Large calderas are among the Earth's major volcanic
features. They are associated with large magma reservoirs and elevated
geothermal gradients. Caldera-forming eruptions result from the withdrawal
and collapse of the magma chambers and produce large-volume pyroclastic
deposits and later-stage deformation related to post-caldera resurgence and
volcanism. Unrest episodes are not always followed by an eruption; however,
every eruption is preceded by unrest.
The Campi Flegrei caldera (CFc), located along the eastern Tyrrhenian coastline
in southern Italy, is close to the densely populated area of Naples. It is
one of the most dangerous volcanoes on Earth and represents a key example of
an active, resurgent caldera. It has been traditionally interpreted as a
nested caldera formed by collapses during the 100–200 km3 Campanian
Ignimbrite (CI) eruption at ∼39 ka and the 40 km3 eruption of
the Neapolitan Yellow Tuff (NYT) at ∼15 ka. Recent studies
have suggested that the CI may instead have been fed by a fissure eruption from the Campanian Plain, north of Campi Flegrei.
A MagellanPlus workshop was held in Naples, Italy, on 25–28 February 2017 to
explore the potential of the CFc as target for an amphibious drilling
project within the International Ocean Discovery Program (IODP) and the International Continental Drilling Program (ICDP). It was agreed that Campi Flegrei is
an ideal site to investigate the mechanisms of caldera formation and associated
post-caldera dynamics and to analyze the still poorly understood interplay
between hydrothermal and magmatic processes. A coordinated onshore–offshore
drilling strategy has been developed to reconstruct the structure and
evolution of Campi Flegrei and to investigate volcanic precursors by
examining (a) the succession of volcanic and hydrothermal products and
related processes, (b) the inner structure of the caldera resurgence, (c) the
physical, chemical, and biological characteristics of the hydrothermal system and
offshore sediments, and (d) the geological expression of the phreatic and
hydromagmatic eruptions, hydrothermal degassing, sedimentary structures, and
other records of these phenomena. The deployment of a multiparametric
in situ monitoring system at depth will enable near-real-time tracking of
changes in the magma reservoir and hydrothermal system.
Introduction
Large collapse calderas are associated with climactic explosive volcanic
eruptions capable of producing a global catastrophe second only to that from
a meteorite impact. On the other hand, many calderas are characterized by
hydrothermal systems that represent a source of “clean”, geothermal energy
production (e.g., Lipman, 2000). Despite numerous scientific and applied
studies, the inner structure and the dynamics of caldera systems are still
poorly known. In many cases, large calderas also host densely populated
urban and agricultural districts. As a consequence, understanding the
caldera structure and dynamics has an immediate effect on the assessment of volcanic hazards and associated risk at a local and global scale.
Large calderas are found on all continents and in different geological
settings. For example, recent restless examples can be found in New Zealand
(Taupo), North America (Crater Lake, Long Valley, Valles, Newberry, and
Yellowstone), South America (Laguna de Maule and Cerro Blanco), Asia and
Oceania (Toba, Tambora, Krakatau, Rabaul, Toya, Shikotsu, and Kuttara), and
Europe (Santorini and Campi Flegrei). Some of these are located close to
coastlines and continental shelves, where hydrothermal and groundwater
dynamics may partly control the expression of volcanism and the distribution
of eruptive products. Deposition of these products occurs in a changing
depositional regime with a high average sedimentary supply. Coastal and partly
submerged calderas on continental shelves thus contain unique stratigraphic
archives of interbedded volcaniclastic and marine deposits with a high
potential for preservation.
The Campi Flegrei caldera (CFc), next to Naples in southern Italy, has been
the world's most restless, non-erupting caldera for the last 69 years,
characterized by episodes of significant ground uplift, enhanced
hydrothermal activity, and seismicity. In addition to these short-term
episodes (e.g., 1950–1952, 1969–1972, and 1982–1984; De Natale et al., 2006;
Del Gaudio et al., 2010; Troiano et al., 2011; Chiodini et al., 2015, 2017;
Kilburn et al., 2017; Moretti et al., 2017, 2018), ground subsidence and
uplift of several meters has been documented since at least Roman times
(e.g., Bellucci et al., 2006; Di Vito et al., 2016). Moreover, as a result of
resurgence over the last ca. 12 000 years, the central part of the CFc has undergone
a long-term, antiformal uplift of ca. 100 m that is partly recorded by La Starza marine terrace's
present-day elevation (ca. 40 m above sea level) that emerged ca. 5000 years ago (Di Vito et al., 1999; Sacchi et al.,
2014).
The cause of ground uplift episodes and phases occurring at the CFc (magmatic vs.
hydrothermal) is still debated and likely consists of periods of shallow
magmatic intrusions accompanied by injections of deep fluids into shallow
aquifers (e.g., De Vivo and Lima, 2006; De Natale et al., 2006; Lima et al.,
2009; Moretti et al., 2018; Troise et al., 2019). Significant hydrothermal
activity is shown on land and in the submerged portion of the caldera by the
discharge of hot gases and liquids (Sacchi et al., 2014; Passaro et al.,
2016; Chiodini et al., 2017; Steinmann et al., 2018). Since offshore
emissions cover an area 4 times larger than the main onshore hydrothermal
site around the Solfatara crater (Passaro et al., 2014; Somma et al., 2016;
Steinmann et al., 2018), the marine portion of the CFc may play a
substantial role in the recent dynamics of the caldera, representing an
underestimated source of degassing and heat flux. The lack of adequate data
on offshore fluid emissions prevents a correct estimate of the fluid release
and gas and heat flow budget at Campi Flegrei. In addition, while the
uppermost 100 m of the submerged part of the CFc have been
intensively studied (D'Argenio et al., 2004; Milia and Torrente, 2007;
Sacchi et al., 2009, 2014; Passaro et al., 2016; Steinmann et
al., 2016, 2018), the deeper portion remains largely unknown.
