The Agnano��Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy)

Ž .Journal of Volcanology and Geothermal Research 91 1999 269–301www.elsevier.comrlocaterjvolgeores

ž /The Agnano–Monte Spina eruption 4100 years BP in the restlessž /Campi Flegrei caldera Italy

S. de Vita a,), G. Orsi a, L. Civetta a,b, A. Carandente a, M. D’Antonio b,A. Deino f, T. di Cesare b, M.A. Di Vito a, R.V. Fisher c, R. Isaia a, E. Marotta a,

A. Necco b, M. Ort d, L. Pappalardo a, M. Piochi a, J. Southon e

a OsserÕatorio VesuÕiano, 80056 Ercolano, Naples, Italyb Dipartimento di Geofisica e Vulcanologia, L. go San Marcellino, 10-80138 Naples, Italy

c Department of Geological Sciences, UniÕersity of California, Santa Barbara, CA, USAd Department of EnÕironmental Sciences and Geology, Northern Arizona UniÕersity, Flagstaff, AZ, USA

e LiÕermore National Laboratory, LiÕermore, CA, USAf Institute of Human Origin, Berkeley, CA, USA

Abstract

Ž . 40 39 14The Agnano–Monte Spina tephra AMST , dated at 4100 years BP by Arr Ar and C AMS techniques, is the productŽ . Ž .of the highest-magnitude eruption in the Campi Flegrei caldera CFc during its last epoch of activity 4800–3800 years BP .

The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two mainpumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the ventarea. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flowsovertopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit issubdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological character-istics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun duringthe course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, andgenerated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereaslater-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that twoisotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the

Ž 3 .eruption. The small volume 1.2 km DRE of erupted magma and the structural position of the vent suggest that theeruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Agnano–Monte Spina tephra; Campi Flegrei caldera; magma; pyroclastic-fallout; pumice

) Corresponding author. Tel.: q0039-81-7777149r150; fax:q0039-81-7390644; E-mail: [email protected]

1. Introduction

Volcanic hazard assessment is an important taskin modern volcanology. To do so, it is necessary toreconstruct the behavior through time, and to deter-

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 99 00039-6

( )S. de Vita et al.rJournal of Volcanology and Geothermal Research 91 1999 269–301270

minate the present state of a given volcano and itsfeeding magmatic system. To hypothesize the type ofexpected eruption, it is necessary to define the typeof eruptions that a specific volcano has produced in

Ž .the past. The Campi Flegrei caldera CFc , a denselyŽ .inhabited, restless, resurgent structure Fig. 1A–C ,

is one of the most dangerous volcanic areas on earth.Its magmatic system is still active, as testified by the

Ž1538 AD Monte Nuovo eruption Di Vito et al.,.1987 , the recent bradyseismic episodes in 1969–

1972 and 1982–1984 that have generated a net upliftof 3.5 m in and around the town of PozzuoliŽCasertano et al., 1977; Corrado et al., 1977; Barberi

.et al., 1984, 1991; Orsi et al., 1999-this issue , andthe widespread fumarolic and thermal springs activ-

Ž .ity Allard et al., 1991 . It has been the site ofstrongly explosive volcanism between 4800 and 3800

Žyears BP Orsi et al., 1996; Di Vito et al., 1999-this.issue , which resulted in the emplacement of pyro-

clastic-fall, -flow and -surge deposits over a widearea outside the caldera, well beyond its morphologi-

Ž .cal margins Di Vito et al., 1999-this issue . Orsi etŽ . Ž .al. 1996 and Di Vito et al. 1999-this issue demon-

strated the relationship among regional tectonism,volcanism and volcano-tectonism in theNeapolitan–Phlegraean volcanic area. They showedthat deformation was produced by a local stress fieldthat tended to follow preexisting geologic features,and that the areal distribution of volcanic vents isstrongly influenced by their orientation and kinemat-ics. In particular, vents active during the last 4800years of activity were mostly located within thenortheastern part of the resurgent block and subordi-nately in the Averno–Monte Nuovo area.

Among the eruptions occurred during the last4800 years of volcanic activity in the CFc, thehighest magnitude is the Agnano–Monte Spina erup-tion; it was characterized by eruptive phenomenathat caused the largest environmental impact. Anarea of about 1000 km2 at present inhabited bynearly 2 million people, was covered by at least10-cm-thick deposit of pyroclastic-fallout, while py-roclastic-flows or -surges invaded an area of about

200 km2, presently inhabited by about 600,000 peo-Ž .ple Fig. 1C .

2. The Campi Flegrei caldera

The volcanic and deformational history of theŽ .CFc has been recently discussed by Orsi et al. 1996 .

The caldera is a nested structure resulting from twomain collapses, related to the Campanian IgnimbriteŽ .CI; 37,000 years BP and the Neapolitan Yellow

Ž .Tuff NYT; 12,000 years BP eruptions, respectivelyŽ .Fig. 1C . The structural boundaries of each calderapartially result from reactivation of earlier regionalfault systems. The more recent caldera is beingdeformed by ongoing resurgence occurred through a

Ž .simple-shear mechanism Orsi et al., 1991, 1996with a maximum uplift of 90 m in its central part inthe last 10,000 years. The most uplifted part of thecaldera floor is the La Starza block, whose geometryis defined by N50W and N30E trending faults.

