The New Decade Volcano Program; #3 Campi Flegrei

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Devil May Care – Campi Flegrei, NDVP #3

Naples, formerly the capital of the “Kingdom of the Two Sicilies” before the unification of Italy in 1859-60, is a city with a rich cultural heritage that goes back more than 4,000 years. Today, it is the third largest city of Italy (after Rome and Milan) vibrant, carefree and, dare it be said, chaotic. 18 km from the vantage point of this photographer, Vesuvius looms over Naples, a constant reminder of danger. However, Vesuvius is perhaps the least dangerous of the three volcanic systems that threaten Naples. Behind the back of the photographer and 25 km away lies the volcanic island of Ischia. But between Ischia and the photographer, less than 5 km away, lies the potentially greatest threat of them all, Campi Flegrei. (city.samondeo.com)

I suppose not many will be surprised by this choice as “googling” Campi Flegrei immediately yields results such as “Campi Flegrei: The Massive Italian Volcano that Will Kill Us All (Or Not)” (Time, Aug 08, 2012) or “Italian Supervolcano Could End Eurozone Crisis the Easy Way” (Smithsonian, Aug 06, 2012). Indeed, both Carl and myself have written on the subject before as the most worrying aspect of Campi Flegrei is not that it is capable of VEI 7 eruptions, but that it is located less than 10 km from the city centre of Naples. This means that the 1 MDE criterion could easily be met by an eruption only 1/1,000th as powerful as a full-scale caldera eruption, something that makes it the most likely of all our choices for the NDVP program to happen within our lifetimes. Of the three volcanic systems in the Naples area, Campi Flegrei is the most dangerous, not only because it shows strong signs of ongoing, continuous activity, but because a large portion of the Naples metropolitan area is built within what in effect is its crater. Any eruption here will put a great number of lives at risk. But let’s not get ahead of ourselves just yet!

Italy is the southernmost part of the European segment of the Eurasian plate where it slides over the colliding and subducting African plate. Geologically, the crust is somewhat similar from the Po Valley/Basin down to at least the ankle of the Italian “boot”. Very approximately; at the bottom of the plate you find the usual old oceanic crust overlain and somewhat intermixed with old “granitic” crust. On top of that is a sedimentary layer some 10 km thick which in turn lies underneath more recent “surface” layer of bedrock of mixed composition.

The area from well north of Rome down to and including Naples is one that has seen a great number of very large, ignimbrite depositing and caldera forming eruptions. The 13 x 19 km Lago di Bolsena caldera (Vulsini Volcanic Complex) formed around 370 kA. The ~8 km wide Lago di Vico caldera (Vico Volcanic Complex) formed about 150 kA, the 10 km Lago di Bracciano caldera (Sabatini Volcanic Complex) ~370 kA. Although the major period of activity at Colli Albano occurred between 480 – 360 kA, the 5 km diameter Lago di Albano maar or caldera formed as recently as 20 kA and there are signs of volcanic unrest to this very day. Monte Roccamonfina suffered a major, caldera-forming collapse about 400 kA. Activity at the latter ended at about 50 kA, seemingly coinciding with the onset of large volcanism at Campi Flegrei at 39 kA. (Author)

From Lago di Bolsena, about 100 km NNW of Rome, and all the way down to Naples lies a series of Quarternary calderas. Although the major eruptive period of the area took place between approximately 0.5 and 0.3 MA, very large, caldera forming and ignimbrite eruptions have continued into what geologically is the present day and of the five volcanic centres to the north of the Naples area, only Monte Roccamonfina may be considered truly extinct. Most worrying perhaps is the low-level activity at Colli Albani which includes the emission of carbon dioxide in doses lethal to grazing animals, proof that magma is still degassing not far below the surface.

The raw terrain of the Campi Flegrei – Naples area where the approximate outlines of the calderas formed by the 39 kA Campanian Ignimbrite Eruption and the subsequent 15 kA Nepolitan Yellow Tuff Eruption have been outlined. The names of some of the subsequent eruptive centres is also rendered. (Based on map found years ago at the now defunct address ars.els-cdn.com, origin unknown.)

The Campanian Ignimbrite Eruption 39 kA

The 39 kA Campanian Ignimbrite (CI) caldera-forming super-eruption is the largest volcanic eruption in Europe in the last 200,000 years. A recent study (Costa & al: 2012) measured 115 sites for the ash layer at distances up to 2500 km from the volcano. Subsequent 3D ash-dispersal computer model analysis yielded figures of. 250 to 300 cubic kilometres of ash spread across 3.7 million square km. This is twice to triple the previous estimate.

