A few days ago, in the VC back-channel, I made mention of something I had seen on a SciChan program. That October is the anniversary of the 79AD eruption. Albert then chimed in with a link to a BBC article detailing the same.
Okay, that’s the second thing about October that is interesting. 25 Oct has other interesting events…
Why Vesuvius? Well, it’s sitting in a volcanic arc that doesn’t really align with the landform, though all could be related to the 41kyr (inconvenient) VEI-7 of Campi Flegrei. (…well, inconvenient to the Neanderthals that were centered around this region.) Technically the two are not interrelated, but having that large a neighbor sitting on your doorstep…
So in celebration of (maybe) October ’79, One of the best authors in our camp;
Is this the most famous volcano in the world? The shape is instantly recognisable: the twin peaks give the appearance of an unintended gap where something has been removed. Naples of course has the reputation that anything that is not securely fixed down can quickly and involuntarily change owner, so perhaps a missing bit of mountain is not such a strange idea. And whole towns have gone missing here. Pompeii and Herculaneum had been completely destroyed, and although the stories of the fatal eruption were immortal, in the writings of Pliny the Younger, the towns themselves had been forgotten. Pompeii was rediscovered in the 1500’s during the building of a canal. The find was documented but not pursued. Herculaneum was re-discovered in 1709. In December 1738, an inscription was found which labeled the excavated ruin as Theatrum Herculanensi. Only now was it realized that this was ancient, lost Herculaneum. The excavation of Pompeii began in 1748, at first rather destructively but later better organised and with great effort. Still, the site is enormous and much remains hidden under the layers of ash. The excavated part is very impressive. The numerous public buildings and original graffiti provide a less-than-flattering view of the morals of Roman life (‘bawdy’ would be a generous way to describe the culture) but bear witness to a practical attitude to town design. The access roads were especially well designed. The ancient bakeries along them provided drive-through food for the people on the carriages. Naples is the original home of the pizza (it is hard to find anything else to eat in its restaurants), and looking at Pompeii it is easy to see how this would have fitted in with the people on the carriages grabbing food from the passing displays using the pizza base as their plate.
In between the mountain and the sea, and amidst very fertile ground, this was a good place to live. Of course now we know better. The mountain is now recognised for the volcano it is, and the area to the west is known to be an active caldera. We wouldn’t build a town there. Except of course, this is where Naples did grow up, the only European city with an historical eruption within the city limits. Everything you see is volcanic, every single hill or depression a memory to some ancient eruption.
The local people are now well aware of their environment. The Vesuvius Observatory is the oldest volcanology observatory in the world, founded in 1841. At the moment the main area of concern is at the opposite side of Naples, at Campi Flegrei, where increased ground heat is present with some inflation. But Vesuvius is not forgotten. At the moment it is completely quiet, its fires spend in the 1944 eruption. This interlude will not last. One day the mountain will rumble into life again. The lava will flow, and it will not find a free channel to the sea. Anywhere it could go, suburbs have sprung up.
To see what the future holds, look at the past. Those who forget their past are condemned to repeat it. Our view of the past Vesuvius is shaped by this single destructive eruption of AD 79. What is a normal eruption like? How long is typical between eruption? History should tell.
Vesuvius began to grow around 17,000 years ago. There is evidence for 8 large eruptions, similar to the one in 79 AD or even larger. The main product from Vesuvius is andesite, known for causing explosive eruptions. Lava flows tend to be comparatively small in volume. The main impact comes from large explosive eruptions and associated pyroclastic flows. The cause of the volcanic is -obviously- underground, a combination of the subducting African plate situated to the east, underneath the Adriatic Sea (it is a pretty mixed-up region, with bits of Africa found in various places around Southern Europe), and extension.
Vesuvius is a grand cone, appropriately called Gran Cono, 1280 meters high. At the top is a round, deep crater where a few weak fumaroles can be seen but otherwise there is little evidence of activity. To the north and east, there is an arc-like rim surrounding Gran Cono, lower than the peak. It is called Somma, and is 1130 metes high. In the valley between the rim and the central cone are some old-looking lava flows. The rim is the remnant of an earlier phase of the mountain, 17,000 years ago when when a larger caldera formed. Not much of that remains. The combination of the rim and cone is called Somma-Vesuvius.
Since Roman times, there were significant plinian eruptions in 472 and 1631, although smaller than the 79 AD event, as well as an unknown number of smaller eruptions. The 1631 eruption signalled a change: after this, Vesuvius had frequent small eruptions, until 1944. Than it changed again, to its current silence. But how well do we know the history before 1631?
By 79 AD, Vesuvius was not considered an active volcano. It had been silent for long enough that it was seen as extinct, insofar that concept existed. There had been earthquakes in the decades before the eruption, and interestingly, the Pompei aquaduct has ceased to work. This may have been caused by local uplift around Vesuvius, causing the water supply to try to run uphill.
