244 Ice in the Sea 



consist of pure water. As the ice crystals grow they withdraw pure water from this 

 enclosed salt solution, which thus becomes more concentrated and of more specific 

 weight; it gradually percolates out between the ice crystals and increases the salinity 

 of the surrounding water. This diffusion process beneath a forming ice layer is pre- 

 sumably the reason why the crystal plates in sea-water are always oriented perpen- 

 dicular to the freezing surface, while in fresh water they are parallel to it. The arrange- 

 ment of the crystal plates is in similar groups and they are oriented approximately 

 parallel, relative to each other, so that the structure of simple sea ice is fibrous; 

 therefore the fracture surfaces of the ice lumps appear perpendicular to the surface of 

 the ice layer. 



The classification and terminology of ice formation and ice forms can be made 

 according to diff"erent viewpoints ; unfortunately there is still no uniform terminology. 

 Drygalski (1930) has given a completely general classification of ice forms based on 

 genetic relationships. The two main forms of ice are shelf ice and sea ice. Shelf ice 

 represents a transitional stage between the forms of ice occurring on the land and those 

 found at sea. It Ues along the coast over the continental shelf and is for the most part 

 a mixture of sea ice and land ice (coastal snow ice). Shelf ice reaches its greatest thick- 

 ness and extent around the Antarctic; a typical example of this type is that found along 

 the northern coast of Grant Land which is known as palaeocrystalline ice (Ureis). 

 Other forms of Arctic shelf ice are found along the east coast of Greenland (Wegener, 

 1902, "floating land ice"). In sea ice there occurs a gradual change of the ice crystals 

 to pap ice (ice mud, ice slush); in calm weather and at low temperatures it freezes 

 together to a hard layer of ice up to 5 cm thick and forms, especially at the surface, 

 a weakly saline top layer. In a rough sea and at still lower temperatures small sheets 

 of ice are formed which grow rapidly and assume a plate-shaped form with upwards 

 bulging edges (pancake ice). The individual plates have a diameter of 0-5-1 m, with a 

 maximum of about 3 m. In calm weather pancake ice and ice slush freezes together 

 to form a solid layer of young ice with a thickness of between 5 and 20 cm, having a 

 greenish blue colour; the surface is wet and still rather plastic. Further growth gives 

 sheet ice, often forming large lumps which are broken and piled up by pressure forming 

 pack ice. 



A detailed terminology of ice forms has been given by Maurstad (1935; see also 

 ZuKRiEGEL, 1935). Sea ice is divided according to age into two groups: winter ice 

 (including young ice) and polar ice. The first is not more than one year old, still rela- 

 tively soft and plastic, and usually occurs in the form of ice lumps. Polar ice, on the 

 other hand, is mostly two or more years old, contains little salt and is therefore hard. 

 Due to ice pressure it soon takes the form of pack ice. 



With reference to its position and movement Maurstad distinguishes between solid 

 ice and drift ice. The first is found for the most part in bays, fiords and above shallow 

 waters. Also winter ice, as long as undisturbed, may remain stationary during the 

 entire winter; however, it is usually broken up by long open cracks and drifts away. 

 Drift ice can take all forms and reaches its greatest extent in the drifting ice fields of 

 polar ice in the Arctic. 



In spring and summer, under influence of the increasing solar and sky radiation 

 and the warm winds, the winter ice begins to melt. The volume of the salt solution 

 enclosed in the ice increases and the inner structure of the pure ice crystals is weakened. 



