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Tales from the Lake Sediments
|by V. K. Joshi (Bijji)|
Landslides are integral part of the Himalayas. In these columns you have read of many devastating landslides that blocked the rivers. In the recent history, the landslide on Birehi Ganga at Gohna is well recorded. On 1893 September an entire spur of the mountain fell across Birehi and blocked the river, leading to a huge lake, locally known as Gohna Tal. In August 1894 part of the 1000 feet high dam broke and the water released submerged quite a bit of area downstream till Haridwar. But the remaining blockade continued till 1971 and the lake remained there.
I must shar with the readers that a lake and a river has different types of sediments. While a river continues to flow over sand, the lake water remains placid over fine clays. A lake is a lake because it holds water still. This is possible because of the fine sediments, that is, clays do not allow water to percolate down. Another character of the clays is that they are plastic in nature, just like the clays you might have used for creating various shapes as a kid!
The plastic nature of the clays makes them change shapes according to pressure on them. A character which comes handy, while studying the clay sediments of a lake. Particularly when they occur as layered deposits of a paleo-lake - a lake that existed in the past.
Earthquakes and our earth have an age-old association. Since the collision of Indian Plate with the Tibetan plate, the former has been continuously pushing under the later. The constant stress thus generated leads to earthquakes. The instrumental records of the earthquake are barely a century old. Information prior to that is based on the historical records. However, for the earthquakes that must have occurred in the pre-history one has to depend upon geological evidences.
The plastic clays of lake deposits are ideal preservers of past tectonic activities. Atargoo is a non-descript place at the confluence of Pin and Spiti rivers in the Spiti Valley, Himachal Pradesh. While looking for marine fossils in Spiti valley in 1976, I used to wonder at the broad valley of the river at this confluence. It is unusually wide valley here. Much later I realized that the U shaped, broad Spiti River Valley is much broader here, and becomes narrow at Atargoo. It was at this narrow neck the River was dammed, by the Spiti fault about 10000 years ago, claims a renowned geologist Dr O.N. Bhargava of Geological Survey of India. This dam must have produced a huge lake-today being referred as Atargoo paleo-lake.
Much later, the seismologists realized that such structures are formed due to liquefaction of sand in the subsurface. Sand is nothing but silica (glass) and it has a property of melting when heated-that is how glass is given different shapes for our use.
Likewise some of the clays have thixotropic properties. They become fluid, when shaken or stirred by any reason and later revert to semisolid state when aloud to stand. Such clays in the event of earthquakes form what the seismologists now term as the seismites. The seismites horizon of Atargoo is bound by rocks which show no deformation. Kotilia and his co-workers claim that liquefaction phenomenon in Atargoo may be linked to active Kaurik-Chango Fault, or even there could be a possibility of several active faults in the area, which might have activated the Spiti fault as well!
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