Map showing some of the sites investigated by Costa et al and their projection of Ash dispersal from the 39 kA Campanian Ignimbrite eruption (Costa et al, 2012)

Their results yielded the following details: The eruption lasted between two to four and a half days and erupted a total of up to 3 x 1011 tons of ash (or 111 km3 DRE at a density of 2.7). The eruption column may have reached an altitude of 37 to 40 km with a mass ejection rate of 1.7 million tons per second. This is the equivalent of a VEI 5 eruption every 9 minutes 48 seconds for 2 – 2.5 days continuously. If the estimate for the “proximal pyroclastic density current deposits” (eg. ignimbrite deposits) calculated by Pyle & al in 2006 are taken into account, the total bulk volume of the Campanian Ignimbrite eruption corresponds to some 180–280 km3 DRE, comfortably within the range of a VEI 7 eruption.

The secondary effects of this eruption would have been a drop in average temperatures of between 1 – 2 degrees for a period of time of two to three years, shortening the growth season and leading to serious food shortages for animal life. Furthermore, the fluorine content in the widely dispersed ash would have led to fluorosis in herbivores, killing a large percentage of the staple diet of carni- and omnivores such as mankind. Acid rain, water contamination, destruction of food sources and ash inhalation would have had a great impact on Early Upper Paleolithic communities in the region. As most of the East Mediterranean area, the Balkans, and part of the Caucasus region were covered by more than 5 mm of ash, 5- 25 mm of ash constitutes “moderate burial”, the ecosystems would have been affected. The recovery of vegetation would have taken at the very least a year in the areas “moderately buried” up to several decades in areas covered by deep layers of ash. The tracts of land covered by ignimbrite deposits would have been sterile and barren for several centuries.

Neapolitan Yellow Tuff Eruption 14.9 kA

The Island of Nisida at the eastern edge of the NYT caldera offers a good view of the Neapolitan Yellow Tuff and in the far background, the mountains that make up part of the CI caldera. (uncredited photograph at comuni-italiani.it)

The Neapolitan Yellow Tuff (NYT) is the ignimbrite product of the largest known trachytic phreatoplinian eruption. The eruption covered an area larger than 1,000 km2 with an estimated volume of about 40 km3 of erupted magma. If the proportion between ignimbrite and distal ash deposits is similar to that of the Campanian Ignimbrite eruption, this would yield a figure of close to 100 km3 DRE for the NYT eruption, which is borderline VEI 7. The NYT-eruption, as it is commonly referred to, resulted in an approximately 11 km wide caldera collapse within the older Campanian Ignimbrite caldera. The resulting structure has influenced subsequent volcanic activity in the Campi Flegrei caldera strongly as it has been contained within the NYT caldera.

Dating the of the NYT-eruption has not been straightforward as the dates obtained have not “converged towards a unique result”, which means that the dates obtained by various studies have been outside the margin of error of the others. Based on a recent set of obtained 40Ar/39Ar dates, Costa and his co-workers place the NYT eruption at 14.9 ±0.4 ka (Costa et al, 2012).

SEM image of Phillipsite crystals of the Neapolitan Yellow Tuff. (Gatta, Cappelletti & Langella)

The 4.1 – 3.8 kA Astroni Eruption

The Astroni crater located 10 km west of Naples city centre is a WWF oasis, a lush nature reserve and popular recreational area. (http://whaton.uwaterloo.ca)

After the NYT eruption at 14.9 kA, there have been several large eruptions within the confines of the NYT caldera. One of these is the eruption which formed what is now the “Riserva Naturali Cratere degli Astroni”, a WWF nature reserve. What is often referred to as the Astroni eruption is in reality a series of seven eruptions with a total output of about 0.45 km3 DRE dated to between 4.1 and 3.8 kA BP. The eruptions were Peléean in nature which means that the eruptions were not continuous but came in a series of pulses. This resulted in vast base surges, similar in nature to those that destroyed Herculaneum 79 AD or the town of “Old Taal” in 1754 AD.

