jump to navigation

The ups and downs of Vesuvius 17 September 2008

Posted by admin in current research, geoscience, Italy, natural hazards, Vesuvius.
Tags: , , ,
trackback

Naples and Vesuvius, August 2003

A new study of Vesuvius suggests that the magma source involved in the eruptions of the last 20,000 years or so has been rising within the volcano, so that the more recent the eruption, the shallower the depth from which its magmas have come. Does this mean, looking to the future, that Vesuvius is more or less dangerous than we thought? Well, that depends…

The research in question was done by a Franco-Italian team of scientists and the results have been published as a letter in the 11 September 2008 issue of Nature under the title ‘Upward migration of Vesuvius magma chamber over the past 20,000 years’ (link is to abstract only). Analysis of samples of material from four of Vesuvius’s main explosive events between 5800BC and 472AD indicates that the pressure to which magmas have been subjected before each event has progressively reduced with each successive eruption: the scientists’ conclusion is that the magma chamber has been moving upward within the volcano, from 7-8km to 3-4km depth between 79AD and 472AD. If data from the Pomici di Base event of 18.5kyr ago and the most recent major eruption of 1944 is taken into account, the total upward migration of the magma chamber is around 9-11km.

The significance of this is explained by Dr Erik Klemetti in his crystal-clear discussion of this research at Eruptions: magma at shallower depths is subject to lower pressure, which means less chance of the explosive degassing that produces violent eruptions. However, he goes on to point out that this research, focused as it is on this ascending and recently tapped magma source, has nothing to say about future eruptions that might be rooted deeper in the system, produced by the influx of fresh magma at depth.

The authors of the Nature study, as is the way of such things, conclude very circumspectly by saying that the apparent upward migration of the magma chamber needs to be incorporated into the predictive models used to forecast the future behaviour of the volcano, and that more research is needed. They don’t make any predictions about the future. Reporters writing up their research are not so coy, so we have stories apparently using the same facts to draw diametrically opposed conclusions: ‘Mount Vesuvius’ destructive power may be diminishing’, ‘Vesuvius magma chamber rising; may mean milder eruption’, and ‘Mount Vesuvius may be less dangerous than predicted’ on the one hand, and ‘Pompeii-style eruption of Vesuvius can’t be ruled out’, ‘Vesuvius still an eruption risk’, on the other. How apocalyptic are you feeling at the moment? Take your pick accordingly.

  • B. Scaillet, M. Pichavant & R. Cioni, ‘Upward migration of Vesuvius magma chamber over the past 20,000 years’, Nature, no. 455 (11 September 2008). Link to abstract.

Image: Naples and Vesuvius, August 2003 (source).

News
Vesuvius magma chamber rising; may mean milder eruptionNational Geographic, 10 September 2008
Mount Vesuvius may be less dangerous than predictedScientific American, 10 September 2008
Pompeii-style eruption of Vesuvius can’t be ruled out – AFP, 11 September 2008
Mount Vesuvius’ destructive power may be diminishingDiscover Magazine, 11 September 2008
Vesuvius still an eruption risk – ABC Science, 11 September 2008

Information
Global Volcanism Program: Vesuvius – summary information for Vesuvius (0101-02=)
Vesuvius Observatory – home page in English

The Volcanism Blog

Comments

1. magmatist - 18 September 2008

I stumbled across this Nature article on tuesday having done a presentation on Vesuvius the previous day. Its good to see Vesuvius in the news again even if the press shake all the science out of the work and leap to wild conclusions.

The magma chambers (size, location, compositon, etc.) involved in the past eruptions of Vesuvius is a pretty hot topic. Classic ideas favoured the model of a principle magma chamber whereby there is a compositionally stratified phonolitic upper part and a convecting phonotephritic to tephriphonolitic lower part. In this scenario the plinian-style magma is wholly erupted from this chamber alone (e.g. the white Pompei pumice from the upper half and the grey from the lower).

While a number of studies indicate the Avellino, Pompei and 472 A.D magmas were located in the carbonate hosted system (shallow chamber), they also showed evidence for crystallization and storage at greater depths. Indeed, the ‘grey magmas’ erupted during the Avellino and Pompei eruptions are a mixture between the shallow magma, a deeper tephritic magma (never erupted unmixed) and a cumulate end member.

I know a few workers believe there to be a polybaric system under Vesuvius whereby magma differentiates at great depths (> 10km) before it forms shallow chambers in the carbonate bedrock/even shallower. I know Cioni favours a polybaric system at least for the 1944 effusive volcanism (doi: 10.1016/j.jvolgeores.2008.04.024). In this scenario you have two principle chambers rather than one (the lower of which is probably a sill network – the upper of which is confirmed seismically). This seems to be further suggested by the lack of crystallization at intermediate depths and the features of the grey pumice. During the plinian eruptions the shallow phonolitic system is tapped before the deeper system. In effusive scenarios the shallow system resides at higher depths (~ 2km b.s.l.). However, during both scenarios the shallow system is perturbed by regular mafic batches. Work even suggests the incoming batches changed the style of volcanism during 1944!