Understanding the mechanisms for unrest and eruptions is of primary
importance for confident hazard assessment. Data on the deeper, submerged
portion of Campi Flegrei are required to constrain forecasts of the type,
intensity, and frequency of future magmatic, phreatomagmatic, and
hydrothermal eruptions. More than 600 000 people are potentially exposed to
pyroclastic flows, rising to about 2 million considering the ash fall, also
emitted from submerged vents (Rossano et al., 2004; Mastrolorenzo et al.,
2006, 2008; Tonini et al., 2015; Sandri et al., 2016, 2018). Traditionally,
calderas have been analyzed through field studies, monitoring observations,
analogue models, and numerical simulations (e.g., Druitt and Sparks, 1984; De
Natale et al., 1991, 2001, 2006; Martí et al., 1994, 2008; Gudmundsson et al., 1997;
Gudmundsson, 1998; Burov and Guillou-Frottier, 1999; Acocella et al., 2000,
2001, 2004; Martí and Gudmundsson, 2000; Roche
et al., 2000; Roche and Druitt, 2001; Folch and Martí, 2004;
Lavallée et al., 2004; Geyer et al., 2006; Gregg et al., 2012, 2013). More recently, offshore
reflection seismic imaging has emerged as a tool to understand the
stratigraphic architecture and shallow structure of collapse-resurgent
calderas in continental margins (e.g., Sacchi et al., 2009, 2014; Passaro et
al., 2016; Steinmann et al., 2016, 2018). However, only deep drilling can
provide conclusive information on the causes and mechanics of unrest and on
the state and evolution of the magmatic–hydrothermal system. These data
represent a fundamental prerequisite for evaluating the caldera-related
hazards (Lowenstern et al., 2017).
Campi Flegrei caldera
Campi Flegrei is an active caldera belonging to the Neapolitan Volcanic
District, which includes the active volcanoes of Vesuvius and Ischia Island.
The caldera contains the westernmost districts of Naples as well as the
towns of Pozzuoli, Bacoli, Baia, and Quarto and several smaller villages.
Half of the CFc is submerged and forms Pozzuoli Bay (also known as the
Gulf of Pozzuoli). This area has represented an active segment of the eastern
Tyrrhenian margin since the Late Quaternary (Oldow et al., 1993; Ferranti et
al., 1996) and may be considered a natural laboratory for studying the
interplay between tectonics and explosive volcanism in the rifted back-arc
margin of the Tyrrhenian Sea and the Adriatic subduction system below the
Apennine fold-and-thrust belt (Fig. 1) (e.g., Milia and Torrente, 1999;
Acocella et al., 1999).
Tectonic sketch map of the Campanian continental margin in SW Italy
with the location of the Campanian volcanic zone. Frame of Fig. 2 is also
indicated.
Digital Terrain Model (DTM) of the Campi Flegrei caldera (CFc)
showing the structural border(s) and the offshore ring-fault–resurgent-dome system associated with the eruption of the Neapolitan Yellow Tuff (NYT)
(∼15 ka), along with the most prominent subaerial and
submerged volcanic morphologies of Napoli Bay. Location of seismic
reflection profiles illustrated in Figs. 10 and 11 is also indicated.
The CFc describes a quasi-circular depression approximately 13 km across.
The present-day shape of the caldera has been conventionally interpreted as
the result of two large collapses related to the eruptions of the Campanian
Ignimbrite (CI, ∼39 ka; Giaccio et al., 2017) and the
Neapolitan Yellow Tuff (NYT, ∼15 ka; Deino et al., 2004) (Fig. 2) with respective volumes of 200 km3 DRE (dense-rock equivalent)
(Rolandi et al., 2003) and 40 km3 DRE (Scarpati et al.,
1993). Evidence of older ignimbrites has been reported in the Campanian
Plain (De Vivo et al., 2001) and in the distal marine archives (e.g., Insinga
et al., 2014). The locations of these eruptions remain poorly constrained
around Campi Flegrei. As described below, an alternative view now emerging
is that the CI was erupted outside Campi Flegrei so that the caldera
was formed only by the NYT eruption.
