On the world scene of the most dangerous volcanoes, meaning those near inhabited areas, the Indonesian volcano Merapi is one of the most formidable. On October 26 2010 it exerted its force in the form of eruptions and the formation of burning clouds highly dangerous to the inhabitants. Unfortunately, there have been up until now about three hundred deaths and two hundred thousand people left homeless.
Merapi, whose name means mountain of fire, produces plinian and sub-plinian eruptive phenomena. This characteristic leads us to compare it to the historic activity of the Neapolitan Vesuvius. Both volcanoes have caused vast numbers of deaths, in the case of Merapi as a result of the population not following the evacuation order. Rescuers found themselves face to face with scenes in many ways similar to those to which the plaster casts of Pompeii so dramatically bear witness following the famous and terrible plinian eruption of the Vesuvius in 79 AD. From this point of view Merapi seems like an Asiatic Pompeii; many inhabitants were surprised in their sleep by the burning clouds and victims were even found in the village of Argomulyo, eleven miles from the volcano.
To understand the similarities better, we asked Professor Giuseppe Mastrolorenzo for his scientific contribution.
a) How would you describe the eruption of Merapi that began on October 26 2010?
It was a mixed type, effusive explosive-eruption characterised in the initial phases by the formation of a lava dome followed by an eruptive column of gas and ashes of a relatively modest height of between about half a mile and four and a half miles. A sequence of pyroclastic surges and flows was propagated on the flanks of the volcano reaching very high temperatures and speeds. This eruption typology is recurrent in the Merapi and other volcanoes fed by highly viscous dacitic magma where effusive activity (lava flows and domes) alternate with devastating clouds of gas and ash that are propagated radially in respect to the crater. An analogous eruption took place in 1902 with the Montagna Pelee on the island of Martique in the French Antilles, causing the total destruction of the city of Saint Pierre and the death off its 30,000 inhabitants.
b) Professor Mastrolorenzo, what geological affinites are there between the Merapi and the Vesuvius?
The Vesuvius and the Merapi are both strato-volcanoes formed of a thick succession of lava flows and pyroclastic deposits of ash and lapilli caused by the alternation in the course of thousands of explosive, effusive and mixed eruptions. These two volcanoes differ, however, in the composition of their magma: phonolitic and tephritic in the Vesuvius and rhyolitic and dacite in the Merapi. This last type, characterised by greater viscosity than the Vesuvian magmas, in some cases results in the formation of viscous lava flows that gently move to form lavic domes, while in other cases are highly explosive which is dangerous in the presence of even small quatities of magma.
c) Is the Merapi monitored in the same way as the Vesuvius?
The Merapi has a network that monitors seismic activity, ground deformation and magnetic anomalies that altogether make it possible to follow the precursory events of an eruption.
It should be emphasised, however, that the monitoring systems on the Merapi, as on the Vesuvius and other active volcanoes in the world, detect changes deep in the magmatic system but cannot give any information about the duration of the precursors nor about the type and size of the eruption. Therefore, in terms of mitigating risk, for a monitoring system to be useful it always needs to be supported by an adequate emergency plan. Obviously, it is also necessary, as soon as precursory events have taken place, for the competent authorities to rapidly decide on what course of action to take, and to evaluate the necessity or not of evacuating an area at risk previously defined according to scientific analysis.
d) Should the deaths recorded in this eruption be blamed on an undervaluation of the danger of the pyroclastic flows and surges?
There is no doubt that the undervaluation of the risks associated with the generation and movement of the pyroclastic clouds is amongst the basic factors that caused the disaster. Infact, although in the early days of the eruption evacuation had been predisposed, the area considered to be at risk was limited to about six miles from the eruptive centre, a decidedly optimistic evaluation of the potentially maximum limit of the propagation of the pyroclastic clouds. This decision was fatal for many, as the high temperature pyroclastic surge caused victims in a radius even beyond ten and a half miles from the volcano. This initial underevaluation made it necessary to carry out a desperate operation to modify the evacuation plan while the eruption was already taking place.
