basal sliding of a glacier occurs because of

Glaciers: How do they form and how do they move? - Geology.com The two bright areas in the 10 lower part of the picture are the ledges seen in (g). 148, 5 September 1969, 10.00 h, 102 mm. 2d; Fig. Also, the larger sliding motion inferred from inclinometry corresponds to the larger motion observed by bore-hole photography. In Figure 4g and h, the large stone coming into view from the left is moving decidedly faster than, and is catching up with, the smaller clast near the top (south) and the gravel mass in which the latter is imbedded. 2; Reference Vivian and BocquetVivian and Bocquet, 1973, p. 441). Figure 9 shows the results in terms of the deformation of the two bore holes: the horizontal components of displacement of the initially vertical bore holes are plotted as a function of depth below the surface. Bore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. The motion in the day since (h) is obvious. 2d); smaller clasts are generally angular. 99, 19 August 1969, 18.20 h, 80 mm. The velocity is now 2.7 cm/d, more than twice that observed earlier. (Hole Z was an exception: photography indicated that water was leaking from the hole at a height of about 1 m above the bottom.). To keep the bore holes open for passage of the camera during this period, the holes were reamed at least once a week with conical thermal drills of maximum diameter 62 or 71 mm. Crack has opened further and partly collapsed as large clast moves out toward top right. The displacement rate at the surface relative to the bore-hole bottom is given in Table III, columns 6 and 7. Les valeurs observes varient de 0,3 3 cm/d. 140, Figure 2h and i, Figure 2k and l (H & K, Fig. We therefore conclude that the indirect method of determining the sliding velocity from surveying and inclinometry confirms the direct measurements of sliding velocity by bore-hole photography, to within the not inconsiderable errors involved in these methods. In fact, no regelation layer was visible by direct visual inspection at the head of one branch of our 1967 tunnel in the Blue Glacier ice fall (Reference Kamb and LaChapelleKamb and LaChapelle, 1968), where the sliding velocity was 1 cm/d and the basal ice was air-free and debris-laden. Visibility of dark markings, interpreted as exposures of ice, is improved. No. Fig. The water levels then fluctuate wildly owing to variations in water input and in flow capacity of constricted basal passageways from the bore holes to the subglacial conduits. Location of bore holes in Blue Glacier. [e1971], p. 25; Reference Souchez, Souchez, Lorrain and Lemmens.Souchez and others, 1973, fig. Side-looking picture taken with bottom of camera touching the bed. 3bf), and in hole Y (Fig. A slight southward motion (toward top) has occurred, and a new clast has appeared at lower right. Fig. WebThis is known as basal sliding and may account for most of the movement of thin, cold glaciers on steep slopes or only 10 to 20 percent of the movement of warm, thick glaciers lying on gentle slopes. Glaciers If the debris is loose and the hole caves, the rate of drill advance is progressively slowed as the hole widens, and becomes limited by the rate of bailing, but from our experience and the capacity of our bailer we judge that if we had been drilling in rock debris of unlimited depth we would have achieved a penetration substantially deeper than 15-20 cm after drilling for a total of 6 h, with frequent bailing. Also, as argued in Section 5c, their rather weak cohesion implies at most only limited ice cementation. (b) With poorer resolution, which often occurs because of technical problems, the distinction between rock surface and sand is less clear, and a number of our photographs present this problem to a greater or lesser extent. The range of observed sliding rates is 0.3 to 3 cm/d, and the observed sliding directions range 30 to either side of the direction of surface motion. The lack of extrusion of such clasts may be a consequence of the gap between sole and subsole drift, which freed the ice locally from the pressure of the ice overburden except to the extent supported by water pressure, which was high in hole X. Conversely, obstructions formed frequently in hole C, where there was no gap and the water pressure in the hole was low. Motions, water pressures, icebed separations, regelation phenomena, state of compaction and cementation, and character of the bedrock surface may be affected. (b)B-52, 20 July 1970, 19.50 h, 193 mm. (g) The compass arrow points toward magnetic south in the photographs, because the compass needle lies immediately below the film in the camera and the image of the external scene is therefore inverted relative to it. (l)Z, A-34, 8 August 1970, 17.08 h, 88 mm. A major question is why the glacier ice fails to invade the subsole drift completely, which it could rapidly do by the action of regelation under the ice overburden pressure. 