Kuenselonline: Environment Large types of glaciers in the Bhutan Himalaya are covered with rock debris at the lower part. Terminus of those glaciers is surrounded by huge moraines, which were formed by rocks flowing down with glacier ice during the Little Ice Age. Glaciers have expanded, and their surfaces have reached at the height of moraine ridges during the Little Ice Age. At present, several debris-covered glaciers in the Bhutan Himalayas have large lakes at the terminus due to glacier shrinkage.
We have to know the glacier mechanisms and expansion process of glacial lakes before understanding the behaviour of glaciers being ready to have large glacial lake. At the upper part of the glacier, snow deposit on the glacier surface and the snow layer change into ice gradually. And the glacier ice flows down by its own weight, which takes a long period (ex. several decades) to reach the lower part, in relatively low warm altitude, glacier ice tends to melt. These phenomena have repeated every year at all glaciers. If the ice supply flowing down from the upper part of the glacier is less than the lost ice by melting at the lower part, the glacier can’t maintain the mass and the glacier would shrink. This situation represents present glaciers’ condition in the Bhutan Himalayas. But, here, what I’m trying to say is “All debris-covered glacier don’t necessary have large glacial lake at their terminus”.
Debris-covered glacier have ice-cored moraine, which dams the glacier melt-water, filling the gap of glacier ice. Therefore, there would be a layer, which is saturated with glacier melt-water as shown in the Fig.1-A. These pond waters often dry away, when glacier surface does not reach the water level. But, once glacier surface reaches to the water level, supraglacial ponds on the glacier can stay around (Fig.1-B). Then, ponds expand rapidly by exposed submerged ice melting, because water surface can absorbs much heat for melting (Fig.1-B~C). In this way, glacial lakes expand at those glaciers with large surface lowering, because those glacier surface levels can reach to the englacial water level.
Slope of glacier surface can be also a good indicator for condition of glacial lake formation. Glacier with gentle slope tends to flow slowly, and only a little mass of ice can be supplied to the lower part. Those glaciers tend to shrink and large surface lowering could occur (Fig.2). Then, perennial supraglacial lakes appear near the terminus after glacier surface lowering to the water level in the glacier. As described above, glaciers with gentle slope and large surface lowering tend to have large glacial lakes in the Himalayas.
By: Dr Akiko Sakai (Glaciologist) Nagoya University
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