40 cm thick. Observations of McMurdo Sound sea ice show that, although there are 

 many crystals of enormous size near the bottom of a thick ice sheet, there are also 

 many crystals that occupy much less area than 1 cm'^. When crystals of all sizes 

 were considered, the average was found to be about 1 crystal per square centimeter 

 for the bottom meter of the ice sheet. 



Platelet Width Versus Depth 



An average platelet width of 0.45 mm, with a range varying from 0.2 to 

 0.8 mm, was determined for arctic sea ice by Weeks (1958, p. 97). Schwarzacher 

 (1959) made about 500 measurements of arctic sea ice to get an average platelet 

 width of 0.902 mm (p. 2359), or about twice that determined by Weeks. Weeks and 

 Hamilton (1962) describe the subcrystal structure in arctic sea ice 30 cm thick and 

 state that one of the apparent variations in structure is the gradual increase in 

 platelet width with increasing depth (p. 954). They further state, however, that the 

 expected variation in platelet width with depth has not yet been demonstrated. 



Figure 15 is a plot of platelet width versus depth in the ice sheet. Measurements 

 were made at various depths from thin section photographs of sea ice at stations 1, 

 2, and 3. Platelet widths were measured in the horizontal plane parallel to the 

 c-axis by counting the number of distinct platelets within a predetermined linear 

 distance and dividing to obtain an average width. Measurements were also made 

 from cores collected on three different dates at station 1, but show no apparent 

 differences in width that can be related to time or temperature changes. The system- 

 atic increase in platelet width from about 0.4 mm near the surface to more than 

 1.0 mm at the bottom is clearly shown in Figure 15. 



Stressed Sea Ice 



A sea Ice specimen oriented in relation to the stress field was collected from 

 the crest of a small pressure ridge near station 2 to study the effect of stress on the 

 crystal structure. The ice in the actively growing pressure ridge was about 2.44 meters 

 (8 feet) thick and had been stressed for at least 8 months by horizontal pressure from 

 the adjacent westward-moving McMurdo Ice Shelf. 



A thin section from a depth of 20 cm photographed in crossed polarized light 

 is shown in Figure 16. The large arrows in photograph 16a show the direction of 

 tensile stress. The small lines on the photograph show the strongly preferred direction 

 of c-axis orientation typical of many stressed materials made up of elongate, plate- 

 like, or tabular crystals. Photograph 16b has been rotated 20 degrees under crossed 

 polarized light to show the wavy extinction in the large crystal in the center of the 

 view. Wavy extinction is typical in thin-section photographs of strained minerals 

 such as quartz or olivine, and is a good criterion that the mineral has undergone 

 stress sufficient to cause crystal deformation. The preferred c-axis orientation and the 

 wavy extinction occurring in stressed sea ice indicate that the stress conditions in a 

 sea ice sheet can be deduced by detailed crystal studies. 



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