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

William A. BrownWilliam A. Brown

ESS 433ESS 433

10/10/0710/10/07

What are glacial surges?What are glacial surges?andand

How do glaciers surge?How do glaciers surge?

Defining “surge-type” Defining “surge-type” glaciersglaciers

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

Variegated Glacier (St. Elias Variegated Glacier (St. Elias Mountains) and Black Rapids Mountains) and Black Rapids

Glacier (Alaska Range), AlaskaGlacier (Alaska Range), Alaska

Medvezhiy Glacier, Pamir Medvezhiy Glacier, Pamir Mountains, TajikistanMountains, Tajikistan

PeriodicityPeriodicity

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

Quiescent phaseQuiescent phase

Geometric evolution during the Geometric evolution during the quiescent phasequiescent phase

The glacial thickness profile of a The glacial thickness profile of a typical surge glacier during its typical surge glacier during its quiescent phase demonstrates an quiescent phase demonstrates an idiosyncratic geometric trajectory over idiosyncratic geometric trajectory over time, separating into:time, separating into: an active thickening zone (“reservoir an active thickening zone (“reservoir

area”) up-glacierarea”) up-glacier a nearly motionless depleting zone a nearly motionless depleting zone

(“receiving area”) down-glacier(“receiving area”) down-glacier

Geometric evolution, cont’dGeometric evolution, cont’d

These two areas are separated by the These two areas are separated by the “dynamic balance line” (DBL)“dynamic balance line” (DBL) The DBL shows approximately no inter-The DBL shows approximately no inter-

annual net gain or loss of ice thickness …annual net gain or loss of ice thickness … … … but it may advance down-glacier as but it may advance down-glacier as

the quiescent phase unfoldsthe quiescent phase unfolds Concomitantly, the slope of the glacier Concomitantly, the slope of the glacier

increases near the interface between increases near the interface between the reservoir and receiving areasthe reservoir and receiving areas

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Quiescent-phase ice Quiescent-phase ice movement and changing movement and changing

geometrygeometry Model 1: “flow models based on Model 1: “flow models based on

parameterization of ice deformation with parameterization of ice deformation with negligible contributions from basal sliding” negligible contributions from basal sliding” (Raymond 1987:9124)?(Raymond 1987:9124)?

Model 2: a difference in basal slip potential Model 2: a difference in basal slip potential between above-DBL and below-DBL areas, between above-DBL and below-DBL areas, affected by spatial variations of basal affected by spatial variations of basal temperature?temperature?

VelocityVelocity

Velocity increaseVelocity increase An inter-annual increase in the velocity of ice movement An inter-annual increase in the velocity of ice movement

affects both zones of the glacier… affects both zones of the glacier…

… … but more pronouncedly in the reservoir zonebut more pronouncedly in the reservoir zone

Minisurges punctuate this gradual velocity increase Minisurges punctuate this gradual velocity increase during the early melt season, announcing the arrival of during the early melt season, announcing the arrival of increased summer flow ratesincreased summer flow rates

Like “normal” glaciers, surge glaciers also demonstrate Like “normal” glaciers, surge glaciers also demonstrate increased flow velocity during the melting season, increased flow velocity during the melting season, decreased flow velocity during the freezing season, w/ decreased flow velocity during the freezing season, w/ increased velocity during the summer, owing primarily increased velocity during the summer, owing primarily to seasonal slidingto seasonal sliding

Basal water dischargeBasal water discharge

High basal water pressureHigh basal water pressure

Low discharge rate …Low discharge rate …

… … Except during mini-surges, during which Except during mini-surges, during which “water flowing along the bed … travels as “water flowing along the bed … travels as a pulse of highly turbid water to the a pulse of highly turbid water to the terminal stream” (Raymond 1987:9123)terminal stream” (Raymond 1987:9123) accompanied by the inclusion of fine rock accompanied by the inclusion of fine rock

debrisdebris

Surge PhaseSurge Phase

Initiation, propogation, Initiation, propogation, slowdowns, and terminationslowdowns, and termination

TimingTiming Surge behavior typically initiates in the winter Surge behavior typically initiates in the winter

and takes pause or terminates in the early to and takes pause or terminates in the early to mid-summermid-summer

Surge behavior pulses, pausing after an initial Surge behavior pulses, pausing after an initial surge season and resuming the following surge surge season and resuming the following surge season, over the course of a few years (<10 yr).season, over the course of a few years (<10 yr).

