a2.3gq3 glacial and quaternary geology mike paul welcome to the module
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A2.3GQ3Glacial and Quaternary Geology
Mike Paul
WELCOME TO THE MODULE
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Module Aims
To describe glacial processes, sediments and geomorphology from a range of modern settings
To study analogous settings at various places around the former West Highland glacier complex of Loch Lomond age
To attempt a glaciological reconstruction of the WHGC in order to explain the contrasts in behaviour seen at the positions studied.
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Weekly Module Topics
1. Glacier dynamics and introduction to the WHGC.
2. Study of highland glacial settings. Field visit to Glen Roy
3. Study of meltwater deposits and tidewater glacial settings
4. Study of lowland glacial settings. Field visit to Menteith area
5. Further study of lowland glacial settings
6. No class - university holiday
7. Synthesis and reconstruction of the WHGC
A2.2GQ3 Glacial and Quaternary Geology
LECTURE 1
BEHAVIOUR OF GLACIERS AND ICE SHEETS
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OVERVIEW
Mass balance
Mechanics of glacier flow
Basal regime
Thermal regime
Patterns of glacier flow
Ice streams
Glacier surges
Mass Balance
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The mass balance of a glacier is the net gain or loss of snow and ice during the balance year.
Clearly the balance will be positive on the upper parts of a glacier and negative on the lower parts.
The area of positive balance is known as the accumulation area and the area of negative balance as the ablation area.
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The boundary between the two is called the equilibrium line. Its height is the equilibrium line altitude (ELA) and is determined by several factors:
the relative sizes of the accumulation and ablation areas;
the annual snowfall vs annual melting rate
the annual temperature and temperature gradient
Determining the ELA provides an important parameter for the reconstruction of an ancient glacier.
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If any of these factors change then the ELA will change in response. The most likely change is in snowfall and/or melting rate:
higher snowfall lowers the ELA since the ablation area must expand to balance the higher imput
lower snowfall raises the ELA since the ablation area will contract until melting once again balances input.
Analogous changes occur if the melting rate decreases or increases.
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The equilibrium line is difficult to measure in the field. Thus many workers use the firn line, the boundary between old compacted snow (firn) and glacier ice.
The firn line is the altitudinal limit of surface melting and corresponds closely with the equilibrium line.
Glacier ice is darker than firn - thus the firn line is usually visible on aerial photographs.
Mechanics of Glacier Flow
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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.
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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.
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Internal deformation is described by Glen’s law of ice flow (with various modifications).
This is a temperature dependant flow law that, on a rigid bed, determines the long profile of a glacier or ice sheet.
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Basal sliding is described by various models, including those of Weertman, Lliboutry, Kamb and Nye.
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.
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Bed deformation is described by Boulton and others’ model of a deforming subglacial layer.
Subglacial pore water pressure is a key feature of this model.
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The basic shape of an ice sheet is a dome. Mountain glaciers theoretically approximate to a tilted dome, with the thickest point in the centre.
The exact form depends on several factors: The rheology of the ice (controlled by temperature)
The topography of the bed
The shear stress at the bed (controlled by water pressure and the strength of the bed itself).
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Profiles of an ice sheet on a rigid bed (field evidence from Antarctica)
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Sliding models produce lowered surface profiles compared with rigid bed models.
Rapid ice streams may be explained by deforming bed models and this mechanism has been validated by field observations.
Thermal Regime
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The thermal regime of the body, usually considered in terms of the vertical temperature profile from bed to surface:
• temperate (isothermal) – the ice is at the pressure melting point throughout the ice body;
• warm-based (basal melting) – the basal ice is at the pressure melting point, although higher layers may be below the pressure melting point;
• cold-based (basal freezing) – the basal ice is below the pressure melting point (as therefore must be the whole of the ice body).
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• 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.
• Models of varying complexity can be built, depending on the pattern of surface temperature and the lateral gain/loss of heat and mass.
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Basal Regime
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The basal regime of the ice bodyis the consequence of the thermal regime and/or the nature of the bed:
sliding bed – some component of movement is derived from relative motion between the basal ice and the bed, colloquially as a result of ‘sliding’;
frozen bed – there is little or no relative movement between the basal ice and the bed, the ice being presumed frozen to the bed;
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deforming bed - some component of movement is derived from relative motion within 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 be both geological and glaciological causes.
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Patterns of Glacier Flow
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A parcel of ice a within a glacier follows a flow-line dictated by the gain and loss of mass, and by the longitudinal velocity gradient.
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In the accumulation area, above the equilibrium line, the ice accelerates, thins and the flow-lines descend into the body of the glacier.
This is termed extending flow.
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At the equilibrium line, there is no longitudinal velocity change and the flow-lines are parallel to the bed.
In the ablation area, below the equilibrium line, the ice decelerates, thickens and the flow lines move towards the surface.
This is termed compressing flow.
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Compressing flow may be associated with more extreme shortening by thrusting and/or folding, that causes stacking of the frontal ice.
Compressing flow is a basic mechanism by which basal ice and its associated debris is brought to the glacier surface and so creates conditions of supraglacial deposition.
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Basal thrust, Breidamerkurjökull
Photo: M.A.Paul
Ice Streams
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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 5-10 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.
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Glacier surges
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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.
56Comfortlessbreen: SpitsbergenNorsk Polarinstitutt photo
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ComfortlessbreenSpitsbergenPhoto: J.D.Peacock
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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.
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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.
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Hilmstrombreen: SpitsbergenNorsk Polarinstitutt photo
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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.
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OVERVIEW
Mass balance
Thermal regime
Basal regime
Mechanics of glacier flow
Patterns of glacier flow
Ice streams
Glacier surges
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THE ENDTHE END
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