Based mostly on the papers of Scandone et al (1991), and Lirer et al (1987)
Campi Flegrei (Phlegrean Fields) is a quaternary caldera located west of the city of Naples in an area of regional extension. The erupted products range in composition from K-basalts to alkali-trachyte, phonolite. The complex has been active since at least 47000 a bp, and it is surrounded by three other quaternary volcanic centers:
According to several authors, activity in Campi Flegrei itself has been dominated by two eruptions that produced widespread ash-flow deposits: the 'Campanian Ignimbrite' (CI) at about 34000 a bp, and the smaller 'Neapolitan Yellow Tuff' ( NYT) about 12000 years ago.
The area and its eruption products have been intensely studied since the XVIII century, and, as a consequence, many theories have been postulated about their origin. Breislak (1798) identified the CI and considered it to be the product of mud eruptions from centers scattered across the Campanian plain.
Scacchi (1848) firts considered that the CI was erupted from Campi Flegrei; later (1890) he changed his mind and approached the view of Breislak. De Lorenzo (1904) subdivided the activity of Campi Flegrei into three periods including the CI in the first, the NYT in the second, and the products of the recent activity inside the caldera in the third. Zambonini (1919) was the first to interpret the CI as a deposit related with an ash-flow mechanism of deposition.
Rittman et al (1950) postulated the existence of a central volcano (the Archiflegreo) which was destroyed by the eruption of the CI and the following caldera collapse; according to Rittman et al. the subsequent activity produced several other collapses of lesser extent.
Rosi et al (1983), and Rosi and Sbrana (1987) have argued for the existence of a ring fracture surrounding Campi Flegrei, and interpret the pipernoid tuff of Camaldoli and some breccia deposits called Museum Breccia (Johnston-Lavis, 1889 ) as the proximal deposits of the Campanian Ignimbrite. They agree with Rittmann about the formation and limits of a caldera in Campi Flegrei. Other authors (Di Girolamo, 1970, Barberi et al, 1978, Di Girolamo et al, 1984, Lirer et al, 1987) believe that the Campanian Ignimbrite was fed through an arcuate fracture on the northern edge of Campi Flegrei; this eruption resulted in the collapse of a large area including Campi Flegrei and part of the gulf of Naples.
According to Lirer et al (1987), the Campi Flegrei caldera was mainly formed after another voluminous eruption which produced another major pyroclastic deposit, the Neapolitan Yellow Tuff. Scandone et al (1991) suggest that the Campanian Ignimbrite was fed through a NE-SW fracture bordering the present Posillipo Hill and continuing into the Acerra depression. These authors share Lirer et al (1987) view that the Campi Flegrei caldera was formed after the NYT eruption.
The Campanian Ignimbrite (CI) has an average composition of a trachyte . Di Girolamo (1970) identified some spatial variation in the chemical composition of scoria and pumice suggesting an eruption from a zoned magma chamber with the emission of progressively more mafic products.
According to Di Girolamo (1970), the mafic products cover areas of lesser extent than the more acid members, suggesting a decrease of explosivity in the course of the eruption. A detailed geological map of the Campanian Ignimbrite has been made by Di Girolamo (1968); isopleth distributions of lithics and pumice have been reconstructed by Barberi et al (1978). Scandone et al (1991) have drawn new isopachs based on the interpretation of data of drillholes in the Campanian plane
Both Di Girolamo (1968) and Barberi et al (1978) consider the CI to be the result of a unique eruption, even if lateral facies variations are observed. The more striking lateral variation is a change in color from a poorly welded grey deposit to a more welded yellow one. This change is explained in terms of a secondary mineralization and welding produced by zeolitizatio n (Di Girolamo, 1968). On average the deposit is made up of pumice and black scoriae, with a different degree of flattening, embedded in an ashy matrix with subordinate lithics and crystals. Columnar jointing and fumarolic pipes are often observed .
