Glaciers are formed in very cold areas, and the annual average temperature is at least below 0℃, so glaciers are only distributed in high mountains and high-latitude continental areas. In addition to the influence of temperature on the formation of glaciers, snowfall is also an important factor, because glaciers are transformed from snowfall year by year, which means that the annual snowfall is higher than the annual melting, otherwise there will be no excess snow. There is a limit to control the accumulation of snowfall, which is called the snow line, which means that the annual snowfall in the snowfall area is equal to the annual melting amount. Above this line, the annual snowfall is greater than the annual melting, and there is snow accumulation in one year, which is a snow-covered area and may form glaciers. The snow line varies greatly in different areas, which is mainly influenced by climate, topography, slope direction and other factors. For example, if the snowfall is heavy, the snow line will be lowered. Therefore, in the Himalayan region, the snow line on the south slope (5500m) is lower than that on the north slope (6000m), and it also decreases from west to east. However, it should be noted that the snow line is the lower limit of the snow-covered area all year round, not the lower limit of the existence of glaciers.
Fig. 6- 1 Process of snow transforming into glacier ice (percentage in the figure is air volume fraction) (modified according to Press et al., 1982).
In the snow-covered area, the snowfall gradually compacts, and after a series of changes, glacier ice is formed (Figure 6- 1). This process is called ice formation. Under the action of gravity, glacier ice began to move, forming glaciers. Icing has different characteristics in different areas. In the arid and low-temperature continental climate zone, the compaction of snowfall is the main factor, and the snow particles are bonded to each other, so the ice formation speed is slow, which is called cold ice formation. Glaciers in China, for example, belong to this type. In the maritime climate area with heavy snowfall and high temperature, the process of snowfall-melting-refreezing is the main process, with the participation of meltwater, and the ice forming speed is relatively fast, which is called warm ice formation, such as continental glaciers near the ocean.
2. The movement of glaciers
The main driving force of glacier movement is gravity. Glaciers on high mountains move from high to low, while continental glaciers at high latitudes move from the center of the ice sheet (where glaciers are thick) to the surrounding areas (where glaciers are thin). Glaciers move in two ways. The first is called basement sliding, which means that glaciers slide forward along the ice bed with the help of lubrication of bedrock surface and floating of molten water. Mountain glaciers mainly move in this form; The second type is called plastic creep, because under the pressure of the glacier, the ice crystals that make up the glacier produce intragranular shear creep parallel to the bottom of the crystal grain, which leads to the forward dislocation of the ice crystals, and its macro performance is that the whole glacier creeps forward slowly. The thicker the glacier, the more obvious this movement is.
Glaciers move very slowly, usually only tens or hundreds of meters forward every year, and a few continental glaciers may reach one or two kilometers. Generally speaking, continental glaciers move faster than mountain glaciers, and thick glaciers move faster than thin glaciers. The moving speed of different parts of the glacier is also different (Figure 6-2). In cross section, the velocity in the middle is higher than that on both sides, while in longitudinal section, the glacier near the bottom moves fastest. Because of the slow movement speed of glaciers, the "marking" method and GPS measurement method are generally used to study the movement law of glaciers.
Figure 6-2 Vertical Zoning of Mountain Glaciers and Glacier Movement (revised and supplemented according to B. J. Skinner and S. G. Porter, 1994)
3. Types of glaciers
According to the scale, shape, landform and geographical environment of glaciers, glaciers can be divided into the following types.
(1) alpine glaciers
Mountain glaciers refer to glaciers distributed in high mountain areas at middle and low latitudes. This kind of glacier is small in scale and its shape is controlled by topography. According to its developmental form, it can be divided into the following types.
Barrel glaciers and cirque glacier refer to glaciers developed in snow-covered depressions near the snow line (Figure 6-3). This depression is called an ice bucket, so it is called a bucket glacier. The scale of ice bucket glaciers is very small, from a few square kilometers to less than one square kilometer. It is the most common type of mountain glaciers and one of the important cradles of mountain glaciers. Most valley glaciers are provided by ice bucket glaciers in the glacier ice. When there is more and more snow in the ice bucket, the amount of glacier in the ice bucket increases, and some glacier ice flowing out of the ice bucket hangs on the steep ridge outside the mouth of the ice bucket, forming a small glacier tongue, that is, hanging glacier. Hanging glaciers are formed in areas where ice and snow are scarce.
