Two issues ago I started a brief review about the geologically hazardous position of Naples, Italy, snuggled between the volcanically active area of Campi Flegrei in the west and the Somma-Vesuvius volcanic complex to the east.
The position of Naples in both space and time is surely a matter of grave concern for Italian volcanologists. The City of Naples is located between two potentially catastrophic eruptive areas and both volcanic complexes have been quiescent for more than half a century. This is compounded by the fact that more than one million people live in Naples and an extra 1.3 million live in an area that could be threatened by a major eruption.
Vesuvius is arguably the best known volcano in the "western" world largely because of the eruption of AD 79. This caused the destruction of two moderately large towns, the best known being Pompeii and Herculaneum, but also Oplontis, Boscoreale and Stabiae, that were situated on the flanks of the volcano and downwind of the eruptive centre. The eruption has provided a name for one on the nastiest types of volcanic outbursts; the Plinian type. The eruptive phase took place in a shockingly brief 30 hours or so and with considerable devastation.
So why should we be so concerned about Vesuvius and what might be the possible results of activity in this complex? In order to examine the potential for disaster we need to look to the past and review what happened in those fateful few days of August in the year 79 AD. Geologists have spent a long time reviewing the petrology of the ejecta from this eruption, analyzing the effects of eruption from physical evidence left behind in the pyroclastic deposits and also re-examining the accurate description left by Pliny the Younger (nephew of Pliny the Elder) as he watched the eruption from Misenum on the west side of the Bay of Naples.
Pliny the Elder was admiral of the Roman Fleet based as Misenum, an ardent "natural historian" and heroic figure, who died in a rescue effort near Stabiae. In early August AD 79 the population of the towns on the flanks of Monte Somma had little idea that they were residing on the slopes of a volcano since the area had been free from eruptions for about 700 years. In retrospect perhaps there had been some warnings; a fairly large earthquake that had rattled the region in A.D. 62, some 17 years earlier, followed by numerous smaller earthquakes that created damage to homes and frescoes on interior walls. There was also a small swarm of localized earthquakes in the days immediately preceding the eruption, but perhaps nothing that would signify to the populace that their lives were soon to be altered forever.
The younger Pliny (Gaius Plinius Caecilius Secundus) recounted the following about his uncle's death in a letter requested by and sent to Tacitus, another famous Roman historian. This account has been modified from the literal translation.
He (Pliny the Elder) was at that time with the fleet under his command at Misenum. On the 24th of August, in the early afternoon, my mother desired him to observe a cloud which had appeared of a very unusual size and shape. A cloud was ascending (from which mountain was uncertain at this distance), the form of which I cannot give you a more accurate description than by likening it to that of a pine tree, for it shot up to a great height, in the form of a very tall trunk, which then spread itself out at the top into what looked like branches. (The cloud) appeared sometimes bright and sometimes dark and spotted according as to how it was more or less impregnated with earth and cinders.
Pliny had ordered a galley so that he could observe the phenomenon more closely. As he was leaving the house a letter from Rectina - the wife of a friend who lived in a villa in or near Herculaneum - was delivered, saying that she needed help escaping the eruption and had to leave by sea. Realising that there were many others in a similar situation he ordered the quadrireme galleys of the fleet to put to sea, and went himself on board with an attention of assisting the towns that lay thickly strewn along the beautiful coast (on the far side of the bay of Naples). Passing vessels that were fleeing the eruption he held course for Herculaneum, but could not approach because of shoals and ash and rocks "blackened and burned and shattered by fire" that were falling into the ships. He was now so close to the mountain that the cinders, which grew thicker and hotter the nearer he approached, fell into the ships, together with pumice stones and black pieces of burning rock.
A rescue at Herculaneum was impossible and so he ordered the ships toward the port at Stabiae (Figure 1) where he joined another commander, Pomponianus, who was waiting to put to sea when the contrary winds diminished. Unfortunately they were now trapped by the northwest winds on this shore.
