STUDY OF ICE FABRICS, THULE AREA, GREENLAND

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2 STUDY OF CE FABRCS, THULE AREA, GREENLAND by George P. Rigsby ABSTRACT Crystal fabric studies were made on glacier ice taken from 11 locations on the Nuna Ramp at:td three locations on the Moltke Glacier, Greenland. From samples taken near the surface of the glacier, 1/16- in. sections of ice were prepared, and the orientation of the optic axis of each ice crystal in the section was determined with a large universal stage with four axes of rotation, mounted between crossed polaroid sheets. The optic axes, when plotted on a Schmidt equal-area projection, often show very strong patterns with concentrations as high as 30% in 1% of the area. ce appears to be very sensitive to shearing forces and the crystals in polar glaciers tend to become oriented so that the basal glide plane is parallel to the shear plane. The strength of the pattern appears to be more or less proportional to the strength of the shearing forces imposed on the ice. Strong shearing forces seem to produce small crystals with strong preferred orientations, while high temperatures tend to produce larger crystals.. NTRODUCTON 1. Since it was first recognized that glaciers move down their valleys under the influence of gravity, many attempts have been made to explain the mechanism of this flow. n a metamorphic rock such as glacier ice, the mass is almost completely crystalline and viscous flow formulas cannot be used, at least not without modification. Therefore, studies need to be made on how an individual crystal can deform and its interaction on the surrounding crystals with different orientations. Crystal fabric studies offer a promising method of attack on the still troublesome problem of "solid flow." The fabric of a crystalline mass is found by the statistical study of the orientation of the individual crystals and their relationship to each other, and studies on several temperate glaciers have demonstrated impressive fabrics within glacier ice (Rigsby, 1951; Meier, Rigsby, and Sharp, 1954). * Strong preferred orientation of the optic axes (crystallographic c-axes) is found, which appears to be!related only to the shear co~ple placed on the ice by differential movement. t is well known that, at the surface o"f a valley glacier, the center moves at a higher velocity than the ice near the sides, and it is suspected that similarly the top moves faster than the bottom. The greatest differential per unit of distance, and therefore the largest shear couple, is found to be near the sides of the glacier where the ice is "held" by the rock walls. The strongest fabric (a more nearly perfect alignment of axes) is also found here. G. P. Rigsby, "Crystal fabric studies on Emmons Glacier, Mount Rainier, Washington," Journal of Geology, vol. 59, pp (1951). ' M. F. Meier; G. P. Rigsby; and R. P. Sharp, "Preliminary data from Saskatchewan Glacier, Alberta, Canada," Arctic, vol. 7, pp ( 1954 ).

3 2 STUDY OF CE FABRCS, THULE AREA, GREENLAND Foliation in glacier ice is a planar structure usually consisting of alternating layers of relatively clear and bubbly ice, and is best developed in areas where the shear stresses are the largest. Many workers in glaciology have attributed foliation in ice to shear deformation, and field observations by the author agree with these conclusions. Well-developed foliation can often be seen in thin section using only unpolarized light. The darker bands shown in Figures 1 and 2 consist of many small bubbles of air which are usually under an atmosphere or more pressure. At the melting temperature the bubble is surrounded by a small pocket of liquid water. 5 em Figure 1. Thin section of ice from Moltke Glacier. Unpolarized light. Cloudy areas in bands are a result of many small air bubbles. Little or no fabric data had been collected from polar glaciers, and it was recognized that some useful results might be obtained from ice which has flowed at temperatures below the freezing point. An opportunity to collect such data came during the summer of 1954 when SPRE sent several field parties to the Thule area in Greenland for ice, snow, and glaciological studies. Field work for this study extended from about June 23 to August 15, The author was assisted by Mr. Glenn Walker, a SPRE employee who was a great help in the field. Appreciation is expressed for his valuable services.. PROGRAM OF WORK 2. The Nuna Ramp in the Nunatarssuak area, Greenland (Figs. 7 and 8, pages 7 and 9) is a gradual incline to the ice sheet and was chosen for this research because there is very little crevassing, which indicates simpler flow conditions within the ice. t was hoped that the strike and dip of the foliation would be easy to obtain at each location established for fabric study, but in many cases it was difficult to find, especially some distance away from land, where only small shearing forces are set up. 5 em Eleven locations for fabric study were established on the ramp within 4 mi of the edge. Three other locations were Figure 2. Thin section of ice from Moltke Glacier near surface, in unpolarized light. Clear ice layer on right side; bubble ice on left. '

