Ice Formation of the Glaciers on the Northern Slope of Bogda, Tian Shan
Xie Zichu, Wu Guanghe,Wang Zhongxiang and Zhang Wenjing (Lanzhou Institute of Glaciology and Cryo pedology, Academia Sinica, Department of Geology and Geography, Lanzhou University)
This article tries to give a description of the peculiarity of the ice formation of the glaciers on the northern slope of Bogda, its unique belt pattern structure and the reflection of climatic changes on ice formation process in the last twenty years. In the area investigated, the temperature is low and the precipitation high, providing cold storage and material sources favourable for ice formation. At the same time, the existence of paleo-peneplain at about 4000 m a. s. 1., close to the snow line of the modern little ice age, is advantageous for the formation of mature cirques or firn basins on it. Because the glaciers are comparatively stable at present, the firn line and the zero-equilibrium line are more suited to the landforms. The surface slope of the glacier at its height is very small, therefore infiltration-congelation belt widely develops with its width far more wider than all other glaciers in the source area of the Urumqi river. The infiltration-congelation belts grow on the gentle passes at about 4100 m a. s. 1., forming inversion phenomemon of ice formation belt. In the past it is believed that the thickness of the firn layer of the infiltration belt in the eastern Tian Shan is less than 5 m. According to data from drilling, the authors found that it may reach 10 m thick, i. e., close to the bottom of active layer. This shows that on the profile of the firn layer there must be many annual layers and that the time of ice formation lasts also much longer than 2-4 years as estimated in the past. Frequent avalanche and avalanche cone and accumulation of snow drift in Bogda Mts. arc important supply source for the glaciers. Melt water from the surface of the avalanche cone can make the temperature of the firn in the active layer reach 0℃ in summer, shewing a thermal infiltration in ice formation. Temperature turns back to negative in the winter and a cold infiltration ice formation appears again. In view of the above, the authors suggest a conception of the ice formation in the form of an avalanche cone.