Variegated Glacier Research Articles

The 1982/83 surge and antecedent quiescent phase of Variegated Glacier: revising the original dataset for application in flow line models

ABSTRACTThe extensive dataset of Variegated Glacier's 1982/83 surge and antecedent quiescent phase is fragmentary and predominantly of flow line nature. Applying the raw centre line data in a flow line model to conduct research into the mechanism behind Variegated Glacier's surge behaviour inevitably entails problems. In this study, the incomplete dataset is extrapolated into the upper and lower parts of the glacier. Furthermore, the centre line data are adapted to account for differences between width-averaged and centre line surface mass balance and ice thickness change, inflow from tributaries, and changes in surface width over time. The modifications to the dataset are backed-up by observations and clear, plausible physical explanations. Moreover, the revised dataset meets the imposed constraints regarding ice volume flux, specific mass balance and net volume change. Hence, the final dataset is considered a satisfying revision that makes the dataset more valuable for future research. Subsequent application of the revised dataset corroborates the idea that glacier evolution during quiescence is basically the growth back towards steady state after the glacier was brought out of balance by the preceding surge.

A note on the water budget of temperate glaciers

Abstract. In this note, the total dissipative melting in temperate glaciers is studied. The analysis is based on the notion that the dissipation is determined by the loss of potential energy due to the downward motion of mass (ice, snow, meltwater and rain). A mathematical formulation of the dissipation is developed and applied to a simple glacier geometry. In the next step, meltwater production resulting from enhanced ice motion during a glacier surge is calculated. The amount of melt energy available follows directly from the lowering of the centre of gravity of the glacier. To illustrate the concept, schematic calculations are presented for a number of glaciers with different geometric characteristics. Typical dissipative melt rates, expressed as water-layer depth averaged over the glacier, range from a few centimetres per year for smaller glaciers to half a metre per year for Franz Josef Glacier, one of the most active glaciers in the world (in terms of mass turnover). The total generation of meltwater during a surge is typically half a metre. For Variegated Glacier a value of 70 cm is found, for Kongsvegen 20 cm. These values refer to water layer depth averaged over the entire glacier. The melt \\textit{rate} depends on the duration of the surge. It is generally an order of magnitude greater than water production by `normal' dissipation. On the other hand, the additional basal melt rate during a surge is comparable in magnitude with the water input from meltwater and precipitation. This suggests that enhanced melting during a surge does not grossly change the total water budget of a glacier. Basal water generated by enhanced sliding is an important ingredient in many theories of glacier surges. It provides a positive feedback mechanism that actually makes the surge happen. The results found here suggest that this can only work if water generated by enhanced sliding accumulates in a part of the glacier base where surface meltwater and rain have no or very limited access. This finding seems compatible with the fact that, on many glaciers, surges are initiated in the lower accumulation zone.

Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge

Abstract. Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-yr dataset, consisting of surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior to and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundsson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wavelength changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodic changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. Our results support that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to propose a scenario for the hydrological history leading to the occurrence of a surge.

Conditions for thrust faulting in a glacier

Dipping, arcuate bands of debris‐rich ice outcropping near the margins of glaciers are often interpreted as thrust faults, assumed to originate in zones of longitudinal compression. Identification of thrusts is typically based either on the geometry and sedimentology of the debris bands or on the crystal fabric of surrounding ice, but the physical processes necessary to generate thrusts are rarely evaluated. Herein, we combine a numerical model of compressive ice flow near a glacier margin with theoretical stress and strain rate criteria for ice fracture and stress criteria for frictional slip to determine the conditions necessary for thrust faulting in glaciers. This model is applied to two different glaciological settings where longitudinal compression has been documented: (1) the transition between warm‐based and cold‐based ice near the terminus of Storglaciären, Sweden, and (2) the downglacier extent of the 1983 surge front of Variegated Glacier where surging ice encountered stagnant ice. Simulations representing the margin of Storglaciären indicate that peak compressive strain rates are six orders of magnitude too small to induce fracture, whereas at Variegated Glacier, strain rates were an order of magnitude too small for compressive fracture. In both groups of simulations, preexisting fractures governed by Coulomb friction are susceptible to slip if they span the ice thickness, are oriented close to the optimal fracture angle, and, in the case of Storglaciären, are subject to water pressures that are a large fraction of ice overburden pressure. Variations about the optimal fracture orientation, low or zero water pressure, high sliding friction coefficient, and limited vertical or lateral fracture extent each tend to suppress thrusting.

