Wednesday, 20 March 2013

The rock on a hill

It is one of the oldest rock formation on Earth and  it is located bang in the centre of a  botanical Garden. Formed millions of years ago, it stands a mute testimony to the events gone by. Over the years, it has become as much an attraction as the garden itself.
Children love to climb it, while lovers search for an isolated spot to sit. A rickety blue board proclaims that thro ck is a geological wonder.
Apart from the board, there is very little information about the rock and people make a beeline to the tower atop the rock to get themselves photographed. They rarely get themselves photographed with the background of the rock.
What people do not relies is that while the garden is a manmade wonder, the rock is a natural wonder and its origin goes back to hundred of million years. The garden, on the other hand, is a little more than 250 years old.
Yes, this rock is the Peninsular Gneiss of Lalbagh of Bangalore.
Peninsular means a mixture of granitic rocks and this type of rock is widely dispersed in the southern plateu of India. The Peninsular Gneiss was first coined by Dr W F Smeeth of the Mysore Geological Department. When the Rock at Lalbagh was dated, it was  declared a national geological monument way back in 1916.
The antiquity of this rock has attracted geologists from all over the world and has given rise to several scientific papers on the evolution of earth.  Geologists from the Mysore Geological Department, Geological Survey of India and scholars have written realms on it. The stone quarry of this Gneiss continues to be source material for research in the various branches of earth science.
Geologist Dr. M.S. Anantha of Mysore says such rocks can help unlock the secret behind the evolution of Earth and also give invaluable information on geological history.
This rock is 2.5 to 3.4 billion years old and this shows that it took shape much before the evolution of man. Such Peninsular Gneissic rocks form the basement of the bedrock in south or peninsular  India.
The Gneiss and Dharwar rock types are classified groups of the Precambrian rocks of India. The precambrian period refers to the time that began with the formation of the Earth around 4500 million years ago and concluded with the Cambrian period about 542 million years ago.
Whew. So this means that the Peninsular Gneiss surfaced during the evolution of the Earth or thereabouts. It is really a natural marvel that we can view above the surface the deepest rock type.
This makes it all the more exciting as generally most of Earth’s bedrock or consolidated rock surface is covered by numerous layers of soil, deposits, vegetation, and gravel. These overlying layers can be as thick as several hundred meters above the bedrock. As a result, it is difficult to access the Earth’s rock surface without extensive drilling. Even then , you cannot see them. Here, you see it free of cost.
When you start climbing the rock, you notice white strips along the rock surface. This indicates the presence of  sedimentary rock called Biolite. Interestingly, Biolite is formed either by living organisms or by their remains or fossil.
Remnants of older rocks are seen in the form of enclaves. Some excellent research information of the geological record of the gneisses is published in the “Geological Monuments of India”, brought out by the Geological Survey of India.
The GSI publication says the Gneiss “is dark grey to black rock called amphibolite containing the minerals plagioclase feldspar and horneblend. Due to interactions with granite fluids, these enclaves have developed a border rich in biotite mica. Later magmatic activity represented by grey poryphiritic granite followed by pink massive granite is also seen at several places. The gneisses and granites have been profusely invaded by still younger veins of coarse grained pegmatite which have cut up the gneisses into several irregular and lenticular shapes”.
The GSI has identified this Gneiss as one of the best exposures of this rock mass, protruding high above the ground as a hillock, dated 2.5 to 3.4 billion years. It has called it the Lalbagh rock.
Generally, such Gneiss is a coarse-grained high grade metamorphic rock formed at high pressures and temperatures in which light and dark mineral constituents are segregated into visible bands.
Kempe Gowda built a tower atop this rock. What have we done. We climb the rock, look around casually and get down, without realising that we are on the cusp of  Earth’s history. We have also littered it in every way that could have been possibly done.

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