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A2.2IE1 Introduction to Environmental Processes Week 8. GLOBAL ICE. 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 .

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overview
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
slide4
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
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).
slide8

The Vatnajokull ice cap, Iceland

Source: Landmaelingar Islands

slide9

The Antarctic ice sheet

Photo: British Antarctic Survey

classification of ice bodies1
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 bodies2
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
slide18

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 bodies3
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 bodies4
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 bodies5
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
slide25

Basal conditions

Source: Benn & Evans 1998

mechanics of glacier movement
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 movement1
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.
slide29

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.
slide32

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.
slide35

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.
slide38
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.
slide40
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.
slide44
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.
slide48
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.
observations of surge type glaciers
Observations of surge-type glaciers
  • Surging glaciers undergo periodic increases in discharge, perhaps by an order of magnitude.
  • Several hundred present-day surge-type glaciers have been identified, either from direct observation or from geological evidence.
  • They appear to be particularly common in certain geographical areas, including Alaska, Spitsbergen and Iceland.
slide58

Comfortlessbreen

Spitsbergen

Photo: J.D.Peacock

observations of surge type glaciers1
Observations of surge-type glaciers
  • During a surge the glacier snout may advance by several kilometres in a few years.
  • This is ~100x faster than quasi-static flow.
  • This rapid movement may be the result of large scale detachment of the ice from its bed, possibly due to the creation of a thick water film that submerges the controlling obstacles.
observations of surge type glaciers2
Observations of surge-type glaciers
  • In comparison with non surge-type glaciers, surge-type glaciers broadly:
    • have higher accumulation rates;
    • experience warmer ambient temperatures;
    • rest on more deformable bedrock lithologies.
observations of surge type glaciers3
Observations of surge-type glaciers
  • Only temperate-based or subpolar (thermally composite) glaciers are known to surge.
  • No instances are known of surging in entirely cold-based glaciers and theoretical glacier dynamics suggests that this type of glacier cannot surge.
slide62
The surge causes extreme deformation to both the glacial ice and to the deposits around the glacial margin.
  • It also produces very large volumes of meltwater.
  • Following the surge the glacier enters a quiescent phase, during which the ice wastes back to around its previous position.
slide64

Sefstrømbreen

Coraholmen

slide66

Glacier Hydrology

  • In temperate glaciers the presence of meltwater at the bed exerts a fundamental control on the rate of movement and the deposition of sediment.
  • The greater part of glacial meltwater originates on the ice surface and finds its way to the bed via internal conduits and by intergranular flow.
slide67

Blomstrandbreen

Spitsbergen

Photo: J.D.Peacock

slide69

Glacier Hydrology

  • At the glacier bed, the water flow may become either channelised or distributed:
    • in basal channels
    • within subglacial sediments
    • as a thin film at the ice-bed interface.
slide71
Subglacial water flows under pressure and so reduces the stress both on the bed and within any subglacial sediment.
  • This can create the conditions for a rapid-flow regime such as an ice stream or surge.
slide72

Glacier Hydrology

  • Catastrophic drainage ( Icelandic: Jökulhlaup) occurs when the ice detaches over a wide area and causes the drainage of a substantial body of water such as a lake.
  • Catastrophic lake drainage may have occurred in the Great Glen during the ice age, with a major outflow via the Inverness area.
  • Such an event recently occurred under the Icelandic Vatnajökull ice cap in May 1996, due to a subglacial volcanic eruption.
  • The discharge occurred via the Skeiderar glacier and thence to the Atlantic.
slide73

Subsidence above Grimsvötn

Vatnajökull, Iceland 1996

slide78
Sea ice is a general name for:
    • pack ice - mobile plates of frozen seawater
    • fast ice - frozen coastal water attached to land
    • icebergs - fragments of glacier or ice shelf
  • Sea ice undergoes seasonal expansion and contraction.
  • Free sea ice drifts with surface currents. Individual fragments can survive for several years.
slide79
Pack ice is frozen seawater. It reaches a maximum thickness of a few metres, except when compressed at pressure ridges.
  • Individal fragments can survive for several years if they become trapped within a circulation such as the Arctic ocean or the Weddell Sea.
  • There is a strong seasonal change in pack ice cover, with a maximum (in n.hemisphere) around February.
slide85
There is also a strong annual change in pack ice cover and thickness.
  • The recent trend is for a thinning of the pack ice and a reduction in thickness, perhaps by up to 30%+ over the past 20 years.
  • The long term effect on regional and global climate is not yet understood.
slide90
Icebergs are broken fragments of glaciers.
  • Those derived from tidewater ice fronts are usually small and relatively unimportant in the context of global ice.
  • They can, however, pose a considerable local hazard.
slide91

Kongsfjord

Spitsbergen

Photo: J.D.Peacock

slide92

Kongsfjord

Spitsbergen

Photo: J.D.Peacock

slide93

Kongsfjord

Spitsbergen

Photo: M.A.Paul

slide94
Tabular icebergs are derived from ice shelves. They are sometimes very large (100s kms)
  • They are mainly found in the southern hemisphere, derived from shelves around the Antarctic ice sheet.
  • There have recently been some spetacular collapses of Antarctic ice shelves, with the production of very large tabular bergs.
slide105
Ice sheets and sea ice form an important component of the earth’s climate system.
    • The extent of snow and ice controls planetary albedo
    • The discharge of cold fresh water controls ocean currents in the North Atlantic
    • The removal of water to form global ice sheets exerts a major control on sea-level on the 1000 year timescale.
    • Due to the time-lag, global ice acts as a climatic buffer over periods of 1000 to 10,000 years.
slide106
In the recent geological past, global ice has been much more extensive than today.
  • The major centres of ice formation were located around the North Atlantic margin, with minor centres over the main mountain belts.
  • The distribution was controlled by snowfall, not by temperature. This is also true today.
slide108
Glaciations have occurred regularly during the past few millions of years, due to periodic changes in the earth’s orbit.
  • There are three orbital components involved:
    • Variation in the elliptical shape of the orbit (eccentricity: 400ka and 100ka cycles).
    • Variation in the magnitude of the axial tilt relative to the plane of the orbit (obliquity: 41ka cycle)
    • Variation of the direction of axial tilt relative to the long axis of the orbit (precession: 19 ka and 23ka cycles).
causes
Causes
  • It is generally agreed that these parameters have provided the primary driver for climate change both during the Quaternary and earlier geological periods.
  • However, these changes have not always resulted in glacial events throughout geological time. Other factors must also be involved.
  • During the Quaternary, changes in ocean currents, particularly in the North Atlantic, provide a non-linear amplification of this effect and generate the mid-latitude ice sheets in response.
  • It is thought that a north-south ocean basin is required for glaciations, also perhaps a polar continent. Thus plate configurations provide a very long-term control.
summary
SUMMARY
  • 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