The CI eruption is Europe's largest explosive volcanic event recorded in the
last 200 000 years. It has been considered a possible cause for the decline of the
Neanderthals, thus implying a potential influence on human evolution
(Fitzsimmons et al., 2013). The CI deposits are widespread in the
Mediterranean, and its ash has been found in the Russian Plain, more than
2500 km away from the source (Pyle et al., 2006; Giaccio et al., 2008). The
CI eruption was followed by the NYT eruption at 15 ka and at least 60
post-caldera eruptions (Di Vito et al., 1999). The most recent eruption
occurred in 1538 after a repose of ca. 3000 years. It produced Monte Nuovo and was
preceded by a century of uplift (Bellucci et al., 2006; Di Vito et al.,
2016). Most recently, non-eruptive unrest episodes occurred during
1950–1952, 1969–1972, and 1982–1984 (Del Gaudio et al., 2010). They have been
characterized by ground deformation (with rates up to 100 cm yr-1), shallow,
low magnitude earthquakes (about 16 000 events with a magnitude up to 4.0 in
1983–1984), and marked geochemical variations in the emitted gases (Berrino et
al., 1984; De Natale and Zollo, 1986; Dvorak and Berrino, 1991; De Natale et
al., 1991, 1995, 2001; Battaglia et al., 2006; Chiodini et al., 2015; Di
Luccio et al., 2015; Moretti et al., 2017, 2018). In fact, the recorded
history of non-eruptive ground movements goes back to Roman times as
revealed by marine incrustations and mollusks on Roman and medieval
buildings (e.g., Bellucci et al., 2006; Troise et al., 2019). Campi Flegrei
thus represents the caldera with the longest record of ground movements not
immediately followed by eruptions. Uplift began again in 2005 after 20 years
of subsidence. It has been characterized by a slow movement of ca. 3 cm yr-1
and less seismicity but a longer duration than previous uplifts (Troise et
al., 2007; Chiodini et al., 2017; Moretti et al., 2018). The long duration of
the ongoing unrest has led the Civil Protection Department to declare the first level on its alert scale (yellow), which implies an increase in monitoring
activities.
Although the CI eruption has been previously considered as the main
caldera-forming event (Rosi and Sbrana, 1987; Orsi et al., 1996), De Vivo et
al. (2001) and Rolandi et al. (2003) presented evidence in favor of a
fissure eruption to the north of the CFc. Recently, new evidence of buried
CI products inside the caldera area at a depth of ca. 400 m beneath the
surface has been found in the pilot borehole of the ICDP (International Continental Drilling Program) Campi Flegrei Deep
Drilling Project (CFDDP) (De Natale et al., 2016). The shallow depth and
modest thickness of the deposit (less than 200 m) raised further questions
about a caldera collapse associated with the CI eruption. These and other
studies highlight the complexity of the caldera system and, hence, the need
for additional in situ information to fully understand the whole framework
and evolution of volcanism in Campania. Due to its partly submerged setting,
Campi Flegrei represents an ideal site to test the potential of IODP (International Ocean Discovery Program) shallow-water drilling on a volcanic continental margin by a multiplatform drilling
program including a land–sea transect, in the frame of a fully integrated
IODP–ICDP Amphibious Drilling Proposal (ADP). The research outcomes derived
from Campi Flegrei may also be transferred to similar partly submerged
volcanic systems, including the Aira, Kikai, Krakatoa, Maug, Santorini, and
Tavui calderas.
The MagellanPlus workshop
During the MagellanPlus workshop held in Naples on 25–28 February 2017, 35
participants from four European countries (Italy, Germany, Spain, and the
UK), the USA, and Japan, gathered to discuss the key scientific issues for a
coordinated IODP–ICDP proposal dedicated to drilling in the CFc. The
workshop built upon previous research and networking activities, including
(1) a coordinated ICDP and ESF (European Science Foundation) Magellan workshop held on 13–15 November 2006
in Naples; (2) an approved ICDP full proposal (Campi Flegrei Deep Drilling
Project – CFDDP) in 2006–2008; (3) a submitted IODP pre-proposal (#671) in
2006–2007 with an indication of resubmission on the basis of an implemented
site-survey package; (4) the realization of two pilot holes, a few meters
apart, 500 and 200 m deep, as a preliminary phase to the ICDP deep
drilling (the 200 m hole has been continuously cored by wireline drilling
and used to install a borehole seismometer); and (5) the acquisition of
new offshore site-survey data (3-D multiscale multichannel and single-channel
reflection seismics, multibeam bathymetry, and gravity core data) between
2008 and 2016 (Fig. 3).
Offshore site-survey seismic data package (high-resolution
multichannel and single-channel reflection seismic profiles) supporting the
IODP component of the Campi Flegrei Amphibious Drilling Proposal. Locations
of the proposed IODP drill sites and the ICDP site and pilot hole drilled
in 2012 are also indicated.
Participants at the MagellanPlus workshop represented a wide range of
disciplines, including volcanology, geology, geophysics, geomorphology,
petrology, geochemistry, and geochronology, as well as numerical and analogue
modeling. The initiative was intended to bring together experts,
early-career researchers, and other representatives from academia and
industry involved in marine and continental research drilling. The aims were
to (1) provide a global perspective on the potential and challenges of
scientific drilling at active calderas, (2) discuss drilling issues on
volcanic continental margin settings, (3) illustrate the new site-survey data,
and (4) define drilling objectives for reconstructing stratigraphic events
associated with the caldera's evolution and the interaction between
magmatism and hydrothermal activity in coastal marine settings.
Participants were asked to contribute to scientific debates on volcanism and
associated hazards over coastal areas and identify problems that can be
addressed by coordinated marine and continental drillings, with reference to
the CFc as a representative case study. The workshop program addressed data
integration, the building of a scientific rationale for drilling strategies,
and scientific partnering through a multidisciplinary approach, by linking
geology, geophysics, volcanology, petrology, microbiology, and
geotechnology. The event is among the first efforts to assess scientific
themes directly related to volcanic hazards in highly populated coastal areas
within the context of fully integrated IODP–ICDP drilling research.