Volcanic activity in the last 12,000 years wasconcentrated inside the NYT caldera. About 60 erup-tions occurred after the NYT eruption during three

Žepochs of volcanic activity Di Vito et al., 1999-this. Ž .issue . During the first 12,000–9500 years BP and

Ž .the second 8600–8200 years BP epochs, ventswere located along the structural boundary of the

ŽNYT caldera. During the third epoch 4800–3800.years BP they were mostly located in a relatively

depressed area in the northeastern sector of the resur-Žgent block Orsi et al., 1996; Di Vito et al., 1999-this

.issue . The Agnano–Monte Spina eruption took placeinside this depression within the caldera deforma-tional framework.

( )3. The Agnano–Monte Spina tephra AMST

The Agnano–Monte Spina unit was first definedŽ .by Rosi et al. 1983 and later described by DiŽ . Ž .Girolamo et al. 1984 , Rosi and Santacroce 1984

Ž . Ž .and Rosi and Sbrana 1987 . Alessio et al. 1971determined a 14C age on a charred wood fragment of

Ž . Ž . Ž .Fig. 1. A Morphological map of the Campanian plain contour lines every 100 m . B Location and numbers of measured sectionsŽ . Ž . Ž .sections sc1, sc2 and sc3 are from Scherillo, 1954 . C Areal distribution of the Agnano–Monte Spina tephra AMST .

https://www.researchgate.net/publication/215755681_The_Phlegrean_Fields_Structural_evolution_volcanic_history_and_eruptive_mechanism?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2

https://www.researchgate.net/publication/223608242_Isotopic_study_of_the_origin_of_sulfur_and_carbon_in_Solfatara_fumaroles_Campi_Flegrei_Caldera?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2

https://www.researchgate.net/publication/215755614_Structural_evolution_of_Campi_Flegrei_Caldera_in_light_of_volcanological_and_geophysical_data?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2

https://www.researchgate.net/publication/223902291_Simple_shearing_block_resurgence_in_caldera_depressions_A_model_from_Pantelleria_and_Ischia_J_Volcanol_Geotherm_Res?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2

( )S. de Vita et al.rJournal of Volcanology and Geothermal Research 91 1999 269–301 271


Fig. 2. Stratigraphic sequences of selected boreholes showing the main thickness variations of the AMST. Vertical movements are shown bythe present elevation of beach and shallow sea sediments. Location of boreholes is reported in the inserted maps.


Ž .Fig. 2 continued .


Table 1Data from selected boreholes in which the AMST has been found. Location of boreholes and unit numbers are reported in Fig. 2. s.l.ssealevel; g.s.sground surface

Borehole Ground surface Depth of the top of Thickness of Underlying deposit Depth of marine Overlying depositŽ .number elevation m the AMST the AMST sediments

Ž . Ž .m from s.l. m from s.l.

2 12 7 )10 r r unit 16 157 152.2 )17 r r unit 1

12 95 82.4 )2.8 r r unit 113 86 74 )8.5 r r unit 114 100 88 )3 r r unit 117 75 69 )9.7 r r unit 121 64 57 6.6 unit 5 r unit 122 62 55 6.4 unit 5 r unit 124 62 45.2 )3.2 r r unit 125 62 48 3.3 unit 5 r unit 131 62.5 51.4 1.50 unit 6 r unit 132 45.4 35.4 3.5 unit 6 r unit 134 41 29.4 2 unit 6 r unit 135 16.4 9.4 32.5 unit 6 r unit 136 3.4 y6.8 )20.6 r r palustrine sediments37 33.5 27.2 )17 r r unit 138 12.2 2.2 17.5 unit 6 r unit 139 75 70.6 13.9 unit 6 r unit 140 85 80 4.5 unit 6 r reworked deposits41 90 80.4 10.4 unit 6 r unit 143 35.2 21.2 )16.5 r r reworked deposit

at the base of unit 144 14.5 3.2 19.2 marine sediments y16 unit 145 9 0.8 22.8 marine sediments y22 reworked deposits46 14 y21.2 25.5 paleosol above unit 6 y61.8 palustrine sediments47 24.7 13.7 29.2 paleosol above unit 5 y22 unit 148 166 140 21.5 paleosol above unit 6 r unit 149 5.2 y3.3 )9.5 r r palustrine sediments51 39.4 28.6 1.6 unit 6 r unit 153 38.5 26.8 3.2 unit 6 21.6 unit 355 43.8 28.2 5 unit 6 22.4 unit 358 34.4 34.4 7.8 unit 6 25.8 r

Ž .4000"50 years BP. Di Girolamo et al. 1984 de-scribed the products of this eruption as a sequence ofpumice- and scoria-flows confined inside the Ag-nano plain, underlain by an associated lag–brecciadeposit, and characterized by the presence of awelded scoria deposit in the upper part. Rosi and

