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An immense volcanic eruption in 1257 A.D. affected our entire planet. But which volcano exploded?

Phil Plait Bad Astronomy getty_rinjani_plume

A titanic volcanic eruption in 1257 A.D. spewed out colossal amounts of ash, sulfur, and glassy pumice, and affected climate over the entire planet. It was one of the largest if not the largest eruption in the past 7,000 years, and evidence of it can literally be found from pole to pole.

But... what volcano erupted?

Weirdly, until recently no one knew just which volcano on Earth had exploded. This had geologists scratching their heads until just a few years ago, when multiple lines of evidence pointed towards the literal smoking gun: The Samalas volcano on the island of Lombok in Indonesia.

The strongest evidence of such a catastrophic eruption came from ice cores, both from the Arctic and Antarctic. Every year, a fresh layer of ice is laid down in these areas, trapping gases and particulates in the atmosphere. They can be dated extremely accurately, making them crucial in determining ages of global climactic events.

Volcanic sulfate deposits spike hard in ice deposits dated to 1257/1258 A.D., indicating an eruption far larger than both Krakatau (in 1883) and Tambura (1815), eight times and twice as powerful, respectively.

The new work looking at the eruption examined evidence from radiocarbon dating, tree rings, the geochemistry of volcanic deposits, and even an ancient historical record called the Babad Lombok, written in Old Javanese on palm leaves, that documents the explosion.

Phil Plait Bad Astronomy getty_rinjani_volcano

The Babad Lombok talks about an eruption from a volcano called Samalas, now part of a volcanic complex that includes Mount Rinjani (which towers over 3,700 meters above sea level) and a huge caldera that is partially filled with water, forming a crater lake called Segara Anak. This complex is still active, with eruptions still occurring now. A developing cone, called Gunung Barujari, has been growing for some time, including via eruptive events in the mid-'90s.

The Babad Lombok puts the eruption in the mid to late 13th century, consistent with the ice cores. In 1258 the northern hemisphere experienced an unseasonably cold summer, with heavy rains and flooding that resulted in widespread crop failures in Europe. This too is consistent with a huge eruption; dark particles can rise in to the stratosphere and block a fraction of sunlight, dropping temperatures.

Compellingly, tephra — pieces of rock and ash blown out by volcanic eruptions — dated to around that time have been found in both northern and southern hemispheres, implying the volcano itself was near the equator.

Phil Plait Bad Astronomy getty_Gunung_Barujari_volcano

Assuming the Babad Lombok did describe this eruption, geologists looked to the Samalas caldera and surrounding areas for evidence. Burnt tree trunks and branches were dated to 1257 A.D. using carbon-14 (a radioactive form of carbon that can quite accurately determine the ages of events that affect living matter), and no young trees were found after that date, indicating catastrophic conditions at the time.

Field work in the Samalas caldera and surrounding islands shows tephra deposits all over the region. Glass shards in the deposits match the chemistry of similar ones found in the ice cores dated to the event as well. Mapping the tephra deposits in the area, the scientists found it totaled up to 7.5 cubic kilometers — as much as 7 billion tons of fallout.

But the total volume erupted was much, much larger: Models of the pre-eruption volcano indicate it rose to a height of about 4,200 meters above sea level (about 13,800 feet — roughly the size of the largest mountains in Colorado’s Rocky Mountain National Park). During the eruption the peak collapsed. Looking at all the components of the event, the scientists find that at least 40 cubic kilometers of material were blown out, and the ash plume would have risen a staggering 43 kilometers into the sky, possibly as high as 50 km. It would have been visible from hundreds of kilometers away.

Phil Plait Bad Astronomy getty_rinjani_plume

Looking at the event in total, it would have rated about a 7.0 on the Volcanic Explosivity Index, a logarithmic scale where, for example, the 1980 Mount St. Helens eruption rates a 5 and the last Yellowstone caldera supereruption 600,000 years ago was an 8. The Samalas eruption was therefore among the largest in the past 12,000 years.

The eruption was apocalyptic, and must have been terrifying to the local population (the capital of the Lombok kingdom, a city called Pamatan, was wiped out, and remains undiscovered; if ever found it may be resemble Pompeii). There were steam explosions as hot magma reacted to sea water (called phreatic eruptions), followed by a magmatic explosion that blasted pumice and rocks out to great distances (several hundred kilometers). After that there were pyroclastic flows; hot ash and gases that can move at hundreds of kilometers per hour and be scorching hot. Deposits on Lombok Island reached depths of 35 meters — the height of an eight-story building.

Mind you, all of the evidence for this until a few years ago was indirect and scattered. This work ties it all together, including the newer climate change effects in the northern hemisphere.

Speaking of which, and given the recent release of the IPCC Sixth Assessment Report on climate change, you might hear some folks saying that volcano eruptions account for much of the carbon dioxide released into the atmosphere; I saw a tweet making this claim myself. This is not only wrong, but utterly wrong; human activities emit more than one hundred times the carbon dioxide of all the volcanoes all over the world. An eruption by something like Samalas would actually cool us down due to dark particulates that blanket the atmosphere, but only a little, only for a little while, and at great, great cost.

Immense events like the Samalas eruption can change the course of human history, but on the scale of the planet can prove difficult to pin down. Human activity, on the other hand, has the ability to change the planet as well, and that can be found everywhere. I was horrified to read about the power of this ancient eruption, but was sobered to remember that we ourselves are doing far worse, and we do so continuously.

Unlike the planet, we have a choice.

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