Eruptions

First off, I want to thank Dr. Behncke for taking the time to answer your questions – and also, thank you to all who sent him some thought-provoking questions. In fact, the questions and answers take up about 12 pages of text, so the Q&A will be divided into two parts.

If you want to see one of the previous in the Q&A series, check out:
Dr. Jonathan Castro.

Question and Answer with Dr. Boris Behncke of the Italian National Institute of Geophysics and Volcanology in Catania.


Dr. Boris Behncke.

Questions for Dr. Boris Behncke

(Bernard Duyck) Qu’en est-il de l’évolution du mécanisme éruptif de l’Etna d’un volcanisme de point chaud vers celui de subduction ?”
(What’s new over the evolution of Etna from a volcanism of a hot point to a volcanism of subduction?)

BB: This refers to a quite provocative publication (Schiano et al. 2001) of Etna changing from a hot-spot to a subduction type volcano and thus becoming more explosive There hasn’t been any further research into this area to my knowledge – but certainly Etna has demonstrated that it has more explosive potential than was believed previously, both in the form of new eruptions – in particular, the very ash-rich 2002-2003 eruption – and in evidence coming from research on Etna’s eruptive history. We thus know that explosive volcanism is quite common (Coltelli et al., 1998, 2000, 2005) during the past 100,000 years, which is pretty much the period during which Etna grew into the large mountain it is now (for an updated geological history of Etna, see Branca and Del Carlo, 2004). As it seems, the current consensus on why Etna is there is based on another 2001 publication (Doglioni et al. 2001), which places the volcano in a context of extensional tectonics between two different (oceanic and continental) lithospheric domains in the collision zone between the Eurasian and African plates. To the east, in the Ionian Sea, oceanic lithosphere making up the northern margin of the African plate is subducted beneath the southern margin of the Eurasian plate (the Calabrian Arc), the volcanoes of the Aeolian Islands being the result of the subduction-related melting processes. To the west, on the island of Sicily, the northern African margin is characterized by continental lithosphere colliding with continental Eurasian lithosphere, and the convergence process is slower than in the subduction setting to the east. So it can be said, the convergence occurs at two different speeds, and the two domains are separated by a major system of tectonic structures, which are also seismically active (producing, among others, the major earthquakes of 1693 in southeast Sicily and 1908 in the Messina strait area). Movement at these structures is believed to have an extensional (rifting) component that opens what Doglioni et al. (2001) call a “mantle window”, and which is held responsible for significant decompression in the underlying mantle, generating magma – in fact, THE magma that feeds Etna. So to my knowledge that’s the currently preferred hypothesis concerning the question why Etna is there, and so Etna would actually be a type of volcano unlike all others, which certainly fits with its extremely complicated and versatile dynamics.
References:
Branca, S., Coltelli, M., Groppelli, G. (2004) Geological evolution of Etna volcano. In: Bonaccorso, A., Calvari, S., Coltelli, M., Del Negro, C., Falsaperla, S. (eds). Mt Etna Volcano Laboratory. AGU Geophysical Monograph Series, 143: 49-63.
Coltelli, M., Del Carlo, P., Vezzoli, L. (1998) Discovery of a Plinian basaltic eruption of Roman age at Etna volcano, Italy. Geology, 26: 1095-1098.
Coltelli, M., Del Carlo, P., Vezzoli, L. (2000) Stratigraphic constraints for explosive activity in the past 100 ka at Etna Volcano, Italy. International Journal of Earth Sciences, 89: 665-677.
Coltelli, M., Del Carlo, P., Pompilio, M., Vezzoli, L. (2005) Explosive eruption of a picrite: the 3930 BP subplinian eruption of Etna volcano (Italy), Geophysical Research Letters, 32, L23307, doi:10.1019/2005GL024271R.
Doglioni C., Innocenti F. & Mariotti S. (2001): Why Mt. Etna? Terra Nova, 13: 25-31.
Schiano, P., Clocchiatti, R., Ottolini, L., Busà, T. (2001) Transition of Mount Etna lavas from a mantle-plume to an island-arc magmatic source. Nature, 412: 900-904.

