The much more diverse external dynamic deformations fracture ice formations, thus causing 

 changes in either their outline and form, as well as hummocks and isostatic phenomena. 



Wind, rough sea, sea currents, tidal phenomena, and the forces of gravity appear to be the 

 chief factors determining the dynamic deformation. All these factors function differently in open 

 sea and along the shore, on floating ice and on fast ice, on winter (cold) and summer (warm) ice, on 

 solid and broken ice. Wind exerts the greatest influence on floating ice in an open sea; however, 

 the rolling sea is the greatest deformative factor on the ice's edge. Along the shore, sea currents 

 and tidal phenomena play a much greater role than the wind. 



In winter, thermal, and especially dynamic, deformations occur with a distant din. 



According to their statements, all polar explorers learn very quickly to distinguish by sound 

 the changes occurring in the ice surrounding them in winter. Thus, at low temperatures, the for- 

 mation of thermal crevices is accompanied by sounds which resemble harsh gunshots. The break- 

 up of ice causes the most diverse types of sounds. 



On the other hand, the quiet which accompanies the summer break-up impressed all the ex- 

 plorers, being sometimes far more grandiose in size than the winter break-up. Huge monoliths of 

 ice broke off, sighed, and plunged, producing almost no noise. Furthermore, this occurred in the 

 complete absence of wind. It is necessary to note that the dynamic deformations, which are deter- 

 mined by the different velocities of the various ice fields, depend on active forces and do not re- 

 quire high velocities for large-moving ice fields. Significant fissures and hummocks are some- 

 times formed, as it were, under completely calm circumstances. 



According to the observations of Brusnev, hummock ridges, stretching along the New Siberian 

 Islands, are found in quiet weather. They are the result of movement of one ice field along another 

 which is immobile. Sometimes the ice hummocks rise to a height of 7 m, although the relative 

 movement of the fields is imperceptible to the eye. 



LITERATURE: 62, 77. 



Section 92. Thermal Fissures 



As we have seen, the temperature of the bottom surface of the ice fields generally remains 

 constant. Consequently, with a temperature change in the upper layers, the bottom surface of the 

 field will strive to preserve its dimensions, while the dimensions of the upper surface may change 

 radically in one direction or another, depending on the salinity of the ice, its temperature, and the 

 direction of the temperature change. Hence it follows that the ice field will sag in one direction or 

 another under the influence of the surface-layer temperature, as long as thermal fissures do not 

 appear on the top or the bottom of its surface (see figure 84). 



If the temperature change forces the upper layers to contract, the fissures will appear on the 

 upper surface. On the other hand, if the surface layers expand under the influence of the tempera- 

 ture the fissures will appear on the lower surface. Since the surface layers of ice are almost 

 fresh, the upper surface of the ice will usually contract with a reduction in temperature and thus 

 become covered with fissures. This phenomenon received the name of "frost cracks. "* 



*The frost crack is also characteristic of icebergs and glaciers. Icebergs also split into 

 monoliths and sometimes, in the winter, disintegrate before your eyes. This is accompanied by 

 harsh noises, which exceed in volume the noise of the break-up of ice fields. In the Alps, the 

 noises in glaciers are always taken as one of the signs of a bad change in the weather. 



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