o The Frog Blog: Bradyseism in the Pozzuoli area north of Naples

Tuesday, 11 November 2008

Bradyseism in the Pozzuoli area north of Naples

The Trattoria Vico Marinai in Pozzuoli just north of Naples provides a pretty good lunch, as I and my colleagues from the Classics and History departments discovered last Summer.

In the days of the Roman Empire Pozzuoli (or ‘Puteoli’) was one of the most important ports in Italy, receiving grain from Egypt and most of the exotic animals destined for the amphitheatres of Europe. Part of the old market place (or ‘macellum’) still remains, a little way inland from the modern ferry port.

When first excavated in 1750 a statue of the Roman god Serapis (a local version of the Egyptian’s Osiris) was found here, and the old market became misnamed ‘the Temple of Serapis’. Interesting as all this may be, it seems strange at first sight that a picture of this ancient ‘temple’ was chosen as the frontispiece of the world’s most influential geology book: Principles of Geology by Charles Lyell (published in 1830).

The reason why this ancient site has become a ‘Mecca’ for geologists lies in the fact that about 4 m above the base of each of the three remaining upright marble columns there is a zone where holes have been bored into the marble by the marine bivalve Lithophaga lithophaga. This implies that although the market place was of course constructed above sea level, at some time in the past 2,000 years, the columns have been submerged to a depth of 4 m or more – long enough for generations of boring bivalves to dissolve away their surface - and have then been raised up above sea level again.

In linking these vertical movements to local volcanic activity, rather than to a major catastrophe such as the biblical flood, Lyell encapsulated his ideas about uniformitarianism, and the present being the key to the past (see notes on Hutton in the blog entry for October 23rd). We now know that Pozzuoli harbour lies directly above a relatively shallow magma chamber which periodically fills and empties – causing the ground above to rise and fall. Such movements are termed bradyseism (“braddy” “sigh” “ism”), from the Greek bradus ­= slow and sism = movement.

The last two major eruptions of this magma chamber were 15,000 and 39,000 years ago, and the next major volcanic activity in the Bay of Naples area may well be centred here rather than further south near Vesuvius. The Pozzuoli area is currently undergoing a phase of slow subsidence, but over the past 20 to 30 years this has been periodically interrupted by relatively short and sharp episodes of bradyseismic uplift ranging from a few cm to a couple of m. Each of these mini-uplift phases can be accompanied by seismic activity. Damage was caused by a major earthquake in 1983, and because of this parts of the old town are still deemed unsafe and remain a no-go area, even today.

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