4 STUDY OF CE FABRCS, THULE AREA, GREENLAND 3 established on the Moltke Glacier using a helicopter, Two of these were near the sides in high shear zones and one in the center, all along a profile about 4 mi above the terminus. ce samples were collected with a 3-in. coring auger. The samples were taken near the surface of the glacier in order that the orientation of the core could be established before removing it. About 1/2-in. sections of ice were cut from the cores, and melted down to about 1/16 in., using a flat-bottomed cast aluminum teakettle partially filled with warm water, Orientation of the optic axis of each ice crystal in the section was determined by means of a large uliversal stage with four axes of rotation, w~ich was mounted between crossed polaroid sheets.. RESULTS OF STUDY Fabric diagrams, 3. The fabric diagrams obtained at the 11 stations on Nuna Ramp are given in Figure 8, with a map showing the location of the stations. Figure 7 shows the three locations on Moltke Glacier and the fabric diagrams obtained. The diagrams show density contours made after plotting the optic axis of each crystal on the lower hemisphere of a Schmidt equal-area projection, as is conventional among petrographers, At location 1 on Nuna Ramp, the density of these points exceeds 30% in 1% of the area of the projection net, Each diagram is in the horizontal plane and oriented according to the map with north at the top. The foliation was usually much clearer after a rainy or a prolonged cloudy period, but it could not be determined at some locations even under the most favorable conditions. At locations 3, 4, 5, 6, and 7 (Fig. 8) foliation could not be determined, but bluebands which in general appear to be parallel to the foliation could be seen nearby. Dips on these bands appeared to be vertical in. most cases. The foliation at location 11 graded into bluebands when followed along the strike away from the nunatak. The strike at this station was generally west but gradually curved around to the south as it changed into broad bluebands before crossing the trail between locations 5 and 6 (Fig. 8), One profile was made, which in general followed the summer trail. Location 1, only 875 ft from the edge of the glacier near the trail, shows the strongest preferred orientation of optic axes, This was in an area of well-developed foliation and undoubtedly in a zone which had been subjected to high shear stress. This diagram shows that the crystallographic c-axis (optic axis) of almost every grain is essentially normal to the foliation plane, which makes the glide planes in the ice crystals almost parallel to the foliation plane. The farther one progresses from the ice edge along the trail, the weaker the pattern becomes. This can be followed in the diagrams in the following sequence: location 2, 0.5 mi from edge of the ice; location 3, 0. 9 mi; location 4, 1.3 mi; location 5, 1.6 mi; location 6, 2.3 mi; location 7, 3 mi; location 8, 4 mi from ice edge. t can be seen that the pattern gets progressively weaker until location 7 is reached, where the orientation appears to be random, The surface ice at this station, only about 0.5 mi below the firn line, probably has never been deeply buried nor subjected to strong shear stresses, Location 8 was above the firn line and also has random orientation of crystals, but this was clearly superimposed ice caused by meltwater from the last winter's snow refreezing on the cold ice surface below the snow. During the early part of the summer, superimposed ice was found at location 6. This was the result of an increase in slope for a short distance, which gave more protection from the sun during part of the day. The orientation of this superimposed ice (shown as location 6A) is random as at location 8. The deeper coarse-grained glacier ice at location 6 was taken in August after the superimposed ice had melted away. Figure 3 illustrates the grain size of the superimposed ice at locations 6 and 8 during midsummer, Figure 4 shows the grain size of the glacier ice beneath the superimposed ice at location 6.

5 4 STUDY OF CE FABRCS, THULE AREA, GREENLAND Scm Scm Figure.3. Thin section of superimposed ice at location 6. Between crossed polaroid sheets. Figure 4. Thin section of glacier ice at location 6. Between crossed polaroid sheets. The ice of the Nuna Ramp flows in a westerly direction into a north-south valley 500 to 600 ft deep along the western side of the ramp. The ice then flows both upvalley to the north and downvalley to the south for a short distance, Locations 9 and 10 were established on the northward-flowing part of the glacier (Fig. 8) about 3/4 mi north of the trail. n general, location 9 shows the axes of the crystals normal to the foliation plane, but the pattern shows a clustering of points into several maxima similar to those found by the author on Emmons and other temperate glaciers. The ice at location 11 was taken only 600 ft south of the edge of First Nunatak in a strongly foliated area, The pattern shows the same strongly oriented crystals as in other ar~as of higher shear stress.. At locations 1 through 6, the shear couple imposed on the ice in the later stages of deformation (deduced from the foliation, direction of flow, and the position of the ice sample on the glacier) was probably one in which the ice to the east was being thrust over the ice to the west at each location. The ice at location 11 was undoubtedly subjected to a shear couple such that the north side was held by the nunatak while the south side moved in a westerly direction (see arrows, location 11, Fig. 8). The fabric diagrams from the Moltke Glacier give maxima similar to those at location 9, especially number 3, which came from the north side of the glacier only a few hundred yards from the edge. The reason for several maxima clustered about the pole to the foliation, but not coincident with it, is not known but may be a recrystallization phenomenon, The first Mc:tltke location was about 3/4 mi from the south side of Moltke Glacier and the grains are still rather strongly oriented. The second Moltke location was near the center of the glacier and no foliation could be found, but the pattern still shows a well-developed preferred orientation of crystals. The shear couple imposed on the two locations nearest the sides of the Moltke Glacier

