The Phlegraean Volcanic District is probably one of the most complex
volcanic structures in the world, due to caldera collapse and repeated
volcanic activity within a limited area. The centre is situated upon a
Pliocene-Quaternary extensional domain with normal faults which trend
NE-SW and NW-SE from the margin of a thrust belt (Apennine belt). The
geological and deformational history of the Campi Flegrei complex has been
subdivided into three periods, based upon the a chaotic ignimbrite (Campanian
Ignimbrite), which marks the end of the first period in 37 ka, a yellow
tuff unite (Neapolitan Yellow Tuff), which marks the end of the second
period in12 ka, and eruptions in the third period which dates from 12 ka
to the present.
Period I: The beginning of this period has not yet been well
defined, though rocks which are older than the Campanian Ignimbrite can be
seen within the cliffs of Mt.Procida, the hill of Cuma and the northern
border of the Quarto and Soccavo plains. An approximate age for the Cuma
lava domes of 37 ka was calculated by Cassignol and Gillot ( 1982) and an
age of >42 ka was deduced from the pyroclastic deposits of Tuff at Torre
Franco (Alessio et. al., 1973). The oldest dated exposures of 60 ka using
40Ar/39Ar isotopes can be seen on the slopes which
border the northern edge of the Quarto plain. This period ended with the
eruption of the Campanian Ignimbrite (37 ka) which covered approximately
30,000 km2 with 150 km3 of magma with a composition
which ranges from trachyte to phonolitic-trachyte; this event is suggested
to have been the biggest event within the Mediterranean area over the past
200 ka (Barberi et. al., 1978), and it has also been suggested that the
epicentre might have migrated during the course of the eruption (Civetta
et. al., 1997).
Period II: Prior to the eruption of the Neapolitan Yellow Tuff
there has been identified within limited outcrops an explosive
hydromagmatic phase. This event can be calculated to have occurred between
37 and 18-14 ka as it has been moulded by the Würmian level surface with
its epicentre being located within the Campi Flegrei caldera depression.
The most important event during this period was the Neapolitan Yellow Tuff
eruption which, it has been suggested, was of phreatoplinian to
phreatomagmatic in its nature. It was the second largest eruption in the
Campanian area, covering an area of approximately 1,000 km2
with 40 km3 of magma with a composition which ranges from
alkali-trachyte to latite. From its stratigraphical, compositional etc.
characteristics is has been suggested that the caldera began to collapse
during the eruption (Wohletz et. al., 1995) though the rim of this caldera
is poorly exposed as it was disturbed during later eruptions.
Period III: This period covers the last 12 ka. Di Vito et. al.,
1998 has defined three epochs of activity between 12-9.5 ka, 8.6-8.2 ka
and 4.8-3.8 ka with the last eruption occurred in 1538 AD They were also
able to recognise 75 volcanic units, 61 of which had vents located within
the Campi Flegrei caldera. The first epoch started soon after the
Neapolitan Yellow Tuff caldera collapse and lasted approximately 2.5 ka,
during which period there were 34 explosive eruptions with a mean
frequency of one eruption every 70 years. The eruptions of this epoch were
explosive, varying from magmatic to phreatomagmatic depending upon the
position of the vents with respect to the coastline. The caldera floor at
this time was on average 60m lower than at present with the major part of
caldera depression being invaded by the sea, only the northern sector
remaining constantly above sea level. The second epoch started with the
eruption of the Fondi di Baia in the western sector at approximately 8.6
ka. Within this epoch there were 6 low magnitude explosive eruptions with
a mean frequency of one every 65 years; all the eruptions with the
exclusion of the first occurred along the north eastern sector of the
Neapolitan Yellow Tuff caldera. After this epoch there was a period of
quiescence which lasted approximately 3.5 ka during which the sea level
rose once more and partly eroded the volcanic features within this area.
During the third epoch there were 16 explosive and 4 effusive eruptions
with a mean frequency of one every 50 years. The majority of the eruptive
vents were located within the north eastern sector of the Neapolitan
Yellow Tuff caldera; only two occurred within the north western sector in
the area where the later eruption of Monte Nuovo took place.
Sites to see
Within this area there are sites of special interest; these include the
Solfatara fumarolic field, Monte Nuovo caldera, Astroni caldera and the
evident bradyseism.
The Solfatara crater has been a site of intense hydrothermal activity
since Grecian times and is the most impressive manifestation of the
current hydrothermal activity of the Campi Flegrei caldera. The
manifestation includes both focused vents with a maximum temperature of
approximately 160°C, and large areas of hot steaming ground. The average
molar composition of the fluids is approximately 82% H2O,
17.5%CO2, 13% H2SO4 with minor amounts of
N2, H2, CH4 and CO. The isotopic
composition suggests an aquifer system, also evidenced by the absence of
soluble acid gases typical of high temperature volcanic gas. At present
the solfatara is the main object of the geochemical surveillance of the
Campi Flegrei. In particular both the chemical and fluid compositions and
the CO2 fluxes from the soil within the crater are monitored.
The most obvious manifestations of bradyseism occur along the coastline
where the Roman coastline can be noted to lie approximately 10m below the
current sea level. One of the areas where this phenomenon is most easily
seen and recorded is in the town of Pozzuoli where within the main square
there is the temple of Serapeo which was constructed between the I and II
century AC The site had to be restored at the beginning of the III century
AC as sea water had invaded its floor. The subsidence continued until the
X century AC when uplift started and continued until the eruption of Monte
Nuovo in 1538. Capocci, 1835, reported an uplift of 5 to 8 m within a few
days before the eruption of Monte Nuovo. This phenomenon was noted several
times up to 1992 since when no great movement has occurred greater than a
few centimetres.
The calderas of Monte Nuovo and Astroni are well preserved
manifestations of the last period of eruptive activity.
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