A view centered on the Astroni crater with the 100% fatality zone out to 10 km and the 50% fatality zone out to 20 km should a similar VEI 4 eruption occur today. It is abundantly clear that unless there is an extensive evacuation of in principle the entire population of the Naples region, 4½ million people, the 1 MDE criterion will be surpassed with horrifying ease. (Author)

The 1538 Monte Nuovo Eruption

A contemporary rendering, well almost, from 1657 of Campi Flegrei and the 1538 Monte Nuovo eruption. Even though the rendering is simplistic, details such as the flames allows us to guess that it may have been a “red eruption” and the rain of stones on all sides that it was Strombolian in nature. (Theatrum illustriorum Italiae urbium tabulae cum apendice celebriorum in Maris Mediterranei insulis civitatum, Amsterdam)

Even more frequent are the smaller eruptions such as the one that began on September 29th 1538 and resulted in the today 123 m high Monte Nuovo cone. The signs of unrest had begun long before. As early as 1502, residents of Pozzuoli noted the emergence of new land from areas formerly occupied by the sea. From the early 1530s, this uplift was accompanied by unusual seismicity that reached a first climax in the spring of 1534. During the next four years, seismic activity continued at a lower level until there was a dramatic increase during September 1538 which culminated with the Monte Nuovo eruption beginning on the 29th. Seven days before the eruption began, the ground began to inflate rapidly, an astonishing six metres in only seven days and once the eruption had begun, it only took another seven days to build the 123 m high edifice of Monte Nuovo. From this description, it is likely that the eruption was a relatively speaking mild, Strombolian affair, far removed from the disastrous future eruption evoked by either Time or Smithsonian online edititions, but this type of eruption is by far the most common, even from a volcano as terrifying as Campi Flegrei.

How Likely Is a Disaster?

The formerly submerged Macellum (market building) of the Roman colony of Puteoli, now known as Pozzuoli. The discolorations visible on the columns indicate the former sea level. (Photograph by Angela Sorrentino)

Like all very large volcanoes where there is no visible volcanic edifice, it took us a long time to realise what Campi Flegrei really was. The Romans had an inkling of its true nature when they named it Campi Flegrei, “The Burning Fields”, but it was not until the 1960s when uplift made the harbour at Pozzuoli unusable that the dotted lines finally began to be connected. There were known volcanic areas such as Solfatara where the high temperature of gas emissions, twice as high as at Vesuvius, indicate that magma is much closer to the surface. As early as the beginning of the 19th Century, excavations of a Roman site in Pozzuoli had revealed marine encrustations several metres up on marble columns. These had been described by amongst others Darwin and Lyell and of them, at least Darwin made the connection between volcanism and fact that salt-water molluscs (Lithophaga) were found several metres above the current sea level. But it wasn’t until a period of uplift in the late 1970s made us realise that the entire area was one large and not many small volcanoes.

Campi Flegrei is what is known as a restless caldera. Bradyseism, the alternating uplift and depression of the ground on a scale of several metres or even tens of metres over a period of time measured in years or even tens of years, is nothing to be alarmed at. It is absolutely and perfectly normal. Since the period of uplift during the late 1970s and 1980s, there have been periods of more rapid uplift followed by slow subsidence. In 2012, slow deflation reversed into inflation and over the period of one year the ground rose by 6 cm. Each inflation event is associated with earthquake swarms, which indicate magma movement at 5 km depth. The general trend is an uplift which since the early 1970s totals about 2 metres.

Temperatures of the gas at Solfatara have been rising in recent years. An eruption is not considered imminent, much may be due to heated water rather than rising magma. But the current behaviour shows similarities to the period of activity that began in 1502 and culminated with the 1538 Monte Nuovo eruption. Over the past two millennia, the bottom beneath Pozzuoli Bay has slowly subsided and the old Roman Portus Julius is now 12 meter below sea level while at the same time, land areas have risen. The fracture in between may be a probable place for a future eruption.

Based on previous eruptions, a new eruption is likely to be preceded by uplift of meters to tens of meters, due to magma refill at depths of 3 km or even less. The eruption period 4,000 years ago that culminated with the Astroni series of eruptions was preceded by as much as 30 meters of inflation. Eruptions could follow within decades to centuries from the onset of such inflation, thus the fact that the inflation periods of the 1970’s and 1980’s were 30 years ago is not a reason for complacency. There is a significant danger of a small eruption, but perhaps not quite as fast as that following on the heels of our Mount Aso article!

