A2.3GQ3 Glacial and Quaternary Geology Mike Paul

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A2.3GQ3Glacial and Quaternary GeologyMike 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

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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

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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.

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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 Glens 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.

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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
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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.

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Comfortlessbreen SpitsbergenNorsk
Polarinstitutt photo
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Comfortlessbreen Spitsbergen Photo 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 END

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