Pliny landed at Stabiae, bathed and dined and eventually went to bed. Meanwhile, broad flames shone out in several places from Mt. Vesuvius, which the darkness of the night contributed to render more brighter and clearer. As the ashfall continued and the villa continued to be rocked by earth tremors he and his companions decided that it would be more prudent to leave in case the building collapsed. Pliny and his colleagues left the villa with pillows tied over their heads as protection from falling rocks. It was now day everywhere else, but there a deeper darkness prevailed than in the thickest night; which, however, was alleviated by torches and other lights of various kinds. They made their way to the sea shore, but again could not leave. Then came a strong smell of sulphur and clouds of ash and flames. His breathing was obstructed by the dust-laden air and he collapsed and died. When the air cleared some two days later his body was found, unharmed, and dressed in the clothes that he had died in. He looked as though he was asleep.
This account, embellished by personal anecdotes from surviving members of Pliny's party remains as an accurate description for what is now called a "Plinian Eruption". Pliny the Younger was forced to leave the villa at Misenum and sheltered on a hill that was rocked by many earth tremors during that day and night, and eventually assaulted by clouds of ash from the eruption.
So what can we deduce from this account and the physical evidence? The most obvious facts first; that the eruption was unexpected by the populace, although we would realise today that earthquake swarms close to a vent are a potential fore-runner to a volcanic outburst.
The eruption was initiated by a phreatomagmatic (steam-generated) explosion that "unbottled" the volcanic conduit. This was perhaps just before midday, on August 24th, when Rectina sent her message to Pliny asking for naval assistance. The initial blast was almost immediately followed by tremendous up-rush of gases and magma that threw ejecta (largely pumice and ash) 30 km into the atmosphere. As the ash reached its maximum height, upper atmospheric winds pulled the cloud out into the characteristic "pine tree" effect noted by Pliny (Figure 2) somewhere about 1 to 3 p.m. The first tephra that descended was white in colour (Figure 3) and attained local thicknesses of 2 - 3 m at Oplontis and Pompeii. People would have survived this phase, but if they decided to take shelter (as many did) they were likely doomed for the worst was still to come. Through that afternoon (when the fleet neared the shoreline) and into the early evening the outbursts continued, with the tephra fall changing to grey pumice (Fig. 3). About 8 p.m. the force of the upward rushing gases diminished and the eruption cloud gradually subsided initiating a third stage that involved the collapse of the main eruption column. The weight of suspended material above the vent allowed what is called a "pyroclastic surge" where ash, hot-gases and large particles fell down and then surged outward from the main conduit. One such surge destroyed Herculaneum, some 8 km from the summit of Vesuvius.
Shortly after midnight the eruption changed to a series of phreatomagmatic explosions - perhaps as many as four to six. Each explosion allowed pyroclastic flows (seen as the ash-flow deposits in Figure 3) to take place. It is likely that Pliny the Elder was caught at the very edge of one such surge. Even Pliny the Younger who was over 20 km to the west of Vesuvius perhaps was caught in this final "base surge" phase. The citizens of Pompeii and Herculaneum as well as other smaller settlements were caught in similar surges much closer to the vent and died in far more gruesome manners.
By the early afternoon of August 25th the eruption was practically over and it ceased in the early evening. The whole region was blanketed with thick pumice and ash deposits and heavy rains associated with steam and storm activity mobilized mudflows (lahars) that spread across the area. Pompeii, Herculaneum, Oplontis, Stabiae and numerous smaller settlements and farms were destroyed and often buried. Forgotten within two or three generations, the cities, settlements and villas remained for 1,600 years before being re-discovered by well-diggers in the 1700's.
The effects of the eruption
Today as you walk through the streets of Pompeii and Herculaneum it is difficult to imagine what happened here almost 2,000 years ago. Our excursion visited Herculaneum, a small "resort" style town in AD 79. This was the town that Rectina lived in or near, and it was the initial destination of Pliny's rescue efforts. Herculaneum was destroyed by the first major collapse of the eruption cloud likely between 8 p.m. and 1 a.m. This sent a searing wall of gases and fragmental rocks down to the coast. pushing the coastline hundreds of metres out to sea.