Ž . Ž .Santacroce 1984 and Rosi and Sbrana 1987 , basedon 15 sections measured over an area of about 400

km2, identified four main phases in the Agnano–Monte Spina eruption. The first phase included avent opening explosion and the formation of a brec-cia deposit. The second phase was characterized by aplinian column that generated pumice-fallout de-posits, dispersed toward the east and northeast. Dur-ing the third phase, the eruption column collapsedand generated pyroclastic-flows that traveled as far

Ž .Fig. 3. AMST type section not in scale . The values reported for eruption column heights and volumes of fallout deposits are calculatedŽ .using the method of Pyle 1989 . Emplacement temperature measurements were carried out on two samples from proximal deposits of layers

A2 and B2, collected at section numbers 102 and near the town of Pozzuoli, respectively. Samples were analyzed on a Molspin spinnermagnetometer at University of California, Santa Barbara, and demagnetization carried out by standard step-wise thermal techniques.



as 5–6 km from the vent area moving over 100–150m high morphological barriers. Explosions driven bymagma–water interactions, which produced pyro-clastic-surges that did not surmount the scarps bor-dering the CFc, characterized the last phase of theeruption. The authors, on the basis of the arealdistribution of the basal breccia deposit, suggestedthat the area of Monte Spina hill, in the Agnanoplain, was the vent area for this eruption. For char-coal embedded in proximal deposits, they obtainedan average age of 4400 years BP

We have measured stratigraphic sequences of theAMST in 80 sections over an area of about 1000

2 Ž .km Fig. 1B . The most distal studied exposure is atMonteforte, 44 km northeasterly of the eruption ventŽ .Fig. 1A–B . The distribution of the unit inside thecaldera has been also investigated by interpreting the

Ž .cores of 67 boreholes Fig. 2 drilled in variablemorphological settings, at depth variable from 15 to125 m from ground surface. The unit has beenrecognized in 43 of the 67 examined logs. Theborehole data reported in Table 1, allowed us todefine the thickness variations of the AMST and thevertical movements of the Agnano plain. The thick-ness of the unit varies from a maximum estimatedvalue of about 70 m in the Agnano plain to fewcentimeters at a distance of 50 km from the vent areaŽ .Fig. 1C . The unit usually overlies a thin, poorlyevolved paleosol that locally contains small frag-ments of charred wood and pieces of pottery, exceptin the Fuorigrotta plain where it occurs directly

Ž .above shallow-sea sediments Orsi et al., 1996 .Variations of sedimentological characteristics andpresence of a significant erosional unconformity haveallowed the subdivision of the AMST in six mem-bers, named A through F, from base upsection.Many of the members have been subdivided in lay-ers due to the occurrence of second-order sedimento-logical variations. The type sequence of the unit andthe main sedimentological characteristics of eachdeposit are reported in Fig. 3. The analysis of thesedimentological characteristics of the AMST de-posits shows some features common to several lay-

Ž .ers. All the pumice-fallout layers e.g., A1, B1, D1are composed of cauliflower-shaped pumice clasts,broken on impact with the ground. Very few pumiceclasts are unbroken, therefore accurate pumice iso-pleth map cannot be made, specially in the more

Ž .proximal areas Figs. 6A, 7B and 10B . Isopleths ofmaximum lithic clast size of the same layers areregularly elliptical in the distal areas, and, instead ofnarrowing to a point in the proximal areas, they

Ž .widen out Figs. 6B, 7C and 10C . This widening isinterpreted to be caused by an abundance of ballisticclasts in the proximal areas. The sedimentologicalcharacteristics of each layer, as well as their arealdistribution, allows constraints for interpreting themechanisms of formation and deposition of eachmember.

The values reported for eruption column heightsand volumes of fallout deposits are calculated using

Ž .the method of Pyle 1989 .The results of our study corroborate the previous

Žhypothesis Di Girolamo et al., 1984; Rosi and San-.tacroce, 1984; Rosi et al., 1983 that the location of

the eruption vent is in the Agnano plain. Lackingmorphological evidence of a vent, we will use thecenter of the plain as a source reference for most ofthe AMST beds.

Member A, subdivided in two layers, named A1Ž .and A2 Fig. 3 , is exposed over an area of about

800 km2 which covers almost the entire outcrop areaŽ .of the AMST Figs. 4 and 5 . The basal ash bed of

layer A1 covers the eastern and northeastern outcroparea of the unit. The areal distribution of maximumsize pumice clasts of the upper coarse bed does not

Ž .allow reconstruction of an isopleth map Fig. 6A . Itonly shows a general decrease of the size of theclasts with the distance along the dispersal axis.