After 79, people (at least the survivors) knew better. There were further eruptions, but quite irregular. The records of these older eruptions are fragmentary. Magnus Aurelius Cassiodorus and Procopius the Cæsarean allude to eruptions in 472 and around 512. The first one was major, with ash falls as far as Tripoli and perhaps Constantinople. The 512 eruption (the date is in question: 532 has also been suggested) was mildly explosive, for which a DRE of the fallout has been estimated at 0.025 km3. Procopius mentions lava flows which occurred during both eruptions, albeit his report is from hearsay as Vesuvius did not erupt during his own four-month visit to the area.
Paulus Diaconus (720-c. 787) mentions Vesuvian eruptions in two documents, and is also the first to suggest that the original Somma crater was related to the 79 AD eruption. He mentions the ash eruption of 685. After this, there is a period of 300 years where we have very few written records. There is a mention of events around 968 and 999. In January, 1037, lava flows reached the sea.
A large eruption in 1139 is stated in one account to have lasted eight, in another, forty days, but little is known about it. It has been argued that many of the lava flows on the side of the mountain, coming from parasitic vents, date from around this time. After this, there is little written evidence until the next big eruption of 1631. It is assumed that the mountain was mostly dormant over that 500 year period.
The 1631 eruption was a significant, plinian affair which caused many casualties. It came as a surprise: the mountain had been dormant since living memory. But living memory is not that long. A description from 1570 mentions highly active fumaroles in the crater with glowing rocks and occasional explosions, something that seems to have been forgotten about 60 years later.
After 1631, a very active period followed, with always something going on. Around 1770, Sir William Hamilton wrote that the mountain had for many years ‘never been free from smoke, nor ever many months without throwing up red-hot scoriae usually followed by a current of liquid lava’. Jospeh Wright made some 30 paintings of the mountain, invariably shown in eruption, even though there were no documented eruptions during his visits, around 1774. Eruptions were long lasting: the 1944 eruption was the culmination of an episode that began in 1913, with a peak in activity in 1929.
The 472 and 1631 eruption are classified as VEI4. The 79 AD eruption was VEI 5. The Avellino eruption, 4000 years ago, was significantly larger than the 79 eruption, and was a VEI 6.
This was during the collapse of the Roman empire. The area had seen widespread destruction caused by the invading tribes, and population was already in sharp decline. Into this stressed life, a major eruption happened. The prevailing westerly winds send the fall-out to the east, away from the sea, the opposite to what happened in 79 AD. The regions affected by the pyroclastics and ash falls remained de-populated for centuries. The coastal areas escaped the impacts, and stayed inhabited. From the locations of the deposits, the eruption column is estimated at 20 km height. It was big, but was not on the scale of 79.
The 1631 eruption is famous. It began in the early morning of 16th December, with incessant earthquakes and ash fall. Mid-morning on the next day, a large eruption column formed. As the ejecta came down, the mountain seemed to disintegrate and material flowed down like water. It reached the coast and caused a tsunami several meters high. Before the eruption, the mountain had been densely vegetated, perhaps even inside of the crater. The mountain remained active for several years, although without the intensity of the first few days. After it, the mountain was desolate and some 3000 people had been killed. There were four more eruptions that century, a much higher level of activity than before.
The 1631 eruption changed our language. The word lava is Neapolitan by origin. It came from the Latin labe (to flow), and was used for volcanoes rather than rivers for the first time in 1633. How much lava was actually involved in the eruption is disputed. The written records mention seven ‘rivers of fire’ of which most were rivers of hot ash. Dating of some lava flows show that three flows on the southwestern slopes date from the 1631 eruption.
The eruption was probably similar in size to the one from 472. The eruption column was 15 km or higher.
The phase of frequent effusive and strombolian eruptions lasted for 314 years, with a slowly decreasing peak energy of the eruptions. It appears that over time, the magma slowly cooled and a plug build up making eruptions more difficult. The last one of the series was in 1944 and is well documented. It took place in the midst of the battles of the second world war and it caused significant damage to the allied bombers. The earthquakes started in early March. The diary of Leander Powers states:
Saturday, March 17, 1944. While we were just finishing supper, someone called to say there were huge red streams of lava flowing down the sides of Mount Vesuvius. It was a sight to behold. Never had we seen such at night, usually a faint red glow at the most. As we watched the streams, like giant fingers flowing down the sides, we could see a glow in the sky. All during the night and Sunday there were quakes of the earth with tremendous roars -similar to thunder- from Vesuvius. The windows rattled, and the entire building vibrated. (http://www.warwingsart.com/12thAirForce/Vesuvius.html)
It is interesting that it mentions the red glows seen before the eruption, showing lava was already present. The diary goes on to describe the lava flows entering Naples and the cinder cones and lava rocks. Finally:
Friday, March 30, 1944. The eruption seems to have abated very slowly during the past few days. Cinders and ashes have been raining down over all the villages in this section, but seem to be slowing up. The smoke from the crater is apparently changing from the intense black to white again. Yesterday, I looked at the Autostrade through my glasses, and it is apparently covered with cinders as is the entire mountainside. Only two weeks ago, I rode up the Autostrade and then walked several hundred yards up toward the crater. Yesterday afternoon, we rode in the ambulance, and, on the way back, we took a shortcut via Pompeii. Bulldozers were plowing the cinders to the side of the road in huge banks. Practically all the gardens and vineyards are covered to a tremendous depth in the area all the way from Vesuvius to Salerno. Many people are homeless and without food, but they seem to take it in stride, just as the Northerners take the snow in winter. After this eruption it’s easy to visualize the destruction of Pompeii – a most amazing and uncanny phenomenon. From my quarters I can still see what appears to be small areas with smoking lava, but the smoke from the crater has abated. Today, the wind is blowing inland, and it appears that cone is much lower than before. Vesuvius is definitely not dead after all these years of inactivity.
