Has volcanic activity been increasing?

We don't think so.

A look at the number of volcanoes active per year, over the last few centuries, shows a dramatic increase, but one that is closely related to increases in the world's human population and communication. We believe that this represents an increased reporting of eruptions, rather than increased frequency of global volcanism: more observers, in wider geographic distribution, with better communication, and broader publication. The past 200 years (see plot below) show this generally increasing trend along with some major "peaks and valleys" which suggest global pulsations. A closer look at the two largest valleys, however, shows that they coincide with the two World Wars, when people (including editors) were preoccupied with other things. Many more eruptions were probably witnessed during those times, but reports do not survive in the scientific literature.

If these apparent drops in global volcanism are caused by decreased human attention to volcanoes, then it is reasonable to expect that increased attention after major, newsworthy eruptions should result in higher-than-average numbers of volcanoes being reported in the historical literature. The 1902 disasters at Mont Pelee, St. Vincent, and Santa Maria (see 1902 arrow) were highly newsworthy events. They represent a genuine pulse in Caribbean volcanism, but we believe that the higher numbers in following years (and following Krakatau in 1883) result from increased human interest in volcanism. People reported events that they might not otherwise have reported and editors were more likely to print those reports.

Additional strong evidence that the historical increase in global volcanism is more apparent than real comes from the lower plot below. Here only the larger eruptions (generating at least 0.1 km3 of tephra, the fragmental products of explosive eruptions) are plotted. The effects of these larger events are often regional, and therefore less likely to escape documentation even in remote areas. The frequency of these events has remained impressively constant for more than a century, and contrasts strongly with the apparent increase of smaller eruptions with time.
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