Di Girolamo (1968) identifies also a gradual vertical facies variation: the lower part of the deposit is made up of a more welded ash matrix with iso-oriented, collapsed black scoriae, a relative high density and high unconfined compressive strength; toward the upper part, the deposit is more incoherent, the scoriae have progressively lesser flattening and are chaotically dispersed in the matrix. Di Girolamo (1968) uses different terms to characterize the different degree of welding and density; they are from the bottom to the top: piperno, pipernoid tuff, tuff, "cinerazzo" (a local name for loose ash).
Outcrops, where the base of the CI is visible, are found along the margin of the Campanian Plain. The occurrence of a pumice-fall deposit at the base of the CI is observed only on the eastern side of the Campanian Plain. About 80 % (volume) of the pumice-fall deposit is made up of angular pumices with the remaining 20% of lithics (predominant) and crystals. The pumices have a light grey colour, sometimes grading to reddish in the upper part. Crystals are mostly sanidine with accessory biotite and piroxene. An increase in the ratio lithics/pumice is observed in the upper part of the deposit. An ash layer (~70 vol.% coarse-ash) of variable thickness (~10- ~100 cm) is sometimes found above the pumice deposit. It is made up by up to 90 vol.% of pumice and glass shards, and about 10 vol% of lithic fragments and crystals (sanidine, biotite, piroxene, and, occasionally, magnetite).
The initial phase of the eruption of the Campanian Ignimbrite was characterized by a sustained eruption column which deposited a pumice-fall deposit to the east of the source area, thus confirming the suggestions made by Thunell et al (1979) ans Sparks and Huang (1980).
The Campanian Ignimbrite crops out along the border and in dwelling and quarries of the Campanian Plain . The middle of the plain is covered by the products of the recent activity of Campi Flegrei and Vesuvius, and by alluvial terrains. As mentioned in the introduction, Rosi et al (1983) and Rosi and Sbrana (1987) suggest that the lithic breccia and the Piperno found in Campi Flegrei are lateral facies of the Campanian Ignimbrite. Lirer et al (1991), Perrotta et al (1993) suggest that these deposits have been produced by later eruptions.
After the eruption of the CI (wherever it occurred) the volcanic activity in the Campanian plain started to localyze on one side at Vesuvius volcano, and onto the other side over a large area extending from the area where presently there is the town of Naples as far as Procida Island.
The island of Procida (fig) is only a kilometer away from Monte di Procida, the western edge of Campi Flegrei. The erupted products of Procida and Monte di Procida range in composition from K-basalts to alkali-trachyte.
The age of the eruptions that occurred within this sector span between 40 and 14 ka corresponding to the products of Vivara and Torre Gaveta volcanoes. The activity covers a relatively long period before and after the eruption of the Campanian Ignimbrite. Scandone et al (1991) tentatively identified the products of the CI as the "S. Martino pyroclastic flow". Rosi et al (1983,1987) identify the CI as the products of the Museum Breccia.
The activity of this sector is mostly characterized by explosive eruptions of limited energy as the products are not deposited over large distances (at most a few kilometers). Pyroclastic flow deposits are strongly controlled by topographic highs and often the products pinch out on reliefs of a few tens of meters.
The oldest volcano is the islet of Vivara which raises above sea-level as an isolated cone. Two lava domes (Punta Ottimo, and S. Martino) were emplaced in the oldest period along with a partially dismantled scoria cone (Miliscola). The areal extent of this scoria cone must have been conspicuous as evidenced by a positive gravity anomaly coincident with it (Cassano and La Torre, 1987). After these eruptions, there was the deposition of several pumice layer of uncertain origin (probably from Ischia). The Fiumicello eruption occurred at about 31000 y BP; the center is probably on Procida Island where are found the most thick deposits with a surge facies. The deposits of this eruption are found on Monte di Procida as sequence of alternating grey ashes and black lapilli. Th flow unit is characterized by black scoriae embedded in an grey pumiceous and ashy matrix; it is strongly controlled by topographic highs and in places (Torregaveta) has been strongly eroded by the overlaying Museum Breccia made up by lithic fragments and pumice lapilli (see detail).
