Researchers at the Department of Earth Sciences have found that volcanic eruptions in many parts of the world show a distinct seasonal pattern. The research was published in the Journal of Geophysical Research.

The research carried out by Ben Mason, a Cambridge PhD student, David Pyle, senior lecturer at the University's Department of Earth Sciences, Brian Dade, now Professor at Dartmouth College, New Hampshire, and Tim Jupp, now a researcher at the nearby Centre for Ecology and Hydrology.

The team’s analysis of the patterns of volcanic activity over the past 300 years was based on the Smithsonian Institution's global catalogue of more than 3,200 dated volcanic eruptions from 1700 to 1999. Up to 18% more volcanic eruptions begin during the Northern Hemisphere's winter season (November to March) than during the summer months. This effect is especially strong in the some of the volcanic regions around the Pacific (including the Andes, Central America and Kamchatka).

Seasonal patterns of volcanic activity are also observed in long datasets from single volcanoes which erupt repeatedly, such as Sakura-jima volcano in Japan, where the eruption rate peaks in the month of November.

To find out why this seasonal pattern occurs, the team ran statistical tests that ruled out any systematic link with Earth tides (deformation of the solid Earth by the movements of the sun and moon). The researchers believe the culprit is the other major driver of seasonal cycles at the Earth's surface - the global water cycle.

Since most of the Earth's continents lie in the Northern Hemisphere, the winter accumulation of snow, ice and rain on the continents, coupled with the removal of water from the oceans (which mostly lie in the Southern Hemisphere) causes the entire shape of the Earth to change from one season to the next.

These annual changes in the Earth's shape can be seen with both satellite 'laser ranging' and ground-based 'global positioning system' measurements, and are vertical movements of only centimeters. These very small periodic movements are probably too small to 'trigger' volcanic eruptions.

The team believes active volcanoes can be thought of as moving towards a crisis point at which will eventually erupt, so the seasonal changes in the stress experienced by the Earth's crust makes it slightly 'easier' for systems to reach the crisis point at certain times of the year. This results in seasonal volcanism.

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