Variegated Glacier, Alaska, USA: a century of surges

AbstractA 1995 surge of Variegated Glacier, Alaska, USA, is discussed in the context of its six 20th-century predecessors, especially the previous surge in 1982/83 which was studied in detail. The average time between surge initiations is 15 years. The 1995 surge was considerably weaker than its predecessors, having a single phase or at most a very weak second phase. The 1995 surge confirms that there is a seasonal cycle, with surge initiation in winter and termination in the first part of the melt season, and a correlation between weather and both surge termination date and surge extent. Two days of record high temperature correlated with the termination of the 1995 surge. The most obvious issue is the absence of a strong second surge phase (as there was in the 1982/83 surge) culminating in a surge extent more in line with that of the predecessors. This is considered in the light of a simple criterion for surge initiation and re-initiation which depends upon the evolving basal shear stress.

Is there a single surge mechanism? Contrasts in dynamics between glacier surges in Svalbard and other regions

During the 1990s, Monacobreen, a 40‐km‐long tidewater glacier in Svalbard, underwent a major surge. We mapped the surge dynamics using ERS synthetic aperture radar images, differential dual‐azimuth interferometry and intensity correlation tracking. A series of 11 three‐dimensional (3‐D) velocity maps covering the period 1991–1997 show a months‐long initiation and years‐long termination to the surge, with no indication of a surge front travelling downglacier. During the surge, the front of the glacier advanced ∼2 km, the velocity and derived strain rate increased by more than an order of magnitude, and maximum ice flow rates measured during 1994 were ∼5 m d−1. The spatial pattern of both velocity and strain rate was remarkably consistent and must therefore be controlled by spatially fixed processes operating at the glacier bed. We combine these results with those published in the literature to construct a typical Svalbard glacier surge cycle and compare this to surge dynamics of glaciers from other cluster regions, especially those of Variegated Glacier in Alaska. The strong contrast in dynamics suggests that there exist at least two distinct surge mechanisms.

The surges of Variegated Glacier, Alaska, U.S.A., and their connection to climate and mass balance

AbstractOne of the questions still unanswered concerning the surge behavior of glaciers concerns their quasi-periodic occurrence. Some results on the phenomenological connection between local cumulative balance and surge initiation of Variegated Glacier, Alaska, U.S.A., are discussed here. Based on climate data from neighboring weather stations, an empirical relation between precipitation, temperature and local mass balance is established and used to reconstruct the annual balance at a location in the accumulation area back to 1905. Between the last four surges in 1946/47, 1964/65, 1982/83 and 1994/95, the ice-equivalent cumulative balance was 43.5 m on average, with a 1σ error of 1.2 m. Although the existence of a surge level cannot be directly interpreted in physical terms, it explains the variable length of the quiescent periods of Variegated Glacier by variations in the accumulation rate prior to the surge. We use the surge level to hindcast former unobserved surges, to compare the results with other surge datings obtained from photographs and to establish a complete surge history for Variegated Glacier for the 20th century.

Deformation histories and structural assemblages of glacier ice in a non-steady flow regime

Abstract Deformation histories of ice exposed at various locations on the centreline in the ablation area of surge-type Variegated Glacier were estimated using a technique based on evaluation of strain-rate fields derived from velocity gradients. The analysis indicates that ice exposed in the upper part of the ablation area a few years after the 1982–83 surge had experienced a relatively simple deformation history that included one surge-related high magnitude deformation event. Ice exposed in the terminal lobe at the same time had experienced six surge phases during its approximately 100 year-long residence time. The histories of accumulation of cumulative strain are complex, and indicate that measures of cumulative strain can mask the effects of large but transient strain events. They also demonstrate that substantial cumulative strain can be ‘undone’ during subsequent deformation to leave a cumulative strain signal that is unrepresentative of earlier cumulative strain states. Structural relationships in general do not reflect the complexity of the deformation histories experienced by the ice. A preliminary analysis of the relationships between the deformation histories and structural assemblages suggests, in particular, that brittle structures are reactivated several times during the history of the ice.