Ž .Isopleths of the maximum lithic clasts Fig. 6Bshows an easterly directed dispersal axis, but theyare elliptical in the distal areas, while distorted andwidened in the proximal areas. This member isinterpreted to be the product of an initial phase of theeruption, during which an efficient water–magma

Ž .interaction caused: a extreme magma fragmentationŽ .and formation of an expanded cloud, and b forma-

tion and collapse of a low eruption column with ahigh particle concentration. In this first phase, wateravailable for interaction with magma was probablyfrom near-surface ground water. The facies variationof layer A2 is interpreted as the product of variableemplacement mechanisms of pyroclastic deposits,according to distance from the vent and topographyat the time of their formation. Within the CFc layerA2 represents mainly the product of deposition of


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Fig. 4. Correlations among AMST measured sections, along a south–southwest–north–northeast and a southwest–northeast directions. Location of the section is reported in theinserted map. Legend as in Fig. 3.


Fig. 5. Areal distribution of member A. Dots indicate location of measured sections. Dots without numbers are relative to sections in whichthe member is not exposed.

pyroclastic currents; outside the CFc, in those siteswhere they were channeled along morphological sad-dles, it is the product of simultaneous deposition ofpyroclastic-surges and -fallout; in more distal areaslayer A2 is represented only by an ash-fallout de-posit.

Member B is subdivided in two layers, named B1Ž .and B2 Fig. 3 . Layer B1 is a pumice-fallout de-

posit, which covered an area larger than 500 km2

with a dispersal axis oriented toward the east–north-Ž .east Fig. 7A . The irregular areal distribution of

maximum pumice clasts does not allow reconstruc-Ž .tion of a detailed isopleth map Fig. 7B , at least in

the more proximal areas. The isopleth map of themaximum lithic clasts shows the same dispersal axis

Ž .as the isopach map Fig. 7C . As in the case of thecoarser part of layer A1, the isopleths in the proxi-mal areas are widened. The variable facies patternshown by layer B2 is related to distance from theeruption vent and topography at the time of theiremplacement. Layer B2 is prevailingly composed ofpyroclastic-flow with subordinate strombolian de-

Žposits inside the lowland of the Agnano plain prox-.imal facies, section numbers 101, 152, 153; Fig. 1B ;

of pyroclastic-flow and -surge deposits at the foot ofthe scarps that border the Fuorigrotta, Soccavo and

ŽPianura lowlands near proximal facies, section num-.bers 151, 159; Fig. 1B ; of pyroclastic-surge and

Žpumice-fallout deposits, above these scarps near.distal facies, section number 157; Fig. 1B ; and of

Ž .fallout deposits in the distal areas distal facies .Pyroclastic-flow and -surge deposits were distributedover an area of about 100 km2, while fallout deposits

2 Ž .blanketed an area larger than 600 km Fig. 8 .Member B is interpreted to be the product of amagmatic phase of the eruption that began withdeposition of layer B1 from a pulsating eruptioncolumn. Layer B2 is interpreted to be generated by

Ž .two interplaying phenomena: a collapse of theŽ .eruption column and b the intermittent water–

magma interaction that generated base-surges, strom-bolian scoria-fallout and pumice-and scoria-flows.During this phase of the eruption, the geothermalsystem may have been partially involved. The maincause for the facies change of layer B2 is the barrierof the slopes that border the Fuorigrotta, Pianura andSoccavo lowlands. In particular, pyroclastic-flowsand -surges did not overtop the Camaldoli hill, to-ward the northeast, the saddle between the Camaldoliand Vomero hills, toward the east, and the Posillipo

Ž .hill toward the southeast Figs. 1A and 8 .Member C, consisting mainly of east-to-northeast

dispersed ash beds, occurs in all distal exposures, butŽ .is missing in proximal areas Fig. 4 . It covered an

2 Ž .area larger than 600 km Fig. 9 . This member iseroded by the overlying pyroclastic-flow deposits, or


Ž . Ž .Fig. 6. Upper coarse bed of layer A1. A Maximum pumice clasts. B Isopleth map of maximum lithic clasts. Dots indicate location ofmeasured sections. Dots without numbers are relative to sections in which the layer is not exposed.

it is cut by other erosional agents. Member C isinterpreted to be the product of fallout of particlessuspended in the atmosphere after the emplacementof member B.

Member C and the upper part of member B areŽcut by a roughly planar, erosional unconformity Fig.

.3 which slopes between 10 and 308. This uncon-formity occurs along the slopes that border the Ag-

Ž .nano section numbers 102, 152, 154; Fig. 1B andŽ .the Pianura section numbers 140, 159; Fig. 1B

plains, where the underlying sequence slopes toward

the lowlands. Wherever the sequence is subhorizon-tal there is no evidence of erosion or soil formationbetween members C and D. The erosional uncon-formity indicates that there was a time break duringthe eruption. During this time break, continued thedeposition of suspended fine ash that forms memberC. Deposition may have been accelerated by heavyrains, which also caused strong erosion along gullies.In some places member C was deposited and erodedbut in some other places it was not deposited, ac-cording to the random distribution of rains. Although



Fig. 8. Layer B2. Thickness of the fallout deposit and areal distribution and thickness of pyroclastic-flow and -surge deposits. Dots indicatelocation of measured sections. Dots without numbers are relative to sections in which the layer is not exposed.

the erosional unconformity was well developed, theŽtime break was probably short a few days or

.weeks? , because there is no evidence of soil forma-tion or erosional unconformities in almost flat orgently sloping areas.