People fleeing the eruption were overwhelmed by this "base surge" which swept from the crater of Vesuvius to the sea at Herculaneum in a matter of minutes. Although only 30 to 40 bodies had been found in the town in the original excavations, one group of almost 300, together with several dogs and a horse, was found huddled under one of the arches probably used for sheltering boats at the water's edge. The gases in the pyroclastic flow were so hot, at about 500¼ C, that their flesh had seared away and the bones were charred. A singularly nasty death!
Nearby excavations uncovered a huge villa along the (old) waterfront just outside Herculaneum known as the "Villa of the Papyri". Slaves were surmised to be evacuating the library and carrying scrolls away from the area when they were killed. In the mid-1700's when excavations began, many scrolls were thrown away during discovery. No-one realised just what they were - they were assumed to be charcoal blocks - and only about 1,750 still exist. There is a possibility that there could be many more still hidden in other parts of the villa. It has also been suggested that many of the scrolls could contain records of the history of Rome and of the works of Aristotle, Sophocles and other Greek writers and philosophers. Multi-spectral digital imaging developed in the last few years allows the possibility that these carbonised rolls can now be read by this new 3-dimensional imaging technique (Gregory, 2004).
Ironically the nature of the eruption, hot gases that carbonized the scrolls followed by massive ash and mud flows, ensured that the scrolls were sealed away in a waterproof and airtight manner. Recent arguments are being made pro and against excavation of the villa; those arguing for, say that priceless literature, long lost to history, might be concealed and should be recovered immediately. Those against state that it would be far better for funds to be spent renovating and repairing what has been already uncovered; that the scrolls have been there for almost 2,000 years and a few decades more will not make any difference. Of course the unknown factor is Vesuvius itself and the nature of the next eruption when it happens!
Herculaneum is a fascinating glimpse into an affluent portion of Roman society silenced in a matter of minutes in one, horrific and eventful day. You can see the house and shop where one of the wine sellers lived (Figure 4) you can walk along narrow streets that were silenced by the volcano (Figure 5) and you can stroll up the ramp through the walls where those hundreds of people fled in the hours before the hot gas-charged particulate clouds raced down from the summit. You can see the charred beams still in place over many doorways (Figure 6) and gaze at the 25m or more of solidified tuff that mantles the sides of the exposed city. Perhaps, if you are really a thinking person, you can also wonder about the people who have now live in Ercolano, on top of Herculaneum, and wonder about their fate in the next paroxysm from the mountain behind them (Figure 7).
Pompeii, some 15 kilometres to the south and east is better known than Herculaneum and involves an almost complete excavation of this city of approximately 20,000. We are unsure just how many died at Pompeii. We do know that over 2,000 bodies have been recovered and the nature of their positions and the materials/objects found near them tell us something about their lives, occupations and perhaps, characters. The reason for this is the nature of the volcanic deposits that entombed the unfortunate individuals. In the middle of the last century, Guiseppe Fiorelli, the director of excavations at the site realised that "hollow" sounds indicated that there had been "something" buried in the ash. A technique where liquid plaster of Paris was poured into the cavity (now replaced by quick drying resins) was developed with some amazing results. In some cases the subsequently revealed casts showed inanimate objects (bed frames or cupboard doors with handles), but in many cases stark and horrifying details of people and animals that had been escaping the eruption when overcome by fumes, hot gases and dust clouds. Figure 8 is part of a scene at the "garden of the fugitives", a vineyard near the Nocera Gate where what appears to be several families were overcome by the eruption. The bodies were covered in the fine ash that hardened and voids were left as the bodies - or other organic materials - decomposed.
This "casting" technique has allowed us to reconstruct the fate of many of the citizens in those last few hours. The families traveling together - one small group that had sheltered beneath a staircase when it collapsed and crushed them - the muleteer with his mule nearby, the dog that someone forgot to release from its chain, the children who were lost or abandoned. The same applies to the 54 gladiators in their barracks and the woman with expensive jewelry who was visiting or hiding with them. And then there were those who had left it too long - people trapped trying to break through walls to get out - or trying to escape with household possessions and sometimes valuable hoards of silver. All of these are preserved where they fell in that black afternoon and night of August 24th, and 25th 79 A.D.
