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 CHAPTER Y 



MOVEMENT AND DRIFT OF ICE 



FACTORS PRODUCING TRANSLATION AND DIFFERENTIAL MOTION 



Sea ice, other than fast ice in shekered bays or along the coast, is 

 continually in motion as a result of the effects of wind, tide, and cur- 

 rent. Although this motion may be the same for a time over a con- 

 siderable area, there is a number of factors tending to produce 

 differential motion of adjacent masses. Cakes, for example, vary in 

 area and thickness, so that effect of wind and current differs on differ- 

 ent masses of ice. Wind and current are also subject to continual 

 local variations, wind from the usual meteorological causes and cur- 

 rent from tidal effects. 



The swinging or turning of floes is due to the tendency of each cake 

 to trim itself to the wind when the pack is sufficiently open to permit 

 this freedom of movement. In close pack this tendency may be pro- 

 duced by pressure from another floe ; but since floes continually hinder 

 each other, and the wind may not be constant in direction, even greater 

 forces result. Thus wind produces rotation as well as translation. 

 This screwing or shearing effect results in excessive pressure at the 

 jutting corners of floes, and forms a hummock of loose ice blocks. Ice 

 undergoing such movement is called screwing pack, and is extremely 

 dangerous to vessels. 



In its motion the ice opens and shuts like an accordion; there is 

 always a certain number of lanes present, otherwise the ice could not 

 move. In summer these lanes remain open, except in very high lati- 

 tudes, but in winter they are soon frozen over with young ice. Swell 

 also tends to break up the ice, as well as the vertical movement of the 

 tide in narrow or shallow waters. As a result of all these agencies, 

 the ice is alternately being broken up, even throughout the winter, and 

 subjected to pressure. The onset of pressure or release of pressure 

 may happen at any time of year, even during the lowest mid- winter 

 temperatures. 



HUMMOCKING 



As moving floes are driven together or pressed against fast ice, 

 bending, tenting, or rafting occurs, according to the degree of pressure 

 and the composition of the ice. Definite ridges may thus be formed, 



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