Member D has been subdivided in three layers,Ž .named D0, D1, and D2 Fig. 3 . Layer D0, exposed

only in few sections in proximal areas, uncon-formably lies above the erosional unconformity thatcuts the lower part of the AMST. Layer D1 is apumice-fallout deposit with a northeastward orienteddispersal axis. Isopachs are quite regular elliptical

Ž .curves Fig. 10A . Layer D1 covered an area of atleast 700 km2. The non-systematic thickness distri-bution of the maximum pumice clasts does not allow

Žthe construction of a detailed isopleth map Fig..10B . Lithic clasts are larger than pumice clasts in

the distal outcrops. The isopleth map of the maxi-mum lithic clasts also for this deposit shows thetypical distortion of the curves in the more proximalareas, up to a distance of about 5 km from the vent

Ž .area Fig. 10C . In more distal areas and toward theeast, layer D1 is the stratigraphically highest exposeddeposit of the AMST. Layer D2 usually thickens in

preexisting valleys of the Campi Flegrei morphologi-cal low and thins toward the slopes that border theAgnano, Fuorigrotta, Soccavo and Pianura lowlands.Some surge deposits overtop the northeastern slopesof the Pianura plain, passing through the saddlebetween the Camaldoli hill and the section number

Ž .134 Fig. 1B . On the northwestern side of theCamaldoli hill these pyroclastic-surges were chan-neled into the gullies that formed the drainage net-work. They continued into the plain, reaching adistance of about 15 km from the vent area wherethey deposited beds with a thickness of about 3 cmŽ .i.e., Aversa; Fig. 11 . The pyroclastic-flow and-surge deposits were distributed over an area ofabout 180 km2. The equivalent distal deposits of thepyroclastic-flows and -surges of layer D2 are falloutlayers dispersed mainly toward the north–northeast.

ŽThey occur at Cancello and near Acerra section.numbers 147, 164, 165, 179; Fig. 1B . Toward the

east, the deposit outside the caldera consists of apumice-fallout, layered for contrasting grain-size.Toward the north and northeast, in more distal areas,layer D2 is the stratigraphically highest exposeddeposit of AMST. Layer D0 is interpreted as the

Ž . Ž . Ž .Fig. 7. Layer B1. A Isopach map. B Isopleth map of maximum pumice clasts. C Isopleth map of maximum lithic clasts. Dots indicatelocation of measured sections. Dots without numbers are relative to sections in which the layer is not exposed.


Fig. 9. Thickness of member C. Dots indicate location of measured sections. Dots without numbers are relative to sections in which themember is not exposed.

product of the flashing of the geothermal system thatformed base-surges. Layer D1 is a fallout depositfrom a sustained eruption column. It represents thebeginning of a new magmatic phase of the eruption,during which a volcano-tectonic collapse also oc-curred. Compared with the underlying, subplinian

Ž .pumice-fallout layer B1 , layer D1 is more north-Ž .ward dispersed Figs. 7A and 10A . Moreover, the

isopachs map of layer D1 clearly shows a lessaccentuated decrease in thickness across the disper-sal axis, compared with layer B1. Such a differentorientation of the dispersal axis and shape ofisopachs, may be related to differences in orientationand strength of dominant winds in this phase of the

Ž .eruption. Layer D2 was possibly generated by: athe collapse of the eruption column previously

Ž .formed, andror b the efficient interaction of waterof the geothermal system and the magma that gener-ated base-surges and pyroclastic-flows.

Ž .Member E Fig. 3 is dispersed northward of theeruption vent, and is exposed over an area of about

2 Ž .200 km Fig. 12A, B and C . The pyroclastic-surgedeposits locally interbedded with the fallout deposits

of Layer E1, rapidly decrease in thickness and disap-pear as they approach the slopes that border theplains of Pianura and Soccavo. They were distributed

2 Ž .over an area of less than 70 km Fig. 12A , whilethe fallout deposits covered an area of about 300

2 Ž .km . Layer E2 Fig. 12B covered an area of about300 km2. The isopachs, although not well con-strained, owing to few exposures, are quite regularellipses that suggest the eruption vent had shiftedabout 2 km northwestward of the source vent of theprevious fallout layers. The data collected on arealdistribution of maximum pumice and lithic clasts ofthe middle, coarser bed, are insufficient to drawisopleth maps. Layer E3 occurs over the entire out-

Ž .crop area of member E Fig. 12C . Toward the northit preserves its internal stratigraphy even thoughthere is a slight decrease in thickness and grain-sizewith distance from the vent area. Layer E3 is ofvariable thickness, down to the outcrop scale. Thick-ness abruptly decreases approaching the slopes thatborder the Soccavo and Pianura plains and thenincreases in the Gricignano plain owing to the over-

Ž .lapping of different flow units Fig. 4 . The pyroclas-

Ž . Ž . Ž .Fig. 10. Layer D1. A Isopach map. B Maximum pumice clasts. C Isopleth map of maximum lithic clasts. Dots indicate location ofmeasured sections. Dots without numbers are relative to sections in which the layer is not exposed.