Peak velocity and topographic peakPeak velocity and topographic peak Slowdowns: affect nearly the whole length Slowdowns: affect nearly the whole length

of the glacierof the glacier (contra the minisurges of the quiescent interval, (contra the minisurges of the quiescent interval,

which affect only the reservoir area)which affect only the reservoir area)

CyclicalityCyclicality

These “progressive thickness changes … These “progressive thickness changes … reversereverse the thickness changes of the surge the thickness changes of the surge and gradually return the glacier to near its and gradually return the glacier to near its presurge state” (Raymond 1987:9122, presurge state” (Raymond 1987:9122, emphasis mine)emphasis mine)

““[S]urge may be described by the rapid [S]urge may be described by the rapid reversalreversal of the geometrical evolution during of the geometrical evolution during quiescence” (Raymond 1987:9124, emphasis quiescence” (Raymond 1987:9124, emphasis mine)mine)

Resetting the geometryResetting the geometry

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Surge-phase ice movementSurge-phase ice movement

Model 1: “… motion is [almost entirely] by Model 1: “… motion is [almost entirely] by sliding and surface mass balance is negligible sliding and surface mass balance is negligible in the thickness changes” (Raymond in the thickness changes” (Raymond 1987:9127)1987:9127) The high availability of water as a lubricant is The high availability of water as a lubricant is

corroborated by borehole tests, showing increased corroborated by borehole tests, showing increased water pressure corresponding with surgeswater pressure corresponding with surges

Lowered basal water pressure and flood evacuation Lowered basal water pressure and flood evacuation of basal water from the terminal stream during of basal water from the terminal stream during slowdown also suggest lowered lubrication slowdown also suggest lowered lubrication potential as the primary slowdown mechanism.potential as the primary slowdown mechanism.

Back to the big questionsBack to the big questions

Mechanisms driving the life Mechanisms driving the life cycle of surge-glacierscycle of surge-glaciers

What drives the establishment of a What drives the establishment of a dichotomized surge-glacier geometry dichotomized surge-glacier geometry during the quiescent phase (i.e., during the quiescent phase (i.e., upglacier bunch-up vs. downglacier upglacier bunch-up vs. downglacier depletion)?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 What controls minisurges during the quiescent interval?quiescent interval?

Mechanisms driving the life Mechanisms driving the life cycle of surge-glacierscycle 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 What conclusively releases this buildup of stored water preceding the

return to the ground/quiescent state? “A critical basal shear stress is 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 reached in the lower part of the reservoir area, where the glacier both thickens and steepens” (Raymond 1987:9131).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 What controls the trajectory of surge propagation and pause; why do surges “pulse” with winter-season regularity over the course of do surges “pulse” with winter-season regularity over the course of a few years?a few years?

Why aren’t all glaciers surge-type glaciers? Why do they lack the Why aren’t all glaciers surge-type glaciers? Why do they lack the critical switch?critical switch?

““The tendency for surges to recur periodically suggests that the The tendency for surges to recur periodically suggests that the geometrical evolution of the glacier has overriding control” (Raymond geometrical evolution of the glacier has overriding control” (Raymond 1987:9130)1987:9130)

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

… … namely the bunching-up of ice overburden in the namely the bunching-up of ice overburden in the reservoir area, eventually reaching a threshold reservoir area, eventually reaching a threshold shear stress level in the lower reservoir area …shear stress level in the lower reservoir area …

… … which results in a dramatic displacement of ice which results in a dramatic displacement of ice through sliding …through sliding …

… … which (a) resets the glacial geometry to its which (a) resets the glacial geometry to its ground state, (b) facilitates the evacuation of the ground state, (b) facilitates the evacuation of the accumulated, high-pressure basal water under a accumulated, high-pressure basal water under a new hydrological regime, and (c) subsequently new hydrological regime, and (c) subsequently resets the drainage system to a constricted regime.resets the drainage system to a constricted regime.

Competing hypothesesCompeting hypotheses

Equifinality?Equifinality? Do all surge glaciers share (and do all “normal” Do all surge glaciers share (and do all “normal”

glaciers lack) a common set of driving glaciers lack) a common set of driving mechanisms and boundary conditions? (e.g., mechanisms and boundary conditions? (e.g., basal temperatures, water pressure, upglacier basal temperatures, water pressure, upglacier flow attributes, etc.) …flow attributes, etc.) …

… … or can the cooperation of different sets of or can the cooperation of different sets of factors produce comparable behaviors factors produce comparable behaviors between surge-type glaciers?between surge-type glaciers?

Uncontrolled variablesUncontrolled variables

The character of basal water systems: The character of basal water systems: source, pressure, drainage system, etc.source, pressure, drainage system, etc.

Voids (e.g. basal fissures), which may Voids (e.g. basal fissures), which may produce an inconstant volume:mass ratioproduce an inconstant volume:mass ratio This interferes with our ability to treat volume as a This interferes with our ability to treat volume as a

proxy of mass when working with models where proxy of mass when working with models where mass mattersmass matters

Nature of the bed: hard rock vs. Nature of the bed: hard rock vs. unconsolidated and potentially saturated unconsolidated and potentially saturated debrisdebris