File picture of the Barren Island volcano

New Delhi, April 12: A pile of sediment plucked from the seabed near Barren Island suggests that India’s only active volcano is at least 1.8 million years old, but its date of birth remains unknown, scientists have said.

The scientists at the Physical Research Laboratory (PRL), Ahmedabad, said their studies of the sediment also indicate that the Barren Island volcano, 70km east of the Andaman Islands, has had seven major eruptions over the past 70,000 years.

Each of these eruptions would have carried volcanic ash to great distances from the island. The PRL scientists examined a four-metre thick column of sediment from the seabed about 32km from the island and observed seven distinct layers of volcanic ash sandwiched between the muddy undersea sediments, indicating seven big eruptions.

“The ocean faithfully preserves a historical record of such events,” said Jyotiranjan Ray, a senior scientist at the PRL.

The lowermost layer of the ash, believed to have been released by an eruption about 61,000 years ago, surprisingly, contained 1.8-million-year-old material.

“The ash layers had unmistakable chemical fingerprints of the Barren Island volcano,” Ray told The Telegraph.

The presence of the 1.8-million-year-old material in the ash suggests that older rocks present in the “plumbing system” of the volcano were brought out 61,000 years ago.

Ray and his collaborators — Neeraj Awasthi at the PRL and Kanchan Pande at the Indian Institute of Technology, Bombay — have published their findings this week in the journal Current Science, published by the Indian Academy of Sciences.

The Barren Island volcano turned active in 1991 after a dormant period lasting about 159 years. The analysis of the ash layers in the sediment suggests that large eruptions had occurred 70,000 years ago, 69,000 years ago, 61,000 years ago, 24,000 years ago, 19,000 years ago, 15,000 years ago, and 10,000 years ago.

Scientists still do not know when the volcano first emerged from the seabed. Barren Island rises about 2km above the seafloor and has an average height of about 300 metres above sea level.

“The island is still growing,” Ray said. In the two decades since it erupted in 1991, he said, the island’s diameter had grown by nearly a kilometre, and the vertical cone-shaped zone had increased by 20 to 30 metres.