mass balance 

  • Compare ablation (m w.e.) with accumulation (m w.e.) over the balance year

glacier volume change (Geodetic methods)

• From repeat topographic surveys or digital elevation models (DEMs)

  • Map contour comparison

  • Terrestrial photogrammetry 

  • Remotely sensed DEMs - e,g from stereographic aerial or satellite radar altimetry  

• Must convert volume change to mass change and then water equivalent volume (i.e. need to know density of ablated/accumulated material

  • Can calibrate surface elevation changes with snow pits and cores

  • Or use average densities for snow, firn and ice

  • Can have change in volume with no change in mass e.g. snow melting but then refreezing within snowpack and firn densification

hydrological method (for valley catchment glaciers)

• Stream gauges used to measure glacier and catchment runoff, R

• Precipitation, P is measured on and off ice

• Evapouration,  is either estimated or measured (usually relatively small)

• Mass balance = P - R - E

mass flux method (for ice sheets)

• Compares the mass flux (depth-averaged ice flow and ice thickness) through a downstream cross-section (often at the grounding line where the ice begins to float) with upstream observed or modelled accumulation and ablation

Weighting ice sheets: satellite gravimetry

• Satellites (e.g GRACE) measure small changes in the earth’s gravitational r=field every 30 days 

• Temporal changes in the regional mass distribution of ice sheets and underlying bedrock can be extracted after removing the effects of tides, changes in ocean circulation and atmospheric loading

• Two satellites send signals to each other/measure the distance between each other - because of the size of the greenland ice sheet is alters the gravitational pull - these satellites can measure this 

glacier mass balance and climate

• Knowledge about mass balance of current glaciers and its relationship with climate can be used to:

  • Reconstruct past climate from paleo glacier extent, shape and behaviour (e.g. Carr et al. (2010))

  • Quantify/predict impacts of changing glacier mass balance on

    • Catchment water resources (e.g. of mass loss of glaciers in the Alps – Farinotti et al. (2016))

    • Sea level (e.g. Arctic Canada – Gardner et al (2011))

• But mass balance is typically difficult and/or expensive to measure, so glaciologists have produced some simpler ‘proxy’ metrics

contemporary mass balance proxies

• Snow line (c. ELA) mapping of contemporary glaciers as a proxy for mass balance

  • For temperate glaciers, ELA estimated by mapping the end of the summer snow line (i.e. when glacier mass balance is at a minimum) 

  • Relative position of snow line and firn line can be used to assess longer-term trends in mass balance

    • Snow line adjacent to bare ice = positive mass balance relative to preceding year (snow cover more extensive than long-term firn cover) 

    • Snow line separated from bare ice by firn = negative mass balance relative to preceding year (snow cover less extensive than long term firn cover)

paleo glacier mass balance proxies

• Maximum altitude of lateral moraines

  • Delineate ELA based on evacuation of debris by ice flow only in the ablation area 

• Accumulation area ratio (AAR) 

  • Modern glacier ELA used to determine relative size of the accumulation area, typically ~0.6 (60 % of total) 

  • Palaeo glacier extent and surface elevation reconstructed from geomorphology and a hypsometric curve constructed 

• Area altitude (mass) balance ratio (AABR) 

  • AAR varies between glaciers based on hypsometry (distribution of area with elevation) 

  • Inclusion of the (mass) balance ratio improves AAR by considering both glacier hypsometry and the ablation and accumulation gradients (Rea, 2009) 

• Complications 

  • Debris cover insulates ice from atmospheric temperatures 

  • Precipitation shadows change based on growth of down-wind glaciers

glacier response times

• Response time is the time taken for a glacier’s geometry to adjust to a new steady state after a change in glacier mass balance (usually caused by a change in climate)

• Response time depends on 

  • Glacier volume & area (represented by thickness in table opposite)

    • A change in climate will produce a relatively larger change in ELA at a smaller glacier

  • Glacier mass balance gradient (represented by terminus ablation in table opposite)

    • Glaciers with steeper mass balance gradients respond faster due to more rapid mass throughput (these usually also have a steeper surface slope)

long term variations in climate and mass balance

• Sustained positive mass balance 

  • Glacier will advance and thicken 

• Sustained negative mass balance 

  • Glacier will retreat and thin 

• Speed of the above changes depends on glacier’s response time and rate of climate change 

• Size of the above changes depends on the elevation range and hypsometry of the glacier 

  • Glaciers with a greater proportion of their area close to the ELA will experience a bigger response to a change in ELA because a larger portion of their area will experience either a mass gain or loss

role of ice flow

move mass from accumulation to ablation area to try and maintain equilibrium - otherwise would be unstable

redistribution of mass: balance velocities

• Balance velocities relate ice flux to glacier mass balance

• For an idealised glacier of constant size and shape (side view, right)

