Figure 84. The formation scheme of thermal fissures.- The forces 

 causing the fissures are shown by arrows. 



The fissures, once formed, are gradually filled from below by sea water, which freezes on 

 contact with the cold ice; and from above by snow and water, which flows over them. In such a 

 manner, owing to the temperature change of the ice, a constant ice accvmiulation and a cause for 

 continuing tension occur. In the presence of a continuous ice cover, these tensions cause thermal 

 jammings and the formation of small hummocks and ridges on the surface of the ice. These phe- 

 nomena have already been pointed out by Nordenskjold. 



Thermal expansion and tension of ice is complicated by the fact that the surface of the ice, 

 which varies according to the structure, is usually covered by snow layers of various thicknesses, 

 which decrease the amplitude of the surface temperatures of the ice differently. Thus, toward the 

 system of fissures, in the coldest (therefore the hardest and most brittle) surface ice layers. This 

 system begins with very fine capillaries, proceeds in all directions, and interlaces, penetrating 

 and separating the large areas of ice. According to Schirschov and Fedorov, after all the snow had 

 thawed in the sxmimer and the melted water had flowed off the ice, the ice field on which the station 

 "North Pole" was built appeared covered with a net of more or less deep-surface fissures. When 

 holes were dug in the ice, deep thermal fissures were repeatedly discovered. 



According to the observations of the Sedov 25 March 1939 at 86° 26.5' north, 109° 41' east, 

 in one 24 hour period the ice was covered by an imusual net of threadlike fissures; at the time the 

 air temperature was about -39°. 



Frost-cracMng of sea ice continues all winter (especially with marked reductions in temper- 

 ature) and is accompanied by a characteristic noise which reminds one of the reverberation of a 

 gunshot. A number of the thermal fissures are filled with snow and water, which freeze on contact 

 with cold ice and are done away with in such a manner. Others remain and prepare the ice, as it 

 were, for a subsequent disintegration intiD separate parts, under the influence of suitable external 

 forces. The presence of thermal fissures in ice should afford an explanation for the comparatively 

 easy break-up of thin (even strong in appearance) ice fields, when sufficient areas of open water ap- 

 pear among these ice floes. Thus, in the middle of February, 1938, I observed how, near the 

 shores of Greenland, the Ermak easily split ice blocks 3 to 5 m thick. 



In some cases, thermal changes in the ice area can also have an immediate practical meaning. 



As Barabanov and Richter point out, in the case of damage done to hydraulic installations 

 from dynamic horizontal pressure of ice in the Neva Bay of the Gulf of Finland (impact of ice 



243 



