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  • Carolyn is a staff writer for Science and is the editor of the News of the Week section.
 

Source of Mysterious Medieval Eruption Identified

1 October 2013 4:45 pm
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F. Lavigne et al., PNAS Early Edition (2013)

Big bang. In 2006, the caldera atop Samalas volcano (top) measured roughly 9 kilometers across and 800 meters deep. After reconstructing the volcanic cone to its pre-eruption shape (bottom), scientists estimated the magnitude of the eruption, one of the most powerful in the modern era.

About 750 years ago, a powerful volcano erupted somewhere on Earth, kicking off a centuries-long cold snap known as the Little Ice Age. Identifying the volcano responsible has been tricky. Now, using geochemical, stratigraphic, and even historical data, a team of scientists has fingered a likely culprit: Indonesia’s Samalas volcano, part of the Rinjani Volcanic Complex on Lombok Island.

The Little Ice Age has been abundantly depicted in contemporary accounts of advancing mountain glaciers that destroyed villages and paintings of ice-skating on frozen Dutch canals or on London’s River Thames, but the date of its actual onset was uncertain. Chilling of the Northern Hemisphere was pronounced: cold summers, incessant rains, floods, and resulting poor harvests, according to medieval records.

That a powerful volcano erupted somewhere in the world, sometime in the Middle Ages, is written in polar ice cores in the form of layers of sulfate deposits and tiny shards of volcanic glass. These cores suggest that the amount of sulfur the mystery volcano sent into the stratosphere put it firmly among the ranks of the strongest climate-perturbing eruptions of the current geological epoch, the Holocene, a period that stretches from 10,000 years ago to the present. A haze of stratospheric sulfur cools climate by reflecting solar energy back into space.

In 2012, a team of scientists led by geochemist Gifford Miller of the University of Colorado, Boulder, strengthened the link between the mystery eruption and the onset of the Little Ice Age by using radiocarbon dating of dead plant material from beneath the ice caps on Baffin Island and Iceland, as well as ice and sediment core data, to determine that the cold summers and ice growth began abruptly between 1275 and 1300 C.E. (and became intensified between 1430 and 1455 C.E.). Such a sudden onset, they noted in Geophysical Research Letters in 2012, pointed to a huge volcanic eruption injecting sulfur into the stratosphere and starting the cooling. Subsequent, unusually large and frequent eruptions of other volcanoes, as well as sea-ice/ocean feedbacks persisting long after the aerosols have been removed from the atmosphere, may have prolonged the cooling through the 1700s. 

Volcanologist Franck Lavigne of the Université Paris in and colleagues now think they’ve identified the volcano in question: Indonesia’s Samalas. One line of evidence, they note this week in the Proceedings of the National Academy of Sciences, is historical records. According to Babad Lombok, records of the island written on palm leaves in Old Javanese, Samalas erupted catastrophically before the end of the 13th century, devastating surrounding villages—including Lombok’s capital at the time, Pamatan—with ash and fast-moving sweeps of hot rock and gas called pyroclastic flows.

The researchers then began to reconstruct the formation of the large, 800-meter-deep caldera that now sits atop the volcano. They examined 130 outcrops on the flanks of the volcano, exposing sequences of pumice—ash hardened into rock—and other pyroclastic material. The volume of ash deposited, and the estimated height of the eruption plume (43 kilometers above sea level) put the eruption’s magnitude at a minimum of 7 on the volcanic explosivity index (which has a scale of 1 to 8)—making it one of the largest known in the Holocene. The eruption, the authors note, was on the scale of the Tambora eruption of 1815, and more powerful than Krakatoa in 1883.

The team also performed radiocarbon analyses on carbonized tree trunks and branches buried within the pyroclastic deposits to confirm the date of the eruption; it could not, they concluded, have happened before 1257 C.E., and certainly happened in the 13th century.

It’s not a total surprise that an Indonesian volcano might be the source of the eruption, Miller says. “An equatorial eruption is more consistent with the apparent climate impacts.” And, he adds, with sulfate appearing in both polar ice caps—Arctic and Antarctic—there is “a strong consensus” that this also supports an equatorial source.

Another possible candidate—both in terms of timing and geographical location—is Ecuador’s Quilotoa, estimated to have last erupted between 1147 and 1320 C.E. But when Lavigne’s team examined shards of volcanic glass from this volcano, they found that they didn’t match the chemical composition of the glass found in polar ice cores, whereas the Samalas glass is a much closer match. That, they suggest, further strengthens the case that Samalas was responsible for the medieval “year without summer” in 1258 C.E.

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