The eruption has allowed us to see a town preserved in time; - from the richly decorated homes, to lavish villas and humble dwellings. You can walk along the narrow streets and broader avenues, and through public and private places. You can see the chariot and cart tracks preserved in roadways (Figure 9). Paintings and frescoes (Figure 10) still adorn walls, daily utensils - pots, pans and glassware (Figure 11) - are all there, as well as art objects such as statues and bronzes (Figure 12) and mosaics, Billboards advertising gladiatorial conflicts, graffiti on walls, the Roman equivalent of "fast-food" diners for people in a busy world (Figure 13), all the life-styles of the empire paralysed in one day.
Pompeii and Herculaneum still can be visited, and you should do so. It took me exactly half a century from the time that my growing interest in geology and volcanoes made me aware of these Roman cities to the day that I set foot onto those sun-drenched streets. The fascinating irony is that now uncovered, they have the potential risk of being destroyed again and perhaps we should briefly look at what he future might hold.
As I mentioned above, Italian volcanologists and other European geologists are extremely concerned about a major, active, eruptive centre in the middle of a very densely populated area (Figure 14). Just what are the risks involved and how can they be mitigated? Civil defence choices are limited to an examination of the past eruptive history and "best guesses" as to what might happen in the next eruption.
The potential risks from an eruption can be placed in three major categories. The first would be a catastrophic Plinian or sub-Plinian (as in 1631) eruption, consisting of major ash ejection, hot gas clouds with glowing magmatic fragments and with potential surges. The second would be substantial ash falls as seen during the 1906 eruption. These could be accompanied by small pyroclastic flows and mudflows. The third would be lava flows similar to those seen in earlier centuries.
Each of these categories poses different styles of risk and generates different responses. Generally it is thought that a truly catastrophic "Plinian-type a la A.D. 79" eruption is unlikely (nothing of that significance has happened in over 2,000 years). There is a higher probability of a sub-Plinian eruption, considered to be a "worse-case" scenario and this is the one on which the emergency plan is based. Here in the "Red Zone" of about 200 km2 the populace immediately around the volcano would have to be evacuated. This is an area that could (based on geologic evidence and computer simulations) suffer from pyroclastic flows, mudflows and major ash fall and gas problems. In 1631 some 20% of this area was affected by pyroclastic flows.
In a "Yellow Zone" extending 50 km eastward and up to 25 km NE and SW of the vent ash thicknesses observed during the 1906 eruption can be plotted with houses that are present today. Isopachs (lines of equal depth) show that ash falling on rooftops in the danger area will vary from 100 to over 600kg/m2. Any thicknesses greater than 100kg/m2 has the potential of collapsing roofs, especially if the ash is water-saturated Based on these figures approximately 6,500 homes would be adversely affected and over 30,000 residents would face potential roof collapses.
A third "Blue Zone" occupies an area immediately north of the volcano and this is in a region in which flooding could be an additional problem.
Unfortunately people were allowed to move back into areas that had been affected by the last (1944) eruption. Had efforts been made to prevent "population creep" into these more risky areas the problems that will have to be faced during the next evacuation would have been considerably simplified. Current fiscal inducements (about 40,000 Euros) for families to voluntarily move from the main danger zones are simply insufficient and the establishment of a "Vesuvius National Park" (my term) as an "unoccupied zone" seems unlikely, at least until the next major eruption forces people away. The Emergency Plan does dictate what sectors around the volcano will be evacuated and where the populace will have to move elsewhere in Italy. Modes of transport - ferries, trains, busses - are also proscribed, but the evacuation, in narrow streets and on crowded roads, depends on time and a well-organised and "passive" populace.
