Has volcanic activity been increasing?
The Global Volcanism Program does not see any evidence that volcanic activity is actually increasing. Data about eruptions compiled by the Smithsonian since 1968 provide context for assessing issues of regional and global volcanism. The following figures and discussion below are modified from an introductory section in Siebert et al. (2010); data is through 2009. Please do not reproduce the figures below without all of the accompanying analysis and proper citation (links are preferred).
There is great value in knowing the recent volcanological record, but its limitations are not as apparent. Readers are strongly cautioned against mistaking the record for the reality. Perhaps the best illustration of these limitations come from the data themselves. The last 600 years of apparent global volcanism can be displayed by plotting the number of volcanoes known to have been active each year (figure 1). At first glance the nearly exponential increase through recent centuries suggests that the planet may soon be overwhelmed by volcanic activity, but this increase tracks the striking growth of global population (dashed line on figure 1) that has spread potential observers of eruptions over much of the Earth and the technological advances that have facilitated reporting of those eruptions.
Another indicator of the growth of the record is the increase in the total number of known historically active volcanoes (upper line on figure 1). These are not newly-formed volcanoes, but already established features (or volcanic fields) that clearly had numerous previous eruptions. If a list had been continuously kept, about 2000 years ago it would have contained only the names of nine Mediterranean volcanoes and West Africa's Mount Cameroon. In the next ten centuries the list would have grown by only 30 names, 18 of them Japanese. Although newly settled Iceland soon added seven volcanoes, the list totaled only 63 by 1400 CE, where figure 1 begins.
A dramatic increase in both the number of historically active volcanoes and recorded eruptions took place about 1500 CE. These resulted in part from the great Spanish/Portuguese marine explorations – the Age of Discovery – around the end of the 15th century, when explorers opened Latin America and much of the western Pacific to European record-keeping. Perhaps equally important was the development and widespread distribution of the printing press in the late 15th century, markedly increasing the likelihood that new volcanological records would survive. Through the 17th and into the early 18th century the recognition of additional active volcanoes generally corresponded with steadily increasing eruption reports. By the 18th century global trade was flourishing, the Industrial Revolution was under way, and the heightened reporting of eruptions noticeably accelerated the discovery of new volcanoes. The list has continued to grow, with several important volcanic regions such as New Zealand, Alaska, and Hawaii being unrepresented until the last 250 years.
The suggestion above is that the apparent trend is closely related to increases in population and communication, and that it actually represents an increased reporting of eruptions, not actual increased volcanism: more observers, in wider geographic distribution, with better communication, and broader publication. The last 200 years (figure 2), with humans distributed over most of the globe and relatively efficient communications, would seem to be the best suited to search for episodic trends. However, even in the last two centuries reporting factors overshadow any real trends in volcanism, as detailed below. Additional strong evidence that the historical trend in volcanism is more apparent than real comes from the lower plot of figure 2. Here only the larger eruptions generating at least 0.1 km3 of tephra (VEI >= 4), 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.The effects of these large eruptions are far reaching and thus they are less likely to escape documentation even in remote areas.
Inspection of data from the past 200 years shows 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 publishers) were preoccupied with other things. Great economic crises, like wars, might also be expected to interfere with the reporting of natural events. One of the most precipitous drops in reported volcanism followed the stock market collapse of 1929 and during the ensuing Great Depression. From the late 1920's to the early 1930's, every volcanic region in the world – except those of Russia, Melanesia, and the West Indies – showed a drop in reported active volcanoes. In the war years of 1941-45 the number reported active in the western Pacific and Indonesia dropped by nearly one-third from the preceding 5 years, while regions less affected by the war showed little change. With more observers in more remote areas during World War II, it seems likely that more eruptions than normal were actually witnessed, but it is easy to appreciate why many of these reports do not survive in the available literature.
If these apparent drops in activity are caused by decreased human attention to volcanoes, then it is reasonable to expect that increased attention after major newsworthy eruptions should result in above-average numbers being reported in the literature. The 1902 disasters at Mont Pelee (about 29,000 victims), St. Vincent, and Santa Maria 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 with more than 36,000 victims) result from increased human interest. People reported events that they might not otherwise have, and editors were more likely to print those reports.
More recent major eruptions have not had this effect, in part due to more systematic cataloging and media focus on volcanic events. The newsworthiness of an eruption depends more on its location and human impact than on its size. The 1980 St. Helens eruption, for example, generated enormous media attention, while the remarkably similar 1956 eruption of Bezymianny in sparsely populated Kamchatka, in which no lives were lost, was hardly noticed by the world press. Similarly, the 1991 Pinatubo eruption, with its many fatalities and effects on nearby military bases, was front page news, but the eruption of Cerro Hudson that year in isolated southern Chile created barely a ripple in international interest.
A sharp rise in reported active volcanoes immediately post-WW II was followed by another steep increase in the early 1950s that has no obvious relationship to historic events. There is then a transition from a sequence of peaks and valleys to a more uniform pattern. This corresponds to the establishment of three venues of global eruption documentation: the beginning of the Catalog of Active Volcanoes of the World series with eruption-rich Indonesia in 1951, publication of the annual Bulletin of Volcanic Eruptions of the Volcanological Society of Japan in 1960, and Smithsonian cataloging beginning in 1968.
Yet another rise in reported activity can be noted in the late 1990s. Several new reporting effects appear to have contributed to this change. These include widespread availability of internet access and the corresponding ability to quickly and easily transmit or publish reports, the increased utilization of remote-sensing satellite data by volcano observatories, the onset of systematic reporting of ash plumes by the Volcanic Ash Advisory Centers (VAACs) to mitigate aircraft-ash interactions, and the deployment of new satellite technology starting in December 1999 with NASA's Earth Observing System (EOS) Terra satellite. Its MODIS infrared sensors have detected volcanic eruptions in sparsely populated or rarely visited regions. These collective "eyes in the skies" have detected eruptions that might otherwise go unreported. Another contributing factor may have been the onset of more systematic weekly documentation of current volcanism by the Smithsonian and USGS in 2000.
Thus, all major trends in our recent volcanological record can be reasonably explained by historical events, technological changes, and exploration influences. The apparent increase in activity reflects increases in people living near volcanoes to observe eruptions and improvements in communication technologies to report those eruptions. The best evidence that these trends are apparent rather than real comes from the record of large eruptions, whose effects are far reaching and less likely to escape documentation even in remote areas. Their constancy over the past two centuries is a better indicator of the global frequency of eruptions than the improved reporting of smaller eruptions.
Global Volcanism Program, 2013. Volcanoes of the World, v. 4.7.1. Venzke, E (ed.). Smithsonian Institution. Downloaded 18 Jul 2018. https://doi.org/10.5479/si.GVP.VOTW4-2013