Fig. 11. Layer D2. Thickness of the fallout deposit, and distribution and thickness of pyroclastic-flow and -surge deposits. Dots indicatelocation of measured sections. Dots without numbers are relative to sections in which the layer is not exposed.

tic currents overtopped the morphological barrier ofnortheastern slopes of the Pianura plain through thesaddle located between the Camaldoli hill and sec-

Ž .tion number 134 Fig. 1B . On the northwesternslope of the Camaldoli hill, intersected by a densedrainage network, these currents were channeled intogullies forming lobes that flowed into the plain to asfar as about 15 km from the vent area, covering an

2 Ž .area of about 170 km Fig. 12C . Overlapping oflobes in the plain produced anomalous thickening of

Žthe deposit, as in the Gricignano area Figs. 4 and.1C . Layer E1 represents the product of a new

phreatomagmatic phase that caused development ofan expanded cloud accompanied by base-surges.Layer E2 is interpreted to be the product of a moremagmatic explosion that determined the formation ofa short-lived eruption column, initially pulsating,which increased its height during the eruption anddispersed without a final collapse. Layer E3 is inter-preted to be the product of a repetitive series ofphreatomagmatic explosions that produced pyroclas-tic-flows and -surges and minor amounts of falloutdeposits from a continuously collapsing low eruptioncolumn. The sedimentological characteristics of thesedeposits and their areal distribution, suggest that they

were erupted from different vents, likely along frac-tures generated during the volcano-tectonic collapseof the Agnano plain.

Ž .Member F Fig. 3 occurs only in the area ofŽ .section number 109 Fig. 1B . In section numbers

Ž .140, 151 and 157 Fig. 1B it is partially reworkedand humified. Occurrence of indurated ash betweensingle beds suggests possible time breaks in thedeposition of the sequence. If so, each bed representsthe product of a single phreatomagmatic explosion,each of which formed an expanded ash cloud. Verylikely during this final phase of the eruption, therewas also the fallout of the ash formed during theprevious phase of the eruption and still suspended inthe atmosphere.

The volume of tephra extruded during the erup-tion has been estimated in the order of 1.8 km3

Ž 3 .about 1.2 km DRE , 0.025 of which are accountedfor the pyroclastic-fallout deposits.

4. Volcano-tectonic collapse

The AMS eruption was accompanied and fol-lowed by a volcano-tectonic collapse that affected

Ž . Ž . Ž .Fig. 12. Member E. A Layer E1. B Layer E2. C Layer E3. Dots indicate location of measured sections. Dots without numbers arerelative to sections in which the layers are not exposed.



Ž .the area of Agnano Fig. 13 . The present Agnanoplain is the remnant of the structure generated by thiscollapse, which has been partially hidden toward thenorthwest by later eruptions of Astroni.

Isopach and isopleth maps of the whole AMSTdeposit and of the single members and layers con-strain the location of the vent in the Agnano plain. Inthis area there is no evidence of any volcanic edificeformed during the eruption. As the younger ventswere located toward the northwest, we can excludethat the AMST edifice was destroyed by later vol-canic activity. If an edifice formed during the erup-tion, it was likely destroyed by a collapse of the area.

The Agnano plain has a northwest–southeastŽ .elongated, polygonal shape Fig. 13 . It is bordered

by rectilinear features, except toward the northwest,where its margin is covered by the southeasternsector of the later Astroni tuff ring. The rectilinearfeatures are high-angle scarps trending northwest–southeast and northeast–southwest. Many of theminclude triangular facets that develop 50 to 70 mfrom the present plain.

The stratigraphic sequence of the borehole drilledŽ .in the Agnano plain number 46 in Fig. 2 shows that

AMST is 25.5 m thick and was deposited in asubaerial environment above the Monte St. Angelo

Žtephra 4400 years BP; Di Vito et al., 1999-this.issue which is 14.7 m thick and overlies a beach

deposit. The base of the AMST sequence is locatedat 46.7 m b.s.l., while the top of the beach deposits isat 61.4 m b.s.l. The sea level 4400 years BP was lessthan 5 m lower than present and did not vary signifi-

Ž .cantly until deposition of the AMST 4100 years BPŽ .Labeyrie et al., 1976 . This implies that the base ofthe AMST in the Agnano plain has been loweredabout 56 m since its deposition. The AMST isoverlain by a sequence of palustrine deposits withthe same sedimentological characteristics over theentire 25 m thickness. In turn it is covered by about2 m of continental, coarse sediments topped by the

Ž .Astroni tephra 3800 years BP . The palustrine sedi-ments must have formed in a continental environ-ment at about sea level. Lack of deposits between theAMST and the overlying palustrine sediments sug-gests that the top of the unit was located at sea levelsoon after its formation. The thickness of the palus-trine deposits suggests that the area subsided; fur-thermore the constancy in their sedimentological

characteristics implies that subsidence and sedimen-tation rates were similar. We do not know the age ofthe continental, coarse sediments and arbitrarily use

Ž .the age 3800 years BP of the Astroni eruption as anupper limit for the end of deposition of the palustrinesediments. It can be inferred that the Agnano plainsubsided about 25 m at a rate of at least 6 cmryr

Ž .after AMST 4100 years BP and before AstroniŽ .3800 years BP eruptions. The average net loweringof the plain is about 60 m, 25 of which wereaccounted for after the AMST eruption. Therefore, itis concluded that a displacement of about 35 moccurred during the course of the eruption.