Spatial, temporal and kinematic characteristics of surges of Variegated Glacier, Alaska

Analysis of five surges of Variegated Glacier, Alaska, indicates that they all terminated with their surge fronts in the terminal lobe of the glacier, and that different surges penetrated into the terminal lobe by different amounts. Of the five surges, the one that occurred in 1905–06 penetrated furthest into the terminal lobe. The 1964–65 surge affected a greater proportion of the glacier than either the 1947–48 or the 1982–83 surge, caused greater total ice displacements than the 1982–83 surge and appears to have resulted in larger total displacements down-glacier than the 1982–83 surge. Overall, the 1964–65 surge was a significantly larger event than either the 1947–48 or the 1982–83 surge.

Spatial, temporal and kinematic characteristics of surges of Variegated Glacier, Alaska

Analysis of five surges of Variegated Glacier, Alaska, indicates that they all terminated with their surge fronts in the terminal lobe of the glacier, and that different surges penetrated into the terminal lobe by different amounts. Of the five surges, the one that occurred in 1905–06 penetrated furthest into the terminal lobe. The 1964–65 surge affected a greater proportion of the glacier than either the 1947–48 or the 1982–83 surge, caused greater total ice displacements than the 1982–83 surge and appears to have resulted in larger total displacements down-glacier than the 1982–83 surge. Overall, the 1964–65 surge was a significantly larger event than either the 1947–48 or the 1982–83 surge.

Structural evolution of Variegated Glacier, Alaska, U.S.A., since 1948

AbstractАn excellent photographic database, comprising 12 sets of vertical aerial photographs taken between 1948 and 1993, has enabled the structural glaciology of surge-type Variegated Glacier, Alaska, U.S.A., tо be analyzed for a period that spans two surge cycles and includes the effects of three surges. The glacier-wide patterns of crevasse development at the end of each of these surges, which occurred in 1947–48, 1964–65 and 1982–83, were remarkably similar, despite differences in surge intensities and extents. At the end of each surge a three-fold longitudinal zonation of crevasse orientation characterised that part of the glacier that had been affected by surge activity. This zonation comprises: (i) transverse crevasses in a narrow upper zone, (ii) superimposed longitudinal and transverse crevasses in an extensive middle zone, and (iii) longitudinal crevasses in a narrow lower zone. The main difference between patterns produced by successive surges was small variations in the extent оf the three zones. During quiescence, the most intense crevasse development occurs in the upper glacier in the latter part of quiescence, and is a function of increasingly steep down-glacier velocity gradients developing in the buildup to the impending surge. Some crevassing also develops in the central part of the glacier in mid-quiescence that is not related to the surge nature of the glacier, but to normal flow processes over bedrock. There is no structural evidence to indicate that any of the tributaries of Variegated Glacier surge. This observation suggests that the bulb-like loops in the medial moraines at Variegated Glacier form during quiescence as a result of the steady flow of the tributaries into the stagnant trunk glacier, rather than as a result of tributary surges.

Structural evolution of Variegated Glacier, Alaska, U.S.A., since 1948

AbstractАn excellent photographic database, comprising 12 sets of vertical aerial photographs taken between 1948 and 1993, has enabled the structural glaciology of surge-type Variegated Glacier, Alaska, U.S.A., tо be analyzed for a period that spans two surge cycles and includes the effects of three surges. The glacier-wide patterns of crevasse development at the end of each of these surges, which occurred in 1947–48, 1964–65 and 1982–83, were remarkably similar, despite differences in surge intensities and extents. At the end of each surge a three-fold longitudinal zonation of crevasse orientation characterised that part of the glacier that had been affected by surge activity. This zonation comprises: (i) transverse crevasses in a narrow upper zone, (ii) superimposed longitudinal and transverse crevasses in an extensive middle zone, and (iii) longitudinal crevasses in a narrow lower zone. The main difference between patterns produced by successive surges was small variations in the extent оf the three zones. During quiescence, the most intense crevasse development occurs in the upper glacier in the latter part of quiescence, and is a function of increasingly steep down-glacier velocity gradients developing in the buildup to the impending surge. Some crevassing also develops in the central part of the glacier in mid-quiescence that is not related to the surge nature of the glacier, but to normal flow processes over bedrock. There is no structural evidence to indicate that any of the tributaries of Variegated Glacier surge. This observation suggests that the bulb-like loops in the medial moraines at Variegated Glacier form during quiescence as a result of the steady flow of the tributaries into the stagnant trunk glacier, rather than as a result of tributary surges.