A problem with very large, ignimbrite depositing, caldera forming eruptions is that they tend to obliterate almost all evidence of previous and lesser volcanic activity. Millenniae of human rural and urban “development” tend to further obfuscate the picture as can be inferred from the open-ended solution given by Costa et al on where the caldera intersects the modern city of Naples. What is certain though are the volcanic scars left by the eruptions subsequent to the latest very large eruption, the NYT eruption at 14.9 kA. Lago Averno, Gauro, Pianura, Soccavo, Agnano, Fuorigrotta, Astroni tell us that such eruptions have have occurred at least seven times in the last 15,000 years or once every two thousand years. If we include the further five that can be inferred from their incomplete outlines, this figure rises to almost one every thousand years. In fact there are indications for a total of 65 separate eruptions over the past 15,000 years. These occurred in bursts or clusters of activity, separated by long periods of quiescence. Once the activity resumes, we can expect a series of eruptions over a few centuries.

One attempt at description categorises three periods – the First Phlegrean Period began with the Campanian Ignimbrite eruption at 39 kA and ended with it’s aftermath at 35 kA. The Second Phlegrean Period began at 35 kA and ended with the NYT eruption at 14.9 kA. We are thus now in the Third Phlegrean Period which is characterised by the eruption of white pozzolana, the material that forms the majority of volcanos in Flegrei Fields. Initially between 10 and 8 kA, activity was in the western part at Bacoli and Baiae, on the western shore of Pozzuoli Bay. Activity then moved to an area centred between Pozzuoli, Monte Spaccata and Agnano (8 – 3.9 kA). Recently – 3.8 to 0.5 kA – activity is supposed to have moved towards the west and resulted in the formation of Lake Avernus and Monte Nuovo. Again, note the human proclivity to regard it as a thing of the past, a closed chapter that ended five hundred years ago, having moved away, and thus implicitly poses little danger today. It is however not unique to Naples but a phenomenon observable everywhere.

Recently, the INGV proposed an explanation model whereby the uplift prior to 2012 could be explained by hydrothermal activity and from 2012 by the periodical emplacement of magma within a magmatic sill (reservoir) centred 0.5 km south of the port of Pozzuoli at a depth of 3 km. (D’Auria et al 2015).

In 2013 came another piece of the Campi Flegrei jigsaw when researchers discovered that the mineral composition of clays extracted from the harbour floor of Pozzuoli was identical to that of the most durable concrete ever mad by man, the Roman variety. It has further been proposed that this mix constitutes the roof of the Campi Flegrei magma reservoir (Vanorio & Kanitpanyacharoen 2015), and if so, it is both a blessing and a curse. It is a blessing because “the concrete” is strong, resilient and flexible, thus it requires great force before it is breached and it can contain the minor fluctuations of the Campi Flegrei Caldera. But it is a curse from the mineralogical point of view as it provides a ready-made mix for water-rich, evolved magmas (dacite to rhyolite), in other words, highly explosive and the very worst possible type of magma that could be erupted in this densely populated area. Due to the nature of the crust, once magma cracks the cement, events may develop very quickly. There is a 60% chance that an eruption will be in the form of small explosions, 11% of an effusive eruption, 25% of a medium explosion and 4% that there will be a major explosive event. The most likely location of a vent is northeast of Puzzuoli, in or just east of Astroni.

The Devil May Care

This is the final ingredient necessary in a recipe for disaster; the attitude of the population in general and of the local authorities. Naples is not only at risk from Campi Flegrei but also from Vesuvius even if the former, and far greater risk, is mostly ignored. From his travels Carl reports that whenever Vesuvius was mentioned, people turned around to give the mountain the one-fingered salute, cursing it and the authorities that have forbidden profitable exploitation of its slopes; housing and industrial development. Not long ago, an attempt at drilling the Campi Flegrei in order to find out the exact stratigraphy was stopped and forbidden from ever being undertaken again with the spurious motivation that this could cause an unwanted caldera eruption.

With this mindset, let me introduce you, good citizens of Naples, to the events of Rabaul town and caldera in 1994. Rabaul is located inside the 8 x 14 km Rabaul caldera, a caldera that resembles Campi Flegrei and one which too is mostly filled by an arm of the ocean, here known as Blanche Bay. Rabaul had been thoroughly destroyed by an eruption as recently as 1937, but as it was the Provincial capital and because the bay provides a magnificent natural harbour; “Simpson Harbour” of great strategic importance in those days, it was rebuilt.

