Slructural Paramefers Versus Depth and Growth Rate 



The variation in structural parameters occurring at different depth levels 

 throughout an ice sheet is related to the growth rate of the ice. This is especially 

 true In the upper levels of a young ice sheet, where temperature fluctuations have 

 a pronounced effect on crystal growth. The sudden increase in crystals per square 

 centimeter at the 40-cm depth in Figure 14 correlates well with the decrease in 

 platelet width at the same depth as shown in Figure 15, and probably indicates a 

 period of extreme cold during the early growth history in McMurdo Sound. 



CRYSTAL STRUCTURE VERSUS STRENGTH 



The relationship of crystal structure to strength is a wei rablished property 

 of many crystalline solids. The obvious variation of certain ci ital and subcrystal 

 parameters with depth suggests that the strength of ice should also vary with depth. 

 The possible reasons for the vertical variation of strength have been discussed in 

 detail by Assur (1958) and by Anderson and Weeks (1958). Weeks and Assur (1963) 

 state (p. 259) that "the principal structural parameter controlling the distribution of 



brine in single crystals of sea ice is the plate width If the plate width changes 



systematically with the distance below the upper surface of the ice sheet, this 

 variation might conceivably account for a change in vertical strength." 



The platelet width determines the primary spacing of brine layers and cylinders. 

 The size, shape, and spacing of brine inclusions vary with temperature, but after their 

 initial formation these inclusions no longer have any relation to crystal structure. 

 If an ice sheet becomes cold, more ice freezes out of the trapped brine and the brine 

 cavities become smaller; as an ice sheet warms up, ice melts and dilutes the brine. 

 The brine cavities and layers then become larger and migrate downward through the 

 ice sheet under the influence of gravity and a temperature gr;;,:.ient. The spacing and 

 geometry of brine drainage features are thought to be the major factors affecting 

 the strength of sea ice (Anderson and Weeks, 1958, p. 632). 



Assur (1958) established a correlation between brine volume and ring-tensile 

 strength and showed that as brine volume decreases, ring-tensile strength increases 

 (p. 130). Further, brine volume is shown to decrease with depth in an ice sheet as 

 a result of increasing platelet width (Weeks and Assur, 1963, p. 267). The data of 

 Graystone (1960, pp. 36-39) show a slight increase of strength with depth in sea ice 

 1.2 to 1.5 meters (4 to 5 feet) thick. However, Weeks and Anderson (1958, p. 643) 

 found no difference in strength when breaking cantilever beams by both push-up 

 and push-down loading of sea ice varying in thickness from 6.3 to 36.8 cm. 



26 