  • Ice flux through a cross-section should equal the sum of accumulation and ablation occurring up glacier of the section 

  • Based on this idea, glaciers with steeper mass balance gradients will have greater balance velocities 

  • ice discharge increases from the head of the glacier to the ELA, and then decreases from the ELA to the snout

• Useful concept to determine, from ice flow observations, whether a glacier is dynamically unstable (i.e. mass in = mass out)

• But this is rarely the case – glaciers are not usually (or ever!) in steady state but reacting to changes in climate over many timescales

• Glacier will move faster if more accumulation and ablation to balance 

key concepts: newtons law of motion

1. An object either remains stationary or moves at a constant velocity unless acted on by a force   

2. The sum of forces, F acting on an object, is equal to the mass, m of the object multiplied by its acceleration, a. 

F = ma (1 N = 1 kg x m s-2 ) 

3. If an object exerts force on another object, the second object exerts an equal and opposite force on the first

key concepts: stress

Stress

• Force per unit area (Units = Pascals: 1 Pa = 1 N m-2 , or kilopascals 1 kPa = 1000 Pa) 

• Same force acting across a smaller area - bigger stress

Different types of stress

• Normal stress (tension or compression)

  • Acts perpendicular to a surface 

  • E.g faster ice flow meeting slower flowing glacier 

  • Pulling material apart 

• Shear stress 

  • Acts parallel to a surface

  • Differential motion in same direction or opposite directions 

  • Antarctica - fast flowing ice stream bounded by ice flowing in same direction but slower

Material properties

• Yield stress: the stress at which a material begins to deform plastically (i.e. a permanent change in shape). 

• Non-permanent deformation is termed ‘elastic’

stresses affecting a glacier

• Weight, W is the force on an object due to gravity – the product of an object’s mass, m and the acceleration due to gravity, g (9.81 m s-2)

  • W = mg

• For a glacier, mass is a function of the density of glacier ice, i (~900 kg m-3 ) (rho ice) and its thickness, h (~100 m)

• If the glacier bed is flat

  • W = ρi gh 

  • W = 883 kPa

• However, if the glacier is represented by a parallel-sided slab of ice on a sloping bed (e.g.  = 6), W is split into normal stress and basal shear stress,  (tau) components

  • τ = ρi gh sin α

  • τ = 92 kPa

• It is basal shear stress (i.e. the down-slope component of gravity acting on an ice mass) that drives glacial flow

• Ice moving under its own weight due wo these stresses

• Stresses increase with ice depth as the weight of overlying ice increases

key concepts: strain

• Strain 

  • The relative deformation of a material experiencing stress (e.g. change in length in a given dimension) (unitless ratio)

• Strain rate 

  • The rate of change of shape of a material experiencing stress (often used in glaciology) (units: yr-1 )

• Most common types of strain associated with ice flow

  • Pure shear: flattening or compressing (e.g. ice being forced to flow through a lateral constriction such as a narrow valley) 

  • Simple shear: shear stress (e.g. where there are sudden changes in ice speed – at the edge of an ice stream)

    • Differential motion

ice flow mechanisms: internal deformation

• Internal deformation/ice creep due to gravity acting on ice mass (driving stress)

  • Results from movement within or between ice crystals 

  • Above yield stress but below brittle failure (not forming cravasses)

• Glen’s flow law (Glen, 1955; Nye 1957) based on laboratory measurements of ice deformation

• Strain rate roughly scales with the cube of the stress (if stress doubles, strain increases by a factor of 8)

• Internal deformation velocity (UF ) profile – increases towards the surface because sum of deformation in each layer

• Rate of most deformation is at the bottom due to more shear stress - lessens at the top

ice flow mechanisms: basal sliding

• Sliding at the ice-substrate interface (basal sliding)

  • Water at the ice-substrate interface lubricates ice flow

  • Basal water pressure reduces normal stress (due to weight of overlying ice) resulting in a lower ‘effective pressure’ (the actual stress between the ice and its bed)

  • Water may be present because

    • Due to the stress of the overlying ice, ice melts at a lower temperature (see thermal regime slides) 

    • Geothermal heat means the substrate is warm enough to melt the ice 

    • Friction caused by ice motion melts the ice 

    • Surface-derived meltwater reaches the bed through moulins and crevasses

  • The rate of sliding depends largely on the basal water pressure (you will learn more about this later in the module) 

  • Leaves behind key evidence for former ice flow directions (more on this later in the module)

    • Rocks it carries - gauges out - striations 

ice flow mechanisms: substrate deformation

• Many glaciers are underlain by a product of glacier erosion called till, a mixture of sediments varying in size from clay to boulders. 