In the evening of September 18th, unrest began with strong earthquakes and on the advice of volcanologists, more than 50,000 people were evacuated to safety. Overnight, the ground rose over 6 metres, and early next morning the volcanoes Vulcan and Tavurvur on opposite sides of the bay (caldera) erupted simultaneously. In the event, only five people lost their lives which was a great triumph of crisis management and cooperation between scientists and local authorities. Rapid inflation is no stranger to Campi Flegrei and let me here present you with a scenario, one which you yourselves must be the judges of how accurate or realistic it may prove:

It is a fine spring evening and tonight, Napoli are hosting AS Milan, a contest that will have a great bearing on the eventual destination of the Scudetto. It is also the run-up to the local elections and tonight, the Mayor entertains a very important gathering of political backers. Over at the Osservatorio Vesuviano INGV, the seismologist on duty is much alarmed by the series of earthquakes , apparently centred near Pianura, 10 km from the city centre and close to the margin of both the CI and NYT calderas. He calls the Direttore and by the time she arrives, the instruments indicate that the ground has begun to inflate, possibly as much as 50 cm as the instruments located at Solfatara, more than 4 km away, indicate 12 cm. The direttore immediately calls the Mayor but is told that he is unavailable. It is not until nearly midnight before she gets through to him.

Over at Stadio San Paolo, no-one has noticed anything as the greatest quakes were no more than M3.3 in strength and in any case, the Nepolitans are too exited. By the time the game ends, Napoli have won three goals to one and the streets are filled with a jubilant throng. Barring an improbable upset away at Siena, the Scudetto will return to Napoli!

Signor Sindaco is not best pleased by the call from Signora Direttori. He too, as well as his backers, is caught up in the jubilant mood and he has no time for unwanted news – it’s time to turn sporting success into political coin – and in any case, if we are to be honest, he is not fit to deal with serious matters effectively at present. He hangs up on the Direttore with the promise of a meeting at 10 a.m. next morning.

During the night, INGV staff have established that the ground is indeed inflating rapidly over a large area centred on the suburb of Pianura, that it inflates at a rate of almost 50 cm per hour which seems to be accelerating and that by four a.m., the ground has already risen more than four metres and it looks as if an eruption may be imminent. Armed with irrefutable data, the Direttore finally manages to get some response and together with the Chief Commissioner decides to begin an evacuation on their own initiative.

At 7 a.m., inflation stands at almost seven metres. Evacuation of the Pianura and Astroni suburbs has begun but it collides with the morning rush. In spite of the efforts of the Carabinieri, by 8 a.m. the roads in and out of Naples are completely congested. The Mayor, woken at five, has made a public appearance and urged calm as well as full compliance with the directions given by the Carabinieri, but few Nepolitans hear this. Most are on a high from last night’s victorious game against AS Milan and furious at the congestion of the traffic.

At 8:53 a.m. there is a series of large earthquakes that most people feel and three minutes later, an ominous cloud rises over where Parco Attianese used to be, one that resembles an acacia tree but is dark, grey, billowing and from which lightning bolts streak towards the ground. It is a cloud that rises higher and higher in the air as well as spreads rapidly in every direction with great walls of darkness through which a glow of lightning can occasionally be seen…

Final Note on the Freedom of Science

At this concluding point, I can no longer refrain from roundly cursing the modern practice of certain publishers to place the fruits of scientific research out of reach behind paywalls. Information that was easily accessible only a few years ago is now unobtainable unless you belong to a faculty or are prepared to pay exorbitant prices, sometimes as much as $39.95 for a single article. Science used to be, and should be, free and open to all. Since it is paid for by the public, it belongs to the public!

Henrik

(My thanks to Albert for checking the article and making several valued contributions!)

Costa, Folch, Macedonio, Giaccio, Isaia & Smith, “Quantifying volcanic ash dispersal and impact of the Campanian Ignimbrite super-eruption”, Geophysical Research Letters, May 2012

D’Auria, Pepe, Castaldo, Macedonio, Ricciolino, Tizzani, Casu, Lanari, Manzo, Martini, Sansosti, Zinno: “Magma injection beneath the urban area of Naples: a new mechanism for the 2012–2013 volcanic unrest at Campi Flegrei caldera”. Scientific Reports; Nature Publishing Group 2015.

Vanorio & Kanitpanyacharoen: “Rock physics of fibrous rocks akin to Roman concrete explains uplifts at Campi Flegrei Caldera”, Science 2015
Dr Boris Behncke, Italy’s Volcanoes: The Cradle of Volcanology, http://www.geo.mtu.edu/volcanoes/boris/mirror/mirrored_html/Italiahome.html

USGS: Volcanic Ash Effects and Mitigation Strategies (well worth a read)
http://volcanoes.usgs.gov/ash/agric/index.html

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About Melly Rocks

20 something living in London while doing my Natural Science BSc with the Open University. Wannabe geologist and trainee volcanologist. Living life to the full and following the rocky road to my dreams...

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