Fundamental questions were discussed on a wide range of topics, such as the
mechanisms and timing of caldera formation and resurgence, ignimbrite
deposition environments, magma transfer processes, and explosive volcanic
activity in submarine and coastal settings, volcano-tectonic coupling,
the dynamics and energy budget of onshore and offshore hydrothermal (geothermal)
systems, subaerial vs. submarine volcanic unrest, and monitoring.
Participants identified the following key questions and objectives that,
which shall be addressed by the Amphibious Drilling Proposal:
Interaction between magmatic and hydrothermal processes. This will be investigated regarding
shallow crustal levels, the mechanism of submarine degassing and hot fluid
discharge and their contribution to deformation, and recent unrest. What are
the source, dynamics, and consequences of the hydrothermal activity in the
marine portion of Campi Flegrei, and how are they related to unrest? Does the
structural framework of the CFc exert control on the ascent of fluids and
magma? Can microbial communities help in tracing hydrothermal fluid paths
and defining thermodynamic environments and facies at depth?
Stratigraphy and structures of the CFc. This will be investigated within the half-graben
system of the Bay of Naples. This investigation includes recovering a representative stratigraphic
record of the caldera fill and borders, down to the upper structural levels
of the caldera floor and reconstructing the distribution of the CI (and older
ignimbritic) deposits across the Bay.
Kinematic reconstruction of caldera collapse structure and resurgence. This investigation includes reconstructing the pattern, timing, and rates of deformation
involving the various structural components of the CFc system.
Eruptive history of the CFc. When was the onset of volcanic
activity, and what was the driving mechanism, e.g., volcano–earthquake
interaction? In what way did the magmatic system change over time? Are there
any major pre-CI ignimbrites originating from the Campanian Plain? What is
the type of post-collapse, submarine volcanic activity? How have large
eruptions changed the submarine morphology, and which are the syn- and
post-eruptive deposition mechanisms?
Difference between terrestrial and shallow marine volcanism.
Why have post-collapse eruptions occurred preferentially in the onshore part
of the caldera? Is the hydrothermal activity in the marine setting driving
overpressure and fluid-saturated subvolcanic intrusions that are
substantially different from on-land subvolcanic or volcanic processes?
Environmental and climatic impact. This investigation refers to large-scale ignimbrite
eruptions such as the CI and NYT events. Did these eruptions influence
climate events? Did the CI eruption influence the decline of the
Neanderthals? Do we recognize significant changes in the abundance and
diversity of faunal and floral species after exceptionally large eruptions?
Establishment of an in situ monitoring systems. This investigation regards the providing of
optimal conditions for the quasi-real-time evaluation of critical parameters
to be used as proxies to define the caldera dynamics.
Rationale of the Campi Flegrei drilling proposal
The outcomes of the workshop provided a conceptual framework for a
full proposal for the drilling of the CFc to be submitted to the IODP and
ICDP (Fig. 4) as an Amphibious Drilling Proposal. The Campi
Flegrei ADP will combine complementary research topics into a general view
on collapse-resurgent calderas located along continental margins. The partly
submerged setting of the CFc provides a unique marine stratigraphic archive
for a detailed reconstruction of the timing and kinematics of individual
structures and components of the volcanic system, under different forcing
factors (internal vs. external) during the past 106 years (Fig. 5).
Flow chart illustrating the history of site-surveying activity and
nurturing of the continental (ICDP) and marine (IODP) components of the
Campi Flegrei Amphibious Drilling Proposal.
Timescales and forcing involved in the Campi Flegrei Amphibious
Drilling Proposal (ADP).
The drilling is important to reconstruct the subsurface 3-D stratigraphic
architecture, identify faults and volcanic or volcano-tectonic features, and
obtain information on the hydrothermal discharge areas and thermal
structure. It also provides valuable information concerning the eruptive
history of volcanoes and the dynamics of eruptions with different intensity.
Previous research in active volcanic areas has shown that drilling can be
fundamental in clarifying and constraining structural interpretations based on
geophysical data alone. For instance, drilling at the Kakkonda geothermal field
in Japan revealed the steep permeability and lithological gradients where
magmatic and hydrothermal regimes interact (Saito et al., 1997; Nakada,
2013). The IDDP-1 (Iceland Deep Drilling Project) well at Krafla in Iceland intercepted rhyolite melt in the
region where geophysics had implied it would be absent (Elders et al.,
2011). In the Long Valley Exploratory Well, a maximum temperature of
∼100∘C was measured where the presence of shallow
magma or at least very hot rock was assumed prior to drilling. Moreover, the
use of selected drill holes as observatories provides an additional
advantage for the in situ monitoring of volcanic and/or hydrothermal
activity.
Drilling into active volcanic areas is not completely without risk, even
though previous experiments in Japan and Iceland (Saito et al., 1997;
Nakada, 2013; Elders et al., 2011) suggest that the chance of triggering a
volcanic eruption is extremely small. The main risks are the hazards of
possible underground pressure blowouts, meeting zones of fluid loss and
material failure, and, more rarely, surface emission of liquefied sediments
and steam (e.g., Sawolo et al., 2009). Documented experience indicates,
however, that these risks can be significantly reduced or prevented by
applying the appropriate mitigation techniques (e.g., blowout preventer
systems) (e.g., Homuth et al., 2010).