The last eruptions on Procida were that of the Museum Breccia (~18000 a BP according to Perrotta and Scarpati, 1993) and Solchiaro (~17000 a BP) which erupted also the less differentiated products (K-basalt). On the mainland, the last eruption before the Neapolitan Yellow Tuff event was that of Torre Gaveta.
A schematic geological map of Campi Flegrei is given in figure. On the eastern edge of Campi Flegrei, on Camaldoli hill, the products of the activity of Campi Flegrei are exposed. The oldest products are the so-called Torre-Franco tuffs that comprise a series, 50 m thick, comprising ash-beds with cross-laminations that alternate with pumice and scoria deposits interbedded with paleosols.
Above another paleosol is the Piperno-Museum Breccia formation. The Piperno is a welded ash with abundant fiamme and some pipe-structures that pass into the overlying breccia. As mentioned before Rosi et al (1983,1987) identify the Piperno -Breccia Museum as the proximal facies of the CI and suggest that the eruption of the Campanian Ignimbrite was responsible of the formation of the Campi Flegrei caldera. According to these authors, after the CI the area was invaded by the sea.
A subsequent activity occurred and filled the caldera depression until the eruption of the Neapolitan Yellow Tuff. On the eastern side of the caldera, above the Piperno there is a ≈70 m thick sequence of Whitish Tuffs (16 ka) overtopped by the Neapolitan Yellow Tuff.
The eruption of the Neapolitan Yellow Tuff (NYT) occurred at ~12000 y BP. Rittman (1950) suggested that the extensive deposits of yellow tuff that crops out around Campi Flegrei and in the town of Naples were the results of different eruptions. Lirer and Munno (1976) and Di Girolamo et al (1984) proposed that all the deposits of yellow tuff outcropping outside and on the rim of Campi Flegrei were the results of a unique eruption that produced the collapse of the Campi Flegrei caldera (Lirer et al, 1987).
Rosi et al (1983), and Rosi and Sbrana (1987) followed the views of Rittman (1950) and suggested that different eruptions were responsible of the deposition of the Neapolitan Yellow Tuff. Recently, Scarpati and Cole (1993) suggested the uniqueness of the Neapolitan Yellow Tuff deposit based on a detailed stratigraphic, granulometric and geochemical study of the different outcrops.
Scarpati and Cole (1993) suggest that the eruption of the Neapolitan Yellow Tuff was firstly characterized by the deposition of a phreato-plinian deposit of alternating pumice and ashes, followed by the deposition of a huge sequence of surge and pyroclastic flows. The deposits of the NYT are extensively found on the rim of the caldera and within it, as evidenced by geothermal and water-well drill-holes. A conservative estimate of its volume is between 10 and 20 km^3 of DRE.
After the NYT eruption, every activity outside the caldera ended (Procida, Monte di Procida, Naples) and the following activity emplaced only within the caldera structure, frequently along its rims and sometimes involving intra-calderic collapses (Lirer et al, 1987).
Scandone et al (1991) suggest that after the eruption of the NYT the inner part of Campi Flegrei was invaded by the sea and all the subsequent eruptions occurred along the border of the caldera. The major volcanic edifices built during this period have been eroded on the side facing the sea.
Several major eruptions occurred between 11 and 9 ka abp. These were the eruptions of Gauro (10 ka bp), Archiaverno and of Agnano Pumices (9 ka), Monte Ruscello and probably the volcanoes along the northern margin of the caldera (Montagna Spaccata, Pisani, etc.) and Nisida, on the eastern flank.
All these eruptions occurred along the rim of the caldera and many of these volcanoes are made up by litified yellow tuff that has been interpreted as an indication of water-magma interaction (Di Girolamo et al, 1984). Other eruptions occurred along the western flank of the caldera, and they were Baia and Fondi di Baia, Miseno and Porto Miseno
An uplift of the caldera floor occurred between 10 and 5 ka bp (Cinque et al, 1985). A marine terrace (La Starza) raised to an height of about 40 m presently borders the northern shore of the gulf of Pozzuoli. The uplift was accompanied and followed by renewed volcanic activity whose centers are slightly shifted toward the center of the caldera.
