Monday, October 19, 2009

250 feet high hill collapsed overnight near Ranchi city of India.


Most of the hills in Jharkhand State are highly weathered.
by
Dr. Nitish Priyadarshi



Fig. Villagers praying the mountain under fear.,

On 8th October in local news paper one news baffled me. The news stated that a 250 feet high hill vanished in thin air overnight. The incident happened some 50kms away from the Ranchi city, the capital of Jharkhand State of India. Being a geologist I have never heard about such phenomenon of vanishing hills suddenly in recent years or in past especially in Jharkhand State which is free from major earthquakes and Volcanism. Volcanism promotes such types of subsidence. The crumbling of a section of the huge ‘Jumbla’ Mountain Range in the Angara Circle of Ranchi district has posed a question mark on vanishing of 250 feet High Mountain. But the incident has occurred, so I decided to collect the information from different sources and media persons who visited the area.

The on-the-spot survey by some of the electronic media person revealed that the mountain section in question had not vanished but definitely caved in.

According to the reports reaching to me, the debris of the said section of the huge mountain range, in form of huge boulders and scattered pieces of rocks in a vicinity of nearly a kilometer from the peak were strewn. The sleepy village of Rasujara had a night-marish night on the 6th of October when the entire village heard the hair raising thunders of rocks rumbling with heavy drizzle orchestrating it.

In the worst of their dreams they did not anticipate the scenario of the next morning. The very mountain which acted as their savoir had ‘vanished’ with scattered debris in its place! The small village of Rasujara with a population of less than 200 people was in a state of panic, thinking that the celestial forces were ‘angry’.

But before moving into any conclusion we should understand the rock types and geology of such hills in and around Ranchi district.

Most of the hills in the area has attained old age and are highly weathered. These hills are one of the oldest part of the earth’s crust. All the slopes below the top consists of soil, sediments and rock fragments. Occasionally there are few rock exposures.

Collapse of the hill may be due to weathering aided by mass-wasting forming ‘talus slopes’.

Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to years.

Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilised the surface. It may also occur at very high speed, such as in rock slides or landslides, with disastrous consequences, both immediate and delayed.

The predominant processes that degrade a rock slope depend largely on the regional climate (temperature, amount of rainfall, etc.) and also through Thermal stresses and Topographic stresses.

From first instance it looks that the hill became the culprit of Debris avalanches. Debris Avalanches are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes.

Other theory which can be implemented here that hill may have collapsed due to underground mine fire in the nearby coalfields of Ramgarh and Gola areas. Underground hollow created by mine fire may have put stressed on the highly weathered hill.

What ever may be the cause but it is true that more incidence like this may happen in future especially in Ranchi city where one of the hill, popularly known as Ranchi Hill, has already threatened the local people last year as part of it was affected with landslides.

Some of the other hills are also affected as they are in regular threat by explosives used by the miners for making stone chips for building purposes.

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