Mystery volcano that cooled Earth in 1831 identified

The first sign of bad weather in the summer of 1831 was an eerie dimming of the sun, which for days appeared bluish green across the Northern Hemisphere. In the ensuing weeks, huge hailstones destroyed crops in Europe. Decreased rainfall during the Indian monsoon led to crop failures and a devastating famine in the eastern Indian state of Madras in 1832 and 1833 that killed about 150,000 people. About twice as many died in a famine that gripped northeastern Japan from 1832 to 1837. The instigator was long presumed to be a major volcanic eruption, but the volcano’s identity had been one of the great unsolved mysteries of volcanology.

Now, nearly 200 years later, scientists have finally identified the 'mystery volcano'[1].

The eruption was one of the most powerful of the 19th century, spewing so much sulphur dioxide into the stratosphere that annual average temperatures in the Northern Hemisphere dropped by about one degree Celsius. While the year of this historic eruption was known, the volcano's location was not.

Examination of the Greenland ice cores revealed that in 1831, sulphur fallout - a sign of volcanic activity - was about six-and-a-half times greater in Greenland than it was in Antarctica. This finding suggested that the source was a major eruption from a midlatitude volcano in the Northern Hemisphere, the researchers reported.

Using geochemistry, radioactive dating and computer modeling to map particles' trajectories, the scientists linked the 1831 eruption to an island volcano in the northwest Pacific Ocean. According to the analysis, the mystery volcano was Zavaritskii (Midoriko Karudera in Japanese) on Simushir Island (Shimushiru-tō in Japanese), part of the Kuril Islands archipelago, an area disputed by Russia and Japan. However, Zavaritskii's last known eruption was in 800 BC.

"For many of Earth's volcanoes, particularly those in remote areas, we have a very poor understanding of their eruptive history," said lead study author Dr. William Hutchison.

"Zavaritskii is located on an extremely remote island between Japan and Russia. No one lives there, and historical records are limited to a handful of diaries from ships that passed these islands every few years," Hutchison explained.

When the scientists compared their results with geochemical datasets from volcanic regions, the closest matches were in Japan and the Kuril Islands.

Volcanic eruptions in nineteenth century Japan were well-documented, and there were no records of a large eruption in 1831. But colleagues who had previously visited volcanoes in the Kuril Islands provided samples that led the researchers to a geochemical match with the Zavaritskii caldera.

"The moment in the lab analysing the two ashes together - one from the volcano and one from the ice core - was a genuine eureka moment," Hutchison said.

Radiocarbon dating of tephra, or volcanic ash, deposits on Simushir Island placed them within the past 300 years. What's more, analysis of the caldera's volume and sulphur isotopes suggested the crater formed after a massive eruption between 1700 and 1900, making Zavaritskii 'the prime candidate' for the mystery eruption in 1831, the authors wrote.

"I am still surprised that an eruption of this size went unreported," Hutchison added. "Perhaps there are reports of ash fall or atmospheric phenomena occurring in 1831 that reside in a dusty corner of a library in Russia or Japan.

[1] Hutchinson et al: The 1831 CE mystery eruption identified as Zavaritskii caldera, Simushir Island (Kurils) in Proceedings of the National Academy of Sciences – 2024. See here.