Quarrying

Like glacial abrasion, the direct observation of glacial quarrying is extremely difficult because it involves digging tunnels through a glacier to access basal cavities. However, a 2 km-long artificial tunnel under the glacier Engabreen in Norway, originally built as part of a hydroelectric scheme, provides direct access to the bed of this temperate glacier some 200 m beneath the glacier surface. A number of important experiments have been conducted here on subglacial processes, including studies of glacial abrasion, glacial quarrying, water pressure variations, ice rheology and debris concentrations. To study glacial quarrying, Cohen et al. (2006) installed a granite step, 120 mm high with a crack in its stoss surface, at the bed of Engabreen and used acoustic emission sensors to monitor crack growth events in the step as the ice slid over it. This is illustrated in the photograph below which shows a rock step that was subjected to water pressure fluctuations in its lee over an 8 day period beneath ~ 215 m of ice at Engabreen. A 31 mm deep, 2 mm wide crack was cut in the rock's upper surface. The crack propagated downward over the 8 days of the experiment, such that upon removal of the step from the bed its lee surface was quarried, as shown. Acoustic emission source locations, shown in the lower figure, show the slow crack propagation with time, which was stimulated by water pressure changes and cavity growth in the step's lee. They also measured vertical stresses, water pressure and cavity height in the lee of the step. By artificially pumping water to the lee of the step they observed that adding water initially caused the lee-side cavity to open. The cavity then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions from the base of the crack increased dramatically. With repeated pump tests this crack grew over time until the lee surface of the rock step was quarried. These experiments confirm that fluctuating water pressure in cavities is important in glacial quarryingbecause it greatly aids the development of cracks in the bedrock.

Ice Initial crack fl

Ice Initial crack fl

SO 25 30 35 40 Pwälfon dong ftow tlirscliai (cm)

SO 25 30 35 40 Pwälfon dong ftow tlirscliai (cm)

Source: Cohen, D., Hooyer, T.S., Iverson, N.R., et al. (2006) Role of transient water pressure in quarrying: A subglacial experiment using acoustic emissions. Journal of Geophysical Research - Earth Surface, 111 (F3), F03006. [Modified from: Cohen et al. (2006) Journal of Geophysical Research - Earth Surface, 111 figure 8 and 9, F03006]

Pressure release as glacial erosion proceeds may generate fractures and joints parallel to the erosional surface. As rock surfaces are unloaded they may expand and fracture. The removal of significant quantities of overlying rock by glacial erosion may cause unloading and the development of such fractures. This process has been used to explain the presence of large sheet joints parallel to eroded surfaces such as valley sides (Figure 5.4). These joints are produced by pressure release due to the unloading effect of glacial erosion. Glacial erosion may therefore generate bedrock fracture and thereby accelerate rates of erosion.

As a glacier moves over an irregular bedrock surface, complex patterns of basal ice pressure are generated (Figure 4.6). This pattern of pressure differences is transmitted to the underlying bedrock beneath, causing stress fields to be set up within the bedrock. These stress patterns are often more pronounced if a cavity exists in the lee of a bedrock obstacle. These stress fields may be sufficient to cause

Figure 5.4 Sheet joints formed by pressure release in granite on the side of a glacial valley in the Cordillera Blanca, Peru. Note how the surface of each sheet is parallel to the side of the valley.

[Photograph: N.F. Glasser]

Figure 5.4 Sheet joints formed by pressure release in granite on the side of a glacial valley in the Cordillera Blanca, Peru. Note how the surface of each sheet is parallel to the side of the valley.

[Photograph: N.F. Glasser]

the bedrock to fracture, although theoretical calculations have suggested that this will occur only where pre-existing joints and weaknesses exist (Box 5.3). A rock mass with such weaknesses is referred to as a discontinuous rock mass. All bedrock

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