Author: Rupkini Sengupta, University of Calcutta
email: [email protected] Glaciers, we all know what they are. From the geological perspective, a glacier is a large mass of ice formed from snow falling and accumulating over years that moves very slowly down along a slope of landforms, under the action of gravity (Fig.1). Glaciers can form and move on either side of the ‘snow line’. In high altitudes, snow line is the line above which snow does not melt even in summer forming a perpetual snow cover, and below which snow can accumulate or melt depending upon climatic conditions. Can you imagine the huge amount of fresh water each of these glaciers contains? That would obviously make them a very important part of the earth’s hydrosphere, right? Well, they are. But, the glacial ice is an equally important component of the geosphere too, as the geosphere (‘geo’: ‘ground’) is that ‘sphere’ of the earth that includes all the rocks, minerals and ground on earth.
What are the main factors for a metamorphic rock to form? A metamorphic rock is formed by recrystallization of an earlier rock (a protolith), by the action of pressure or temperature or both. Let us see if the conversion of snowflakes to ice follows these conditions.
Glacial ice, though not all types of ice, can be considered as a type of metamorphic rock because it has flowed under pressure. The principal agent of metamorphism in this case is the action of pressure. It forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. The metamorphism process is driven by the weight of the overlying snow. During metamorphism, hundreds, if not thousands of individual snowflakes recrystallize into much larger and denser individual ice crystals. The pressure generated by new snow piling above the old snow increase the pressure on the latter causing partial melting. The melting takes place at the points of contact- specifically the elongate arms of the hexagonal ice crystals. As the tiny projections melt, surface tension pulls the water inward towards the center of the crystal. As soon as it is away from the point of contact, it recrystallizes. The transformation steps are shown in the figure 5.
When observed under microscope, glacial ice, like metamorphic rocks, show euhedral (bounded by straight, regular faces) to subhedral (one or more irregular faces) polygonal grains with distinct triple point junctions, forming granoblastic texture (Fig.6). This is important because granoblastic texture is a typical metamorphic texture, which is formed due to recrystallization of the grains as they rearrange and reduce their grain boundaries to minimize the surface energy.
Moreover, it has been documented that glacial ice, like many metamorphic rocks, shows foliated fabrics: “Much of the recrystallized surface ice shows a pronounced Layered structure. The name foliation is appropriate for this structure.” (Meier, 1960, p. 52), or, “Glacier ice commonly shows a regular planar structure, usually consisting or alternate layers of relatively clear and bubbly ice…. Although the word 'banding' is also frequently used for this planar structure in ice, ‘foliation' gives the true implication or the three-dimensional structure that actually exists” (Rigsby, 1960, p. 590).
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