Drilling at Campi Flegrei
The workshop was successful in identifying a number of relevant topics and
questions, whose response may solve fundamental problems related to the
caldera volcanism.
The Campi Flegrei caldera represents an ideal example of an active caldera located in a shallow-water setting (<200 m water depth). Other ODP Legs (Ocean Drilling Program) (e.g., ODP Leg 157: Gran Canaria and Madeira Abyssal
Plain) and IODP Expeditions (e.g., IODP Expedition 340: Lesser Antilles
Volcanism and Landslides) have focused on volcanic islands in deep oceanic
settings. Campi Flegrei provides a unique opportunity to obtain a
high-resolution stratigraphic archive of explosive, effusive, and extrusive
volcanism, volcano-tectonic dynamics, and unrest. Moreover, the proximal
marine setting of the CFc documents the primary deposition and reworking of
pyroclastic currents and fall deposits as components of the continental
shelf slope system.
Campi Flegrei is a primary site to unravel the timing, structure, and evolution of caldera resurgence and unrest based on the geological record of marine strata. The mixed marine
siliciclastic–volcaniclastic depositional architecture of the caldera fill
provides a unique opportunity to document the pattern, timing, and rates of
deformation related to resurgence since the Late Pleistocene. The last two
millennia of documented unrest also provide further constraints in
reconstructing time series of deformation onshore and offshore.
The Campi Flegrei caldera generated the largest explosive eruptions in Europe during the Late Quaternary. The results of drilling
and well logging will have a high potential impact on
paleoenvironmental–paleoclimatic reconstruction. The coupling of proximal
drill sites off Pozzuoli Bay with the results from the drilling of the
distal stratigraphic record will also help in reconstructing the dispersal
and erosive patterns of co-ignimbritic tephras. Also, this record could
provide some insights into the puzzling issue of the apparent causal
relationships between the environmental effects of the CI eruption and the
final decline of Neanderthals.
Drilling off the shore of the Campi Flegrei caldera will help investigate the interaction between the magmatic and hydrothermal systems and the occurrence of a wide range of subaerial-to-submarine features from monogenic volcanoes to hydrothermal vents. The apparent difference between
the onshore and offshore evolution may be related to changing magma–water
interactions under saturated conditions within the mixing zone between the
phreatic and marine pore waters, and this can only be investigated in detail by an
onshore–offshore drilling transect. Long-term borehole monitoring of
physical, chemical, and microbiological parameters may additionally provide
a chance to identify the potential precursors to eruptions for the purpose of risk
mitigation.
Amphibious drilling
The half-submerged setting of the CFc provides an opportunity to integrate
results from offshore and onshore drillings and available marine geology and
volcanological data. A deep, onshore borehole (∼3 km) will
allow the processes responsible for the recent unrest to be investigated at
depth through the determination of rock physical properties, magma–water
interaction, and water fluid chemical and physical exchanges. For instance,
(1) extrapolated temperature measurements can be used to detect the depth of
magmatic intrusions and the hydrothermal system; (2) the in situ chemical
composition of fluids will provide information on rapid changes in the
magmatic–hydrothermal system; and (3) deep monitoring systems will be
deployed and incorporated in the existing network of the INGV-Napoli (Istituto Nazionale di Geofisica e Vulcanologia)
(Osservatorio Vesuviano) to enable real-time tracking of such changes.
At the same time, a robust site-survey database, consisting of several
multi-frequency (even 3-D) reflection seismic datasets (both single-channel
and multichannel) has been acquired since 2008, yielding high-resolution
images of the uppermost 100 m of the crust as well as new images to
a depth of 1–2 km. Such a comprehensive database will enable a precise
selection of offshore drill sites and guidance regarding deviated onshore drilling.
The recent discovery of previously unknown volcanic structures and
hydrothermal vents offshore offers a high potential for an integrated
stratigraphic reconstruction (e.g., Sacchi et al., 2009, 2014; Passaro et
al., 2016; Steinmann et al., 2016, 2018). The combined observations and data
call for a joint offshore and onshore drilling program in order to (a) obtain an improved chronostratigraphic correlation between intra-caldera and
extra-caldera products and (b) understand the origin of caldera collapse
and the mechanisms of resurgence. An ideal drilling strategy at Campi
Flegrei would therefore include the following onshore and offshore
coordinated components:
Onshore drilling and well logging down to a depth in the order of 3000 m to investigate the caldera deep structure and associated deep magmatic–hydrothermal system. The drilling will provide the
opportunity to investigate the processes responsible for the recent unrest
episodes at depth, thereby allowing for a reliable evaluation of the hazard.
This component includes (1) the acquisition of physical–chemical parameters
of the geothermal system over the entire depth; (2) stress measurements (size
and direction) at depth; (3) the permeability measurements at depth; (4) the extrapolation of temperatures in the supercritical layer to detect the depth
of the magmatic temperature and locate magmatic intrusions; and (5) the determination
of the physical, mechanical, and rheological parameters of deep rocks.
