Sunday, May 24, 2009
Mount Vesuvius in action!
Well, is it time? With all the pompous weight of scientific certainty, I can now say…uh, maybe. It is instructive to look at the recent history of eruptions for a clue. “Recent” is relative. We can take the last 400 years or so because in geologic terms that is but a heart-beat.
Working back from the present, the last eruption of Vesuvius was in March of 1944. It happened in full view of the Allied armies, which had taken the city of Naples a few months earlier. WWII was still raging further north in Italy when Vesuvius went into what is called an “effusive” eruption (less violent than an “explosive” eruption, but nevertheless dangerous and potentially deadly). That eruption destroyed a number of nearby towns and a U.S. B-25 bomber group parked at the Capodichino airport in Naples. (The volcanic ash rendered the planes useless.)
Eruptions count as major or minor (and everything in between) depending on the extent to which they are explosive or effusive, how much ejecta they produce and the extent to which they change the profile of the volcano, blowing bits and pieces away, adding new craters, new lava flows, etc. Thus, the earlier eruptions of 1929 and 1926 were minor, but they did, for example, add a few new craters and damage nearby structures.
The eruption of April, 1906, was massive and attracted worldwide attention. It killed 100 persons and buried nearby towns. The initial rumblings, however, caused little alarm and locals joked that “the mountain” was just preparing a royal welcome for British King Edward, due in Naples for a visit shortly. He made it just in time for an eruption that dropped the ridge on the main cone some 250 meters. The eruption covered the city of Naples, itself, with ash, and made the roads near the volcano impassable. Residents of destroyed villages fled to Naples, itself, or to nearby towns such as Castellammare. The eruption was followed by heavy rains that produced what geologists now call a lahar (an Indonesian word)—massive mud and ash slides that buried everything in their path. (Some sources reported at the time that it was the most massive eruption since the great explosion that destroyed Pompeii and Herculaneum in 79 AD. That may be an exaggeration, since the 1872 and the 1631 eruptions were likely to have been at least as powerful.)
The year 1872 produced a massive eruption classified as “explosive/effusive.” It had been preceeded by minor eruptive activity in 1861, 1858, 1855, 1831 and 1824. That time frame spans the foundation of the geological observatory, itself, on the slopes, in 1841. The institution was the brain-child of Macedonio Melloni (1798-1854), who became the first director. It survived the political upheavals that came with the conquest of the Kingdom of Naples and its absorption into the modern nation state of Italy. The directorship then passed to Luigi Palmieri (1807-96), who was on duty constantly during the 1872 eruption. You can see the observatory today and from a distance notice that it sits on a handy knoll with the lava flow of the ‘72 eruption going around it! There were scientific heroics as the director, Prof. Palmieri, refused to leave so he could man the instruments. Palmieri was totally cut-off and alone, but he survived.
A major eruption occurred in 1807; in the 1700s, there were two notable eruptions, 1794 and 1737, both of which destroyed local villages. The 1794 eruption opened crates at relatively low levels on the slopes—at 480 and 320 meters. (The current height of Mt. Vesuvius is 1280 meters.)
The modern cycle of eruptions of Mt. Vesuvius started on Dec. 16, 1631 with an eruption classified as “explosive” (as opposed to the less violent “effusive” or “explosive/effusive”). The volcano had been quiet for some centuries and then simply blew its top. Most sources cite this eruption as the “greatest since Pompeii.” It followed the familiar behavior of an exploding volcano: lava fountains as high as 4 km and an ash column as high as 15 km, which then collapsed onto the slopes producing what is now called a “pyroclastic flow.” It was followed in 1637, ’49, ’52, ’54, and ’60 by lesser eruptions. Some of those were accompanied by earthquakes; indeed, even the dreaded bubonic plague showed up in 1656, lending credence amongst believers to the rumor that the world was coming to an end. It didn’t, of course, and it won’t after the next one. (Uh, I didn’t say that aloud, did I?)