The unconformity that intersects the AMST frommember C downsection suggests that there was apause during the eruption. The general increase inlithic content and the occurrence of lithic-rich brec-cias in member D deposited after the formation ofthe erosional unconformity, suggest reactivation offractures. The areal distribution of the pyroclastic-flow deposits above and below the erosional uncon-formity is here interpreted as due to migration of thevent along fractures. Such a migration is also sup-ported by the shifting of the vent of the fallout bedsof member E relatively to those of the other falloutbeds. The beginning of the collapse of the Agnanoarea apparently took place during the probable pausein the eruption. Activation of fractures with conse-quent decompression of the magma reservoir mayhave occurred already before the pause, as seems to

Ž .be suggested by the petrological data see Section 6 .The volcano-tectonic collapse affected an area of

about 6 km2 and was of about 60 m. Therefore, thevolume of the collapsed area is of about 0.4 km3,smaller than the estimated volume of erupted magmaŽ 3 .1.2 km DRE .

5. Geochronology

5.1. 40Arr39Ar dating

Sanidine phenocrysts extracted from pumice lapilliŽ .sample OCF-MS 106B , collected at section number

Ž . 40106 Fig. 1B in layer B1, were dated by the Arr39Ar technique, both by laser single-crystal total-fu-sion analyses, and bulk laser incremental-heating

Žtechniques see Deino and Potts, 1990; Deino et al.,

https://www.researchgate.net/publication/248792977_Single-crystal_40Ar39Ar_dating_of_the_Olorgesailie_Formation_Southern_Kenya_Rift?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2


Ž .Fig. 13. Morphostructural map of the Agnano plain modified by Di Vito et al., 1999-this issue .

1990; Sharp and Deino, 1996 for details of the.analytical procedure . Eleven single-crystal analyses

were performed on grains in the 14- to 30-mesh sizeŽ .range 1.2–0.55 mm . One incremental-heating ex-

periment was performed on approximately 80 mg ofsanidine phenocrysts. Table 2 summarizes of the

analytical data pertaining to all of the 40Arr39Aranalyses.

The single-crystal total-fusion experiments wereconducted to identify the age distribution of individ-ual crystals from the tephra, and to identify anoma-lous components such as inhomogeneously dis-

https://www.researchgate.net/publication/285078245_CO2_laser_heating_of_samples_for_Ar-Ar_geochronology?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2


tributed excess argon, xenocrysts, or cryptic alter-ation. The symmetrical Gaussian-like distribution ofthe 11 analyses suggests that the sanidine populationis indeed homogeneous in age, and as well, in chem-

Žical parameters CarK, and the percent of radiogenic40 U .argon, denoted as % Ar ; Fig. 14A . We realize

that 11 analyses are insufficient to identify rareranomalous components, if present. The weighted-mean age of the single-crystal analyses is 5500"700years BP. An inverse-isochron analysis of these dataŽ36 40 40 39 .Arr Ar vs. Arr Ar yielded an age of 5200"

Ž40 36 .900 years BP, an Arr Ar intercept compo-trappedŽsition of 296"4, and a MSWD mean sum of

.weighted deviates of 1.3. Because the ‘trapped’40Arr36Ar composition of the sanidine matches that

Ž40 36 .of air Arr Ar(295.5 within analytical uncer-tainty, the presence of an excess argon componentŽ40 36 .Arr Ar)295.5 is not suspected. The incremen-tal-heating analyses relies on a far larger sampling ofgrains and provides an average measure of the argonsystematic of the sample. The release spectrum ischaracterized by law radiogenic steps in the first10% of the total 39Ar release, followed by overallincreasing %40ArU to as high as 75% in the final stepŽ .Fig. 14B . Other than the second step, all ages fallwithin a 95% confidence interval of each other, anda plateau is defined as extending from step C throughL, encompassing about 95% of the total 39Ar. Theplateau age is 3900"400 years BP, within error ofthe integrated age of 4700"900 years BP. An in-verse isochron of the incremental-heating data yielded

Ž40 36 .an age of 3900"400 years BP, a Arr Ar trapped

intercept composition of 302"5, and a MSWD ofŽ40 36 .0.75. Again, the Arr Ar composition istrapped

within that of the air value so that the presence ofexcess argon cannot be demonstrated.

