Winter lava. A new study may explain why Pacific rim volcanoes such as Alaska's Cleveland Volcano erupt more often in winter than at other times.

Volcanoes' Favorite Time of Year

Nobody knows why volcanoes erupt when they do. But a team of researchers has discovered a pattern that may bring us a step closer to solving the mystery.

Past efforts to uncover the triggers of volcanic eruptions have focused on things such as earthquakes and tides. But geologist Ben Mason and his colleagues at Cambridge University, U.K., think more subtle forces may be at work. A statistical analysis of the Smithsonian Institution's global catalog of volcanic eruptions during the last 300 years revealed that volcanoes all over the globe are 18% more likely to erupt during the northern winter months than at any other time of year. The pattern is particularly strong for volcanoes along the Pacific rim , where winter eruption rates in some places are up to 50% higher than average, they report in the online 27 April issue of Geophysical Research Letters.

What's behind this striking pattern? Japanese researchers have suggested that the weight of snow could be causing the predilection for winter eruptions in Japan. That can't explain why volcanoes in the Southern Hemisphere are also more likely to erupt during the northern winter months as well.

Mason's team tried to match the eruptions to tidal cycles and couldn't find a link there, either. But when they looked at the global water cycle, they found a potential culprit. Because there is more land in the Northern Hemisphere, more rain and snow end up being stored on land than during the southern winter. This lowers global sea level about a centimeter during the northern winter. The change in sea level causes a small, steady change in stress on coastlines and islands that could trigger eruptions.

"The statistics are fairly convincing," says geologist Michael Rampino of New York University. And the idea that sea level is the trigger fits with Rampino's observations that in the last 85,000 years, more volcanoes seem to have erupted when the sea level changed relatively rapidly for a few thousand years, such as during an encroaching ice age.

Mason's findings could have implications for future sea level changes as well: "If we change the climate, we might change the frequency of volcanic eruptions," says Rampino.

Related sites
Alaskan Volcano Observatory
Cambridge Volcanology Group