Offshore drilling down to a maximum depth of∼1000 m to investigate the shallow structural levels of the caldera fill and resurgence as well as to unravel the mechanisms of magma–water interaction as a function of depth. This component provides the
opportunity to study an undisturbed sedimentary archive without the
challenges posed on land by intense subaerial erosion or urbanization (i.e.,
inaccessibility). This implies a much higher potential for structural,
geochronological, petrographic, and geochemical reconstruction. Hence, marine
drilling will provide a complete high-resolution stratigraphic record
which will (1) improve the chronostratigraphy of volcanic and sedimentation
events and unlock the timing and structural style of the deformation
associated with inner-caldera resurgence, (2) understand the climatic
effects and the environmental impact of ignimbrite eruptions on life and
ecosystems, and (3) investigate the impact of magmatic–hydrothermal
processes with respect to hydrothermal vents and shallow degassing
structures as well as submarine monogenetic volcanoes and intrusions.
Drilling objectives and borehole logging and monitoring
strategies
The workshop participants suggested that the IODP component of the ADP
proposal should address the integrated stratigraphy of the caldera fill and
resurgence, petrology, fluid geochemistry, and architecture of shallow
structural levels (<1000 m depth), whereas the ICDP component
should focus on rock–fluid properties, physical–chemical processes, and the
geothermal system at greater depth (<3000 m). The proposed drilling
strategy includes one major onshore drilling, complemented by an amphibious
drilling transect extending from the Campi Flegrei shoreline towards the SE
border of Naples Bay, together with distal drill sites in the Adriatic and
Ionian Seas (Figs. 2, 6–12 and Table 1). Down-hole logging and long-term
borehole monitoring at selected drill sites of primary physical and chemical
parameters, along with microbiological analysis of rocks and fluids, within
a depth range with a maximum of 0.5–1.0 km, have been also included in the planned
operations.
Location of onshore and offshore drill sites included in the first-draft plan of the Campi Flegrei Caldera Amphibious Drilling Proposal
discussed during the MagellanPlus workshop.
Illustrated section of the Campi Flegrei caldera structure indicating
the targets of the proposed onshore (ICDP) drill site (CFDDP-01). The
reconstruction is mostly based on geophysical and geological data, affected
by large uncertainties. The depth limit critical water temperature is
constrained by previous drillings in the area (AGIP, 1987). Modified after
De Natale and Troise (2011).
Synthetic lithostratigraphy and facies interpretation of the
succession sampled at the CFDDP pilot borehole (500 m) on the shore of Bagnoli in 2012
(modified after De Natale et al., 2016). Red asterisks indicate the depth and
40Ar/39Ar age of sampled K feldspars. See Figs. 2–3 and 6 for the
location of the borehole site.
Schematic reconstruction of the shallow structure (<1 km)
of the collapse-resurgent caldera associated with the eruption of the
Neapolitan Yellow Tuff (NYT) and the location of the proposed offshore (IODP)
drill sites CF-01 and CF-03.
Interpreted high-resolution multichannel seismic profile across
the CF caldera center and proposed location of drill sites CF-01 and CF-03.
M1–M4 are the inner-caldera marine siliciclastic units; CI is Campanian Ignimbrite;
NYT is Neapolitan Yellow Tuff.
High-resolution single-channel seismic profile across the
southern slope of the NYT resurgent dome and proposed location of drill site
CF-02. Unconformities labeled as uplift 1–5 are interpreted as the result
of a series of seafloor deformation phases associated with phases of the
deformation of the resurgent structure. Correlation of tephra layers sampled
by gravity core C32 is after Sacchi et al. (2014).
Proposed site CF-14 (Ionian Sea) for distal tephrostratigraphic
reconstruction of Late Quaternary volcanism of the Campanian region (Crocitti
et al., 2018; Di Donato et al., 2019). (a) Location map with an indication of the
main DSDP (Deep Sea Drilling Project), ODP, and IODP archives (yellow for marine and green for
terrestrial) of the central Mediterranean region; (b) dispersal maps of
plinian and sub-plinian events (Bronk-Ramsey et al., 2015); (c) dispersal
maps of moderately explosive eruptions (Sulpizio et al., 2014; Crocitti
et al., 2018). Please note that the years given in AD in Fig. 12c correspond to those same years in the CE notation system.
Summary of proposed onshore and offshore drill sites for the Campi
Flegrei Caldera Amphibious Drilling Proposal (ADP).
