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It might look like a picture-perfect setting today, but around 800 years ago this was the likely site of a volcanic eruption so humongous that it set a 7000-year record for volcanic sulfur release and left its explosive imprint not only in medieval chronicles, but also in tree rings and even polar ice. For decades, the geographical origins of this eruption have been a volcanological mystery that experts grappled to solve, but now a new study claims to have found a definitive answer ... and it points here: the Rinjani Volcanic Complex on Lombok Island in Indonesia.

To get to the bottom of the mystery, a large international team of scientists from several different disciplines searched for clues in sources as diverse as ancient texts recorded on palm leaves and slices of ice core drilled from the earth's ice sheets. They tell the BBC that finding the answer to the riddle felt like solving a whodunnit : “We didn't know the culprit at first, but we had the time of the murder and the fingerprints in the form of the geochemistry in the ice cores, and that allowed us to track down the volcano responsible.”

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Image credit: Lavigne and others (2013)

So just how massive was this eruption? Take Krakatoa (one of the biggest geologic disasters of modern times) and multiply it by eight – at least if you go by the volcanic emissions that were spewed out into the stratosphere. The climatic upheavals that followed were felt around the world, making their mark in archeological records and the historical writing of the Middle Ages. “Medieval chronicles highlight an unseasonable cold summer with incessant rains, associated with devastating floods and poor harvests,” the authors write. Signs of this massive fallout were also etched into tree rings and even deep in the ice of both the Arctic and the Antarctic, pointing to a colossal volcanic eruption around A.D. 1257 or 1258. But the eruption's chemical imprint in polar ice was not enough to go on when it came to pinpointing the exact location of the blowout. 

For that, the researchers turned to an ancient Indonesian historical text known as Babad Lombok.Written on palm leaves, it describes a catastrophic eruption of a volcano known as Mount Samalas in the Rinjani region and suggests the eruption occured before the end of the 13th century, a possible timing match for the A.D. 1257-8 disaster. To find out whether this was the mystery volcano they'd been searching for, the team conducted extensive fieldwork in the area and on neighbouring islands, tracking volcanic ash and rock deposits, and sampling tree rings for potential clues. They also matched up these samples with the chemical clues found in the polar ice cores. All the evidence, the authors claim, conclusively points to Samalas as the culprit. The ancient volcano would have been located within the modern-day Rinjani Volcanic Complex, somewhere within what is today a large caldera filled by the crater lake known as Segara Anak.

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Image credit: Lavigne and others (2013)

The researchers believe Samalas's massive eruption would have spewed out about 40 cubic kilometres (10 cubic miles) of rock and ash up to 40km (25 miles) or more into the sky. The resulting devastation would have rendered Lombok Island, Bali and other surrounding areas uninhabitable for generations – and may even have buried an ancient city.

“The Babad Lombok indicates that the eruption of Mount Samalas destroyed Pamatan, the capital of the Lombok kingdom. We speculate that this ancient city lies buried beneath tephra deposits somewhere on the island. Should it be discovered, Pamatan might represent a 'Pompeii of the Far East,' and could provide important insights not only into Indonesian history but also into the vulnerability, adaptation, and resilience of past societies faced with volcanic hazards associated with large-magnitude explosive eruptions,” the authors write. 

The findings are published in the Proceedings of the National Academy of Sciences.