• Just like basal ice, the till experiences stress from the weight of the overlying ice can deform 

• The strain rates depend on various factors such as the pore water pressure in the till, and the mechanical strength of the till (affected by whether it is frozen solid)

• Evidence:

  • Differential motion of segmented rods in till beneath a glacier in Iceland (Boulton et al., 1979) 

  • Deformed palaeo subglacial sediment

valley glacier velocity profiles

• Ice velocity in a section across a valley glacier is greatest at the top and centre of the glacier

  • Cumulative deformation of underlying ice and basal motion (sliding and/or deformation) 

  • Least friction from valley sides 

  • Can be seen from surface patterns (e.g. ogives) which deform with differential ice motion

typical ice flow velocities

• Most valley glaciers flow at a rate of 10 – 100 m yr-1 (‘normal’ glaciers) by a combination of internal deformation and basal motion

• Many tidewater glaciers and ice streams flow at a rate of 100 - >1000 m yr-1 (‘fast’ glaciers) and move mostly by basal motion up to ~12 km yr-1 (Jakobshavn Isbrae, Greenland) 

• Typically ice flowing > 100 m yr-1 (~0.25 m d-1 ) can be assumed to have a significant basal motion component (see example ice flow speed map of the Greenland Ice Sheet) 

• Surging glaciers (more later in the module) can temporarily flow at > 22 km yr-1 (>60 m d-1 )

limits on ice flow

• Ice is ‘slippery’, and the gradient of glaciers’ beds is often steep, so why don’t glaciers simply slither off downslope? 

• Bed roughness

  • Ice must be transferred around obstacles in the bed

  • There are two main ways this can occur

    • Regelation sliding (bumps < 1 m): Higher stresses on the upstream side of basal obstacles reduce the pressure melting point allowing ice to melt and resulting meltwater to flow to the downstream side of the obstacle where the lower pressure allows the water to freeze back on to the ice. 

    • Enhanced creep (bumps > 1 m): The same higher stresses that cause the ice to melt also result in enhanced ice deformation around basal obstacles

  • Adhesion by freeze-on

    • Requires ice below pressure melting point and variations in effective pressure 

    • Can resist high basal shear stresses 

    • Limits ice motion because basal motion is absent or minimal (i.e. only internal deformation in the layers above)

  • Friction from debris carried by the ice

    • Proportional to the force pressing particles to the bed (i.e. proportion of weight of overlying ice or normal stress) 

    • Pressing down hard on sandpaper and trying to move it across a piece of wood is a useful analogy

quantifying ice flow

• Movement of stakes drilled into ice

  • Traditional & laser surveying methods 

  • GPS antennae 

    • More recently 

  • Very high temporal resolution but only at relatively few points

• Matching of patterns in sequential aerial photographs or satellite images - can measure over a much broader area 

  • So-called ‘feature tracking’ (cross correlation of brightness patterns) 

  • Can cover very large areas at reasonable temporal spacing (days to weeks)

what is glacier thermal regime?

• Thermal regime: variation of temperature of glacier ice 

• Glaciers are not uniformly cold

  • Typical valley glacier close to 0C Parts of Greenland Ice Sheet -20 C 

  • Parts of the Antarctic ice sheet -40 C

• Thermal regime characterises the temperature of a glacier’s ice and its behaviour 

• Pressure melting point of ice (PMP)

  • Melting point of ice decreases with pressure 

  • Beneath 2000 m of ice (typical for Antarctic and Greenland ice sheets) ice melts at -1.27C

controls on ice temperature

• Heat exchange with the atmosphere

  • Temperate regions

    • Seasonal variations in near-surface firn temperature related to atmospheric temperature

    • Winter ‘cold wave’ can penetrate down from surface

  • Cold regions

    • Firn and resulting ice will be close to the annual mean air temperature

• Geothermal heat flux 

  • Heat transfer from Earth’s gradual radioactive decay

  • Varies depending on tectonic setting 

  • Global mean value sufficient to melt 6 mm of ice (at the pressure melting point) per year

• Frictional heat generated by ice (and water) flow

  • Derived from differential ice motion (shear) within the glacier, basal sliding & water flowing in contact with ice (‘cryohydrologic warming’) 

  • Typically, only significant for ice close to the ice base 

  • Heat is equivalent to melt 100 times as large as typical geothermal heat for fast flowing glaciers

Glacier thermal regimes

• Polar glacier (cold based)

  • Air temperature well below zero all year round 

  • ice-bed interface is below freezing too (i.e. ice not thick enough to reduce PMP under high pressure)

• Temperate glacier (warm based)

  • Air below zero in winter, but above in summer; surface-derived meltwater may get to the ice-bed interface 