(Mike Don) 1. I saw a recent news item that there seems to be magma present under the entire area covering the Bay of Naples and its environs. Is this verified, and does that mean that the three ‘historic’ volcanoes (Vesuvius, Campi Flegrei and Ischia) are now thought of as semi-independent centers in a single volcanic system?
2. As a corollary, is the historic fact that Ischia and Campi Flegrei have apparently tended to erupt during long repose periods at Vesuvius (eg Arso and Monte Nuovo) anything more than coincidence?
3. What is the accepted explanation for Vesuvius odd (silica-poor, leucite-bearing) magma? Has interaction between (already alkalic) magma and the limestone/dolomite country rocks underlying the region got anything to do with it? Are Vesuvius’ eruptions associated with unusually large volumes of CO2?

BB: Answer to (1) and (2): The question of how much the different Italian volcanic areas are linked to each other is a popular and intriguing one. In the case of the Neapolitan volcanoes, it seems that in many senses they are independent, each having its own peculiar repertoire of chemical compositions, eruptive behavior, and type of edifice. But it is true, when one looks at the historical record, it may appear that Campi Flegrei is more active when Vesuvius is in repose, and the unrest at the earlier in recent decades falls into the current, quite long, repose period at the latter. Similarly, the latest eruptive events in the Campi Flegrei – a hydrothermal explosion at La Solfatara in 1198 and the small Monte Nuovo eruption in 1538 – coincided with the ~500 years-long repose period of Vesuvius prior to its catastrophic 1631 eruption. However, the historical record is too short to be certain that this happens as a rule, and at other times the different volcanoes appear to have erupted contemporaneously. So I fear much further research – and time – will be required to better understand how much or little connection there is between these volcanoes.
It should be noted, however, that the clustering of seismic and volcanic events in relatively small areas are now beginning to receive more attention – such as the sequence of earthquakes and eruptions in Sicily in the fall of 2002, which started with an earthquake in Palermo on 6 September, and was followed by the large Etna eruption on 27 October, a submarine hydrothermal explosion near Panarea in the Aeolian Islands on 5 November, and finally by a major eruption at Stromboli on 28 December. A recent publication (Walter et al., 2009) suggests that these events are indeed linked: the Palermo earthquake induced significant stress changes that affected the three volcanic systems, which were already in a “critical state” and would possibly have produced increased activity anyway but maybe at a later time.
Answer to (3): Vesuvius is maybe the archetype of a volcano whose magmas supposedly show significant interaction with host rocks – a concept proposed already early in the 20th century (Rittmann, 1933). This concept has been more or less rejected by some scientists (Savelli, 1967-1968) and essentially accepted by others (Marziano et al., 2008). I think a pretty good overview is presented by Peccerillo (2005), who seems to broadly accept the assimilation-of-crustal-rocks hypothesis.
I don’t know how much the CO2 emissions of Vesuvius are known – I guess one difficulty lies in the fact that the volcano last erupted long before methods for the measurement of such emissions were developed. The only CO2 related study about Vesuvius I have come across is concerned with soil CO2 emission rather than with eruptive CO2 emission.
References:
Marziano, G.I., Gaillard, F., Pichavant, M. (2008) Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes. Contributions to Mineralogy and Petrology, 155: 719-738.
Peccerillo, A. (2005) Plio-Quaternary Volcanism in Italy: Petrology, Geochemistry, Geodynamics. Springer, Berlin Heidelberg New York (Chapter 6: The Campania Province, Pontine Islands and Mount Vulture, pp. 129-171.
Rittmann, A. (1933) Die geologisch bedingte Evolution und Differentiation des Somma-Vesuvmagmas. Zeitschrift für Vulkanologie, 15: 8-94.
Savelli, W. (1967-1968) The problem of rock assimilation by Somma-Vesuvius magma. Part I: Composition of Somma and Vesuvius lavas. Contributions to Mineralogy and Petrology, 16: 328-353; Part II: Composition of sedimentary rocks and carbonate ejecta from the Vesuvius Area. Contributions to Mineralogy and Petrology, 18: 43-64.
Walter, T.R., Wang, R., Acocella, V., Neri, M., Grosser, H., Zschau, J. (2009) Simultaneous magma and gas eruptions at three volcanoes in southern Italy: An earthquake trigger? Geology, 37: 251-254.

