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Fracturing and sediment deposition at the glacier surface by jökulhlaups: a common occurrence

Published online by Cambridge University Press:  08 September 2017

Wilfred H. Theakstone
Wilfred H. Theakstone*
Affiliation:
Department of Geography, University of Manchester, Manchester M13 9PL, England
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Abstract

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Type
Correspondence
Information
Journal of Glaciology , Volume 47 , Issue 158 , 2001 , pp. 517 - 518
Copyright
Copyright © International Glaciological Society 2001

The Editor,

Journal of Glaciology

Sir,

Reference Roberts, Russell, Tweed and KnudsenRoberts and others (2000) reported jökulhlaups which generated basal water pressures in excess of ice overburden, thereby fracturing overlying ice and allowing sediment to be emplaced at high elevations within the glacier as water discharged through fractures onto the surface. Similar circumstances characterized drainage of the lake Kalvtjørna, at the glacier Austre Okstindbreen, Norway, on 5 August 1977 (Reference TheakstoneTheakstone, 1978). Water, with a high sediment load, burst up through the glacier surface at a point 2 km from the lake basin, and fractures resembling low-angle thrusts were formed (Fig. 1). The emerging water covered a large area at the glacier surface, and sediments were deposited for a distance of > 100 m from the fracture zone (Fig. 2). Subsequent examination revealed that sediments had been deposited on the walls and roof of the 5 m high conduit immediately up-glacier of the point at which the water emerged through the glacier surface.

Fig. 1. Water draining from the lake Kalvtjørna burst through the surface of the glacier Austre Okstindbreen on 5 August 1977. The glacier surface was fractured, and sediments were deposited by the emerging water.

Fig. 2. Sediments were deposited over a large area below the fracture zone at the surface of the glacier Austre Okstindbreen as water drained from the glacier-dammed lake.

Short-lived events causing surface fracturing, water spouts at the glacier surface and related phenomena have been surprisingly widely reported over a long period (Table 1). When water flow beneath a glacier is interrupted, or when water is supplied to conduits more quickly than it can escape, high water pressures may develop. Because the resultant discharge of water through the glacier surface may be short-lived, the probability of its being observed is low, even if fieldwork is being conducted on the glacier.

Table 1. Observed events related to water bursting through a glacier surface

Reference GeorgeGeorge (1866, p. 123–125) described “a fountain of very considerable volume rising out of the ice” and published what probably was the first photograph of such a feature. He concluded that water “meeting with some obstacle in its subglacial course had forced its way to the surface through a fissure in the ice”. Reference Sturm, Benson and MacKeithSturm and others (1986) reported that, during jökulhlaups at Drift Glacier, Alaska, U.S.A., water poured out from the base of a rock plug which projected through the lower glacier, and Reference LawsonLawson (1986) attributed water fountains discharging at an active glacier margin to an impermeable dam formed by frozen bed materials and stagnant ice. Reference McKenzieMcKenzie (1969) described a sudden release of water from stagnant ice. Several accounts of water released from subglacial cavities penetrated by tunnels or boreholes have been published (Reference MillerMiller, 1952; Reference Haefeli and BrentaniHaefeli and Brentani, 1955; Reference FisherFisher, 1963; Reference Paterson and SavagePaterson and Savage, 1970).

Reference NordenskjöldNordenskjöld (1881, p. 181) reported a column of water above the surface of a glacier, which “like a geysir … rises to a great height”. Reference GlenGlen (1941) noted that water sometimes “attains such a high pressure that it literally bursts its way up through the ice, sending up a small water spout which may continue for as long as an hour”. Reference WyllieWyllie (1965) reported water spouts which persisted for several days, whilst Reference RucklidgeRucklidge (1956) described one which lasted only a few seconds but was repeated six times at exactly 10 min intervals. Reference WisemanWiseman (1963) observed a water spout which rose rapidly to 5–7 m, and Reference Ewing, Loomis and LougeayEwing and others (1967) saw one which reached a maximum height of 4–5 m and issued for 7–10 s. Reference BaranowskiBaranowski (1973) observed a series of water spouts, which occurred with striking regularity for an hour. Reference Sturm and CosgroveSturm and Cosgrove (1990) reported significant super-elevation of water discharging from a glacier pothole. Reference Sturm and BensonSturm and Benson (1985) noted that, during the early stage of a jökulhlaup, water flowed from under the ice at supraglacial pools.

Fracturing associated with the discharge of water through the glacier surface has been reported from many areas. Reference MathewsMathews (1949) described a 15–30 m diameter circular fracture in firn, and a trail of sediment deposited by the water which had burst through the surface. Reference Reid and ClaytonReid and Clayton (1963) suggested that thrusting of ice blocks in the glacier marginal zone was responsible for the opening and closing of subglacial drainage. Reference Holmlund and HookeHolmlund and Hooke (1983) described surface fracturing induced by a rapid rise then fall of water pressures; cracking of the ice was heard before the rise of water level. Reference GoodwinGoodwin (1988) reported that, over a period of weeks before a jokulhlaup, fracturing of the ice was coupled to a domeshaped uplift; chemical analyses suggested that the water, which erupted as a 1–2 m high water spout from the major crevasse, had been active in chemical erosion of the glacier bed. Reference Warburton and FennWarburton and Fenn (1994) noted arcuate post-event fractures associated with moulins, after water had flowed up them during “a highly unusual sequence of glacier flood events”. Reference Skidmore and SharpSkidmore and Sharp (1999) reported that audible fracturing of the ice preceded the onset of upwelling of water at a glacier surface, and that longitudinal fracturing occurred in the vicinity of a fountain which rose 1–2 m above the surface. Reference Bennett, Huddart and WallerBennett and others (2000) considered that fractures, opened during the 1991 surge of Skeiðarárjökull, Iceland, facilitated the escape of subglacial water to the glacier surface.

Reference Roberts, Russell, Tweed and KnudsenRoberts and others (2000) suggested that there is a continuum of glacier responses to sudden influxes of water to the bed, when the rate of water supply exceeds the rate at which it can escape. Ephemeral events involving water discharging through a glacier surface and depositing sediment there may be relatively common, although their short-lived nature means that they may be neither observed nor reported.

14 September 2001

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Figure 0

Fig. 1. Water draining from the lake Kalvtjørna burst through the surface of the glacier Austre Okstindbreen on 5 August 1977. The glacier surface was fractured, and sediments were deposited by the emerging water.

Figure 1

Fig. 2. Sediments were deposited over a large area below the fracture zone at the surface of the glacier Austre Okstindbreen as water drained from the glacier-dammed lake.

Figure 2

Table 1. Observed events related to water bursting through a glacier surface