  • Ice-bed interface is at or above pressure melting point

• Polythermal/subpolar glacier (mixture of cold and warm-based)

  • Typically temperate in interior, but cold at the margins

Typical ice temperature depth profiles

• Typical ice temperature-depth profiles show an increase with depth due to influence of shear heating, basal friction and geothermal heat flux 

• Even in cold regions, basal ice may be at the pressure melting point if the overlying ice is sufficiently thick to reduce the PMP

effects of thermal regime on ice flow

• Cold based

  • Ice-bed interface is strong and can support high shear stresses, so internal deformation dominates ice motion

  • Some regelation and enhanced creep may occur, but rates are slow 

  • Generally low velocity

• Warm based 

  • Ice-bed interface is weak and ice slides readily over the substrate 

  • Deformation of subglacial sediment may also occur 

  • Generally high velocities

Characteristics of glaciers with different thermal regimes

• Cold based

  • Relatively thin, shallow-sloped and slow flowing glaciers where winter air temperatures are very low (e.g. Arctic Canada, Antarctic Dry Valleys) 

  • Minimal potential for landscape alteration (erosion and deposition – more later in the module)

• Warm based 

  • Relatively steep and fast flowing glaciers in more temperate climates (e.g. European Alps, New Zealand)

  • Huge potential for landscape alteration (erosion and deposition – more later in the module)

Glacier behaviour: mass balance, flow and thermal regime 

• Mass balance drives a glacier system 

• Ice flow attempts to redistribute the mass and maintain a steady state surface profile 

• Ice flow is influenced by thermal regime (variations in ice rheology & internal deformation, amount of basal sliding etc.) 

• Ice flow can also affect ice temperature (e.g. shear heating) 

• Together mass balance & thermal regime determine the form, flow and size of a glacier and its tendency to (and rate of) advance or retreat

Mass balance sensitivity to thermal regime

• Dawson et al. (2022) used numerical modelling to explore how warmer basal ice temperatures could affect Antarctic Ice Sheet mass balance 

• Areas of thawing bed would cause new loci of mass loss – more sliding, sediment deformation could lead to faster ice flow into the ocean or to lower, warmer regions. 

• So important to look at the physical processes that could cause basal thawing 

• For example, external atmospheric and ocean forcing driving thinning, recession and acceleration (frictional heat at the bed, impact of warmer air on snow, firn and ice temperatures)

supraglacial definition

water flow on top of ice masses

englacial definition

flow within the glacier e.g crevasses, moulins etc

subglacial definition

water flow beneath the ice

importance of glacial hydrology

crucial to understanding of glacier motion and hazards

water sources

snow and ice melt at the glacier surface

• most important

• up to 10 cm per day - typically less that 10m per year

basal melt

• frictional heating due to differential motion at the ice bed (few 10s of cm/yr)

• geothermal heating (few cm/yr)

• basal melt induced by rainfall and meltwater

groundwater and runoff from subaerial sediments and bedrock

• very few estimates, probably quite small

liquid precipitation (ie, rain)

• may be significant

• depends on climate, altitude, location etc

primary permeability definition

the permeability of an intact block of snow, firn or ice

secondary permeability definition

permeability of fractures, holes, tunnels etc

primary and secondary permeability of snow

primary

• high to low

• water can move through linked spaces around snow crystals

• but dense wind pack is less permeable and ice lenses can be locally impermeable

secondary

• can have rills

• but fewer passageways form due to high primary permeability

primary and secondary permeability of firn

primary

• high to low

• depends on density

• water can move through linked spaces around compressed snow crystals

• dense firn can be almost impermeable and ice lenses can be locally impermeable

secondary

• can have rills but fewer passageways form due to high primary permeability

primary and secondary permeability of glacial ice

primary

• low (if warm ice)

• water can flow along boundaries of ice crystals if pressure gradient is steep

• negligible (if cold)

• air pockets are isolated and no water can flow

secondary

• high for warm ice where there can be many moulins, englacial passageways and subglacial conduits

• lower for cold ice although crevasses can provide an englacial pathway for melt water

variability of meltwater production

• surface melt is largely controlled by incoming (shortwave) radiation (insolation)

• but also by air temperature which is especially important for raising snow and ice to the melting point

spatial and temporal variation of melt rate

spatial

• cloud cover, albedo, debris cover, slope, aspect, shadows, proximity to rock walls

temporal

• sun elevation, air temperature, cloud cover, albedo (snowfall and snowmelt)

spatial variability of meltwater production

• potential direct clear sky insolation is affected by aspect, slope, cast shadows

• insolation changes relative to a flat glacier in the same geographical position

example

• shaded relief, cast shadows e.g mountains, slope and aspect e.g south facing slopes - where sun is in sky in parts of world