(Aldo Piombino) Last year I wrote a post on my blog about Mount Marsili, the giant volcano deep in the Thyrrenian sea. (http://aldopiombino.blogspot.com/2008/04/il-monte-marsili-un-gigantesco-vulcano.html). Why this volcano is so poorly known and what do you think about his history? Why this volcano is completely ignored by the INGV?
BB: Well, it is not exactly true that Marsili is ignored by the INGV, although I agree that it has so far received relatively little attention. There are three main publications discussing different aspects of this volcano, one on its volcanic and petrological evolution by Trua et al. (2002), one on its presumed hydrothermal activity (Uchupi and Ballard, 1989), and – most recently – a report on seismic studies carried out by the INGV in 2006 (D’Alessandro et al., 2009). From this it appears that the volcano is active, if not erupting. However, the INGV’s main mission is to deal with volcanic hazards and volcano surveillance for the sake of Civil Defence, which grants much of the funding for the institute, and Marsili is not to be considered among the really, really hazardous volcanoes in Italy, considering that we’ve got to deal with pretty monstrous examples such as Vesuvius, Campi Flegrei, Vulcano, and possibly even the Colli Albani. This is why Marsili is not given much priority, although I believe all of us find it a pretty intriguing object of study.
References:
D’Alessandro, A., D’Anna, G., Luzio, D., Mangano, G. (2009) The INGV’s new OBS/H: Analysis of the signals recorded at the Marsili submarine volcano. Journal of Volcanology and Geothermal Research, 183: 17-29.
Trua, T., Serri, G., Marani, M., Renzulli, A., Gamberi, F. (2002) Volcanological and petrological evolution of Marsili Seamount (southern Tyrrhenian Sea). Journal of Volcanology and Geothermal Research, 114: 441-464.
Uchupi, E., Ballard, R.D. (1989) Evidence of hydrothermal activity on Marsili Seamount, Tyrrhenian Basin. Deep Sea Research Part A. Oceanographic Research Papers, 36: 1443-1448.

(Damon Hynes) 1. Question about one difference in the locations of eruptions from Etna and Piton de la Fournaise: Both volcanoes erupt basic lavas, and both have had sector collapses. However, Fournaise’s historic eruptions have been limited to a sector roughly bounded by the two remparts to the north and south. But eruptions have occurred on every radii from Etna’s summit, and there have been eccentric eruptions on Etna that Fournaise hasn’t experienced. Looking at the question the other way, Valle Del Bove doesn’t seem to exert the same control over eruptive locations that the two Fournaise remparts do. In Hawai’i, when the rift zones get pinched off as the volcanic pile gets ‘squeezed’ by the next volcanoes in the chain, the eruptions move to either summit, subterminal or circumferential locations (recent example is Mauna Kea). Etna, erupting through the similar mass of Sicily, still has radial eruptions and the odd eccentric one. Reunion, with a smaller mass, would in my opinion, lead to a similar ‘scatter’ of eruption sites. Are the eruption-site locations just a function of Fournaise’s smaller size compared to Etna, or there there other geologic controls / stress fields on Etna that aren’t evident from observations of topography?
2. It seems to me that Vesuvius has entered a quiet state similar to the period between ~1139 to 1631. If there had been eruptions in the interim, it appears that by the descriptions they were small and probably phreatic. Predicting phreatic eruptions using methods useful for magmatic eruptions has an uneven track record, and precursors for a magmatic eruption on the scale of 1631 just aren’t there, in my view. Since 1944, Vesuvius has given the impression of a rapidly cooled, quickly compacted pile of rock that has reached equilibrium except for the odd fumerole.
I’m not intending that geological research and volcano monitoring can turn its back on Vesuvius until the year 2400 (!) but it would seem to me that the historical record of precursors leading up to the 79 eruption (roughly 10-15 years of earthquakes) would allow plenty of time to revisit evacuation and refugee plans rather then the sort of “Decade Volcano” labelling that Vesuvius has received.