Local collapse during this phase resulted in the Agnano caldera which was formed by several eruptions. Other major eruptions whose products are well exposed, occurred at Cigliano, Agnano-Monte Spina (4000 abp), Astroni (3700 abp) and Averno (3700 abp), Solfatara and Monte Olibano . The last eruption in the area occurred in historical time and was that of Monte Nuovo (1538 AD).
Since 1800, sea-level measurements made in the ruins of a roman market (Serapeo) have indicated a slow sinking of the area. (These slow movements of the ground have been locally called "Bradisismo" by the greek bradi=slow, seism=movement). Already in roman times there was evidence of sinking of the ground. The Serapeo, built near the sea-shore, had its floor elevated, two centuries after its first construction, because of the invasion of the sea.
Levellings made at the beginning of this century showed that the maximum sinking was occurring in the city of Pozzuoli and regularly decreased eastward and westward along the coast. This slow sinking of the ground continued until 1968. In the periods 1970-1972 and 1982-1984 two important episodes of inflation occurred in the Pozzuoli area (Berrino et al, 1984).
These episodes produced 170 cm (inferred with respect to the previous levelling) and 182 cm, respectively, of uplift at the point of maximum deformation (located in Pozzuoli). The inflation geometry is the mirror image of the slow sinking observed until 1968; it has a circular simmetry around Pozzuoli and regularly decreases toward the margin of the caldera.
The inflation which occurred in 1970-72 had a partial recovery of some 20 cm; similarly, a deflation of about 70 cm has occurred since the end of 1984. One particular feature is the constancy of the areal extent of the deformation.
Repeated levellings (Berrino et al, 1984) showed that the bell-shaped form of the deformation did not change appreciably during 1982-1984: altough it displayed a marked vertical variation, its horizontal extent remained the same.
This pattern is not compatible with magma migration toward shallow depth, for which a decrease of the areal extent of the inflation would be expected. Scandone et al (1991) suggest that the deformation is strongly controlled by the caldera structure and this explains the constant planimetric geometry for inflation and deflation episodes.
The slow sinking of the ground observed until 1968 is possibly related with the compaction of the loose pyroclastics that make up the caldera floor. In this sense it represents the normal dynamics of the area. On the contrary the sharp inflation episodes must be considered as anomalous events related with a pressure increase either below the caldera structure or within the loose pyroclastics.
A seismic crisis began in 1983, some months after the beginning of an inflation of the ground. Earthquakes occurred mostly in the coastal region around Pozzuoli, only a few, deeper events occured within the gulf; however they did not extended outside the border of the Campi Flegrei caldera. Hypocenters were located between a few hundred meters until ~5 km depth. The maximum observed magnitude was 4.0.
A distinctive areal difference was observed in the pattern of seismicity. Earthquakes with the shallowest foci occurred mostly as swarms and were located in an area west of Pozzuoli; earthquakes located in the eastern area, however, had higher magnitudes, occurred as single or double events, and generally had deeper hypocenters. The events occurring in the gulf resembled those of the eastern area, but with generally lower magnitudes.
Aster and Meyer (1988) made a tomographic study of the crustal structure of the caldera by a simultaneous tridimensional inversion of velocity and hypocenters of earthquakes. They found that the central part of the Pozzuoli caldera has an anomalously high vp/vs ratio and low vp and vs, indicating an incompetent highly fractured medium, saturated with liquid water, and that areas of anomalously low vp/vs occur on the borders of the caldera depression.
The relocations of the deepest earthquakes showed an invard-dipping elliptical zone of hypocenters, interpreted as a ring fault. The hypocenters of earthquakes occurring in the gulf of Pozzuoli have not been plotted in figure because they fall far from the trace of the structural profile; however they are generally deeper than 3 km and possibly lie on the downward continuation of the south-western fault zone of the caldera so suggesting a funnel-shaped caldera.
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