• glaciers typically form on north facing slopes

influence of temperature and albedo on ablation

high temp = faster melt

low temp, high albedo, high humidity = snowfall

snowfall leads to accumulation and then slower melt until snow is ablated

higher albedo = snow accumulation and then slower melt

typical pattern of melt rate

runoff decreases with altitude

• temperature and albedo

runoff is much greater from areas of snow free ice because it has a lower albedo an is exposed at lower elevations

for a given glacier, runoff is greatest later in the melt season when surface snow has melted from the ablation zone and temperatures and insolation remain high

influence of debris cover on surface meltwater production

debris cover complicates melt rate

• albedo: thin layer of surface debris reduces glacier albedo and increases melt rate and runoff

• insulation: thick debris insulates the underlying ice resulting in low melt rates despite the lower surface albedo

meltwater flow theory

• flow of water is governed by hydraulic potential - the energy of water available at a particular time and place

• hydraulic potential is controlled by the hydraulic elevation head (elevation potential) and the hydraulic pressure head (Water pressure)

• water flowing on a glacier surface (i.e where water pressure = atmospheric pressure which is even across the glacier) is only affected by the elevation potential and water flows downslope just like on land

• within and beneath a glacier, the pressure head (where water pressure is controlled by the thickness of overlying ice) is also important and water flow routing is determined by a combination of bed elevation and pressure head

• Flow of water governed by hydraulic potential 

  • Energy water has at a given time and location 

  • Hydraulic elevation potential and hydraulic pressure head 

  • Water can flow uphill under glaciers due to variations in ice thickness therefore influencing pressure

meltwater routing

snow and firn: present year round in the accumulation area and snow in the ablation area in winter

• meltwater generated at or near the surface percolates downwards through the snow

• it may refreeze in the snowpack and not runoff, or if meltrate exceeds refreezing rate, water accumulates

• this ponded water can form slush zones in topographic lows on shallow slopes

• if sufficient melt occurs on steeper slopes, meltwater flows literally via rills (narrow and shallow channels) that may occur at the surface or within the snowpack

• percolation depth is limited by the low permeability ice beneath

• water from the snowpack flows onto the glacier surface at the firn line (assuming negative mass balance relative to long term mean)

ice is exposed at the surface in summer in the ablation area (but is present beneath snow and firn across the whole glacier)

• glacial ice has low primary permeability, so in the absence of fractures etc. meltwater quickly accumulates into the supraglacial streams

• streams thermally erode the ice surface faster than ambient ablation due to energy from turbulence

• water flow in streams is rapid compared to percolation

• meltwater may penetrate the top few 10s cm of the ice as solar radiation inducing melting enlarged veins along ice grain boundaries. This forms, a pale, crunchy weathering crust

temporal evolution

• as melt season progresses and snow melts, its extent and thickness reduce

• this reduces percolation times and exposes more ice

• the combined effect is to reduce the time lag between variations in melt production and runoff

supraglacial drainage networks

supraglacial streams

• shallower, lower discharge, transient, tributary segments of a supraglacial drainage network

supraglacial rivers

• deeper, larger discharge, perennial, mainstem segments of a supraglacial drainage network

stream morphology

thermal erosion

• frictional heat dissipation in turbulent water flow

• solar heating due to lower albedo

• after initial erosion, ablation along channel walls widens the stream

• in a wider channel, streams tend to meander due to variations in water flow caused by differences in bed roughness

• once initiated, meandering typically increases because faster flow around the outside of bends is more turbulent and generates more heat to melt the channel wall

• streams tend to be straighter where the local ice surface is steeper and meander more where it is shallower

supraglacial ponds

• form in topographic lows on shallow slopes on a melting glaciers surface

• most common on debris covered glaciers where they significantly enhance local ablation rates due to heat absorption, ice melting and calving

• many larger lakes form annually in ablation areas of ice sheets

englacial drainage: crevasses and fractures

• fractures, including crevasses, caused by tensional stresses within the ice provide pathways for surface water to enter the glacier

• locally generated meltwater flows into crevasses and can fill them up, acting as a temporary store of water

• because water is denser than ice, the additional stress at the base of a water filled crevasse deepens the crevasse delivering water to the ice bed, either directly, or by intersecting an englacial conduit

englacial drainage: moulins

• where a supraglacial stream or river intersects a crevasse, frictional dissipation of heat in the turbulent water flow locally enlarges the crevasse forming a quasi vertical shift called a moulin

• ablation by insolation and relatively warm air enlarges the moulin, particularly near the ice surface