BB: Answer to (1): Yes, Etna seems to have more of those “eccentric” eruptions than other, broadly similar volcanoes (from their structural framework) such as Piton de la Fournaise and the Hawaiian volcanoes. This may be due to the fact that Etna, unlike those other volcanoes, sits on continental lithosphere, which always renders things a bit more complicated. Secondly, it is fed by what is believed to be a quite extensive magma source (below the base of the lithosphere), and magma sometimes rather than rising through the central conduit seems to follow tectonic lines of weakness and pops up somewhere on the sides of the volcano, or even at its base, in what we call “eccentric” eruptions. But let’s face it, by far the majority of Etna’s recent eruptions have followed very much the same pattern as those of Piton de la Fournaise, in that they are concentrated along the two main trends (northeast and south-southeast). And then note that Fournaise has occasionally produced eruptions not only outside the caldera (1977, 1986, 1998), but also on a third trend that goes westward from the summit, and a number of young pyroclastic cones lie on the slopes outside the caldera to the north and south. So the historical record can be very misleading about the potential of a volcano to erupt in areas that have not shown activity since human observations are available. In any case, the structural setting of any of these volcanoes and the control of adjacent volcanic edifices seems to be a very important factor in determining the distribution of flank vents.
Answer to (2): Uhh, here we’re touching very sensitive territory, though this is one of the most intriguing and challenging issues in modern volcanology.
Vesuvius might well be in a repose period that could turn out to last for centuries, this is something it has done repeatedly in its lifetime, the about 800 years of quiescence preceding the AD 79 Pompei and the 500 years of calm before 1631 being the most recent examples. That means, people currently living at Vesuvius, volcanologists working on it, and authorities and Civil Defence staff responsible for emergency planning will likely not see Vesuvius erupt. This is a good thing on one side, but obviously, if there will be no eruption for generations to come, how will people living in the area feel in, like 200 years? There will have been centuries of talking about the risk of Vesuvius’s next eruption, and none will have occurred. Maybe people will have gotten a grip on these things in the meantime and simply be ready (or try to be) once the volcano starts stirring back to life.
But today’s reality shows that there are really two challenges to face. One is the volcano and its behavior. The current emergency plan for the Vesuvius area is based on the assumption that clear warning signs of an imminent eruption will be plainly available at least two weeks before the start of the eruption. If we look at the known history of Vesuvius, it seems plausible that there will be warning signs, maybe weeks before an eruption. So the area is evacuated (which in itself is a challenge for imagination alone) successfully, can we be certain that the volcano will erupt on schedule? What if it behaves like Redoubt in Alaska at the beginning of this year? Remember Redoubt seemed to be set for eruption in late January, when it essentially gave the same signs it provided about 24 hours before its previous eruption in 1989. But this time it did not erupt 24 hours but two months later. In Alaska that was not too much of a problem, no one had to be evacuated. But if you evacuate more than half a million people from an economically and culturally significant area in Italy, I doubt you can keep them away from their homes and their work and their everyday life for two months without going into a serious economic and political crisis. And here’s the second challenge, it’s the people, many of whom might actually be reluctant to leave (as were quite a few in Chaitén, Chile, even when the volcano in their backyard was producing pyroclastic flows to within a couple of kilometers of their town, itself partly devastated by mudflows). And then, finally, it’s back to the volcano – can we really be sure that Vesuvius will ALWAYS give clear warning signs early enough to allow (or justify) the evacuation of half a million people? Chaitén (again) teaches that silicic magma can rise to the surface surprisingly fast. I would not really like to see something like this happen at Vesuvius, or at Vulcano, or Lipari, which by the way is rhyolitic like Chaitén.
So I fear volcanology and related sciences have still a long, long way to go – if ever it will be possible to produce watertight eruption forecasts or predictions and carry out evacuations smoothly without causing too much distress. And, as far as the current emergency plan for Vesuvius is concerned, there’s an interesting article that is in the press on JVGR:
References:
Rolandi, G. (2009) Volcanic hazard at Vesuvius: An analysis for the revision of the current emergency plan. Journal of Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2009.08.007

