Thursday, 3 April 2014

The age of Barren Island.

Barren Island is an uninhabited volcanic island belonging officially to India's Andaman Islands (making it India's only active volcano), though it is about 100 km to the east of the main Andaman Island group and about 450 km west of the coast of southern Myanmar. The island is about 3 km in diameter and rises 354 m above sea level. It is the only active volcano in the region, though it forms part of a group with a number of dormant volcanoes including Narcondam Island and Alcock and Sewell seamounts.

Satelite image of an eruption on the Barren Island volcano captured by NASA's Earth Observing 1 satellite on 25 September 2010. Earth Observatory.

The volcanoes sit on the Burma (or Burmese) Plate, a small tectonic plate underlying the Andaman Islands, part of the eastern Indian Ocean and the western part of Sumatra. To the west of the Andaman Islands this plate is being subducted beneath the Indian Plate, but to the east the situation is more complex. The Burma Plate is being pushed northward relative to the Eurasia and the Sunda Plate (which underlies eastern Sumatra, Java, southern Southeast Asia, most of Borneo and the western Philippines) by the northward movement of the Indian Plate, but there is an area of seafloor spreading beneath the Andaman Sea (separating the Andaman Islands from Southeast Asia), which in turn causes stresses within the Burma Plate, leading to a zone of faulting upon which the volcanic islands and seamounts are situated.

In 1991 Barren Island began erupting after 159 years of dormancy. As an active volcano in the Anderman Sea it presents a direct potential threat to settlements in the Anderman and Nicobar Islands, and potentially the nations of Southeast Asia, as well as a tsunami risk to nations around the Indian Ocean, and a threat to aviation, making an understanding of the eruptive history of the volcano a priority for Indian vulcanologists. Previous studies have revealed a series of seven ash and tephra (volcanic material) layers on the island, the oldest of which dates back to 70 000 years ago, although the island and its volcano are thought to be somewhat older.

In a paper published in the journal Currant Science on 10 April 2013, Jyotiranjan Ray of the Physical Research Laboratory, Kanchan Pande of the Department of Earth Sciences at the Indian Institute of Technology Bombay and Neeraj Awasthi, also of the Physical Research Laboratory, describe the results of an Argon-Argon isotope study of minerals from tephra close to Barren Island.

Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystalize. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallization. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.

Ray et al. studied plagioclase grains (plagioclase is a common mineral in volcanic rocks) from two tephra layers, previously dated to no more than 61 000 years old. They obtained dates of around 1.8 million years for the minerals. They infer that the minerals were formed within the volcano at around this time, and ejected around 61 000 years ago (this is not unusual for volcanos). From this they conclude that active volcanism has been occurring on the Barren Island volcano for at least 1.8 million years (though the island itself is not necessarily this old.

Backscattered X-ray images of two typical lithic clasts. Mineral grains (phenocrysts) and vesicular matrix are marked. Plag, Plagioclase; T-Mt, Titaniferous magnetite; Ol, Olivine; Cpx, Clino-pyroxene. Ray et al. (2013).


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