Figure 6-3 Schematic Diagram of Mountain Glaciers (According to Peking University et al., 1965)
Valley glacier refers to the glacier moving along the previously formed valley, which is distributed in a strip shape (Figure 6-3). Under favorable terrain and climate conditions, with abundant snowfall, ice bucket glaciers will continue to flow out to replenish suspended glaciers, which will continue to advance to the valley and flow along the valley, thus forming valley glaciers. Valley glaciers vary in length, and some are tens or hundreds of kilometers long.
Foothill Glacier When the climate is cold and the snowfall is abundant, the glaciers in the valley continue to flow forward, and the outflow valley expands at the foothills, forming foothill glaciers, also known as foothill glaciers. For example, the Malasi Pingshan Glacier in Alaska is formed by the confluence of 12 glaciers. The area of foothill glacier is 2682km2, and the thickest glacier is 6 15m. Glaciers in the foothills vary in scale. When piedmont glaciers expand or multiple piedmont glaciers are connected, they will develop into ice sheets or ice sheets. This is a type of transition from mountain glaciers to continental glaciers.
(2) Continental glaciers
Continental glaciers refer to large glaciers developed in high latitudes. Continental glaciers can be divided into ice sheets, ice caps and ice sheets according to their size and shape.
The flat or sunken ice body on the surface of the ice sheet is called an ice sheet, which is formed by the expansion and connection of glaciers in a relatively flat watershed and plateau, or in high latitudes. The ice sheet is the smallest continental glacier type, and the expansion of mountain glaciers can develop into ice sheets.
With the increase of snow, the ice sheet will further expand, and its surface will begin to uplift and develop into an ice sheet. The scale of the ice sheet is larger than that of the ice sheet, with a maximum of more than 50,000 square kilometers and a center thickness of several hundred meters to several thousand meters. A large area (tens to thousands of square kilometers) thick glacier is formed on the gentle plateau, which is called plateau glacier or highland glacier, also called ice cap or flat-topped glacier, such as Dunde ice cap in Qilian Mountain (the thickest is 167m) and Gu Liya ice cap in the west of Qinghai-Tibet Plateau (the thickest is 350m, and the area is 376 .. Plateau glacier is a transitional type between mountain glacier and continental glacier. The ice surface is slightly convex or flat, and sometimes individual peaks are exposed. Glacier tongues often protrude at the edge of glaciers.
Ice sheet When the glacier area exceeds 50,000 square kilometers, it is an ice sheet. The thickness of the ice sheet is huge, reaching two or three kilometers. For example, the center of the Antarctic ice sheet is currently 3400 meters thick. The shape of the ice sheet is shield-shaped, so it is also called the ice dome. Glaciers flow around from the center, and the fluctuation of basement topography has little effect on their movement. In Quaternary history, ice sheets have appeared in Northern Europe, Siberia and North America. Because of the huge amount of ice preserved by ice sheets, their existence has a great influence on global climate and sea level change.
The scale of glaciers is changing, which is related to the accumulation and melting of ice and snow. Except for the ice bucket glacier, other glaciers have obvious ice and snow accumulation areas and glacier melting areas. An important index to measure the change of glaciers is called the mass balance ofglaciers, which is the ratio of the accumulated amount of ice and snow in the accumulation area to the melted amount of ice and snow in the ablation area. The mass balance of glaciers reflects the change of ice quantity through the mechanism of ice body movement, which is called glacier fluctuation. If the accumulation of ice and snow is greater than the melting amount, it will continue to provide ice, and the glacier will move forward, which is called glacier advance; If the accumulated amount of ice and snow is less than the melting amount, less ice is provided and glaciers retreat, which is called glacier retreat; If the two are equal, the position of the glacier front (glacier tongue) is stable, but it does not mean that the glacier is motionless, and the glacier just moves to this position and melts. Because the melting amount of glaciers is not easy to determine, but the melting area and accumulation area of glaciers are relatively easy to determine, people use the ratio of accumulation area to melting area (AAR) to determine whether glaciers are expanding or retreating. When the value is greater than 0. 6. The glacier moves forward; Between 0.3 and 0.6, the position of glacier tongue is stable. When it is less than 0. 3. Glaciers retreat. According to the change of this value, we can also study the relationship between glacier change and climate and sea level, which is also an important indicator of climate change.