Volcanism in the Neapolitan area

The Neapolitan volcanoes are located within the Campanian Plain which is bordered by the Southern Apennines (Fig. 1) and they are mostly related to Quaternary extensional tectonic phases. The Apennines mountain chain is made up of Mesozoic carbonate and Mio-Pliocene terrigenous sequences, overlain by Quaternary continental deposits, including volcanic sediments. The Campanian Plain, in which lie the active Neapolitan volcanoes, made up of a sequence of Plio-Quaternary sediments, mostly continental and subordinately marine, intercalated with volcanic deposits. It is underlain by a graben formed during activation of NW-SE and NE-SW trending normal faults which, at least during Quaternary times, have downthrown the western Apennines. The regional stress regime, leading to the formation of the plain, has also favored the generation and ascent of the magmas that have fed recent and active alkaline volcanism. The active volcanoes of the Neapolitan area, within the Campanian Plain, are the Campi Flegrei (Phlegraean Fields, literally ‘burning fields’) and the Somma-Vesuvio. A third active volcanic field is the island of Ischia, located 20 km to the south-west of the city of Napoli, at the north-western corner of the gulf of Napoli. Between Ischia and the Campi Flegrei is the island of Procida, another volcano whose last eruption occurred about 18 ka bp. The majority of the volcanic rocks in the continental portion of the Neapolitan area have been generated by Campi Flegrei and Somma-Vesuvio volcanic systems, only very few by the Ischia volcano. Deposits related to the long history of human inhabitation in the area are also widespread.

Fig. 1. Geological sketch map of the Campanian Plain (from Vitale and Isaia, 2014).

The city of Napoli partially lies, with its western sector, within the still active and restless Campi Flegrei caldera, while its eastern periphery is at the foot of the western slopes of Somma-Vesuvio. The two volcanoes, separated by the Sebeto alluvial plain, are very different. The Somma-Vesuvio is a strato-cone 1,281 m high and visible from all the Campanian area, while the Campi Flegrei is a more complex caldera system, composed of many hills that are either remnants of volcanoes predating the main caldera collapses or younger monogenetic volcanoes.

Vesuvio is a young volcanic cone that has built up within the caldera of an older edifice, the Mt. Somma developed mostly in the last 20 ka.

Fig. 2. The inhabited area around Vesuvio volcano in a 3D perspective view from West; DTM overlaid with digital color orthophoto (Laboratory of Geomatics and Cartography, INGV-OV)

 

The Somma-Vesuvio volcanic history is characterized by long periods of repose/inactivity with obstructed closed conduit, interrupted by violent explosive eruptions of either Plinian (Volcanic Explosivity Index, VEI=5) or Sub-Plinian (VEI=4) type. Four Plinian eruptions occurred between 18.3 ka and the year 79 AD (the famous Pompei eruption described by Pliny the Younger) (Fig. 2) and each produced a collapse that modified the dimensions and shape of the Mt. Somma summit caldera. Plinian eruptions were preceded by repose periods lasting from several centuries up to nearly one thousand years (Cioni et al., 2008 and reference therein); shorter but still very long were the repose periods preceding the Sub-Plinian events (about 500 years before the last of these eruptions occurred in 1631). Two Sub-Plinian eruptions occurred in historical time, after the Plinian Pompei event of 79 AD: the so-called Pollena eruption in 472 AD and that of 1631. Plinian and Sub-Plinian events were characterized by the same eruptive and syn-eruptive phenomena but differ for both volume of the emitted magma and energy of the eruptions, that are lower for the Sub-Plinian. . Both kinds of eruptions emitted evolved potassic tephro-phonolitic or phonolitic magmas from compositionally zoned shallow magma chambers (Cioni et al., 1998 and references therein). In all Plinian and Sub-Plinian events the eruption begins with a phreatic or phreatomagmatic vent opening, followed by a sustained eruptive column with widespread downwind ash fallout (mostly toward the eastern sector) and then by a column collapse phase generating pyroclastic flows. Dangerous lahars, with flooding in some morphological lows, are generated by rain mobilization of loose ashes on the steep slopes of the cone and the Apenninic relieves.

After the occurrence of a major Plinian or Sub-Plinian eruption, the volcano usually begins a new phase of activity. This is characterized by an open conduit, high frequency of eruptions of basic magmas producing lava flows with associated subordinate explosive events (VEI up to 3), some of which generated by explosive magma-water interaction (Cioni et al., 2008 and references therein). After the last Sub-Plinian eruption of 1631, Vesuvio underwent such an open-conduit phase that lasted until 1944. Since then the volcano has been in a new phase of quiescence.

