The Telegraph
Since 1st March, 1999
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Vandals prolong Andamans puzzle
- Mission soon to find out if islands have inched closer to mainland

New Delhi, Jan. 2: Port Blair may have lurched a trifle closer to Chennai as a result of the earthquake off Sumatra last week, say earth scientists who now plan to measure the change in the distance.

The earthquake, caused by a section of the Indian Ocean plate slipping beneath the Burma plate, is expected to have changed the relative distances between sites on the Indian landmass and on the Andaman and Nicobar Islands, the scientists said.

But India may have to wait for some time to find out by exactly how much the distance has shortened.

A short concrete slab erected by Indian scientists at Port Blair six years ago, specially to study the relative motions of the two plates with the help of precision satellite measurements, was vandalised by miscreants two years ago.

'My heart broke when I heard that the concrete structure was gone,' said Vinod Gaur, a senior geophysicist at the Centre for Mathematical Modelling and Simulation in Bangalore. 'Imagine, someone just walked away with pieces of stone.'

'Had it been there today, it would have told us how Port Blair moved during the earthquake,' Gaur said. As the Indian Ocean plate dipped eastward last week, the Andaman landmass would have shifted west.

Two years after setting it up, Gaur and his colleagues had used the concrete slab to calculate the rate at which the India plate is pushing against the Burma plate ' about 1.5 cm a year.

The movement during the earthquake would be the result of the release of decades of accumulating tectonic stress.

So, said Gaur, Port Blair may have shifted closer to the Indian landmass by 50 cm to one metre.

Geologists from several institutions in India plan to fly to Port Blair next week to find out how last week's earthquakes have changed the crust beneath the Andaman and Nicobar Islands.

Scientists say that even as the Indian plate pushes north and slips against Tibet, an eastern section of this plate pushes eastwards 'subducting' under Burma.

The rate at which it is going under Burma is faster than the rate at which it is going under Tibet, said Shyam Sunder Rai, a scientist at the National Geophysical Research Institute in Hyderabad.

The subduction itself, geologists say, represents a routine 'life and death' cycle of the Earth's crust that transforms the surface of the planet, changing the orientation and shapes of continents and oceans over tens of millions of years.

At the plate boundary zone, the crust of the lower plate slips beneath the upper plate, pushed down by massive tectonic forces. As the rocks move downwards, they encounter higher and higher temperatures and increasing pressures that causes them to melt.

At a depth of say 50 to 100 km beneath the Indian-Burma plate boundary, rocks of the Indian plate melt. The newly-formed molten material circulates deep underground, waiting to come up again, usually as a volcano, producing new crust. 'It's a life-and-death cycle of the planet's crust,' says K. Sree Krishna, a geophysicist at the National Institute of Oceanography.

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