• moulins can fill and overflow from the moulin base

• moulins are often abandoned by water flow as they are advected down glacier

• a new crevasse opens up up stream and a new moulin captures the water flow

• ice flow parallel lines of relict moulins can often be seen down glacier of active moulins

• spatial patterns of stress control where crevasses and thus moulins form, and therefore influence glacier hydrology

englacial conduits

• evidence from boreholes drilled into glaciers suggests that englacial voids and conduits exist to depths of at least 200m

• theoretically, ice walled conduits can form from water flow along ice crystal boundaries if closing rate from the pressure of surrounding ice compared to the water pressure in the conduit is balanced or exceeded by melt caused by heat dissipation in the flowing water

• theoretical steady state conduit water flow suggests that

• 1. bigger conduits enlarge faster

• 2. bigger conduits have lower pressure

• 3. bigger conduits therefore capture water from smaller ones

• 4. water flow is down the steepest hydro potential gradient

• englacial conduits do not tend to follow the steepest hydro potential gradient

• instead, they form from overhanging closure of supraglacial channels (cut and closure), or existing fractures and crevasses that may have been exploited by hydrofracture (excess stress created by water presence or flow)

• connectivity between fractures, moulins and channels remains poorly understood, but is likely complex

• hydro potential field controls large scale water flow within temperate glaciers, but that water exploits all available structural weaknesses meaning that local water flow does not necessarily conform to theory

subglacial hydrology

• flow of water at the base of the ice

• at most temperate glaciers glaciers, the water comes mostly from the surface melt via the englacial system

• basal inputs - from frictional melting caused by ice sliding over the substrate, from geothermal heat flux, ground water and additional melting caused by relatively warm surface derived water

• for some polythermal and cold ice masses, there are no surface inputs and basal inputs dominate

• delivers water to the ice margin where it exits via a portal into a proglacial river, lake or into the ocean

basal environment - hard or soft bed

• glaciers underlain by a product of glacier erosion - till

• till cover - deep and extensive - soft bed

• till cover - shallow, patchy, or none - hard bed

• type of substrate is an important control on how water flows and how the overlying ice behaves

subglacial water flow routing

• beneath a glacier, the pressure head (water pressure controlled by the thickness of overlying ice) and elevation head are both important for hydropential

• the resulting hydro potential surface, can be used to estimate the path of subglacial water down the steepest hydro potential gradient using gridded inputs of ice surface and bed elevation

steady state + equilibrium

• steady state refers to a case where water flux into the system is equal to water flux out of the system

• there is no change in the volume of water within the system

• equilibrium refers to a state where the rate of conduit closure by ice creep from the pressure of surrounding ice compared to the water pressure in the conduit is balanced by melt caused by heat dissipation in the flowing water

subglacial hydraulic efficiency

• subglacial hydraulic efficiency describes how well a subglacial hydrological system can accommodate an increase in water flux

• in an efficient system, a large increase in water input (high melt day or intense rainfall) can be accommodated with minimal impact on water pressure

• in an inefficient system, the same increase in water input results in a larger and longer duration increase in water pressure

• efficient systems may have unused storage capacity or they can increase their hydraulic capacity rapidly to accomodate the additional flux

controls on subglacial water pressure

1. glacier thickness: the greater the weight of the overlying ice, the greater the subglacial water pressure

2. the rate of water supply: inputs of large amounts of meltwater may increase the subglacial water pressure as water backs up in the system

3. the rate of water discharge: an efficient subglacial drainage system will reduce subglacial water pressure

4. the nature of underlying geology: permeable bedrock or sediment will allow water to drain and therefore reduce subglacial water pressure

subglacial drainage configurations

typical hard bed configuration

inefficient drainage - linked cavities

• distributed system of cavities and connections with slow water flow and high water pressure

• higher water flux - higher water pressure - lower basal friction

efficient drainage - rothlisberger channels

• a few main channels with fast water flow and low water pressure

• higher water flux - lower water pressure - higher basal friction

typical soft bed configuration

inefficient drainage - porous water flow through till

• distributed slow water flow through subglacial till at high water pressure

• higher water flux - higher water pressure - more sediment deformation

efficient drainage - hybrid channels

• a few main hybrid channels with fast water flow at low pressure

• higher water flux - lower water pressure - less sediment deformation

methods for observation of the seasonal evolution of subglacial drainage

fluorescent dyes

• injected into supraglacial streams at moulins and detected in the proglacial river

• the time taken for peak dye concentration to be detected, and the shape of the dye return curve provide information about the flow speed and hydraulic efficiency of the subglacial hydrological system

• earlier in the melt season there is a greater lag between injection and detection, and the dye is released from the glacier more slowly

• later in the melt season the dye is transported more rapidly and is more concentrated

• this suggests an evolution in subglacial hydraulic efficiency (from a distributed to a more channelised system) as the melt season progresses

