When Earthquakes Rocked the Ice!

It was freezing cold and the night temperature used to dip as low as minus ten degrees Celsius, the scientists of the Physical Research Institute (PRL) Ahmedabad and Indian Institute of Geomagnetism (IIGM), Mumbai were braving the cold in their tent on Shalang Glacier. This glacier is a part of Gori Ganga basin in the Central Himalayas of Uttarakhand. 

The scientists knew that the glacier is endowed with rich information about the climates of the past and they were trying to dig that only in that freezing environment. Not only climates, they came across some interesting evidence about the earthquakes that must have rocked the frozen terrain in the pre-historic days.   

The higher Himalayas and the Tibetan Plateau have had considerable influence on the global climate and R.K. Pant a scientist of eminence from PRL, Ahmedabad and his colleagues, have strong reasons to believe that these topographic highs may have played a key role for the onset of glaciation in the last chapter of the earth's history that is Quaternary. Glaciers, like the rivers leave a trail or marks behind them. Thus even eons later geologists are able to reconstruct their trails and work out the climates of the past. To do so they try to work out a detailed history of the Last Glacial Stage (LGS) is a terrain known for past glacial activity. Similarly past temperatures and precipitation are estimated with the help of equilibrium line altitude (ELA) of the glaciers.

The material carried by glaciers, known as moraines give the cue about the presence of a past glacier. Equilibrium Line Altitude of a glacier is associated with material dumped on valley sides or the lateral moraines and defines the boundary between the zone of accumulation of ice from that of ablation. Scientists are able to work out past climate changes with the help of altitudinal differences between the past and the present ELAs.

The Quaternary or the last chapter of the earth's history started about 1.75 million years ago (Ma). The older part of the chapter is the Pleistocene, the period of Ice ages and the Holocene which started some 10,000 years ago and continues even today, is a period of warmth. With the help of the available evidences Pant and his colleagues were able to work out the glacial history and past climates and also the impact of Trans Himadri Fault (THF) on the late Quaternary glaciation processes.

Imagine a criminologist. He visits the site of crime and collects blood stains, hair, cigarette stubs, finger prints and the like. With the help of these he reconstructs the crime scene. Aided by computer images drawn with the help of descriptions of the suspects given by often it is possible to nab the culprits. But while reconstructing the past that too in terms of thousands or millions of years for a geologist the evidences are vague. For example Pant and his colleagues knew that a glacial valley is 'U' shaped. But why the valley of Shalang Glacier abruptly terminated northwest of Rilkot near a straight course of Goriganga River was inexplicable. Rivers normally seldom flow along a straight course. They rather follow a sinuous course. They found that the Trans Himadri Fault ran parallel to the river course or in other words it was THF that guided the course of the river in that part. This fault runs parallel to the Goriganga (northwest-southeast) till Martoli and then turns in northwest direction. Trans Himadri Fault contained the glacier in the northwest part only as confirmed by the typical 'U' shaped valley and the southeastern part was a river valley confirmed by the 'V' shaped valley. 

Yet another interesting observation of Pant and his team was that the Shalang Glacier flows close to the steep western flank of the valley that is closer to the THF. On the other hand about 1000 m east of the present glacier there are marks in the form of polished surface and striations due to the ancient glacial activity. The oldest moraine also has rocks that are found at the base of the relict glacier. 

Pant and his colleagues have actually confirmed that the THF which was till recently considered active till 11 Ma that is till late Miocene period of the Earth's chronological history has been active even in the last chapter that is Quaternary. Incidentally the present day activities are part of this chapter only. They have taken cue from the presence of knick zones, perched glacial valleys, deformed lacustrine sediments, incised outwash gravels and scree cones in the vicinity of THF, as suggestive of tectonic activity during the Quaternary.

Using Radiometric dating methods they found that even in Gangotri glacier the valley glaciation was active till about 64 Ka. Similarly in Garbyang they found moraines at identical elevations at the base of lake sediments abutting against the THF. This led them to conclude that it was active THF that cut the glacial regime and many times caused lakes to be formed and also created deep gorges close to glacial valleys. In Garbyang they succeeded in getting optical luminescence dates of lake sediments and glacial gravels which indicated three major tectonic events during 22-17 ka, 14-13 ka and <11 ka. 

Fault movements now as per the seismologists have been behind some of the great earthquakes. In other words as late as nearly 11,000 years from present the higher Himalayan was being rocked by earthquakes that re-carved the topography, caused the glaciers to abruptly halt and also influenced the climate.

If such huge earthquakes have rocked the Himalayas in the past can we rule them out for the future?

Therefore, the evidences put forth by Pant and his colleagues are not merely for academic archives. They have a great meaning for the present society. The Himalayan state of Uttarakhand is poised for big development. The government is leaving no stone unturned for making the state a developed and prosperous state. The society and the government must consider the risk of natural hazard lurking in the womb of the Himalayas before embarking upon developmental activities involving huge sums. The construction must take into account the earthquake history else the results could be disastrous.  


More by :  V. K. Joshi (Bijji)

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