Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Raymond, C. F.; Malone, Stephen D.

doi:10.1017/s0022143000015495

Cited by 44 publications

(69 citation statements)

References 13 publications

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“…The strongest extension feature is in Figure 18c; it is a weak but definite extension peak, followed by some extra compression before the final state of steadily accumulating compression is attained. To the extent that an extension peak is recogniz able in the strain curves of Figure 18, the curves approach the form portrayed by Raymond and Malone (1986, fig. 5) for event 80-S at Km 7.0.…”

Section: Long-interval Strainmentioning

confidence: 73%

“…Superimposed on the summer-time "normal" are high peaks of daily motion in the range 80-1 40 cm/d; these are the mini-surges, and they are accompanied by the several characteristics described below, and by Raymond and Malone (1986) and Humphrey and others (1986). Records of seismicity and of daily motion from automatic camera photographs indicate that outside the time windows of Figures 2 and 3 there was one additional mini-surge in 1980, on 16 August, and none in 1979.…”

Section: Km 2�mentioning

confidence: 99%

“…Raymond and Malone (1986) described this seis micity, on the basis of seismometer measurements. Figures 7a, h, 8a, h, lOa, 15a * Water-pressure wave not definitely associated with a mini-surge distinct from event 79 -3 a, for which no propagation speed is available.…”

Section: Seismicitymentioning

confidence: 99%

“…The present paper deals with water-pressure measurements in relation to short-term measurements of glacier motion. A second paper (Raymond and Malone, 1986) deals with strain -meter and seismicity measurements, and a third paper (Humphrey and others, 1986) deals with observations of water flow and turbidity in the terminal outlet stream. A fourth paper (in preparation by Kamb and others) will interpret the observations in terms of a theoretical model of mini-surge propagation, and a fifth paper will concentrate on observations in 1981 related to initiation of the mini surges.…”

Section: Introductionmentioning

confidence: 99%

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Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

1987

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ABSTRACT. Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-8 1 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These "mini-surges" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-1 1 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal out flow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The mini-surge wave propagation is controlled by the propaga tion of the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longi tudinal gradient (up-glacier increase) in hydraulic conductivity of the basal water system in response to the up-glacier increase of the basal water pressure in the mini surge wave. The mini-surge waves are initiated in a succession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial *Contribution No.3734,

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Section: Long-interval Strainmentioning

confidence: 73%

Section: Km 2�mentioning

confidence: 99%

Section: Seismicitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

1987

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“…Lliboutry, 1968Lliboutry, , 1979Budd and others, 1979;Fowler, 1986) which predict ub in terms of basal shear stress and water pressure. Toward this goal we will focus primarily on the winter distributions, in recognition that during summer the sliding velocity and underlying physical controls are tied to melt cycles dominated by short time-scale transients (Harrison and others, 1986;Humphrey and others, 1986;Raymond and Malone, 1986;Kamb and Engelhardt, 1987), which are not completely documented by data or easily approachable theoretically.…”

Section: Changes In Basal Sliding Velocitymentioning

confidence: 99%

Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

Raymond

Harrison

1988

J. Glaciol.

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ABSTRACT. During the decade prior to its 1982-83 surge, Variegated Glacier experienced progressive changes in geometry and velocity. It thickened in the upper 60% and thinned in the lower 40% of its 20 km length. Thickness changes were up to 20%. Annual velocity increased by up to 500%, reaching a maximum of 0.7 m d-1 in the year before surge onset. Amplitude of seasonal variation in velocity increased up to 0.3 m d-1 by 1978, but did not increase markedly after that. The changes in velocity were larger than predicted from changes in deformation rate caused by changes in shear stress and depth. This anomalous velocity was especially large after 1978 in the zone of thickening on the upper glacier. If it is assumed to arise from basal sliding, the inferred pattern of sliding shows qualitative features consistent with a direct effect from basal shear stress and an inverse effect from effective normal stress. A drop in effective normal stress in a zone of decreasing surface slope up-glacier from the largest thickness increase may have been significant in the initiation of surge motion in 1982.

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Towards a Hydrological Model for Computerized Ice-Sheet Simulations

Alley

1996

Hydrol. Process.

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Ice-sheet modelling typically uses grid cells 10 km or more on a side, so any hydrological and sliding model must average or parameterize processes that vary over shorter distances than this. Observations and theory suggest that basally produced water remains in a distributed, high-pressure system unless it encounters low-pressure channels fed by surface melt. Such distributed systems appear to exhibit increasing water storage, water transmission and water lubrication of sliding with increasing water pressure. A model based on these assumptions successfully simulates some aspects of the non-steady response of mountain glaciers to externally forced channel-pressure variations; it merits testing in ice-sheet modelling.

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Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Cited by 44 publications

References 13 publications

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

Towards a Hydrological Model for Computerized Ice-Sheet Simulations

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