ADPProposed drillStructuralDrilling targetsDrillingRemarkscomponentsitesectordepth (m)ICDPCFDDP-01Caldera marginStratigraphically reconstruct and well log through the hydrothermal system down to the brittle and ductile zone∼3000Deviated well, directed from the eastern border of the caldera towards the caldera center at depthIODPCF-01Caldera centerSample the stratigraphic succession of NYT caldera fill and penetrate the structural caldera floor∼900Deep offshore well within the caldera collapse area; maximum drilling depth to be agreed on with safety panelsIODPCF-02Flanks of the resurgent domeDrill the post-15 ka caldera fill to reconstruct the timing of deformation and uplift of the caldera resurgence∼50Unique place to study the timing of the deformation of a caldera resurgent structureIODPCF-03Caldera collarDrill the subsurface magmatic intrusion and hydrothermal vent off the shore of Bagnoli (12–8 ka)∼120Subsurficial intrusion (6–4 ka); magma-water interaction; implications for volcanic hazardIODPCF-04Caldera peripheryDrill the CI deposits in the shallow subseafloor of Procida Channel∼100Proximal facies of the CIIODPCF-05Caldera peripheryDrill the stratigraphic succession of the peri-caldera monogenic volcano of Miseno Bank (> 120 ka)∼80Pre-caldera volcanismIODPCF-06Caldera peripheryDrill the stratigraphic succession of the peri-caldera volcanic apparatus of Penta Palummo Bank (> 120 ka)∼80Pre-caldera volcanismIODPCF-07Caldera struc- tural borderDrill the stratigraphic succession of Nisida Bank (ca. 18–15 ka)∼100Caldera-related volcanismIODPCF-08Caldera peripheryDrill the subsurficial intrusion and hydrothermal vent of Mt. Dolce–Pampano Bank (8–4 ka)∼250Subsurficial intrusion (18–15 ka); magma-water interaction; implications for volcanic hazardIODPCF-09Caldera exter- nal peripheryDrill the volcaniclastic diapirs and mounds associated with hydrothermal venting at Montagna Bank (15–5 ka)∼150Soft-sediment deformation and volcaniclastic diapirism triggered by overpressured fluidsIODPCF-010 CF-011 CF-012 CF-013Proximal caldera exteriorDrill the Upper Quaternary mixed siliciclastic–volcaniclastic succession of Naples Bay for a stratigraphic purpose∼200∼200∼200∼200Drilling transect for the recovery of a composite stratigraphic section; proximal stratigraphic record of Campi Flegrei eruptionsIODPCF-014Distal caldera exteriorSample distal products of major explosive events from Campi Flegrei and other eruptive centers of the Campanian district∼100Distal tephrostratigraphyProposed on-land drill site (ICDP component)Caldera margin to center
By drilling a ∼3 km long deviated well from the eastern
caldera margin towards its center (site CFDDP-01), we will be able to obtain
a reference stratigraphic succession of the CFc fill to the basement floor
and conduct well logging through the hydrothermal system down to the
brittle and ductile zone (Figs. 2, 6–7 and Table 1).
Another important component of the on-land drilling will be the deployment of
a network of in situ monitoring stations at depth to provide real-time
insights into changes in the hydrothermal–volcanic system. Such information
is crucial to understanding the ongoing unrest as well as to reliably assessing
hazards and risks. The drilling of site CFDDP-01 will rely on the
results of the 500 m deep pilot hole and associated well log data acquired
by the INGV-Napoli in 2012 (Figs. 2, 6 and 8).
Proposed offshore drill sites (IODP component)Caldera center – caldera fill, resurgent dome, and structural floor
The caldera fill represents a high-resolution archive of the post-caldera
volcanic succession, as well as a record of the ground deformation caused by
caldera resurgence. Hence, drilling the caldera fill will facilitate (1) the discovery of new
insights regarding post-caldera volcanic history, (2) the reconstruction of the
timing, duration, and conditions of caldera resurgence, and (3) understanding the caldera's subsurface structure. Penetrating the floor of
the caldera (site CF-01) will provide conclusive information on the
pre-caldera phase and caldera formation processes. Site CF-02 is designed to
recover the stratigraphic succession that formed over the flanks of the
resurgent structure, in order to provide ages and timing of volcano-tectonic
deformation since the NYT caldera collapse (last 15 000 years) (Figs. 2, 6, 9–11 and
Table 1).
Caldera collar – fractured, permeable zone
The annular depression (“collar”) between the structural border of the NYT
caldera and the inner-caldera resurgent dome is a highly fractured zone,
characterized by ascending fluids and locally shallow magmatic intrusions.
The area represents a remarkably permeable segment of the caldera structure
and is a key location to study the interconnection between the deep
magmatic–hydrothermal system and the surface and its role during caldera
unrest. Site CF-03 is planned to drill through the shallow structural levels
of the ring-fault zone of the NYT caldera collar down to small
laccolith-like intrusion off the shore of Bagnoli (Figs. 2, 6, 9–10 and Table 1).
Drilling operations will be limited to shallow depths (<150 m) and
will be only realized after stepwise monitoring of temperature and fluid
pressure.
Caldera structural border – pre-NYT caldera vents and intrusions
The outer border of the CFc is characterized by a number of offshore vents,
shallow magmatic intrusions, and ignimbrite deposits ranging in age from
∼120 to ∼18 ka. These provide a spectrum of
volcanic features produced by significant magma–water interaction. They
include most of the volcanic banks of the southern periphery of Pozzuoli
Bay and CI ignimbrite deposits occurring at shallow depths beneath the
seafloor, mostly between Procida and the mainland. Drill sites will aim to
characterize the nature of these volcanic centers and units and their role
in the onset of pre-CFc volcanism and the overall fluid circulation (as, for
instance, Nisida Bank represents an active fluid vent). Drilling at site
CF-04 (Procida Channel) will focus on the recovery of a proximal succession
of the CI. Drill sites CF-05 and CF-08 have been proposed to recover stratigraphic
successions from a series of volcanic banks, namely at sites CF-04 (Miseno
Bank), CF-05 (Penta Palummo Bank), CF-06, (Nisida Bank), and CF-07 (Mt. Dolce–Pampano Bank) (Figs. 2, 6 and Table 1).
