1. C. F. Raymond, “How Do Glaciers Surge? A Review” William A. Brown ESS 433 10/10/07

2. What are glacial surges? and How do glaciers surge?

3. 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 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

4. Variegated Glacier (St. Elias Mountains) and Black Rapids Glacier (Alaska Range), Alaska

5. Medvezhiy Glacier, Pamir Mountains, Tajikistan

6. 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. 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.

7. Quiescent phase

8. 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: 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 an active thickening zone (“reservoir area”) up-glacier a nearly motionless depleting zone (“receiving area”) down-glacier a nearly motionless depleting zone (“receiving area”) down-glacier

9. Geometric evolution, cont’d These two areas are separated by the “dynamic balance line” (DBL) 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 … 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 … 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 Concomitantly, the slope of the glacier increases near the interface between the reservoir and receiving areas

13. Quiescent-phase ice movement and changing geometry Model 1: “flow models based on parameterization of ice deformation with negligible contributions from basal sliding” (Raymond 1987:9124)? Model 1: “flow models based on parameterization of ice deformation with negligible contributions from basal sliding” (Raymond 1987:9124)? Model 2: a difference in basal slip potential between above-DBL and below-DBL areas, affected by spatial variations of basal temperature? Model 2: a difference in basal slip potential between above-DBL and below-DBL areas, affected by spatial variations of basal temperature?

14. Velocity Velocity increase Velocity increase An inter-annual increase in the velocity of ice movement affects both zones of the glacier… An inter-annual increase in the velocity of ice movement affects both zones of the glacier… … but more pronouncedly in the reservoir zone … 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 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 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

16. Basal water discharge High basal water pressure High basal water pressure Low discharge rate … 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 1987:9123) … Except during mini-surges, during which “water flowing along the bed … travels as a pulse of highly turbid water to the terminal stream” (Raymond 1987:9123) accompanied by the inclusion of fine rock debris accompanied by the inclusion of fine rock debris

17. Surge Phase

18. Initiation, propogation, slowdowns, and termination Timing Timing Surge behavior typically initiates in the winter and takes pause or terminates in the early to mid-summer 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 (<10 yr). Surge behavior pulses, pausing after an initial surge season and resuming the following surge season, over the course of a few years (<10 yr). Peak velocity and topographic peak Peak velocity and topographic peak Slowdowns: affect nearly the whole length of the glacier Slowdowns: affect nearly the whole length of the glacier (contra the minisurges of the quiescent interval, which affect only the reservoir area) (contra the minisurges of the quiescent interval, which affect only the reservoir area)

19. Cyclicality These “progressive thickness changes … reverse the thickness changes of the surge and gradually return the glacier to near its presurge state” (Raymond 1987:9122, emphasis mine) These “progressive thickness changes … reverse the thickness changes of the surge and gradually return the glacier to near its presurge state” (Raymond 1987:9122, emphasis mine) “[S]urge may be described by the rapid reversal of the geometrical evolution during quiescence” (Raymond 1987:9124, emphasis mine) “[S]urge may be described by the rapid reversal of the geometrical evolution during quiescence” (Raymond 1987:9124, emphasis mine)

20. Resetting the geometry

21. Surge-phase ice movement Model 1: “… motion is [almost entirely] by sliding and surface mass balance is negligible in the thickness changes” (Raymond 1987:9127) Model 1: “… motion is [almost entirely] by sliding and surface mass balance is negligible in the thickness changes” (Raymond 1987:9127) The high availability of water as a lubricant is corroborated by borehole tests, showing increased water pressure corresponding with surges 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. 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.

22. Back to the big questions

23. 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 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 roles do ice deformation and slip play? What role does basal water play? What role does basal water play? What controls minisurges during the quiescent interval? What controls minisurges during the quiescent interval?

24. Mechanisms driving the life cycle of surge-glaciers Switch mechanisms: Switch mechanisms: What causes the buildup of stored water preceding/triggering surges? 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 1987:9131). 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 1987:9131). What neutralizes surges once set in motion? 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? 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 aren’t all glaciers surge-type glaciers? Why do they lack the critical switch? Why aren’t 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 1987:9130) “The tendency for surges to recur periodically suggests that the geometrical evolution of the glacier has overriding control” (Raymond 1987:9130)

25. In short, drainage constriction leads to a distinctive geometric-evolutionary trajectory … 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 … … 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 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. … 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.

26. Competing hypotheses Equifinality? 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.) … 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? … or can the cooperation of different sets of factors produce comparable behaviors between surge-type glaciers?

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