Notes on the Formation of glacial ice

“Glaciers originate in snowfields or neves through transformation of snow into ice”. (Thornbury). Conditions necessary for the growth of a glacier are the following: (i) abundant snowfall, (ii) very low temperatures; and (iii) a summer period not long enough to melt all the snow received in the previous winter.

In other words, when annual snow received is more than the snow that melts in mountainous regions, it will go on accumulating from year to year. In this way the snow fields go on increasing through a long period of years. At this point it is pertinent to explain certain technical terms that have been used in the following discussions.

Snow line:

“The lower or outer edge of a snow field above which all the snow does not melt even during summer is the snow line”; (Worcester).

According to Salisbury, snow line occurs at any altitude at sufficiently high latitude, and at any latitude at sufficiently high altitude. Thus, there may be a snow line at the equator at an altitude ranging from 4000 meters to 6000 meters.

In the Western Himalayas the snow line varies between 4500 m and 6000m, and in the Eastern Himalayas the same may be found at an altitude varying from 4000 m to 5800 m. On the contrary, the snow line in the Arctic or Polar regions comes down to the sea level.

The height of the snow line in the Alps is 2750 m. In Assam it is at an altitude of 4420 m, while in Kashmir it is at 5800 m. In the high mountain peaks of the equatorial regions, the height varies from 5200 m to 5500 m in Africa and the Andes Mountain respectively.

However, the altitude of a snow line depends both on climatic factors and on the nature of the terrain. The permanent snow line is found at the level where melting of snow due to summer heat fails to remove the winter accumulation.

In such circumstances, however, when an exceptionally small amount of snowfall in winter is followed by a very hot and dry summer, the snow-line in certain area may be higher than usual.

Ne’v’e:

Where snow is deposited in a basin, the successive layers of snowflakes undergo a gradual change. Snow is changed into a more compact form. It retains air between its particles and forms a mass of whitish granular ice which is called neve in French and Firn in German language.

Remember that firn, or ne’ve’, is an intermediate step in the conversion of snow to glacier ice. It is granular and loose unless a crust is formed by it. Moreover, when during the warm season there is lot of melting on the surface of a snow-field, water thus produced seeps into the ne’v’e where it re-freezes (called regelation) and forms a more compact mass.

Thus, granular ice is formed. But there are still some bubbles of air present in it giving it an opaque white colour. With further decrease in the size and number of air bubbles, the colour of ice undergoes a change from opaque white to clear blue.

But this blue colour is confined only to particular bands of ice. From the foregoing discussion it is clear that glacier is composed of dense ice that is formed from snow and water through a process of compaction, re-crystallization, and growth.

A glacier is, therefore, dynamic body that moves down-slope and modifies the landscape through which it flows. A glacier is an open system, with inputs of snow and moisture and outputs of melting ice and evaporation.

In fact, a glacier’s budget consists of net gains or losses of glacial ice, which is certainly the deciding factor whether the glacier expands or retreats.

Remember that it takes a very long time for the formation of glacial ice. It is quite similar to the formation of metamorphic rocks. Like rocks, snow and firn are subjected to great pressure so that they are re-crystallized into glacial ice.

In Antarctica, the climate being dry and the input of snow and moisture very low, glacial formation is completed in about 1000 years. On the contrary, in humid climates this time is much less-just a few years only.