A government scientist says his detailed new analysis disproves the theory that volcanoes are more likely to erupt at certain times of the year. Historically, volcanoes have erupted slightly more often in some months than others, leading to disputes over whether the variation was enough to constitute a periodic pattern. Richard B. Stothers concluded that it isn't in a report in the May issue of Geophysical Research Letters, published by the American Geophysical Union. Stothers, of the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md., compared the dates of recorded strong volcanic eruptions between 1500 and 1980. During that time 265 eruptions were recorded in the first half of each calendar year and 236 in the last six months, a difference Stothers said is not statistically significant. Breaking down the eruptions by month and area, Stothers did find some clustering, but this was explainable, he concluded. For example, nearly all eruptions south of 50 degrees south latitude occurred between November and April. This is almost certainly because that is summer in that part of the world, the time of year the nearly uninhabited region is most often visited by outsiders who would record eruptions, Stothers said. No similar pattern was seen in the far north, where residents are always present to notice eruptions. Stother's monthly analysis found a peak average of 52 eruptions in February between the years 1500 and 1980 and a minimum of 36 eruptions in the months of April and December during that period. The average number of eruptions in other months fell in between those two, a variation within the range that statisticians would expect to occur in a random sample. Stother's analysis rebuts several studies done in the late 1800s and early in this century that indicated a seasonality to volcanic eruptions. At least one 1917 report speculated that eruptions would cluster around the equinoxes in the spring and fall as a result of changes in the angle of the sun. Periodic springtime eruptions of some volcanoes have been associated with melting snow seeping into the ground and reacting with molton lava below the ground, scientists have suggested. But seasonality occurring at individual volcanoes disappears when they are averaged in with many others around the world, according to Stother's analysis.
NASA scientist Richard B. Stothers , who studied the dates of strong activity between 1500 and 1900 , has disproven the theory that volcanoes are more likely to erupt at certain times of the year . Nearly all recorded eruptions south of 50 degrees south latitude occurred between February and April because outsiders who can record eruptions only visit that uninhabited region during its summer . No similar pattern is seen in the far north , where residents are always present . The peak monthly total of eruptions was 32 for February and 36 for both April and May , a statistical variation expected to occur randomly .