This is the only description I have found mentioning physical changes in the appearance of the volcano! The effusive event started on March 18. Lava fountaining happened on 21 March. Explosive activity happened on 22 March with ash falls at 500 km from Vesuvius. After April 7, there was no further activity. The cooling magma plug has now blocked the conduit. Nothing more has happened up to the present day.
The most notable feature of Vesuvius is its twin peaks. Because the second peak is actually a partial ring, it is seen on different sides of the main cone depending where you are looking from, although it always looks double. But this aspect is not mentioned in the oldest descriptions.
There is a written description from Strabo, less than a century before the 79 AD eruption. It states: “Above this locality Mount Vesuvius stretches. It is entirely occupied by beautiful fields, except for the top. The top itself is almost entirely flat but is quite barren, the land having a cinerous appearance; there are cracked cavities, opening on rocks sooty as if consumed by fire. We could suppose that this field previously burnt, and later its fire craters died out as the material burnt completely.”
This does not sound much like the current mountain: where is the mountain and where is the deep crater? It has been taken as evidence that instead of two peaks, there was only one mountain. It makes sense that the 79 AD explosion must have removed a fair amount of mountain, and therefore the appearance may have been quite different. But exactly what it looked like, and how tall it was, remains unknown. The description may indicate that the younger cone wasn’t there, but was a plateau at the level what is now the saddle between the twin peaks. Or the description may relate only to the central cone, and indicate it was taller than Somma and had no central crater.
There is one Pompeii fresco which shows only a single mountain, before the 79 AD eruption. This together with the absence of any mention of a second peak during this time, may suggest the younger peak did not exist. But the case is not clear, as the fresco does not show the full mountain. The drawing from 1500 looks very much like the current view. By 1631, the current mountain view was largely there, including the central crater which deepened by 450 meter during the eruption. However, the pictures show the old rim (Somma) to be considerably higher than the central cone, while nowadays the cone is higher. Could the younger cone have begun to grow during the early middle ages? It is not impossible – volcanic cones can grow surprisingly quick.
The next eruption
After eruptions in 1906, 1929, and 1944, Vesuvius has gone eerily quiet. The size of the eruptions had slowly been diminishing since 1631. The likely interpretation is that in 1631, an open conduit was created which since has slowly been blocking up as the magma cooled. Since 1944, it has remained blocked. What does this imply for the future?
Vesuvius seems a fairly unpredictable volcano. It has gone through phases of frequent lava flows, and times of infrequent explosions. We may now be at one of those times where it changes from one to the other. The effusive eruptions may not have completely ended: there is still a chance of another 1944-like eruption, with lava flows endangering the upper suburbs of Naples. But if the conduit is really blocked, the next eruption will be explosive. It will come from the summit: historically, this is where explosions happen. There will be warnings, with earthquakes, lively fumaroles and probably inflation within the crater and perhaps without, lasting for months. A large tectonic earthquake could increase the chance of an eruption a few years later, judging from the 79 AD event. Perhaps this can weaken the blocking plug, and create new channels for magma to rise, albeit at a leisurely rate.
To see how a new eruption might announce itself, the events of 1631 provide a clue. 60 years earlier, highly active fumaroles and glowing rocks were seen, indicating that a failed eruption took place. A few years later these signs had disappeared again. Explosions and landslides in the crater started several months before the eruption. Loud underground explosions were heard two months before, and at this time wild animals disappeared from the summit area. A few weeks before the eruption the vegetation was drying out, indicating degassing. Two weeks before the eruption seismic activity became strong enough to be felt by residents. Finally, the bottom of the crater began to uplift and a few days before the eruption, it had reached the rim. If this happens, it is time to swing the emergency plans into action.
How long would we need to wait for such an eruption? Based on its history, it could be a long time. The time of eruptions every 20 years is over. The dormancy before 79 AD may have been centuries. Before 1631, it was as long as 500 years, although there may have been smaller eruptions in the intervening period. We don’t have a long enough sequence to look for repeating patterns, and we don’t know what is normal and what isn’t. The current length of silence is not unusual, and could easily last for a century or (much) more.
The crater of Vesuvius is deadly quiet, apart from the buzz of the tourists walking around the rim. The fumaroles are hardly visible and nothing seems to threaten an eruption. This tourist heaven could last a long time, or Vesuvius could suddenly decide to rumble back into life. The only thing that can be predicted is its unpredictability. Vedi Napoli e poi muori! What a place to build a city.