(Robert Fowler) If the top of the volcano is removed, would there be an opportunity to generate power from lower heat sources? (N.B. from EK: I think he was trying to imply that by removing the tops of volcanoes, we reduce the pressure, thus “stopping” eruptions. The next step would be to look at how to exploit the “stopped” volcanoes.)
BB: I fear that with the current knowledge of volcanic systems and the presently available technology I’d rather refrain from trying to do something like this, at least if you were intending active volcanoes. By the way, reducing pressure from a volcano would rather facilitate eruptions – decompression causes expansion of gas in magma and makes it rise and foam (or explode). I think there’s no way to stop a volcano from erupting, because it’s something way too big and powerful.
But if we rather talk about dormant or extinct volcanic systems, I’d rather be concerned about the environmental impact of cutting off a part of a volcano – they are important and often beautiful landmarks that merit protection. In the Eifel volcanic field in Germany, not far from where I grew up, a number of Quaternary scoria cones have been nearly entirely removed due to quarrying of the volcanic material, and while such activity has provided precious insights into the inner structure of such volcanic features (such as the discovery of very frequent phreatomagmatic phases during basaltic, scoria-cone building eruptions), it has also destroyed some of the natural landscape.
But then there are quite a lot of volcanic systems where hot rock is not far below the surface, such as in Iceland and New Zealand, to name just two, and where geothermal energy is generated. I don’t know whether it would be worth the effort of removing (and destroying) large volcanic edifices – which still would be some sort of scratching the surface – in order to get closer to their presumably hot cores. I think where there’s hot rock relatively close to the surface, localized drilling can do the job equally.

Part 2 will arrive later this week!

Comments

  1. #1 doug
    November 25, 2009

    Wow! Great questions and fascinating answers. Really made my arm chair volcanologist’s day. You should see if you can get on the National Geographic Lecture circuit. I’d pay to see the whole Etna story told in person.

  2. #2 The Bobs
    November 25, 2009

    A significant eruption of Karkar volcano in PNG. So far only reported on John Seach’s site. link

  3. #3 aldo piombino
    November 25, 2009

    thank you for your kindly answer(s) (and time you have dedicated us).
    1. Clearly, I am not an expert like you and the INGV team, but I believe that Marsili is potentially a tsunami-trigger volcano (almost teorically)

    2. You have remembered the 2002/2003 little volcano – tectonic crises (with a focal point in the Tyrrenian sea and a second main one just in the Etna area!), at the end of wich also the Molise and Algeria quakes occurred. Your point of view is really interesting: at these times I have said that while Etna magma was not related to the quake crisis, the eruption was a consequence of the stress field modifications.

    3. INGV wrote about the Etna breathe, with phases of magma chamber filling wich trigger an uplifting of the volcano and phases in wich magma is only get up during wich the building subsides. Can be these conditions related to the much or less explosivity index of the magma?

    4. And, finally, what do you think about the teory of a double magma chamber for Vesuvius, in wich a plinian eruprion il linked to the formation of a new, upper magma chamber that feeds the varipus eruption of a single magmatic cicle?
    thank you

  4. #4 mike don
    November 25, 2009

    Many many thanks to Dr Behncke for patiently answering what were perhaps rather naive questions of mine. It’s a pity that there has been no opportunity to measure CO2 at Vesuvius; I’d hoped that might settle the assimilation-of-country-rock hypothesis: assimilation of carbonate rocks (limestone and dolomite) would surely make eruptions CO2-rich. And Perret’s account of the 1906 eruption vividly described a phase of continuous ‘blowing off’ at the climax which he stressed was not an explosion but a steady jet of gas (composition unknown, it wasn’t something you could stick a sampling flask into (!))

    The trouble with these answers -not that I’m complaining- is that they provoke even more questions. As Aldo has already discovered. For instance, there is a report that Etna’s latest lavas contain hornblends, for the first time in a long time; obviously significant, but significant of what?

    But once again, thanks Boris, and thanks Erik, for giving us volcanophiles something to get our metaphorical teeth into

  5. #5 Jimmy
    November 26, 2009

    Absolutely, thankyou Dr. Behncke, Dr. Klemetti and the questioners! Fascinating area the Thyrrenian Basin and environs and always of interest to me.

  6. #6 Diane
    November 26, 2009

    Thank you for such thought provoking answers. I will have to read them again to really get the gist of what is going on because I don’t know very much even though I did take geology. That was 30 years ago.

    I think Doug has the right idea about a program with NG or a lecture series on Etna and the whole area around Sicily. Diagrams help me, too.

    Thanks again and I look forward to the rest of the Q&As.

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