INGV

The Campi Flegrei Caldera GIS Database

Geologic History


Volcanic and deformation history of the Campi Flegrei

The Campi Flegrei is a nested and resurgent caldera (CFc) (Fig. 1a) resulting from two major collapses related to the Campanian Ignimbrite (CI; 39 ka; Barberi et al., 1978; Rosi et al., 1996, 1999; Fisher et al., 1993; Civetta et al., 1997; Ort et al., 1999; 2003; De Vivo et al., 2001; Pappalardo et al., 2002) and Neapolitan Yellow Tuff (NYT; 15 ka; Orsi et al. 1992, 1995; Scarpati et al. 1993; Wohletz et al., 1995; Deino et al., 2003) eruptions, respectively (Orsi et al., 1996). Its magmatic system is still active with the last eruption occurring in 1538 AD (Monte Nuovo; Di Vito et al. 1987), widespread fumaroles and hot springs activity (Allard et al. 1991), and the unrest episodes in the last 30 years, with a maximum net uplift of about 3.5 m in the Pozzuoli area (Orsi et al. 1999a and references therein). The volcanic hazard of the caldera is extremely high also because of its explosive character and the occurrence of high-magnitude eruptions (Orsi et al., 2004). Close to 1.5 million people live within the caldera, with about 350,000 people living in its active portion. Due to the high volcanic hazard and the intense urbanisation of both the active portion of the caldera and its surroundings, the volcanic risk is very high.

CFc map
 CfC chronology

Figure 1. a) structural map of the Campi Flegrei caldera; b) vertical ground movements at the Roman market (Macellum) in Pozzuoli.

  Figure 2. Chronogram of volcanic and deformational history of the Campi Flegrei caldera.

Rocks older than CI are only exposed along sea cliffs and high-angle scarps related to the CI caldera collapse (Fig. 2). The oldest detected age on these rocks is of about 60 ka. The CI eruption, the largest of the Mediterranean area over the past 200 ka, extruded not less than 200 km3 of trachytic to phonolitic-trachytic magma. The caldera collapsed after this eruption, affected the area which presently includes the Campi Flegrei, the city of Naples, the bay of Pozzuoli and the north-western portion of the bay of Naples (Orsi et al., 1996). Fedele et al. (2002; 2003) have pointed out the coincidence between this eruption and the coeval bio-cultural modifications in Old World prehistory, including the Middle to Upper Palaeolithic cultural transition and the supposed change from Neanderthal to "modern" Homo sapiens anatomy, a subject still debated in the literature. The authors have hypothesised a positive climate-volcanism feedback, hence an impact of the CI on the atmosphere and consequently at the global scale. The volcanism which followed this event was concentrated within the CI caldera. The NYT eruption, the second largest of the Campanian area, was phreatoplinian to phreatomagmatic and extruded about 40 km3 of magma ranging in composition from alkali-trachyte to latite. The caldera related to this eruption was nested within the CI caldera and centred on the present Campi Flegrei. Volcanism of the past 15 ka, concentrated in three epochs of activity separated by quiescence (Di Vito et al., 1999), has generated mostly explosive eruptions, variable in magnitude and generally characterised by alternating magmatic and phreatomagmatic explosions. During each epoch, eruptions occurred at intervals of about 60 years, on average. During the I epoch (12.0 – 9.5 ka), out of 34 explosive eruptions, only the Pomici Principali (10.3 ka, Di Vito et al., 1999; Lirer et al., 1987) was a high-magnitude event. The II epoch (8.6 – 8.2 ka) generated 6 low-magnitude explosive eruptions. The III epoch (4.8 – 3.8 ka) produced 16 explosive and 4 effusive eruptions. During this epoch the only high-magnitude event was the Agnano-Monte Spina eruption (4,1 ka; Rosi and Santacroce, 1984; de Vita et al., 1999; Dellino et al., 2001). The first two periods of quiescence lasted 1.0 and 3.5 ka, respectively, while the last, begun at the end of the III epoch, has been interrupted only by the Monte Nuovo eruption, the last eruption of the caldera, in 1538 AD (Di Vito et al., 1987).

Volcanism and quiescence are strictly related to formation and deformation of the caldera. During the I and II epochs, magma reached the surface through the marginal faults of the NYT caldera. Between the II and III epoch, a change in the stress regime occurred in the caldera. Before onset of the III epoch, the La Starza block, which had been uplifted at variable resurgence rate with alternating periods of emersion and submersion, definitively emerged (Fig. 2). During the III epoch, magma was able to reach the surface almost only along the faults of the sector of the resurgent block under tensional stress regime (Orsi et al., 1996).

The whole CFc is subsiding, while the central part of the NYT caldera is affected by resurgence since at least the second period of quiescence (Fig. 1a). Resurgence occurs through a simple-shearing mechanism (Orsi et al., 1991) which has disjointed the NYT caldera floor in blocks (long-term deformation) (Orsi et al., 1996). The most uplifted block includes the La Starza marine terrace and has been displaced of about 90 m (Fig. 1a). Vertical ground movement has been well documented for the past 2000 years (Fig. 1b) (Parascandola, 1947). In the past 40 years, unrest episodes have affected the caldera in 1969-72, 1982-82, 1989, 1994 and 2000 and have generated uplifts of 170, 180, 7, 1, and 4 cm, respectively. The deformation occurred during these episodes (short-term deformation) (Orsi et al., 1999a) has been interpreted as the result of ductile (expansion and deflation of the geothermal system) and brittle (fracturing of the magma chamber roof rocks) components, both generated by increase in pressure and temperature within the magma reservoir due to arrival of small magma batches, less evolved and hotter than the resident (Orsi et al., 1996, 1999a). The area deformed during the unrest episodes has a polygonal shape and its boundaries correspond to the structural features bordering the resurgent block inside the NYT caldera (Orsi et al., 1999a). Such a correspondence suggests that the long-term deformation results from the summation of many short-term deformation events.

The magmatic system of the CFc includes a shallow, large-volume trachytic reservoir which has been periodically refilled by new magma batches rising from a storage zone located between 10 and 15 km depth (D’Antonio et al., 1999; Pappalardo et al., 1999). The shallow reservoir has been the site of differentiation processes (mainly crystal fractionation and mingling/mixing, and subordinately contamination). From 60 to 44 ka, the reservoir was growing due to input of new magma batches, while from 44 to 39 ka, it was an isotopically homogeneous, large-volume, zoned system, whose evolution culminated in the CI eruption (Civetta et al., 1997). Arrival of new magma batches formed an apparently independent, large-volume reservoir which fed the NYT eruption (Orsi et al., 1992). In the past 15 ka, three isotopically and geochemically distinct magmatic components were erupted as either homogeneous or mixed magma batches (D’Antonio et al., 1999). One component is similar to the CI trachytic magma, the second is similar to the NYT latitic-alkalitrachytic magma, the third is a trachybasalt never erupted before. D’Antonio et al. (1999) have hypothesised that the CI and NYT components represent residual portions of older, large-volume magma reservoirs which have fed eruptions since about 60 and 15 ka, respectively. This hypothesis is supported by thermal (Wohletz et al., 1999) and magnetic (Orsi et al., 1999b) modelling that concur to suggest the existence of a large volume of molten magma beneath the caldera structure. The least-evolved component, erupted through vents located on a NE-SW regional fault system, likely represents the deeper seated magma tapped by regional faults.


References
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- The Campi Flegrei Caldera GIS Database - July 2005 -