Elephanta Island near Mumbai is a popular tourist destination. Geologically, Elephanta and Mumbai are related. Both are products of the Deccan flood basalt volcanism within the Panvel Flexure Zone, on the western continental margin of India. The Mumbai volcanic sequence dates back 62.5 million years and is geochemically and stratigraphically different from the 66–65 million year old Western Ghats sequences
to the east. However, when did the Elephanta volcanic sequence take place and what is its stratigraphic status relative to the Western Ghats?
Researchers from the St Xavier’s College, Mumbai, IIT Bombay and the Bhabha Atomic Research Centre, Mumbai decided to find out. They collected eight fresh representative
rock samples for argon–argon dating. The samples were crushed, cleaned, sieved and exposed to neutron irradiation for about 100 hours to convert stable potassium atoms into radioactive argon. The team extracted and measured argon from the irradiated
samples using a mass spectrometer.
The radioactive decay of potassium produces isotope argon 40 with a half-life of 1.248 billion years. The researchers measured the isotopic ratio of 40Ar/39Ar and determined the age of the samples. They found that two of the Elephanta lava flows and five of the dykes representing pathways of solidified magma rising to the surface are about 66.6 million years old, like the Western Ghats sequence.
Another dyke gave an age of about 66.1 million years making the total duration of Elephanta magmatism 3.5 to 6 million years, noting analytical uncertainties.
Thus rapid and intense magmatism occurred at Elephanta Island 65 to 66 million years ago, at the same time as the Western Ghats eruptions, but up to 3-4 million years earlier than volcanism in Mumbai.
Mumbai volcanism was linked to the continental breakup event between the Seychelles and Laxmi Ridge-India, and succeeded the Western Ghats and Elephanta volcanism by several million years, say the researchers.
So next time you go to Elephanta, remember that you are standing on rocks 66–65 million years old!
J. Volcanol. Geothermal Res., 379: 12-22 (2019);
*This is a revised version of a item that appeared first in Science Last Fortnight, a science news column in Current Science, 2019 July 25th issue.
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