Crystal Length Versus Depth 



In Figure 12 the crystal length in a horizontal plane is plotted against depth 

 in the ice sheet. Measurements were made from thin section photographs at various 

 depth levels and from two locations. These measurements show a systematic and 

 definite increase of crystal length with depth as related to the growth rate of the 

 ice. This increase probably applies to length both horizontally and vertically, 

 although crystal dimensions in a vertical plane are difficult to obtain and are not 

 presently available. 



Crystal Length-to-Width Ratio 



The ratio of crystal length to width in a horizontal plane is shown in Figure 13. 

 The true ratio is probably not accurately represented for ice near the bottom because 

 of the restricting effect of the 7.62-cm-diameter specimen. The length was measured 

 normal to the c-axis direction, and the width was measured parallel to the c-axis. 

 It was observed that the length-to-width ratio changed considerably with depth and, 

 for this reason, the data were arbitrarily divided at the 1.2-meter depth and were 

 plotted separately to emphasize this difference. The change in the length -width 

 ratio is not abrupt but is probably gradational downward through the ice sheet. 



For the 0- to 120-cm depth, the length -width ratio is approximately two to 

 one, which compares closely with data by Weeks and Hamilton (1962, p. 953) from 

 arctic sea ice 30 cm thick. However, as the ice sheet becomes thicker, the length - 

 width ratio becomes more nearly five to one, as shown in Figure 13. The scatter in 

 both plots was caused by crystals having a width greater than their length. It is also 

 obvious that there is only a slight increase of crystal width versus depth, and that 

 this parameter is of little significance. 



Crystals Per Square Centimeter 



The number of complete crystals in a measured area on the photograph of a 

 thin section were counted and plotted against depth in Figure 14. More data from 

 closely spaced depth intervals were available from station 1 than from station 3 and 

 show strong fluctuations in number of crystals per unit area in the upper 91 cm of 

 the ice sheet. The large number of crystals per square centimeter at the 40-cm level 

 probably indicates a period of extreme cold, when rapid ice growth formed small 

 crystals. As the ice sheet became thicker, growth rate decreased and crystal size 

 increased accordingly. Such an increase in crystal size with depth is discussed by 

 Weeks and Hamilton (1962, p. 951), who compare it with similar phenomena occurring 

 in metal castings. Dykins (1966, p. 25) shows a strong increase in crystal size with 

 depth in sea ice grown under laboratory conditions. In the reports cited above, it 

 is shown that there is less than 1 crystal per square centimeter in ice no more than 



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