On this subject, I have been drawing attention for years in scientfic research carried out with other colleagues as well as in conferences and interviews given to the national and international mass media to the paradoxical dangerousness of the present Vesuvius emergency plan. I have underlined, infact, how the sub-plinian scenario adopted by the Civil Protection on the recommendation of the Commission for Great Risks is utterly inadequate in the event of a plinian eruption. Research carried out both by my own group and by others has shown the strong likelihood of a plinian eruption with its extreme dangerousness and would put at risk atleast three million people who live in a radius of about twelve miles from the volcano. The present emergency plan provides for the preventive evacuation of only 600,000 people including those who live in the red zone less than 6 miles from the volcano.
It is obvious that the present situation of the Vesuvius is very similar to what was tragically experienced in the recent eruption of the Merapi. Given, as revealed in parliamentary questioning, that the risk management system is incapable of providing an adequate emergency plan in the light of scientific evidence, it is essential that a revision of the entire risk management system for the Vesuvius area be carried out immediately.
A few months before the eruption of the Merapi, my research group published the results of a study of the eruption of Pompeii in 79AD and of the victims caused by it. We showed that the exposure to high temperatures and not suffocation, as had been previously erroneously, was the main cause of death in the Pompeiian population. Studying the Merapi victims from the photographic material available, it can be seen that their postures are identical to those of the Pompeii plaster casts, so much so as to make us consider the Merapi disaster to be a new Pompeii.
In October 2010, when the eruption of the Merapi had already begun, the Journal of Geophysical Research published a paper written by me together with the volcanologist Lucia Pappalardo of the Ossevatorio Vesuviano, on all the possible eruptive scenarios of the Somma Vesuvius. We set forth the results of the most advanced numerological simulations applied to volcanology, calling world attention to researchers and the authorities of the necessity of adopting the worse case scenario as the basis for an emergency plan which would be the only way to guarantee the survival of the population at risk in the event of an explosive volcanic event. We also showed how any other optimistic choice constitutes not only a limit to the efficacity of a preventive protective intervention but also a further cause of risk since it creates an unfounded perception of safety to the community.
d) How soon did the Indonesian authorities order the evacuation of the population living around the volcano?
In the case of the eruption of the Merapi, the first clear precursory phenomena occurred September 1, but the commencement of the eruption was observed, although in a limited way, on September 12. The evacuation order was given on September 25, more than 40 days after the effective beginning of the eruption and only a day before the beginning of the most intensive explosive phase. Such a long wait before giving the evacuation order did not cause a disaster on the Merapi only by good luck….whereas such a hazardous decision in the case of the eruption of the Vesuvius would result in a catastrophe since it would obviously be impossible to evacuate millions of inhabitants in the area at risk in a very short period of time.
e) The Merapi like the Vesuvius is on a short list of highly dangerous volcanoes. But what is the most dangerous volcano in the world?
Unfortunately, as I have already underlined in other circumstances, our volcanoes, the Vesuvius and the Phlegraean Fields compete for the title of most dangerous volcano in the world. This is obviously in relation to normal volcanic activity and the effects on the territory on a regional scale (millions of lives at risk). In relation to possible planetary catastrophes, events which are little talked about as they take place only every hundreds of thousands of years, without doubt the most dangerous volcano in the world is the Yellowstone caldera in the United States which 600,000 years ago erupted well over a hundred thousand cubic miles of magma in a few days with serious consequences for the climate on a global scale.
The figure on the left shows a diagram of the deep structure of the Somma-Vesuvius.
The results of a recent study (Lucia Pappalardo and Giuseppe Mastrolorenzo, Earth and Planetary Science Letters 296 2010, 133-143),
indicating the presence at a depth of 6 miles of an extensive phonolitic magma chamber (full of silica and gas) ready to feed any typology eruption not excluding a plinian eruption..
The figure on the right is the image of an electron microscope scan of bone from a victim of the eruption of Pompeii in 79AD. The micro fractures are caused by the heat from the pyroclastic surge.
(The editors of Hyde Park would like to thank Professor Giuseppe Mastrolorenzo, as always, for kindly giving us his time to clarify scientific questions.)
Translation: by Lisa Norall