• removal of snow cover and exposure of relatively low albedo ice in the ablation area is the key source of increased surface water flux

seasonal evolution of subglacial drainage theory

in equilibrium (where the channel or cavity is initially neither expanding or contracting)

• for a system of linked cavities an increase in water discharge, Q, leads to cavity expansion, hydraulic jacking and lower effective pressure, N

• for an efficient channel, a similar increase in discharge, Q, leads to enhanced channel wall melting via turbulent heat dissipation, an increase in channel cross section area and higher effective pressure

at a critical discharge, related to the amount of heat generated by the water flow and for a given ice thickness, ice viscosity and bed roughness, wall melting becomes sufficient to outweigh creep closure and an efficient channel can form

water exchange between drainage types

• in equilibrium, water pressure in a channel drops with increasing water flux

• but channel growth can take days to weeks, whilst meltwater supply can vary over hours, so true equilibrium is mostly theoretical

• rapid increases in meltwater supply can overwhelm channels - water pressure increases

• rapid decreases in meltwater supply can lead to open (Atmospheric pressure) flow in channels - water pressure decreases

• diurnal water pressure fluctuations in channels drive exchange of water with surrounding linked-cavities across a variable pressure axis (hubbard et al 1995)

spatial variability in drainage type

connection to surface meltwater sources:

parts of the subglacial drainage system that are directly fed by surface derived meltwater are well connected:

• experience greater water flux overall

• have greater variability in water flux over diurnal to seasonal timescales

• are therefore more likely to be channelised, though channels can be over pressurised daily

over pressurization of channels leads to exchange of water with poorly connected parts of the bed that

• experience smaller water flux overall

• have less variability in water flux

• are therefore more likely to remain inefficient throughout a melt season

remaining unconnected parts of the bed may be hydraulically isolated from surface water

• experience very small water flux as water is generated solely by basal processes, frictional and geothermal melting

• have limited variability in meltwater flux

• are very likely to remain inefficient

case study - small valley glacier, Yukon territory, Canada

• What and where: 8-year dataset of borehole observations on a small, alpine polythermal valley glacier in the Yukon Territory, Canada

Aim: to test how well current theory describes water pressure variations at this glacier

Findings

• Theory of increasing subglacial hydraulic efficiency during a melt season explained most aspects of the data

• Much of the bed becomes disconnected from surface meltwater inputs during melt seasons

• Subglacial drainage is concentrated along narrow pathways adjacent to channels

• In each year, subglacial drainage shuts down soon after snowfall remains for a few days on the glacier (rather than when temperature falls sharply) highlighting the important role of surface albedo

• Dense borehole array on study glacier. Inset shows location (Rada and Schoof, 2018)

subglacial water storage

water can be stored beneath ice masses

the volume and duration of storage varies significantly

cavities:

• form downstream of bedrock bumps and large boulders

• part of a hard bed distributed drainage system

• water flow through this system is determined by the hydraulic capacity of connections between cavities

• water may remain in cavities for days to months

lakes (livingstone et al 2022):

• water accumulated in lows in the subglacial hydro potential surface (e.g. behind very prominent bedrock ridges, or areas where the ice is frozen to the substrate)

• may be hundred of meters in diameter at valley glaciers but can extend to 10s km beneath ice sheets (e.g. lake Vostok in antarctica)

• some lakes form part of the well connected drainage system and can drain over a few days and fill over a few days

• other lakes are isolated from surface meltwater inputs and have remained undisturbed from millenia

influence of bed topography on subglacial water flow

• subglacial water flow direction is determined principally by the ice surface slope and aspect (and consequent variations in the pressure head)

• however, steep adverse bedrock slopes can reduce the subglacial hydraulic gradient

• this reduction hinders development of subglacial channels and encourages distributed water flow

• presence of near terminus distributed drainage is confirmed by the relatively insensitive relationship between proglacial discharge and sediment concentration

glacier change and water resources: european alps

Farinotti et al. (2016)

• glaciers can be considered as stores of water which release melt during the summer when precipitation is generally lower

• modelling of glacier mass balance in european alps over the next 100 years under a moderate emissions scenario shows:

• an initial increase in water supply from glaciers as they melt faster

• but then a reduction as their overall volume decreases

glacier lake outburst floods

Melting glaciers frequently form proglacial lakes in glacier-eroded overdeepenings, dammed by moraine (poorly-sorted glacially-eroded sediment) or ice

• The moraine or ice dam can suddenly fail resulting in a GLOF: ‘a sudden release

of a significant amount of water retained in a glacial lake’ (Emmer, 2017, pg. 1)

• GLOFs

• Are a pervasive global natural hazard (Carrivick and Tweed, 2016)