Fig 3. Chronogram of Somma-Vesuvio activity. Arrows refer to explosive eruptions, length and colour reflect the estimated VEI. Blue boxes show recorded or inferred periods of persistent mild Strombolian and effusive activity, marked by VEI 2-3 explosive eruptions. Orange-dashed arrows mark eruptions of uncertain source. Breaks in the chronogram mark changes of time-scale (modified from Cioni et al., 2008).

The Campi Flegrei volcanic system was active from more than 80 ka, and is currently characterised by an irregular landscape including different volcanic landforms, plains, lakes, and coastline (Fig.4).

Fig. 4. A 3D perspective view from SW of the Campi Flegrei caldera (Lab. Cartography, INGV-OV)

 

The main volcanological feature is a caldera structure formed during the two main events of the Campanian Ignimbrite (e.g. Fisher et al., 1993; Rosi et al., 1996; Giaccio et al., 2008; Costa et al., 2012) and Neapolitan Yellow Tuff eruptions (Orsi et al., 1992; Scarpati et al., 1993) occurring at 40 and 15 ka, respectively. During the last 15 ka, within the caldera several volcanic edifices were built up and destroyed as the result of about 70 eruptions. Volcanism was mainly concentrated in discrete periods which alternate quiescence periods of variable lengths (Di Vito et al., 1999; Isaia et al., 2009). Eruptions often occurred in short time intervals, varying from a few decades to centuries. After about 3,400 years of rest the last eruption of Monte Nuovo occurred in 1538 AD (Fig. 5).

Fig. 5. Schematic time log showing main deformation and volcanic events occurring in Campi Flegrei from 80 ka to the present (from Vitale and Isaia, 2014)

 

The volcanic activity at the Campi Flegrei was mainly explosive with phreatomagmatic and magmatic eruptions while only few effusive events were recorded. During the last 15 ka, only two high-magnitude eruptions occurred, while medium and low magnitude events were predominant. The magnitude variability of the events are testified by the differences in deposits areal distribution and volume of emitted magma, which only during the high magnitude eruptions exceeded 1 km3 (Orsi et al., 2004; Smith et al, 2011).

The sites of the eruption vents have changed over time, with the more recent one largely in the central-eastern sector of the caldera. Contemporaneous eruptions within the two different sectors were also recorded (Isaia et al., 2009; Pistolesi et al., 2016). The emitted magmas were trachytic in composition except for a few latitic and trachybasaltic products (e.g. D’Antonio et al., 1999). The eruptions were generally dominated by phreatomagmatic explosions alternated with magmatic phases and minor Strombolian events, which represented as the ending phase of more powerful eruptions. Pyroclastic density currents were generated during the phreatomagmatic events and formed deposits whose areal distribution was related to the eruption magnitude and vent location. Tephra fallout followed the magmatic phases, dispersed products mostly downwind toward the East, from Plinian and sub-Plinian columns, and in variable directions, though prevailing northeast, from low eruptive columns.

The caldera was affected by ground deformation phenomena that led to a total uplift of about 100 m in its central part, during the last 12 ka. Recent slow ground movement events, called bradyseisms, have occurred periodically. The significant unrest crises of 1970-72 and 1982-84, accompanied by hundreds of earthquakes and 3.5 m of ground uplift, forced the inhabitants of Pozzuoli to be evacuated. An ongoing unrest phase, mostly involving the central sector of the caldera, has prompted the Civil Protection to move the Campi Flegrei volcano from base level (green) to warning (yellow) alert levels from the end of 2012.

 

Volcanic hazard and Risk in the Neapolitan area

The active volcanoes in the Neapolitan area may produce effusive and explosive eruptions, the latter being associated with higher hazard. The most severe risk in the foreseeable future arises from pyroclastic flows associated with a sub-Plinian eruption involving the ejection of a large ash column with partial collapses. Pyroclastic flows are considered so destructive that the only countermeasure is a complete evacuation of the risk zone at the very onset of the volcanic crisis and before the eruptive phase occurs. In addition, ash fallout may cause the collapse of the roofs in the urbanized area around the volcanoes. At this latitude, stratospheric winds mostly blow eastward. This considerably reduces the hazard of ash fallout in Napoli in the event of an explosive eruption of Vesuvio, this volcano being located in the eastern sector of the city. However, Napoli is highly exposed to ash fallout produced by an eruption of Campi Flegrei, located in the western sector.