While the shallow magma chamber may have migrated upwards through time the numerous subplinian eruptions between Avellino, Pompeii and since suggest there is no simple relationship between chamber depth and explosivity/risk. The upper chamber remained in the same position between the Avellino and Pompei eruptions yet only produced minor volcanism during the interplinian activity. What is crucial is the composition and mobility of the upper chamber – plus the obvious, its size! Regardless, the 1944 activity would suggest there has not been a large enough repose for such a Pompei-sized eruption.

Ideas about Vesuvius have changed dramatically over the past few years … it is an exciting time.

2. PE - 18 September 2008

In a layman’s opinion, I think there is another scenario which could be taken into account….

The shallower the magma chamber, the more it degasses, the less pressure builds, and the eruptions become less and less explosive. OK.

But, as the magma chamber moves closer to the surface, it could mean that the overlying rock layers, which supports the mountain, could become more and more thin, and therefore more fragile.

So fragile, that maybe it won’t be able to withstand the weight of the volcano. In my mind,that only means one thing… caldera formation.The cone would collapse or fracture under its own weight and leave the magma chamber wide open.
This could mean quite a big pyrotechnic show…

Sorry for my bad english, hope it’s understandable…

3. PE - 18 September 2008

I really don’t understand much about this volcano…

It seems like, this volcano is the only one of its’kind… It erupts phonolite and tephrite, as Magmatist said, but I don’t really know much about these kinds of magma…

Given the past behavior of Vesuvius, we could imagine that those lavas are quite fluid and hot, like basaltic lava (as Vesuvius usually produces lava flows hich can reach quite far..).

I’ve never heard of any type of dome growth on the Vesuvius, so it’s Plinian eruption seem not to be triggered by viscous magma.. It could be more like on Etna, Llaima or Mayon volcanoes, which are all basaltic type.

In their case, subplinian eruptions seem to be linked with large pockets of gas-rich basalt which undergo explosive decompression when they reach the surface, giving birth to what is basically lava-fountaining on steroids…

I know I could be completely wrong..

4. magmatist - 19 September 2008

Caldera formation was associated with the four largest plinian eruptions – Base, Mercato, Avellino and Pompei. The large Monte Somme Caldera Rim which Mt Vesuvius nests (in fact Mt Vesuvius – the stratovolcano – was only constructed after Pompei). Thats why its known in the literature as Somma-Vesuvius. No caldera formation has been associated with shallower chambers – most of which are probably a dyke type structure, hence the small volumes.

Tephrite evolves through differentiation to phonolite, in the same way as basalt can do to rhyolite. The style of eruption does occur in cycles though, switching between effusive (think Stromboli erupting tephritic magmas) to plinian-type (think Mt St Helens erupting phonolites). If the two main reservoir theory is correct this can be explained by how many conduits (connecting the parts of the system) are open and how long the magma has had to evolve.

Something I just noticed in this paper is they only analysed the plinian-type eruptions prior to the 1631 cycle. There were also numerous interplinian eruptions, which emitted more mafic compositions in effusive/Strombolian to Vulcanian type scenarios! The only non-plinian-type eruptions they have is 1906 and 1944. This means that, for example between Mercato to Pompei (for around 9,000 years) the magma chamber didnt move higher, yet the eruptive style switched numerous times between a scale larger than Pompei to smaller than the 1631 eruption! Yet, you wouldnt know this is you read their paper .. you would think deeper chamber, more explosive! Effusive more mafic magmas have been common in between most of the major plinian eruptions… even the smaller plinian eruptions.

eg.
Mercato, large plinian phonolitic event
Avellino, large plinian phonolitic to tephriphonolitic event
AP eruptions, very small subplinian phonolitic to phonotephritic events
Pompei, large plinian phonolitic to tephriphonolitic event
post-pompei, more mafic interplinian (e.g. tephritic to phonotephritic) effusive
472 AD. small subplinian phonolitic
post 472 A.D., more mafic interplinian (e.g. tephritic to phonotephritic) effusive
1631, semi-mafic subplinian phonotephritic
1631 to 1944, effusive tephriphonolitic to tephritic

It would be interesting to see at what depth the smaller interplinian eruptions were sourced from (e.g. where the very small AP eruptions sourced from the same reservoir that generated the devestating Mercato, Avellino and Pompei eruptions?…. if so, a deep chamber does not mean big eruptions!).


Sorry comments are closed for this entry