Volcano Detectives Uncover Monster Ancient Eruption (Op-Ed)

Robin Wylie, University College London | November 12, 2013

Samalas caldera and Segara Anak lake
Credit: Céline M. Vidal, Institut de Physique du Globe de Paris (IPGP)

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Robin Wylie is a doctoral candidate in volcanology at University College London. He contributed this article to LiveScience's Expert Voices: Op-Ed & Insights.

Evidence of the explosion was scattered around the world — the column of ash it threw up had apparently reached the stratosphere. That much was clear, but not a lot else. In fact, one solitary line of evidence seemed, incredibly, to be the only remnant of one of the most gigantic natural disasters since the Stone Age: A volcanic eruption which dwarfed anything on record — and had barely left a trace.

Three decades ago, the frozen deserts at our planet's poles released a long-kept secret. When scientists first drilled into the vast ice sheets covering Antarctica and Greenland, amid the countless yearly growth layers, one horizon shone out like no other. It seemed that around the year 1258, the winter snows had carried with them an unusually large amount of sulfuric acid.

The ash that occurred alongside the acid revealed its source; it was already known that these kinds of deposits were linked to particularly large volcanic eruptions. But the sheer amount of debris in the 1258 layer hinted at a blast with no precedent in recorded history. The ice screamed of a cataclysm — yet history fell strangely silent.

It was possible, though, to tease one final clue from the polar ash. Because it had fallen simultaneously at opposite ends of the Earth, it could be calculated, using knowledge of global wind patterns, that the eruption which ejected the ash had occurred somewhere in the middle: the anonymous volcano was lurking in the tropics.

Tracking down an eruption

Professor Franck Lavigne joined the hunt for the mystery eruption with about as much insight as you have now. The volcanologist, based at the Panthéon-Sorbonne University in Paris, was faced with a multitude of possible culprits dotted around the equator. Luckily though, the majority of the world's tropical volcanoes are crammed into a relatively small area. (Lucky, of course, unless you happen to live there.) Alongside an international team of researchers, Lavigne headed for the brightest spot on the ring of fire.

The islands of the Indonesian archipelago host the densest population of active peaks on Earth. Even here, though, there was no known eruption which could explain the 13th-century ash; but that didn't mean, of course, that there hadn't been one. So, Lavigne and his colleagues began to take a closer look at some of the more obscure islands in the chain.

One of these was Lombok, the second landfall east of Java. Its volcano, Mount Rinjani, is an odd one. Its small eruptive cone rises above the turquoise waters of a crater lake, called Segara Anak — a colossal dent in the landscape measuring almost a kilometer from top to bottom.

Segara Anak is a caldera —the kind of scar left by only the largest volcanic eruptions, when so much magma drains from the subsurface that the Earth literally swallows the surrounding ground, forming a huge depression. The time that this one had formed was unclear; prior to the 17th century, Lombok's history was frustratingly hazy. All that changed, however, when Franck Lavigne read the Babad Lombok. Oddly enough, the team's journey into the volcano's past would begin not at the mountain itself, but in a Dutch library.

Echoes of Samalas

In the 13th century, something had apparently happened on Lombok which was worth writing about. The "Babad"is a horrifying record of the last days of a kingdom. In the ancient manuscript, Lavigne came across a familiar name — Rinjani. It was followed by the Old Javanese word for avalanche. "Rinjani avalanched, and Samalas collapsed." Today, there is no Samalas.

Whatever Samalas had been, its demise was catastrophic. In the aftermath of the "collapse," the anonymous historian describes "large flows of debris" and a "noise coming from boulders." Another sentence suggests the humanitarian impact of the disaster, and is all the more powerful for the things left unsaid: "All [the] houses were destroyed and swept away, floating on the sea, and many people died." A better historical account of a pyroclastic flow would be hard to find. (These same "avalanches" of superheated gas and rock were what wiped Pompeii off the map in A.D. 79).

It doesn't take fifteen volcanologists to figure out that the "Samalas" mentioned in the Babad Lombok was probably a volcano. The fact that it doesn't exist now, combined with Lombok's conspicuous crater, left a tantalizing possibility: Was Segara Anak caldera all that was left of Samalas? To find out, Lavigne and his colleagues had to go back to the medieval ice.

Volcanic ash, though it resembles a gas when released, is actually a lot more solid under a microscope. And sharper. As erupting lava vaporizes to form an ash column, it freezes into minute particles of glass. Formed from a complex mixture of metal oxides and dissolved gases, the precise composition of this glass is unique to each eruption, and gives the ash an unmistakable chemical fingerprint.

The tiny shards in the 1258 layer had been the nails in the coffin for an Ecuadorian volcano, Quilotoa, as the source of the eruption; although it seemed to have been active in the right time frame, a disparity in the amount of aluminum in its ash had emphatically ruled it out. So when Lavigne and his colleagues did the same analysis for Lombok, there must have been a certain sense of finality. The researchers took a pinch of the debris scattered around the Segara Anak crater, bombarded it with a beam of electrons to reveal its elemental makeup — and took a huge step towards a discovery. The results were simply revolutionary: The ash from the caldera matched the 1258 layer like nothing before.

The chemical match seemed too close for coincidence. However, one final piece of corroborating evidence was needed to turn this likelihood into a bona fide discovery. For this, the team consulted another buried witness to the eruption. The explosion that had hollowed out Segara Anak may well have killed every living thing on the island, but some of them are still there. Packed in the thick volcanic deposits spread across the caldera's flanks are the cremated remains of trees, the former inhabitants of the mountain that collapsed to form the crater. Their trunks and branches had instantly turned to charcoal in the scorching ash of the pyroclastic flows — but had also started a clock.

After death, the amount of ¹⁴C, a radioactive isotope of carbon inside an organism, can reveal the time that has passed since the organism's cells stopped dividing. Lavigne and his colleagues picked through the cinders to see when the Segara Anak trees had died. The results showed a variety of ages, indicating an expected mixture of living and fallen trees. But amongst the jumble, researchers found a crystal-clear line: Not one of the trees had lived past the year 1300.

And with that, all doubts evaporated, and a picture hidden for the best part of a millennium rose back into view: The apocalyptic death of Samalas, the collapse of the Segara Anak caldera, and the wispy bands of ash in the polar ice, were all relicts of the same colossal eruption, which after 750 years of obscurity — and a pioneering blend of history, volcanology and ingenuity — had a name; Mount Samalas, though now just a memory, finally had the infamy it deserves.

http://www.livescience.com/41170-ancient-volcanic-eruption-discovered.html
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