OVERVIEW • Introduction to global ice • Glaciers and ice sheets • Types of ice body • Thermal conditions • Basal conditions • Ice movement • Mechanisms • Styles • Glacier hydrology • Sea ice • Global ice in the climate system
Global ice is the collective name for glaciers, ice sheets, sea ice and snow. • Together, they form an important component in the global climate system due their ability to reflect sunlight, to store global water and to buffer atmospheric heat flow. • They also play a central role in the behaviour of deep ocean currents in the thermohaline circulation.
Classification of ice bodies • We normally divide ice bodies into four different types: • ice sheets and ice domes, which flow under their own weight without the influence of confining rock walls • valley glaciers which flow downslope and are contained within confining rock walls • ice-shelves, which float without influence from the bed. • Pack ice, which is mostly frozen sea (some snow).
The Vatnajokull ice cap, Iceland Source: Landmaelingar Islands
The Antarctic ice sheet Photo: British Antarctic Survey
Classification of ice bodies • An ice body can also be classified on the basis of its thermal regime • This describes the vertical temperature profile from bed to surface • The key point is whether all or part of the ice body is at the pressure melting point
Classification of ice bodies • Temperate – the ice is at the pressure melting point throughout the ice body • Warm-based – the basal ice is at the pressure melting point, although higher layers may be below the pressure melting point • Cold-based – the basal ice is below the pressure melting point (as therefore must be the whole of the ice body) • Polythermal – some parts of the basal ice are at the pmp., other parts are below
Classification of Ice Bodies • More realistic models allow the thermal regime to vary across the ice sheet. These regimes are termed polythermal. • In such a regime some parts of the basal ice are at the melting point, other parts are below. • The exact distribution depends on ice thickness, basal heat generation and surface temperature.
Classification of ice bodies • Ice bodies on land can be considered in terms of their basal regime • This regime is a consequence of the both thermal regime of the ice and and the nature of the substrate (bedrock or sediment) • Three different regimes are distinguished:
Classification of ice bodies • frozen bed – there is little or no relative movement between the basal ice and the bed, the ice being presumed frozen to the bed • sliding bed – some component of movement is derived from relative motion between the the basal ice and the bed, colloquially as a result of ‘sliding’
Classification of ice bodies • deforming bed - some component of movement is derived from relative motion within the the bed below the basal ice, as a result of internal deformation within the substrate • The opposite of a deforming bed is a rigid bed. • The distinction may have both geological (hard orck) and glaciological (cold ice) causes
Basal conditions Source: Benn & Evans 1998
Mechanics of Glacier Movement • Three basic mechanisms exist by which ice is able to flow relative to its bed. These are: • internal plastic flow; • basal sliding; • subglacial bed deformation.
Mechanics of Glacier Movement • The operation of, or relative importance of, any particular mechanism(s) depends largely on basal conditions. • They are not mutually exclusive: many ice bodies flow by more than one mechanism and they may switch in importance both spatially and temporally.
Mechanics of Glacier Movement • Internal deformation is described by Glen’s law of ice flow. • This is a temperature dependant flow law that, on a rigid bed, determines the profile of a glacier or ice sheet.
Mechanics of Glacier Movement • Basal sliding is described by various models. All involve regelation and invoke the concept of a controlling size of bedrock obstacles. • The differences lie in the mathematical model used to decribe the obstacles and the ice flow around them.
Mechanics of Glacier Movement • Bed deformation is described by Boulton’s model of a deforming subglacial layer. • Subglacial pore water pressure is a key feature of this model.
Observations show that there are three general styles of ice movement: • Slow quasi-static flow • Rapid ice streams • Glacial surges • These styles are probably controlled by ice temperature, subglacial hydrology and the deformability of the subglacial bed.
Some glaciers show only quasi-static movement. Typically these are smaller bodies, often mountain glaciers, and rest only on hard rigid bedrock. Velocities are a few metres or 10s metres/year. • It is controlled mainly by internal plastic flow, with some contribution from basal sliding. • It leads to a characteristic parabolic surface profile in the direction of ice flow. Many glaciers show such a profile. • Sliding produces a flatter surface profile compared with internal flow, which produces a steeper parabolic profile.
Large ice sheets usually have some areas that drain by quasi-static flow and other areas that drain via rapid ice streams. • An ice stream is a narrow zone of ice that flows at about 10 times the rate of the surrounding quasi-static area. • They are often located over areas of soft sediment or in areas into which large volumes of basal meltwater are channelled.
It is believed that many (all?) of the large Quaternary ice-age ice sheets drained via ice such streams. • The routes of these ice streams are now marked by eroded lowlands, ice-moulded or drumlinised landscapes, deformed subglacial sediments. • In Scotland, the main east coast firths now mark the position of former ice streams. • Others are known from the Norwegian Trench and the St Lawrence channel.