Alan V. Morgan
One of my most treasured books is a first edition of Lyell's "Principles of Geology (being an attempt to explain the former changes of the Earth's Surface - by reference to causes now in operation)." Published in 1830 - in two volumes, although a third was added an 1833 - this was the same edition that Charles Darwin took with him on his round the world voyage of the Beagle.
When you open the book the frontispiece is an intriguing woodcut by T. Bradley, entitled "Present state of the Temple of Serapsis at Puzzuoli." This summer I was finally able to get to the so-called "Temple" of Serapsis on the western edge of Naples. Why should such a building be of interest to geologists? Lyell (quoting a Signor Carelli) pointed out in his text that the building was nether a "temple" nor was it devoted to Serapsis (since worship of this Egyptian God had been prohibited by the Roman Senate). Lyell concentrated instead on the "legible characters (inscribed) by the hand of nature" that are preserved on the columns.
"The pillars are forty-two feet in height (12.8m); their surface is smooth and uninjured to the height of about twelve feet (3.66m) above their pedestals. Above this is a zone, twelve feet in height, where the marble has been pierced by a species of marine perforating bivalve - Lithodomus, Cuv." Lyell goes on to remark on the numbers of holes and the remains of the molluscs that were still preserved in situ. He further points out that "the platform of the Temple is at present (in 1828) about one foot below high-water mark..." From these observations he concludes that the upper part of the perforations (formed by marine molluscs) are at least twenty-three feet (7m) above high-water mark. Finally he correctly elucidates that since "...the temple could not have been built originally at the bottom of the sea, it must have first sunk below the level of the waves, and afterwards have been elevated." The lack of molluscan borings in the lower part of the pillars he attributed to sediment and building debris covering approximately the basal 3.5m. To support subsidence and re-emergence in the region Lyell mentions that nearby there are ruins of the Temple of the Nymphs, the Temple of Neptune and two Roman roads under water. In a most enlightened statement he comments that "(People) will be very much astonished if future researches fail to bring to light similar indications of change in all regions of volcanic disturbances."
That brings me to my visit in 2004 on one of the field trips of the 32nd International Congress. We spent a little while at the small park in Pozzuoli that now surrounds the "Temple of Serapsis", as part of a journey that looked at some of the volcanic hazards of the Naples area. Unfortunately you cannot descend to walk on the pavement as Lyell did in 1828, hence my images above are not precisely matched. However, you can make some additional observations. The floor of the "temple" - now re-interpreted as a market place - is relatively dry. The port facility, barely a hundred metres away, has had some considerable problems in recent years because the harbour floor is rising at a remarkably rapid rate.
Boats used to moor here until quite recently. A line of bollards have been left "high and dry" on the old, elevated, quayside. A new roadway has been constructed below with a second line of mooring bollards (adjacent to person walking in the centre). The drop from the old surface to the roadway is indicated.
This area has experienced several episodes of uplift since 1970 and the elevation of the harbour floor is an indication of this ground inflation.
Why were we visiting this area and why should these "ups and downs" of the land be a matter of concern? The Pozzuoli region is subject to small earthquake swarms that accompany land inflation and deflation. The area is volcanically active with three volcanoes, Solfatara, Astroni and Monte Nuevo, being active over the last four thousand years and the latter having erupted on September 29th, 1538. What is of even more concern is that these two vents are part of a far larger caldera complex known as Campi Flegrei, or "The Phlegreaean Fields".
Does this area have the potential to erupt again? Looking at the geological evidence I would find it very hard to say "no". The Campi Fleagri caldera has a long (~50,000 year) history of eruptions including two catastrophic ignimbrite (glowing avalanche) flows. One of the earliest, some 37,000 years ago, ejected the Campanian Ignimbrite (approximately 150 km3 that spread over 30,000 km2) and the second, some 12,000 years ago, deposited the Neopolitan "yellow" tuff from an eruption of some 50 km3 that covered about 1,000 km2. Since that time there have been large eruptions from various centres at 10,300, 9,500, 8,600, 4,800, 4,100, and 3,800 years B.P. The 1538 Monte Nuevo eruption that was preceded by major ground inflation followed a 3,000 year episode of quiescence.
A digital elevation map (or DEM, seen below) illustrates some of the view over the southern part of Campi Fleagri. TheCaldera and eruptive features north of Pozzuoli. Astroni crater can be seen in the middle centre. The
drop faulted areas and lava domes are shown. view shows the island of Ischia in the distance.
Does the present ground inflation indicate that an eruption is imminent in this area? The Italian authorities point out that the two periods of inflation during the past three decades have been separated by deflation, and the rapid inflation of 4 m seen prior to the 1538 eruption has not yet been experienced. However, bearing in mind the tremendous population growth of this part of Italy, over two million people could be potentially affected should Campi Fleagri enter another eruptive phase. This is a matter of significant concern to the Italian Civil Defence Authorities, especially since estimates for evacuation indicate that at least one week would be required. And of course Campi Fleagri is only on the west side of Naples. In many ways a similar, and yet quite different volcanic threat awaits on the east side of Naples; - that of the Somma - Vesuvius centre, and the subject of another article in the next issue of What On Earth.
Alan V. Morgan.