11. The stem was raised and dropped with the help of a standard cat-head, driven by an electric or gasoline motor, with power output of about 375 or 2 000 W, respectively. The ice is separated from the bed by several cm, as indicated by the shadow along the boundary. , B-122, 22 August 1970, 14.46 h, 82 mm. In others, the bottom consists of generally larger fragments (up to at least 10 cm, the approximate diameter of the bore hole at the bottom), with little or no matrix (Figs 3a, g; 4b, l; 6h, i; H & K, fig. The force of gravity on the mass of glacier ice, b. Intermolecular forces between polar water molecules, c. Weak magnetic forces between Earth's magnetic poles and the ice crystals, d. Tectonic forces lifting mountain ranges and changing the slope of the valley underneath. 15. Fig. (d) Turbidity of the basal water, which results from rock abrasion in the sliding process, is actively generated in the subsole drift and is introduced from the drift into the bottom of bore holes prior to the onset of water outflow from them. 99, 19 August 1969, 18.20 h, 80 mm. In a real sense, the meltwater of a glacier has a tendency to work its way underneath the glacier so as to cause the glacier to float in its own meltwater. (c)B-196, 20 August 1976, 18.20 h, 200 mm. The zone of debris-laden basal ice is thus essentially equivalent to the material represented as "light to moderate debris" in Table I. X, B-81, 6 August 1970, 15.10 h, 93 mm. Bottom photographs of bore holes and Z. Except in areas of sufficiently great basal stress concentration, this pressurization will cause the ice to separate from the bed and will thereby open up passageways along which water can leak away from the hole. No. The relationship of the area studied to the overall flow pattern, structure, and areal mass budget of Blue Glacier can be ascertained by reference to the earlier studies of Reference LaChapelleLaChapelle (1959), Reference Allen, Allen, Kamb, Meier and Sharp.Allen and others (1960), and Reference Meier, Meier, Kamb, Allen and Sharp.Meier and others (1974). (c) Basal sliding | glacial process 4. (c)B-55, 21 July 1970, 16.35 h, 177 mm. (j) 4k, l). . * To the quoted error in this figure must be added an additional uncertainty due to the fact that an initial inclinometer survey, just after bore-hole completion, was not made for this hole, so that the initial hole must be assumed vertical. Motion between this and the previous photograph amounts to 0.6 cm/d. Some cohesion of the sub-sole gravel can be attributed to its clay content: the clay-containing compacted till in nearby lateral moraines shows a similar ability to stand at least temporarily in steep or vertical banks, even under water. It remained in this position until 25 August, when we pushed it out of the way with a small cable t:0'.(e)No. A slight southward motion (toward top) has occurred, and a new clast has appeared at lower right. It involves three mechanisms: relatively rapid creep in the basal layers; pressure melting , whereby ice under pressure melts on the up-glacier side of a small obstacle and the released water freezes on the down-glacier side; and slippage over a layer of water at the bed. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. 2. The same result was obtained for the bottom seen in Figure 6e. 13. 2),X (Fig. In contrast to the narrow separation gap seen in holes V and X, a cavity 1 to 2 m high was revealed by photography in a hole that penetrated a subglacial cavern in the ice fall 0.5 km up-stream from the present study area (H & K, fig. The cavity is now seen to be a gap between the debris bank on the right and a large angular rock that has entered from the left. Reference Allen, Allen, Kamb, Meier and Sharp. Cite this article Pick a style below, and copy the text for your bibliography. In the short time since (h), a rounded pebble has appeared in upper center, just left of the black dot. The distinction is clear when the hazy, mottled or banded appearance of irregularly bubbly or bubble-foliated ice is displayed (Photograph B-246; H & K, Fig. Light cable-tool drilling has produced the depressed circular area at right. In a bore hole of average diameter to cm (estimated from Fig. We surmise that the leakage passageways that ultimately make a connection from the bore holes to the subglacial water conduits, through the sealed regions of the bed, open up initially by invasion of pressurized bore-hole water along routes across the bed where the compressive stress across the