common geological origin
The age of the earth is about 4 billion years according to modern geologists and astronomers. Much information about the earth has been recovered from the geological records enshrined in the ancient rocks and stratified formations laid down in successive ages as mud, sand, gravel, and shells in the beds of oceans, lakes, and rivers.
During the greater part of this incalculably long period of the Earth’s History, that small portion of the earths’ land surface constituting the Island of Sri Lanka, at the tip of the Indian Sub-Continent, did not form a separate geographical entity.
The dismemberment of the Gondwanaland continent by continental drift into several units including the Indian Sub-continent Southern Africa and Australia is believed to have taken place towards the close of the Mesozoic Era, but Sri Lanka still continued to be a part of the Indian Sub-Continent, and many more millions of years had to elapse before it became a separate geographical entity in the Miocene period of the Tertiary Era.
During the Miocene period, a belt of sea much wider than the Palk Strait of today flooded the land between Tamil Nadu and the Puttalam-Jaffna coast, thus creating the Island of Sri lanka. At the bed of this sea, shells and other remains of marine organisms accumulated. Subsequently fringes of this sea were brought up above the sea-level on both sides and this led to the formation of Miocene limestone outcropping along the north-west coast of Sri lanka and the Jaffna Peninsula on the one side and the Karikal region on the other.
Further evidence that Sri Lanka and Peninsular India had been one geographical unit until geologically recent times,is that they both stand on the same platform or shelf. The average width of this continental shelf is about 12 miles (20 km) around the Island, where the mean depth of water is only about 36 fathoms (216 ft or 65 metres), and beyond which there is a drop abruptly to 500 fathoms (3,000 ft.or 900 metres) in two miles (3.2 km.), and 1,000 fathoms (6,000 ft. or 1,800 metres) in about 10 miles (16 km),plunging eventually to a steep descent of 3,000 fathoms (18,000 ft. or 5,400 metres) and over.In the Palk Strait the sea is barely 15 fathoms (90 ft.or 27 metres) deep.
An event of great importance in the geological history of Sri Lanka was the unwrap of the central massif that took place in the post-Jurassic period, and had an immense bearing on the landscape and the climate of the Island.
Almost nine-tenths of the Island is composed of igneous and metamorphic rocks of Archaen or pre-Cambrian age. The igneous rocks were formed at great depth through the solidification of molten rock material (magma) beneath the outer shell or crust of the Earth. The source of the present day gems both precious and semi-precious are the igneous rocks of the pre-cambrian age.Thus almost 90% of the land surface of Sri Lanka is gem-bearing. The igneous and sedimentary rocks were gradually transformed to metamorphic rocks by the action of heat, pressure, magmatic intrusions etc. The metamorphic rocks formed were gneiss and schists, leptynites and granulites, khondalites, quartzites and quartz-schists, crystalline limestones, dolomite, etc.
In the Jaffna Peninsula, the adjoining islands, and the north-western coastal strip from Kalpitiya to Mullaitivu,the Archaen or pre-Cambrian rocks lie buried under a layer of sedimentary limestone of the Miocene period. The surface of this limestone region is generally flat and not much above sea-level except in certain areas like Keerimalai where sea-cliffs of about 50 ft.in height are formed.You are welcome to discuss this post/related topics with Dr Shihaan and other experts from around the world in our FORUMS (forums.internetstones.com)
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