Showing papers on "Accumulation zone published in 1987"

Propagation of a glacier surge into stagnant ice

Abstract: The propagation of the surge of Variegated Glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a “stagnant zone” of thin (≈40 m), nearly motionless ice near the terminus, across a “front zone,” to a “surge zone” of thicker (≈100 m), rapidly moving (>20 m d−1) ice above. Within the front zone there were large angles of upward motion (>30°), compressive longitudinal strain rates (>0.1 d−1), and upward vertical velocities (>5 m d−1). Within the surge zone the ice motion was nearly parallel to the ice surface, and longitudinal gradients were small. The deformation pattern produced microcracking, exfoliation, and buckling of surface ice, longitudinal cracking, longitudinal chasms, and an increase in glacier volume. The front zone propagated at an average speed of about 40 m d−1 or roughly twice the surge speed of the ice above the front. This factor is determined by the thickness jump across the front. Finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth. The velocity in the surging zone occurs almost entirely by sliding. The sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone. A comparison of basal velocity and shear stress distributions indicates a highly “lubricated” area beneath the upper part of the front zone and a “locked” area beneath the lower part of the front zone. Normal stress at the bed is not significantly different from overburden, and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone.

Columbia Glacier, Alaska: changes in velocity 1977-1986

Abstract: The Columbia Glacier, a grounded, iceberg-calving tidewater glacier near Valdez, Alaska, began to retreat about 1977. Drastic retreat occurred in 1984, and by early 1986, retreat amounted to 2 km. The glacier has thinned more than 100 m since 1974 at a point 4 km behind the 1974 terminus position. Between 1977 and 1985 the lower glacier ice velocity increased from 3–8 m/d to 10–15 m/d. Ice velocity in the region 0.5 km above the terminus was highest near the time the glacier was most receded (late fall), and lowest near the time of maximum length (early summer), for years 1977–1982. Velocity in the region 52–57 from the head of the glacier was highest in mid-spring, and lowest in early fall from 1977 to 1985. Through the years 1983–1985, the dates of maximum and minimum velocities within 0.5 km of the receding terminus tended toward the dates of the 52–57 km maximum and minimums. This occurred because as the terminus receded, it was no longer strongly influenced by the reverse slope of the terminal moraine shoal. Velocities near the terminus fluctuated by 2–3 m/d during summer and fall, when liquid water input was variable, and were relatively constant during winter. Hourly variations in ice velocities are controlled by liquid water input to the glacier hydraulic system and tide stage. Velocity increases near periods of high surface water input and decreases during periods of high tide as a result of hydrostatic back pressure.

Accelerated ablation at a glacier ice-cliff margin, dry valleys, antarctica

Abstract: Cliffed margins of cold glaciers are common in polar regions and are an important source of meltwater. Because of low sun angles, the cliff face receives more solar radiation than does the upper glacier surface and therefore melts at a faster rate. Ablation of an ice-cliff is particularly enhanced, and melt is initiated early in the season where the cliff impinges against a steep (rock) slope. On subdued ice cliffs which do not calve, differential ablation can form ice terraces, which in turn increase ablation by increasing the area of ice-cliff faces.

Continued decrease of ice-flow velocity at Lewis Glacier, Mount Kenya, East Africa

Abstract: Our earlier monitoring program on Lewis Glacier, Mount Kenya, indicated a slow-down of the ice flow to January 1982, while based on the numerical modeling of the ice dynamics a further drastic decrease of the ice flow was predicted from the 1978 to the 1985 datum. This paper presents velocity measurements over the years 1982-83, 1984, and 1985 . Changes of ice-flow conditions from 1978 to the mid 1980s are characterized by a velocity decrease by nearly half; a decrease of the maximum mass flux by more than half; a flattening and up-glacier shift of the velocity and mass-flux maxima' an up-glacier displacement of the transition bet~een prevailingly longitudinal crevasses in the lower glacier and transverse crevasses in the upper glacier; and a terminus retreat by about 50 m. In consequence of the very weak ice flow remaining in Lewis Glacier, thinning and terminus retreat of the glacier are now primarily controlled by the ill-situ net balance.

Two types of intraglacial meltwater regime for the Bertil glacier, Svalbard

Abstract: Data are presented from hydrological observations from the englacial stream of the Bertil Glacier and from the terminal section of the glacier. Two types of englacial runoff regime during the ablation season have been identified. During the course of the first type of runoff, a rapid type, observed in the 1960s and 1970s, the volume of intraglacial discharge comprised 40% of total discharge. The second, slow type, whereby the volume of intraglacial discharge comprises only 10–20% of the total, dominated in the 1980s. Complete freezing of the central intraglacial discharge tunnel was recorded by the end of the cold period. Water entering the glacier in the spring of 1984 and 1985 produced a high head of pressure and resulted in the occurrence of geysers on the lower part of the glacier surface. It is assumed that the changed nature of intraglacial discharge is associated with the progressive freezing of the glacier and with changes in its hydrothermal regime. If climatic conditions remain the same...