GLACIAL SURGING NADRAH HUSHIN ESS 433

DEFINITION “ A behavior characterized by a multiyear, quasi-periodic oscillation between extended periods of normal motion and brief periods of comparatively fast motion.” (Meier and Post, 1969) Bivachny Glacier, Tajikistan

SURGE CYCLE characteristics Occurs repeatedly Constant quiescent interval between surges. (10-100 years) Short phase (several years) Ice speed >= 10 more than during quiescent stage. Accumulated displacement during quiescent stage is smaller than during surge. 4-ice is drained rapidly from up glacier reservoir area down to receiving area. [during surge] 5- ice accumulates in reservoir area and lost in the receiving area. [during quiescent]

QUIESCENT PHASE COMMON DATAS (FROM 2 GLACIERS) SHOW: Filling of reservoir area and depletion in receiving area Increase in velocity in reservoir area. Dynamic balance line[DBL]: boundary between thickening and thinning of glacier on annual basis.

QUIESCENT PHASE Medvezhiy (1963-1973) glaciers Dominated by annual advance of DBL; separates stagnant ice and uplift the active ice behind. Velocity increases greatly, but annual changes not progressive. Great seasonal variation of velocity. Annual advance of DBL occurs as sequence of ‘wavy surges’- leaves behind severely cracked active zone.

QUIESCENT PHASE 2. Variagated Glacier (1973-1981) DBL moved very slight downglacier Change in elevation (progressive) and steepening of glacier length. Faster motion in summer than winter = seasonal sliding contribution. Increasing velocity in winter between 78-81 is most likely due to sliding Minisurges !

Mini surges in Variegated Glacier Normal occurrence in thickening reservoir area (upper part of glacier). An abrupt increase in speed over few hours. Then, decay slowly ~1 day to near background speed. Speed changes lead to: Increase in seismic activity constant longitudinal strain rates surface elevation changes variation in basal water pressure Propagates downwards. Occurs in early melt season (sequence of 4 – 6), spaced at several days – 2 weeks. Affects reservoir zone only!

Subpolar Condition Dominated by Basal Temperature Distribution. Example: Trapridge Glacier DBL is by boundary between zones of distinct basal slip conditions where ice can slide or not. MAY NOT be affected by this factor, but do have temperate wet bases. Example: Steele Glacier and Black Rapids Glacier.

Surge Phase A) Velocity variation during surge: Day by day variation Started on upper and lower parts of glacier in midwinter 1982, accelerated in Spring 1982. Terminated in late June and early July. Reoccurred in winter 1983, and terminated in early Summer 1983, and spread downglacier almost at the full length of glacier. B) Propagation of surge motion 1. Initiation. 2. Spreading of surging zone by downglacier propagation of velocity and topo disturbance. 3. Leading edges of disturbances coincided, velocity peaks and elevation increases.

Slowing events during surge Regular oscillation ~2 days period on upper glacier. Less regular oscillation (shorter period) on lower glacier. Sequences of slowdown events separated by 4 / 5 days. Accompanied by flood discharges in terminal stream. Results in : Drop in water level in borehole in the central reach of surging ice. - assoc. with hydraulic waves propagation along the bed. Termination of surge episodes.

Surge Periodicity Seasonal Timing ‘The Surge Period’- Separation of surges by relatively constant time interval. Medvezhiy Glacier’s interval between surges shorten from 14 to 10 years (over the last 50 years) Variegated Glacier’s interval ~20 years (although there is deviations) Periods ranging ~10 to 50 years. Seasonal Timing Generally : Initiates in winter, terminates in early summer. Minor exceptions: May initiate in summer terminate in winter.

Surge Motion Mechanisms Deformational motion on surging Variegated Glacier is ONLY small fraction of total speed. Speed is accomplished by rapid sliding on water-lubricated base. Proof: Measurement of water level in borehole. High basal water P during surge; 4 to 1.5 bars of overburden. Lower basal water P after surge; 16 bars below overburden. Flood discharge in terminal streams assoc. to slowdown in motion proves: High water P & sliding speed is related to water storage in glacier. Dye tracing experiment (during and after) shows build up of water storage and high basal P. During surge: slow water discharge ( mean velocity ~0.02 m/s) in large area (as numerous, individual small passageways of mm diameter) After termination: faster discharge ( ~0.7 m/s) (as passageway of m diameter)

Unresolved Questions Bed Structure, Sliding Process, and Basal Water Flow Glacier Sliding (Models) on: Hard bed [interface between ice and rigid bedrock] Soft bed [deformation of an unlithified bed] Structure of basal zone in modelling water flow along base of glacier in: Tunnels melted in ice. Layerlike network along bed Permeable bed material Surges and Other Fast Flow Relationship between surge and continuous fast motion in ice streams and tidewater glaciers

Sources Kamb, W. B., C.F. Raymond, W.D. Harrison, H. Engelhardt, K.A. Enchelmeyer, N. Humphrey, M.M. Brugman, and T. Pfeffer, Glacier surge mechanism: 1982-1983 surge of Variegated Glacier, Alaska, Science, 227 (4686), 469-479, 1985 Meier, M. F., and A. Post, What are glacial surges? Can. J Earth Sci., 6(4), 807-817, 1969. Raymond, C., 1987. Journal of Geophysical Research. Vol 92. N B9. P 9121-9134.