Graphs of the yearly number of volcanoes in eruption from 1900 through 1968 for 21 volcanic areas of the circum-Pacific belt show fluctuations with statistically significant amplitudes in eight of the areas, and some indication of auto-correlation and periodicity in 14 of them. Thus, in a total of 16 out of the 21 areas studied, these volcanic pulses appear to be statistically significant. They are best defined in the New Guinea-Solomons area, Santa Cruz-New Hebrides-Matthew Island area, and the West Indies where the numbers of volcanoes in eruption have significant fluctuations in amplitude, are auto-correlated and tend to be periodic. The periods (around 17 years) of the western Pacific areas are, with few exceptions, much less than those of the eastern Pacific areas suggesting an overall first-order cause for these volcanic fluctuations which, in each area, seem to be due to the volcanoes' responding to regional stress pulses. These may be caused by variations in stress owing to sea-floor spreading on the East Pacific Rise, the dichotomy of periods being due to a different mantle-flow regime on either side of it, resulting from the proximity of the Rise to the eastern Pacific margin. An examination of the years for which the maximum or one less than the maximum number of volcanoes are in eruption suggests that large pulses of volcanic activity tend to migrate southwards. There are six main pulses in the western Pacific and three in the eastern. The latter are better-defined and more widely spaced in time. The pulse occurring in the first decade of this century is simultaneous on both sides of the Pacific, a reflexion of the global tectonic instability of this period. The reason for this southward migration of the volcanic pulses is not yet known. It may be a strain-release phenomenon.
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