• Often caused by ice calving, or slope failure into the lake resulting in wave overtopping the dam and subsequent incision

• Earthquakes can trigger the above mass movements

• Characterised by extremely high peak discharges giving high erosional and sediment transport capabilities (Grinsted et al., 2017)

• Can induce other types of mass movement (e.g. GLOF-induced debris flows)

• GLOFs have been recognised by the United Nations Development Program as a significant global natural hazard

importance of glacier hydrology proglacial rivers

• glaciers store solid precipitation that falls in winter and release it as melt in the summer

• glaciers moderate stream flow variability because glacier derived meltwater runoff correlates with temperature and is higher in the summer, while runoff from the rest of the catchment scales with precipitation and is higher outside of the summer season

glacier fed river flow compensation

• this moderating influence of glaciers is called the glacier compensation effect (GCE)(van tiel et al 2019)

• GCE theory suggests:

• interannual streamflow variability can be predicted based on proportional coverage of glaciers in a catchment

• there is an optimum coverage which results in minimal interannual variability

• based on data from 50 partly glacierized catchments in the european alps, scandinavia and western canada (van tiel et al 2021) found that GCE during anomalously warm and dry periods varied considerably

• antecedent conditions, snow cover, glacial water storage and presence of lakes complicated the simple theoretical relationship

distinct glacial meltwater drainage components

The time taken for glacial meltwater to reach the proglacial river varies considerably depending on where melt occurs and how it is routed

•In the accumulation area, melt percolates into the snow and firn, may be temporarily stored, and then slowly flows laterally (1-2 m/d) through the snowpack before entering fractures

•In the ablation area, once surface snow has melted, melt flows rapidly (0.5-2 m/s) across the low permeability ice surface and reaches the glacier bed via crevasses or moulins

• This results in two distinct meltwater flow pathways: quick and delayed

proglacial river hydrograph evolution

Discharge from heavily glacieriszed catchments

displays strong diurnal variability

superimposed onto a slowly varying base flow

• Diurnal variability (solid lines, opposite) is

driven by glacier surface melt via the quick

pathway, though there is a lag between peak

melt and peak discharge

• Baseflow (dashed lines, opposite) is from

groundwater flow, basal melt water, release of

stored meltwater and surface melt via the

delayed pathway

• Peak daily discharge may be double the

baseflow at small glaciers, but diurnal

variations will be relatively smaller at larger

glaciers

• Peak discharge is normally in late July or early

August in the northern hemisphere

The time lag between peak melt and peak discharge decreases as snow thins and recedes and a greater proportion of the runoff is transported via the quick pathway

why study the links between glacier hydrology and motion?

• subglacial hydrology is the most important influence on short term ice velocity fluctuations at temperate glaciers and in many polythermal glaciers and parts of ice sheets too

• subglacial hydrology mainly affects basal sliding

• basal sliding is important because

• it is often the largest component of glacier motion

• it controls glacier erosion and sediment transport

mechanisms of glacier flow

• ice sheets, ice caps and glaciers flow due to the effect of gravity on their mass

• this so called ‘driving stress’ induces three mechanisms that can contribute to surface ice motion

• internal deformation of ice

• sliding between ice and its substrate

• deformation of unfrozen substrate

first detailed observations of ice motion and melt

muller and iken (1973) observed a close association of daily resolution between surface melt rate and ice velocity at four stakes drilled into white glacier - canada

the ice flowed faster on days with more melting

iken (1972) also saw hints of a relationship between water height in moulins and local ice velocity

these observations indicate a causal connection between meltwater inputs to the subglacial system and ice velocity

first detailed observations of coincident horizontal and vertical ice motion

In 1983, Iken and colleagues used repeat photogrammetry and theodolite surveys to show that increases in horizontal ice velocity at Unteraargletscher, Switzerland (lower panel) coincided with uplift of the ice surface (upper panel)

Interestingly, peak horizontal velocity coincided with the greatest rate of uplift, rather than at the point of maximum uplift

This suggested that basal sliding is controlled by subglacial water pressure rather than by the total volume of water stored at the bed

such behaviour occurred multiple times throughout the 1974-1976 melt seasons

regelation sliding and enhanced creep

regelation sliding (weertman, 1957) - small obstacles

• ice melts under pressure due to higher stresses on the upstream side of basal obstacles

• refreezes on the downstream side due to lower pressure

enhanced creep - larger obstacles

• ice is plastically deformed around the feature

• larger obstacles result in increased stress and therefore enhanced ice deformation over a larger area

But neither mechanism could explain the association of faster ice flow with more surface melt, nor of faster horizontal ice motion with vertical uplift, so researchers looked to basal water pressure…