Unit 5.4 Cryosphere Flashcards
Understand the meaning of the terms (a) accumulation, (b) ablation, (c) mass balance, (d) equilibrium line, (e) water-equivalent.
(a) accumulation: processes that add mass to a cryospheric reservoir.
(b) ablation: processes that remove mass from a cryospheric reservoir.
(c) mass balance: the sum of accumulation and ablation (the latter a negative quantity), measured in metres of water-equivalent per year. Winter mass balance is usually positive, and summer mass balance negative. Equilibrium is extremely rare.
(d) equilibrium line: a conceptual line, lying between the zones of accumulation and ablation, at which point there is no net change in mass.
(e) water-equivalent: because the densities of ice and snow are so variable, all mass balance quantities are standardised to their equivalent in water, so as to allow comparisons.
Describe the different forms of frozen water on Earth.
Snow is highly seasonal, falling at sea-level poleward of latitudes 35o N and S (approximately 25% of the surface of the Earth).
Sea-ice is also seasonal.
Glaciers are perennial, covering about 10% of the surface. The two largest are Antarctica and Greenland.
Understand the processes that influence (a) accumulation, (b) ablation, (c) mass balance, (d) equilibrium line.
(a) accumulation: snowfall, avalanching or snow drifting, condensation, freezing of seawater onto floating ice.
(b) ablation: melting, calving (breaking off of icebergs), snow blowing, evaporation.
(c) mass balance: changes in the relative rates of accumulation and ablation.
(d) equilibrium line: changes in the mass balance (therefore, changes in the relative rates of accumulation and ablation).
Identify the difficulties in defining the sensitivity of ice masses to climate change.
Defining the sensitivity of mass balance to climate change, in terms of quantity of change for a given change in (usually) air temperature, is an important concern of cryospheric science. However, defining such a quantity is complicated by the dynamic response of ice masses, in which the slow flow of ice gradually adjusts to accommodate mass balance changes in an attempt to achieve equilibrium, giving significant lags between climate and mass balance change.
E.g. the 20th century retreat of the Glacier d’Argentière in France was a response to the end of the Little Ice Age (1350-1870).
Identify those variables that determine melt rates and account for variations in melt rates.
The amount of energy available for melting snow ( Qm) is given by: Qm = Qnr + Qs + Ql + Qg
Qnr is net solar radiation:
- typically the most important component of energy balance
- can be predicted, using e.g. time of year, location, aspect
- influenced by incoming solar radiation and the surface albedo, which change only slowly over the course of a melt season
- albedo especially important: energy available from solar radiation at the surface is larger by a factor of three where snow is absent
Qs is sensible heat and Ql is latent heat:
- together referred to as ‘turbulent heat fluxes’
- strongly influenced by conditions in the atmosphere close to the surface, particularly wind speed
- tend to be responsible for most day-to-day variations
Qg is heat derived from heat conduction through a snow cover
Melt rate = Qm / latent heat of fusion (the quantity of energy required to turn a given mass of snow or ice into water).
Describe and account for the differences between distributed and channelised drainage systems.
Glacier drainage systems are generally very variable in space and time, and respond to fluctuations in water input and ice configuration.
Distributed: at the very lowest meltwater fluxes (those resulting from basal melting alone, which may be caused by geothermal heating or pressure of the ice) a water film incised downward into sediment, are likely to occur. These are efficient, and high water flux.
Figure 2.5 is useful in visualising these.
Outline the mechanisms of glacial flow by internal deformation, and identify when this is likely to occur.
Ice deforms under its own weight as a plastic or a highly viscous liquid.
The rate of flow due to internal deformation is proportional to the driving stress. This is the product of:
- the density of ice
- the thickness of ice
- gravity
- the surface slope of the ice
If mass is added to the glacier in its accumulation area and removed from its ablation area, the surface of the ice mass will steepen. This cannot continue indefinitely, some process will act to restore the equilibrium of the glacier. Ice flow can therefore be seen as a response of the glacier to changes in ice thickness and slope caused by accumulation and ablation.
Glaciers that experience high winter snowfalls and warm summers generate a higher rate of mass turnover than glaciers that experience small winter snowfalls and cold summers, and will correspondingly be more actives, exhibiting faster rates of ice flow.