6 STUDY OF CE FABRCS, THULE AREA, GREENLAND 5 was such that the ice toward the center moved more rapidly toward the ocean, while the rock sides tended to hold the ice at its edges. This couple is drawn on the diagrams at locations Moltke 1 and 3. The shear couple at location Moltke 2 cannot be discerned with the information at hand. Other observations. 4. t was observed in several instances that the crystals near the surface, which at this time of the year are at the melting temperature, were considerably larger than those only 2 or 3 ft deeper in the glacier, where the ice was still below freezing or had risen to the melting point only very recently. This was especially noticeable in the almost bubble-free layers of the foliation. t appears that recrystallization of small grains into larger crystals (up to 7 em across) occurs very rapidly in the clear ice areas when the temperature rea-ches the melting point, perhaps in only a few weeks or months, which coincides with 5 em Figure 5. Same as Figure 2 between crossed polaroid sheets. Shows difference in grain size between the clear and bubbly lamina. laboratory findings. The bubbles apparently inhibit the growth of crystals even at the melting temperature. Figure 2 (page 2) is a picture of a thin section from Moltke Glacier location 1 in unpolarized light showing a portion of a bubbly layer and a portion of a clear layer of ice. Figure 5 is the same section between crossed polaroid sheets, and shows the difference in grain size in the two folia. This section was taken within 6 in. of the surface. Deeper in the ice the grain size was small and more uniform, similar to the fine-grained portion of Figure 5. Why the bubbly layers do not recrystallize as rapidly remains a mystery, but perhaps the many small bubbles interfere with the migration of crystal boundaries, or possibly the strains are relieved readily by migrating to a bubble-ice boundary so that there is no need to recrystallize. n general the ice in the active shear zones was less than 1 em in diameter and even smaller in the most active areas (Fig. 6). Grain growth apparently is extremely slow in ice several degrees below the freezing point. 5 em V. CONCLUSONS Figure 6. Thin section, location 1 between crossed polaroid sheets. 5. ce appears to be very sensitive to shearing forces and the crystals in polar

7 6 STUDY OF CE FABRCS, THULE AREA, GREENLAND glaciers tend to become oriented so that the basal glide plane is parallel to the shear plane. The strength of the pattern appears to be more or less proportional to the strength of the shearing forces imposed on the ice. n a few patterns, the glide planes tend to be oriented in three or four directions close to, but not coincident with the shear plane. Strong shearing forces seem to produce small crystals with strong preferred orientations while high temperatures tend to produce larger crystals. The orientation patterns may be preserved during recrystallization, but previous work on temperate glaciers indicates that recrystallization under melting conditions may in some cases tend to change the strong orientation of grains from a single maximum with optic axes normal to the foliation plane into three or four maxima, none of which may coincide exactly with the pole to the foliation plane. With these and future fabric studies, it may be possible to predict something of the nature and direction of the forces to which a sample of ice has been subjected.

8 LOCATON MOLTKE GRANS LOCATON MOLTKE GRANS MAP CONTOURS, 2,4,6,10, 15 4 PER 'Y.AREA POLE TO FOLATON CE O'Y.-15% PLANE OUTLNE OF CE 6,.- 10% ~ DEPRESSON CONTOURS o-1"41nsde BARE ~ S'Y.-20'Yo GLACER D '--'---'---;;5 ';- TA~T;-;-U;-;T -;:- E --:-M~L;-;: 0 E:-;;-s--'1 m ORENTATON OF CE OPTC AXES PLOTTED ON SCHMDT EQUAL - AREA NET, LOWER HEMSPHERE. DAGRAMS N HORZONTAL PLANE. LOCATON MOLTKE GRANS Figure 7. Map and fabric diagrams, Moltke Glacier. Nunatarssuak area (in box) is mapped on a larger scale in Figure 8.

9 200 GRANS... _.. _... / / SHAWS''.. J,..NUNATAK -... / MAP HLL... / _..- / DAGRAM CONTOURS 1,2,4,6,10,15,20,25,30% PER 1% AREA MORANE AND DEBRS m BARE GLACER CE ~ ~ OUTLNE OF CE TRAL OVER 30% 25%- 30'Y. 20% - 25% 15% -20% ~ ~ D 10%-15% 6%-10% DEPRESSON CONTOURS 0-l'o NSDE POLE TO FOLATON PLANE ORENTATON OF CE OPTC AXES PLOTTED ON 2 ; LACER CE 200 GRANS STATUTE MLES SCHMDT EQUAL- AREA NET, LOWER HEMSPHERE. DAGRAMS N HORZONTAL PLANE LOCATON GRANS Figure 8. Map and fabric diagrams, Nuna Ramp, Nunatarssuak.