Several stratigraphic studies of the deposits of Vesuvio and Campi Flegrei, have enabled estimating the occurrence probability of the different sized eruptions, ranging from lava flows to sub-Plinian and Plinian eruptions. It is however worth noting that the monogenic nature of the past Campi Flegrei eruptive vents, introduces additional uncertainty in the location of the future eruptive vent inside the Campi Flegrei caldera.

This knowledge, coupled with numerical simulations of different eruptive scenarios, formed the basis for the hazard maps from pyroclastic flows, lahars and ash fallout of the Neapolitan area. It was adopted by the Civil Protection in Italy in order to define the emergency plan, the hazard zones and the appropriate countermeasures.

The vulnerability of the Neapolitan area is another important parameter making the volcanic risk extremely high. Moreover, the vulnerability of the buildings increases with the earthquakes and ground deformations associated with the volcanic eruptions. In the Neapolitan area, there is an extraordinary range of building types that have been developed over many centuries. These have generally made use of the volcanic rocks and soils, and adopted forms that are suited to the climate, the economic conditions and the available building technology of the period. The older buildings have undergone a process of continuous adaptation to changing economic and social needs. In the second half of the 20th century, the Neapolitan area experienced a period of rapid industrial development and a substantial increase in the number of buildings constructed on the volcanic areas. In particular, until the beginning of the last century, most of the buildings were no more than three storeys high and built using rubble stone masonry for the walls. Later, there was a gradual decline in rubble stone masonry in favour of cut-stone masonry and, eventually, reinforced concrete, but building heights did not increase. However, in the second half of the last century, the dramatic increase in population created a huge surge in building, mainly in apartment blocks, up to 10 storeys in height. Clearly, this also augmented the vulnerability to earthquakes and, in general, volcanic risk in the area. Recent studies, based on stratigraphic data from past volcanic events, numerical simulations of pyroclastic flows and ash fallout, and analysis of the vulnerability of the different types of building, have allowed compiling detailed maps for the volcanic and seismic impact in the Neapolitan area.

Due to the uncertainty of the expected events and the high population density in the Neapolitan area, the hazard and risk maps and the Emergency Plans are extremely critical. Continuous updates are needed to take account of the scientific progress and the evolution of cities. It was estimated that about 680,000 people living in the “red zone” of Vesuvio (25 municipalities of the Vesuvian area and 3 quarters of Napoli), the zone subject to hazard from pyroclastic flows and heavy fallout and 500,000 people living in the “red zone” of Campi Flegrei (7 municipalities and 11 quarters of Napoli), the zone subject to hazard from pyroclastic flows . Both these areas are at the highest risk and need to be evacuated in case of a volcanic crisis. In addition, about 3 million people live in the so-called “yellow zone”, subject to ash fallout that may cause roofs to collapse.

The National Emergency Plans for Vesuvio and Campi Flegrei have recently been updated by the Dipartimento della Protezione Civile in collaboration with the Campania Region, based on the recommendations of the scientific community. In particular for Campi Flegrei, the red zone, which must be evacuated in case of resumption of the eruptive activity, was redefined. Also the yellow zone, namely the area outside the red zone exposed to significant fallout of volcanic ash and pyroclastic materials, has now been delineated. The Vesuvio Emergency Plan is also continuously updated, since the first version dating back to 1995.

Fig. 6. Red zone included in the 2001 Campi Flegrei Emergency Plan (above), redefined red zone approved in the new 2015 Emergency Plan for Campi Flegrei (below; Legal measures of 24th Jun 2016: provisions to update the emergency plan for the Campi  Flegrei volcanic risk).

 

Figure 6 gives an example of the recent changes in the risk zoning at Campi Flegrei carried out by the Civil Protection authorities on the basis of the inputs received from the scientific community, particularly those produced by the “Eruptive scenarios and alert levels” working groups established in 2009 by the Dipartimento della Protezione Civile. The most relevant outcome is the significant widening of the red area with the inclusion of new municipalities and a larger sector of Napoli.

Alongside the activities for updating emergency plans, Civil Protection authorities together with the scientific community and local authorities have carried out risk mitigation initiatives with the involvement of the local community. Information campaigns and simulation exercises have been organized to educate and instruct citizens and test the organization and intervention models in order to improve emergency planning effectiveness. The Osservatorio Vesuviano, in collaboration with the Dipartimento della Protezione Civile promotes education initiatives to disseminate knowledge of volcanic risk and the emergency plans, as well as on behaviour guidelines during a crisis. Other training projects have been addressed to specific stakeholders who are directly involved in the development of civil protection plans and emergency management and who have the chance of becoming, during a future crisis, authoritative interlocutors for the citizens.

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