Civil defence estimates are that a minimum of one week will be needed to move the population at risk. Will the volcano provide one week of warning? The short answer is possibly. The area around Vesuvius is heavily monitored with GPS stations, gravimetric stations, tiltmeters, levelling benchmarks, tidal gauges, analog and digital seismic stations and geochemical analytical collection positions. It is assumed that gas content changes and/or magma movements from 6 to 8 km below the vent will give advance notice. Such activity moves the alert status from "Normal" to "Attention!" and the technical monitoring is reinforced. If further variations in monitoring are detected, the warning rises to "Pre-Alert" and simulation of events commences. Further activity, such as ascending earthquake swarms moves the alert to "High" status. Monitoring is continued by automatic stations. It is in this range that evacuation would begin on the assumption that an eruption could take place within weeks.
I do not envy the geologists that have to advise in such a situation. Call it correctly and thousands if not hundreds of thousands of lives could be saved. Call it incorrectly and two scenarios emerge. One that substantial numbers could be killed if the warning is left too late, or that politicians do not react in time since the evacuation order has to be triggered by (likely) the Prime Minister. For example, people will be limited in what they can take with them (remember the citizens of Pompeii overwhelmed when trying to escape with precious household items). If you leave houses full of expensive objects such as plasma screen TV's, computers, and art works who wants to stay behind to "serve and protect"? The second scenario would inevitably cause deaths (heart attacks; road accidents; possible drownings) but with no eruption! This then leads to questions about the science, the cost of the evacuation, and likely a resistance by sectors of the populace to take the next warning seriously; thus increasing the likelihood of an even more catastrophic loss of life "next time"!
And there will be a "next time". Our knowledge of volcanic behavior has improved dramatically in the last few decades, particularly with Mt. St Helens and Pinatubo as examples. But every volcano is different and one thing that Vesuvius can illustrate is that the eruptive styles can be quite different, and, in some cases, truly terrifying. We can only wait and see!
Andronico, D., Avino, R., Brown, R., Caliro, S., Chiodini, G., Cioni, R., Civetta, L., D'Antonio, M., Dell'Erba, F., Fulignati, P., Granieri, D., Gurioli, L., Marionelli, P., Santacroce, R., Sbrana, A., and Sulpizio, R. 2004. In: The Neopolitan Active Volcanoes (Vesuvio, Campi Flegrei, Ischia): Science and Impact on Human Life. Eds. Orsi, G., Vita, S.D., Di Vito, M.A., and Isaia, R. Pre-Congress Field Trip Guidebook B28. 32nd International Geological Congress, Florence Italy 20-28, 2004. Italian Agency for the Environmental Protection and Technical Services, Rome.
Carapezza, M.L. 2000. The Vesuvius: 2,000 years of observation. Osservatorio Vesuviano. Dipartimento Della Protezione Civile.
Grant, M. 1974. Cities of Vesuvius: Pompeii and Herculaneum. Book Club Associates. Butler and Tanner, Ltd. Frome and London, England.
Web citations (URL's worth visiting)
Gregory, A.P., 2004. Digital Exploration Unwrapping the Secrets of Damaged Manuscripts. At: http://www.research.uky.edu/odyssey/fall04/seales.html
Anonymous, 2005. The Friends of the Herculaneum Society. http://www.herculaneum.ox.ac.uk/news.html
Dobbins, J.J. 1994. Pompeii Forum Project. http://pompeii.virginia.edu/forummap.html
(There are many other references on the web, including travel information. I suggest using Google and enter Pompeii and Herculaneum).
Although this article provides a review of observations made on the International Geological Congress Pre-Congress Excursion B28, much of the information is from data provided by Giovanni Orsi and the co-leaders of this excursion (Andronico, et al., 2004), as well as from the additional references and web URL's provided above. Any errors should be attributed solely to the author. One final comment is a suggestion to read a fascinating (non-fictional) account of the eruption and its aftermath by Robert Harris. "Pompeii" is the story of a young aqueduct engineer, Marcus Attilius Primus, in the week leading up to and including the eruption of Vesuvius.
Alan V. Morgan