Proximal extra-caldera area – Bay of Naples
Significant fluid venting in the Bay of Naples is not restricted to the ring-fault zone of the CFc, but it also occurs outside the structural border of the
caldera itself. This is the case of Montagna Bank, a sub-circular
seafloor region SE of Pozzuoli Bay that was formed by the dragging and
rising up of volcaniclastic diapirs (consisting mostly of unconsolidated
pumice), due to pore fluid overpressure at depth and associated fluid
migration towards the seafloor (Passaro et al., 2016). Site CF-08 has been
designed to drill through the volcaniclastic diapirs of Montagna Bank to
the roots of the unconsolidated sediments involved in this process. The
Naples Bay half-graben also represents an expanded, undisturbed sedimentary
succession with interbedded massive ignimbrite deposits (NYT and CI). An offset
drilling (CF-10 to CF-13) is an opportunity to cover a large time span of 1 million years (exceeding the entire time span of volcanic activity) with a transect of
relatively shallow (∼200 m) drillings (Figs. 2, 6 and Table 1). This will provide novel insights into the overall eruption history of
the entire Campi Flegrei area and its tectonic evolution. Also, by drilling
large ignimbrite units from top to bottom (i.e., contact zone of ignimbrite
and siliciclastic units), their environmental impact and subsequent the
recovery of life after major eruptions can be investigated.
Distal extra-caldera area – Ionian Sea
A distal drill site (CF-14) has been proposed to recover an undisturbed
pyroclastic fallout archive allowing for an integrated tephrostratigraphic
analysis of the entire eruptive history of the Campi Flegrei area and other
eruptive centers in the Campanian region (Figs. 6 and 12).
Down-hole logging and borehole monitoring strategies
The well logging plan incorporates a wide spectrum of down-hole measurements
which are designed to acquire maximum in situ information on petrophysical
and geomechanical properties, as well as enhance monitoring of the
strain–stress conditions, active seismicity, and the hydrothermal system at
depth. The main parameters to be measured include (1) natural gamma rays,
radioactivity, and spectrometry; (2) resistivity; (3) spontaneous
potential redox; (4) sonic log; (5) magnetic susceptibility; (6) hole diameter
(caliper); (7) temperature; (8) oriented microresistivity; and (9) acoustic and
ultrasonic borehole images. The use of long-term borehole observatories
(e.g., down-hole broadband seismic stations equipped with newly developed
opto-electronic strain sensors and advanced monitoring systems that
incorporate multiple seals allowing zoned measurements of in situ physical,
chemical, and biological properties) may be considered for sites CFDDP-01
and CF-01.
Concluding remarks
Every eruption is preceded by unrest, but not every unrest culminates in an
eruption (Acocella et al., 2015; Newhall and Dzurisin, 1988). Understanding
the driving forces of volcanic unrest and the role of magmatic–hydrothermal
processes is thus crucial for reliable hazard assessment. During the
MagellanPlus workshop, all participants agreed that the CFc represents an
ideal natural laboratory to study the interaction among volcanic,
hydrothermal, marine, and volcano-tectonic processes. The amphibious Campi
Flegrei drilling project, involving a deep ICDP and shallower IODP
drillings, will address fundamental aspects including phreatic and
hydromagmatic volcanism, caldera formation and subsequent structural
resurgence and post-caldera volcanism, fallout and ignimbrite stratigraphy,
hydrothermal-magma interactions, mechanisms of volcanic unrest, and
volcano-tectonic coupling. The results will significantly advance our
understanding of the most complex forms of volcanic structures on Earth.
Data availability
The onshore data supporting the work presented in this report are available at the
INGV-Napoli (Giuseppe De Natale: giuseppe.denatale@ingv.it); offshore data are available at the Faculty of Geosciences of the University of
Bremen (Volkhard Spiess: vspiess@uni-bremen.de) and CNR-ISMAR, Naples (Marco Sacchi: marco.sacchi@cnr.it).
Author contributions
MSa, GDN, VS, and LS jointly organized the workshop. MSa, GDN, LS, CK, and SDS drafted the paper. MSa, LS, and DI created the figures. All co-authors jointly contributed to the formulation of the concepts, scientific questions, and drilling/logging strategies discussed in the paper, according to their expertise: volcanology (SDS, NG, CS, HUS, MSu, and GV), petrology (LF), physical volcanology and volcanic hazards (GDN, VA, CK, AF, SP, RS, and CT), integrated stratigraphy (MSa, FM, and MV), structural geology of volcanic margins (VA, GV, and FP), tephrochronology (DI, PP, and ST), marine geophysics (VS, LS, SP, FP, and MC), and borehole logging (MJJ).
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This report summarizes the results of the MagellanPlus workshop “Structure and
Evolution of Magmatic and Hydrothermal Systems in offshore
collapse-resurgent calderas – Development of an IODP Drilling Proposal at
Campi Flegrei (Eastern Tyrrhenian Margin) linking to active ICDP Drilling
Initiatives” (25–28 February, Naples, Italy). The authors wish to thank the two anonymous reviewers for their
critical comments and suggestions on the early version of the manuscript.
Financial support
This research has been supported by the European Consortium for Ocean Research Drilling (ECORD) through the ECORD/ICDP MagellanPlus Workshop Series Programme (grant no. 2017.2376/4/AF).
Review statement
This paper was edited by Tomoaki Morishita and reviewed by two anonymous referees.
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