icebed interface is a minimum. No. Glaciers They subsequently become enlarged by melting of overlying ice as a result of heat dissipation by water flow through them, and because of this enlargement, they tend to be carried down-stream by the moving ice, especially when the bore-hole water pressure drops to the point where it alone is no longer sufficient to keep the passageways open at their original location on the bed. T, No. It now appears to consist of ice (dark) almost completely coated with fine rock debris. T, No. The ice wall of the bore hole fills the lower left half of the picture. The first "ledge" also moves slower than adjacent large clasts as shown in the sequence of Figure 4ci. Ce dcollement peut provenir d'une sparation lit/glace due la mise en pression du lit par l'eau de sondage. , B-122, 22 August 1970, 14.46 h, 82 mm. If the inherent hydraulic permeability of the subsole drift were high, the natural pore pressure in the subsole drift would be the same as the pressure in the subglacial conduits, since only a small source of pore water is available in the subsole drift, from bottom melting. This process is known as regelation. Markings have moved southward (toward top), indicating some sliding motion. 5i; stone and plastic ball with thread near south side in Photograph No. White veinlets (probably quartz) visible on bottom in (e) and (f) suggest an underlying rock surface, but this does not seem confirmed in (g). The direction of sliding found by bore-hole photography (Table III, column 12) is generally within about 20 of the direction of surface motion (Table III, column 3), the maximum deviation being about 30. Bore hole C closed at a rate of about 2 cm/d (contraction of diameter) near the bottom. This movement very much depends on the temperature of the area, the slope of the glacier, the bed roughness, the amount of meltwater from B-73, 3 August 1970, 12.50 h, 184 mm. Tests of the penetrometer at the surface show that penetrations of 30 cm (full penetration) are readily achieved in both loose and compacted granular materials containing abundant pebbles up to 5 cm in size. Rock at left moved at speed 0.9 cm/d between (k) and (l). We report here the results of a method that gets around the difficulties stated, allowing us to observe and measure the sliding as seen in a bore hole reaching the glacier bottom, and to do so on a day-to-day basis. Large clasts and finer debris pass across the bore-hole bottom from lower left to upper right as the glacier slides over its bed. 151, 5 September 1969, 15.20 h, 80 mm. It now appears to consist of ice (dark) almost completely coated with fine rock debris. The glacier Enlarged map of bore-hole study area, showing initial positions (circles) of bore holes discussed in paper and of four nearby earlier bore holes (, BORE-HOLE CHARACTERISTICS AND PHOTOGRAPHIC OBSERVATIONS. Further motion has occurred, with appearance of new stones from beneath bore-hole wall.(d)No. Glacial advance involves basal sliding but not plastic flow. White veinlets (probably quartz) visible on bottom in (e) and (f) suggest an underlying rock surface, but this does not seem confirmed in (g). The relationship between an objects Photographs (k) and (l) are rotated 45 clockwise relative to (g)-(i), in order that they can be viewed as a stereo pair. Glacial advance involves basal sliding but not plastic flow. In holes V (Fig. Probably these clasts rolled or were pushed into their positions in Figures 2d and 6i from somewhere else on the floor of the hole or from the gap between sole and bed. basal sliding The process by which a temperate glacier moves over its bed. Further southward motion. (i)X, B-81, 6 August 1970, 15.10 h, 93 mm. Side view looking north-east, with camera set 10 cm above bottom. Large clast has rotated so that face on left is nearly vertical. (2) Clasts can be caught and rolled between moving sole and stationary bed only around the periphery of (or outside of) the bore hole, whereas in the absence of the hole this rolling process can happen anywhere. The meltwater functions as a lubricant allowing the glacier to slide more readily over bedrock and sediments. (l) The maximum depth to which the water level was observed to drop in each hole is given in column 6, and the minimum depth to which it later recovered is given in column 7. The lighter areas of the bed are silt in patches among the rock fragments. Glaciers Compass needle stuck in erroneous orientation. In the debris-laden basal ice, the penetration rate was much less, and highly erratic, the erratic penetration being caused probably by varying sizes and quantities of rock clasts encountered in the ice.

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