C. F. Raymond,

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C. F. Raymond, How Do Glaciers Surge? A Review

• William A. Brown
• ESS 433
• 10/10/07

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What are glacial surges?andHow do glaciers
surge?
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Defining surge-type glaciers

• Surge glaciers are defined primarily on the basis
  of their distinctive flow pattern, oscillating
  quasi-periodically between pronounced events of
  accelerated movement and relatively long periods
  of normal/slow movement

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Variegated Glacier (St. Elias Mountains) and
Black Rapids Glacier (Alaska Range), Alaska
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Medvezhiy Glacier, Pamir Mountains, Tajikistan
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Periodicity

• Surge-type glaciers demonstrate a cyclical
  evolution, characterized by a loosely constant
  rhythm alternating between a typically
  inter-decadal quiescent interval and a typically
  brief (2-6 yr) surge phase.

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Quiescent phase
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Geometric evolution during the quiescent phase

• The glacial thickness profile of a typical surge
  glacier during its quiescent phase demonstrates
  an idiosyncratic geometric trajectory over time,
  separating into
• an active thickening zone (reservoir area)
  up-glacier
• a nearly motionless depleting zone (receiving
  area) down-glacier

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Geometric evolution, contd

• These two areas are separated by the dynamic
  balance line (DBL)
• The DBL shows approximately no inter-annual net
  gain or loss of ice thickness
• but it may advance down-glacier as the
  quiescent phase unfolds
• Concomitantly, the slope of the glacier increases
  near the interface between the reservoir and
  receiving areas

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Quiescent-phase ice movement and changing geometry

• Model 1 flow models based on parameterization
  of ice deformation with negligible contributions
  from basal sliding (Raymond 19879124)?
• Model 2 a difference in basal slip potential
  between above-DBL and below-DBL areas, affected
  by spatial variations of basal temperature?

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Velocity

• Velocity increase
• An inter-annual increase in the velocity of ice
  movement affects both zones of the glacier
• but more pronouncedly in the reservoir zone
• Minisurges punctuate this gradual velocity
  increase during the early melt season, announcing
  the arrival of increased summer flow rates
• Like normal glaciers, surge glaciers also
  demonstrate increased flow velocity during the
  melting season, decreased flow velocity during
  the freezing season, w/ increased velocity during
  the summer, owing primarily to seasonal sliding

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Basal water discharge

• High basal water pressure
• Low discharge rate
• Except during mini-surges, during which water
  flowing along the bed travels as a pulse of
  highly turbid water to the terminal stream
  (Raymond 19879123)
• accompanied by the inclusion of fine rock debris

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Surge Phase
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Initiation, propogation, slowdowns, and
termination

• Timing
• Surge behavior typically initiates in the winter
  and takes pause or terminates in the early to
  mid-summer
• Surge behavior pulses, pausing after an initial
  surge season and resuming the following surge
  season, over the course of a few years (lt10 yr).
• Peak velocity and topographic peak
• Slowdowns affect nearly the whole length of the
  glacier
• (contra the minisurges of the quiescent interval,
  which affect only the reservoir area)

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Cyclicality

• These progressive thickness changes reverse
  the thickness changes of the surge and gradually
  return the glacier to near its presurge state
  (Raymond 19879122, emphasis mine)
• Surge may be described by the rapid reversal
  of the geometrical evolution during quiescence
  (Raymond 19879124, emphasis mine)

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Resetting the geometry
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Surge-phase ice movement

• Model 1 motion is almost entirely by
  sliding and surface mass balance is negligible in
  the thickness changes (Raymond 19879127)
• The high availability of water as a lubricant is
  corroborated by borehole tests, showing increased
  water pressure corresponding with surges
• Lowered basal water pressure and flood evacuation
  of basal water from the terminal stream during
  slowdown also suggest lowered lubrication
  potential as the primary slowdown mechanism.

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Back to the big questions
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Mechanisms driving the life cycle of
surge-glaciers

• What drives the establishment of a dichotomized
  surge-glacier geometry during the quiescent phase
  (i.e., upglacier bunch-up vs. downglacier
  depletion)?
• What roles do ice deformation and slip play?
• What role does basal water play?
• What controls minisurges during the quiescent
  interval?

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Mechanisms driving the life cycle of
surge-glaciers

• Switch mechanisms
• What causes the buildup of stored water
  preceding/triggering surges?
• What conclusively releases this buildup of stored
  water preceding the return to the
  ground/quiescent state? A critical basal shear
  stress is reached in the lower part of the
  reservoir area, where the glacier both thickens
  and steepens (Raymond 19879131).
• What neutralizes surges once set in motion?
• What controls the trajectory of surge propagation
  and pause why do surges pulse with
  winter-season regularity over the course of a few
  years?
• Why arent all glaciers surge-type glaciers? Why
  do they lack the critical switch?
• The tendency for surges to recur periodically
  suggests that the geometrical evolution of the
  glacier has overriding control (Raymond
  19879130)

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• In short, drainage constriction leads to a
  distinctive geometric-evolutionary trajectory
• namely the bunching-up of ice overburden in the
  reservoir area, eventually reaching a threshold
  shear stress level in the lower reservoir area
• which results in a dramatic displacement of ice
  through sliding
• which (a) resets the glacial geometry to its
  ground state, (b) facilitates the evacuation of
  the accumulated, high-pressure basal water under
  a new hydrological regime, and (c) subsequently
  resets the drainage system to a constricted
  regime.

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Competing hypotheses

• Equifinality?
• Do all surge glaciers share (and do all normal
  glaciers lack) a common set of driving mechanisms
  and boundary conditions? (e.g., basal
  temperatures, water pressure, upglacier flow
  attributes, etc.)
• or can the cooperation of different sets of
  factors produce comparable behaviors between
  surge-type glaciers?

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Uncontrolled variables

• The character of basal water systems source,
  pressure, drainage system, etc.
• Voids (e.g. basal fissures), which may produce an
  inconstant volumemass ratio
• This interferes with our ability to treat volume
  as a proxy of mass when working with models where
  mass matters
• Nature of the bed hard rock vs. unconsolidated
  and potentially saturated debris