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temperature, by unequal loading, or by pressure. Heavy snow de- 

 posits may, to a large extent, protect the underlying ice from sudden 

 thermal changes. 



Although cracks are due to the relief of stress within the pack, they 

 also allow movement of the pack. Blocks and floes are the product 

 of cracks and, under the influence of the wind, they constantly shift 

 their relative position, thereby producing leads which make the pack 

 navigable. Such openings, however, permit the production of pres- 

 sure and formation of hummocks, which make the passage through 

 pack dangerous to vessels. 



Pressure set up in the pack produces bending, tenting, and rafting. 

 The first stage, bending, occurs in thin and very plastic ice. In 

 heavier floes, which are less resilient, the ice bends up until a crack 

 is formed perpendicular to the direction of pressure, resulting in a 

 tent-like structure. Other radiating cracks usually occur and, if the 

 movement is continued, the blocks so formed pile up into a pressure 

 ridge. Kafting is the overriding of one floe on another and is the 

 most common effect of pressure. 



Pressure ridges attain their greatest height when newly formed and 

 before settling to a position of equilibrium. Ridges seldom reach a 

 height above 20 feet; greater height indicates the existence of land 

 which obstructs the free drift of the ice. 



Pressure produces cracks in the pack which have been classified as 

 hinge cracks, shock cracks, and torsion cracks. Hinge cracks are longi- 

 tudinal fissures in front of a pressure ridge. Old ice, which is no 

 longer plastic, will not bend under the weight of a heavy pressure ridge 

 piled on top of it. The result, when the breaking strain is reached, 

 is the formation of a crack where the ice is pressed down by this 

 heavy loading. The crack opens like a hinge. Radial cracks will also 

 develop in front of the pressure ridge, resulting in new breaking up 

 of the floes which, in turn, creates favorable conditions for further 

 hummocking. Between the pressure ridge and the hinge crack the 

 surface of the ice is often depressed below the level of the sea allow- 

 ing a pool of concentrated brine to form. Shock cracks are produced 

 by the impact of a moving floe against a floe relatively inert and in a 

 state of tension. These cracks are produced transverse to the ad- 

 vancing pressure ridge. Torsion cracks result from shearing and 

 screwing and produce chains of pools and zigzag leads. 



The age of floes may often be judged by the presence of colored bands 

 at their edges. During the summer, diatoms adhere to the underside 



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