Quiescent-phase evolution of a surge-type glacier: Black Rapids Glacier, Alaska, U.S.A.

AbstractBlack Rapids Glacier, a surge-type glacier in the Alaska Range, most recently surged in 1936–37 and is currently in its quiescent phase. Mass balance, ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994. The annual speed has undergone cyclical fluctuations of as much as 45% about the mean speed. Ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation, which indicates that they are being drinven by changes in basal motion. The behavior of Black Rapids Glacier during this quiescent phase is significantly different from that of Variegated Glacier, another well-studied surge-type glacier in Alaska. The present medial-moraine configuration of Black Rapids Glacier indicates that a surge could occur at any time. However, ice velocity data indicate that the next surge may not be imminent. We belive that there is little chance that the next surge will cross and dam the Delta River.

Quiescent-phase evolution of a surge-type glacier: Black Rapids Glacier, Alaska, U.S.A.

AbstractBlack Rapids Glacier, a surge-type glacier in the Alaska Range, most recently surged in 1936–37 and is currently in its quiescent phase. Mass balance, ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994. The annual speed has undergone cyclical fluctuations of as much as 45% about the mean speed. Ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation, which indicates that they are being drinven by changes in basal motion. The behavior of Black Rapids Glacier during this quiescent phase is significantly different from that of Variegated Glacier, another well-studied surge-type glacier in Alaska. The present medial-moraine configuration of Black Rapids Glacier indicates that a surge could occur at any time. However, ice velocity data indicate that the next surge may not be imminent. We belive that there is little chance that the next surge will cross and dam the Delta River.

The structural geology of a surge-type glacier

The range of deformation structures found in surge-type Variegated Glacier, Alaska, is similar to that found in many thin-skinned thrust belts, and includes foliations, folds, dip-slip and strike-slip faults, thrust faults and fractures. The development of structural relationships and the overall structural assemblage can be related to the deformation histories of quiescent and surge phases of motion. The surge phase dominates the formation of brittle structures because of the large stresses and strain rates associated with it. The relationship between downglacier-verging overturned folds with hinge lines transverse to flow, and a family of transverse arcuate thrust faults is characteristic of structural development in the lower part of the glacier during the surge. Longitudinal foliation develops mainly from sedimentary stratification in the upper part of the glacier in a marginal transpressive deformation regime during quiescent phase flow from broad accumulation basins. The structural relationships in this surge-type glacier may aid in the interpretation of structures in other gravity-driven tectonic settings.

The 1987–88 surge of West Fork Glacier, Susitna Basin, Alaska, U.S.A.

AbstractA surge of West Fork Glacier, a temperate glacier in the Susitna Basin of the Alaska Range, began soon after the end of the 1987 melt season and terminated on 6 July 1988. Reconnaissance measurements of balance, elevation and speed had been made from 1981 to 1983. Daily measurements of surface speed at two points 9 km apart and of the characteristics of the stream draining the glacier were begun during the surge and continued through the following year. The maximum displacement of the ice during the surge was about 4 km; the maximum change in surface elevation was about 120 m. Between the time of the start of detailed observations on 12 February 1988 and the onset of a complex termination phase during the last month of the surge, the speed was almost constant, and the water discharge was totally free of turbidity, indicating that no basal water was escaping from the glacier. During the termination phase, sharp changes in speed occurred, almost simultaneously at the two observation sites; each deceleration event was accompanied by high sediment concentration and high water discharge. This behavior is similar to that observed on Variegated Glacier during its 1982-83 surge. The mechanism of triggering (related to surface water input and the disruption of the internal drainage system) and the cause of the fast motion were probably the same for both surges, even though there are substantial differences in size and mass-balance characteristics.

Hydrology, erosion and sediment production in a surging glacier: Variegated Glacier, Alaska, 1982–83

AbstractOutlet streams of Variegated Glacier, Alaska, U.S.A., were observed before, during and after the surge of 1982–83. Measurements of discharge, suspended sediment and dissolved load in the outlet streams are presented for the years 1982–84, and comparisons are made with data from previous years. The data are interpreted to yield characteristics of the basal hydraulic system. The surging region of the glacier was underlain by a basal hydraulic zone of low water velocity and high water storage, inferred to be a distributed-flow system. The ice down-glacier of the propagating surge front was underlain by a high-velocity, low-storage zone, inferred to be a conduit system. The volume of water stored above the surge front was the major hydraulic control on the surge. Basal bedrock erosion during the surge was extreme in comparison to non-surging glaciers. The sediment output was directly proportional to the basal sliding, with a dimensionless erosion rate (meters eroded from the bed divided by meters of sliding) of order 1.0 × 10−4. Total erosion during the 20 year surge cycle was on the order of 0.3 m of bedrock, with approximately two-thirds occurring during the 2 years of the surge peak, and the bulk of this during the peak 2 months.