Outline the mechanisms of glacial flow by basal sliding, and identify when this is likely to occur.
Basal motion is intimately connected with the flow of water at the base of the glacier. Basal water pressure can either offset the pressure from the overlying ice, or it can weaken subglacial sediments, allowing them to deform beneath the glacier. Water can also submerge bed roughness, reducing friction between the glacier and its bed.
Distributed drainage systems generate greater basal flow, as they are less efficient than channelised systems, and so a greater proportion of the bed is occupied by water.
Outline the mechanisms of glacial flow by subglacial sediment deformation, and identify when this is likely to occur.
Subglacial sediments can deform more readily than ice.
High basal water pressure increases sediment deformation.
Identify the difficulties in the calculation of the sediment yield from a glacier.
Calculations of sediment yield typically rely on measurements of the mass of sediment transported for the catchment by suspension in turbulent meltwater. It is simple to acquire and analyse such samples, but much more difficult to ensure that these samples are representative of the sediment transport regime. In particular, meltwater discharge and sediment transport can change enormously over a few hours, or often less, while there can often be significant differences in rates of sediment transport from one melt season to another. Because most glaciers are relatively remote, the sort of intensive, long term monitoring required to quantify this variability is rarely implemented.
Provide a definition of periglacial environments at the Earth’s surface in terms of MAAT and state.
MAAT: mean annual air temperature.
Environments strongly affected by sub-surface freezing are usually described as periglacial. Generally, there are two important features of a periglacial environment: intense frost action, and a perennially cryotic ground (permafrost). The periglacial environment can be defined as that where the MAAT is less than +3 oC.
Outline the differences between the ‘freeze-thaw’ model of frost weathering and the ice segregation model.
The ‘freeze-thaw’ model is of in-situ mechanical breakdown of rock by repeated freezing and expansion of water within confined pore spaces, joints and bedding planes.
This simple model may be inadequate: laboratory experiments have shown that freezing-induced weathering occurs at temperatures between -3 oC and -6 oC, in the absence of freeze-thaw.
In the segregation ice model, expansion is primarily the result of water migration to growing ice masses, and only secondarily to volume expansion. Weathering then results from the progressive growth of void spaces wedged open by ice growth.
Outline the reasons that salt can exacerbate damage caused by freezing.
- salts accumulated in the outer layers of rocks by evaporation may block pores and seal the surface, preventing freezing pressures from being released
- salt crystals may grow with ice crystals
- the depressed freezing point of solute-rich water allows more time for susceptible materials to hydrate, expand, and disintegrate, while slower rates of freezing may allow larger ice crystals to grow.
Identify features of the landscape that result from periglacial and permafrost conditions.
Felsenmeer: from the German ‘sea of rock’ (fig 3.5)
Frost cracking: fracturing of the ground by thermal contraction at sub-freezing temperatures. Frost crack polygons are typically 5-30cm across (fig 3.8)
Nivation: localised denudation of the ground surface by a combination of processes in association with snow patches, leading to the formation of nivation hollows (fig 3.9)
Wind: steep atmospheric gradients from the centres to the margins of ice sheets produce consistently strong winds. At the same time, low levels of precipitation and low temperatures support minimal vegetation cover, and finely ground glacial debris is abundant: the result is thick blankets of loess, which spread to cover large areas in North America and Eurasia located on the southern margins of former ice sheets.
Describe features of the hydrology of permafrost regions, including the seasonality of runoff, and the role of taliks and icings.
Although surface water in periglacial environments is frozen for much of the year, and precipitation levels are generally low, river processes still dominate periglacial landscapes. River regimes are highly seasonal, with very large discharges being sustained for short periods during spring snowmelt. The impermeability of permafrost causes a very high percentage of rainfall and snowfall to contribute to surface runoff.
A brief interval of high runoff yields higher river energy than a more extended period of lower runoff. Despite low annual precipitation totals, seasonal rivers in periglacial regions can therefore erode and transport sediment very effectively.
A talik is a pocket of unfrozen water which lies between the permafrost and the active layer.
Icings are sheet-like masses of ice which form at the surface in winter where water issues from the ground. Groundwater icings are commonly generated by taliks, which can sustain perennial water flow.
