Solved: The 200-Old Mystery Behind a Colossal Volcanic Eruption

“The sun, observed through the dense fog, appeared as a pale whitish-blue disc,” wrote one observer in Palermo, Sicily, on August 8, 1831. Two days later, a person in Saint-Sever, France, noted that “the sun appeared round and white like a moon; that is to say, emitted no apparent rays, and could be steadfastly regarded without dazzling or in any manner affecting the eyes. An hour afterwards, it appeared of a pale blue color,” while the sky turned blood red. A reporter in Norfolk, Virginia, was more succinct: “Sky blue and pea green.”

The sudden changes in the sun’s apparent color were accompanied by unusual weather conditions across Europe. The German composer Felix Mendelssohn, on a summer trip through the Swiss Alps, noted “desolate weather; it has rained again all night and all morning; it is as cold as winter.”

Scientists have long theorized that the odd atmospheric conditions that Mendelssohn and others experienced were the result of a gigantic volcanic eruption that may have lowered global temperatures by as much as 1°C. Where this 1831 mega-eruption took place, however, was a mystery. The Babuyan Claro volcano in the Philippines was once thought to be a candidate; a 2021 paper in the journal Climate of the Past suggested the more likely culprit was Ferdinandea, a volcanic seamount to the southwest of Sicily that was observed to break the surface of the ocean in 1831. 

Now, a new study by researchers at the University of St. Andrews in Scotland points to a different, more remote, and more powerful eruption that took place on the Kuril Islands north of Hokkaido, Japan. The sparsely populated, subarctic island chain is currently occupied by Russia.

The study, published in Proceedings of the National Academy of Sciences, focused on ice cores from Greenland. The researchers were able to analyze the chemical signatures of volcanic materials deposited in the ice and pinpointed those that fell in 1831. They then extracted microscopic shards of the ash and began comparing them to numerous samples from volcanic sites in the Kuril Islands, looking for similarities. Eventually, the chemical analyses turned up a perfect match with the Zavaritskii volcano on tiny Simushir Island.

“The moment in the lab when we analyzed the two ashes together, one from the volcano and one from the ice core, was a genuine eureka moment,” lead author Dr. Will Hutchison said in a statement. “I couldn’t believe the numbers were identical. After this, I spent a lot of time delving into the age and size of the eruption in Kuril records to truly convince myself that the match was real.” He also thanked Russian and Japanese colleagues for providing the ash samples for testing.

The researchers conclude that the 1831 Zavaritskii eruption had a magnitude of 5 to 6, similar in power to the 1991 eruption of Mt. Pinatubo in the Philippines, one of the largest volcanic eruptions of the 20th century.

Understanding the Zavaritskii eruption is about more than solving a centuries-old mystery. Researchers said the findings may help detect hotspots of future blasts. “Identifying the sources of these mystery eruptions is crucial,” the statement noted, “as it allows scientists to map and monitor the regions on Earth most likely to produce climate-altering volcanic events.”

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