Hydrology, erosion and sediment production in a surging glacier: Variegated Glacier, Alaska, 1982–83

AbstractOutlet streams of Variegated Glacier, Alaska, U.S.A., were observed before, during and after the surge of 1982–83. Measurements of discharge, suspended sediment and dissolved load in the outlet streams are presented for the years 1982–84, and comparisons are made with data from previous years. The data are interpreted to yield characteristics of the basal hydraulic system. The surging region of the glacier was underlain by a basal hydraulic zone of low water velocity and high water storage, inferred to be a distributed-flow system. The ice down-glacier of the propagating surge front was underlain by a high-velocity, low-storage zone, inferred to be a conduit system. The volume of water stored above the surge front was the major hydraulic control on the surge. Basal bedrock erosion during the surge was extreme in comparison to non-surging glaciers. The sediment output was directly proportional to the basal sliding, with a dimensionless erosion rate (meters eroded from the bed divided by meters of sliding) of order 1.0 × 10−4. Total erosion during the 20 year surge cycle was on the order of 0.3 m of bedrock, with approximately two-thirds occurring during the 2 years of the surge peak, and the bulk of this during the peak 2 months.

The 1987–88 surge of West Fork Glacier, Susitna Basin, Alaska, U.S.A.

AbstractA surge of West Fork Glacier, a temperate glacier in the Susitna Basin of the Alaska Range, began soon after the end of the 1987 melt season and terminated on 6 July 1988. Reconnaissance measurements of balance, elevation and speed had been made from 1981 to 1983. Daily measurements of surface speed at two points 9 km apart and of the characteristics of the stream draining the glacier were begun during the surge and continued through the following year. The maximum displacement of the ice during the surge was about 4 km; the maximum change in surface elevation was about 120 m. Between the time of the start of detailed observations on 12 February 1988 and the onset of a complex termination phase during the last month of the surge, the speed was almost constant, and the water discharge was totally free of turbidity, indicating that no basal water was escaping from the glacier. During the termination phase, sharp changes in speed occurred, almost simultaneously at the two observation sites; each deceleration event was accompanied by high sediment concentration and high water discharge. This behavior is similar to that observed on Variegated Glacier during its 1982-83 surge. The mechanism of triggering (related to surface water input and the disruption of the internal drainage system) and the cause of the fast motion were probably the same for both surges, even though there are substantial differences in size and mass-balance characteristics.

The character, structure and origin of the basal ice layer of a surge-type glacier

AbstractThe basal ice layer of surge-type Variegated Glacier, Alaska, appears to have formed by a combination of (i) open-system freezing of subglacial meltwaters over both rigid and unconsolidated substrates; (ii) apron over-riding during surge-induced glacier advance; (iii) incorporation of glacier ice by recumbent folding, thrust-faulting and nappe over-riding during down-glacier propagation of a surge front; and (iv) post-formational metamorphism involving recrystallization, partial internal melting and squeezing out of meltwaters and dissolved gases. Structural evidence and the characteristics of debris entrained in ice facies formed by basal freezing suggest that the layer includes a lower element formed under surge conditions and an upper element formed during the quiescent phase of a surge cycle. The lower element is depleted in comminution products and enriched in medium gravel, while the upper element contains comminution products but virtually no medium gravel. This distinction is attributed to the efficiency of bedrock fracture and meltwater flushing of comminution products under surge conditions. The basal ice layer thickens from <1 m to >13 m down-glacier in a manner consistent with the magnitude of horizontal shortening induced by the 1982–83 surge. Thickening is largely tectonic in origin, and the style and intensity of folding and thrust faulting change down-glacier as the magnitude of horizontal shortening increases. Tectonic processes associated with the down-glacier propagation of surge fronts therefore appear to be capable of creating thick basal ice layers which allow extensive supraglacial sedimentation of subglacially derived debris.