The isochron ages are taken as more reliable agesthat straight weighted-mean calculations of ages froma series of analyses because the isochron analysisinherently accommodates any departure from theatmospheric argon ratios in the sample material. Theisochron ages for the single-crystal and incremental-heating analyses are statistically indistinguishable,and yield a combined weighted-mean age of 4100"

400 years BP. That this result strongly favors theoutcome of the incremental-heating experiment isappropriate, in that the individual steps of this exper-iments yield much higher %40ArU contents that the

single-crystal experiments, were of larger gas yield,on average, and so less affected by machine blank,and were based on a much larger statistical sampling.

5.2. Radiocarbon dating

The radiocarbon age for the AMST is quoted atŽ .4400 years BP by Rosi and Sbrana 1987 , based on

six dates ranging from 4065 to 4660 years BP, forcarbonized wood embedded in AMS deposits. Scan-

Ž .done et al. 1991 assessed the same results, plusdates of 4000"50 and 4070"50 years BP oncarbonized wood recovered from AMS pumiceŽ .Alessio et al., 1971, 1973 and assigned a signifi-cantly later date of 4000 years BP. Table 3 summa-rizes dates in literature for charcoal which receivedacid–base–acid treatment to remove contaminatinghumics. We consider bulk paleosol dates to be lessreliable, as soil carbon fractions can have 14C ‘re-

Žservoir ages’ of hundreds years at burial Vogel et.al., 1990 , or alternatively, may be affected by infil-

tration of recent humics in groundwater.For this study, we used accelerator mass spec-

Ž .trometry to radiocarbon date small mg-sized char-coal fragments picked from the AMST, underlyingpaleosols, and an archaeological site. The results areshown in Table 4.

Our attempt to date the AMST failed, as smallcharcoal-like black flecks recovered from the basalash yielded no measurable carbon after pretreatment.However, our data do constrain the AMST age.Charcoal sample OCF-MS 130, from a paleosol un-

Žderlying the AMST south of Qualiano section num-.ber 130; Fig. 1B , must have been deposited before

the eruption, so the date of 4130"50 represents anupper limit for the event. Likewise, sample OCF-157

Ž .Pa 4170"40 years BP recovered from a paleosolunderlying Paleoastroni 2 tephra in section number

Ž .157 Fig. 1B , must have been laid down before thateruption, which in turn preceded the AMST, andhence also gives an older limiting date. Similarly,paleosol and an archaeological site at section number

Ž .109 and Orta di Atella Fig. 1A, B underlie theŽ .AMST and provide further limiting ages Table 3 .

Since at least some of these samples were probablydeposited several decades before the AMST, we take4100 years BP as an upper limit for the radiocarbonage of the event.


Table 240Arr39Ar analytical data. Sample OCF-MS 106B. ‘CarK’ is calculated from 37Arr39Ar using a multiplier of 1.96. ‘40ArU ’ refers toradiogenic argon. ‘mol39Ar’ refers to the estimated total moles of 39Ar released during fusion based on spectrometer sensitivityconsiderations. ‘t.f.’ indicates ‘total fusion’. ‘MSWD’ is mean sum of weighted deviates. Errors in age for individual analyses are 1sanalytical uncertainty. Integrated age is the combined age of all gas fractions weighted on the basis of the 39Ar abundance, with anuncertainty calculated as the square root of the sum of the squares of the individual errors, weighted also on the basis of 39Ar abundance.The plateau age is calculated as the inverse-variance weighted-mean of the plateau steps, with an error of one standard error of the

Ž .weighted-mean Taylor, 1982 . The weighted-mean age of the single-crystal analyses is also calculated using weights equal to the inversevariance of the analytical uncertainties. The error in the weighted-mean age of the single-crystal analyses is calculated by the method of

Ž . Ž .Samson and Alexander 1987 , as this yields a greater uncertainty than the conventional method e.g., Taylor, 1982 due to incorporation ofŽ .excess measured dispersion in the data set MSWD)1 . Error in the integrated age and plateau age of the incremental-heating sequence,

and the weighted-mean age of the single-crystal, total-fusion analyses include uncertainty in the neutron fluence parameter J. The neutronŽ .fluence monitor is sanidine from the rhyolite of Alder Creek Turrin et al., 1994; Renne et al., 1998 with a reference age of 1.194"0.012

Ma. Samples were irradiated in the Cd-lined CLICIT facility in the core of the Oregon State University TRIGA reactor at a power level of 1y5 y7 Ž36 37 . y4 y6 Ž39 37 .MW for 3 min. Js1.699=10 "1=10 . Isotopic interference corrections: Arr Ar s2.64=10 "1.7=10 . Arr ArCo Ca

y4 y6 Ž40 39 . y4 y4 y10s6.73=10 "3.7=10 . Arr Ar s7=10 "3=10 . 1s5.543=10 ryrK

U U U36 39 40 39 39 40Lab ID Power CarK Arr Ar Ar r Ar Ar % Ar Apparent age15Ž . Ž . Ž .Wans moles=10 years BP"1s

Incremental-heating analysesA 3 0.278 0.44908 2.194 0.4 1.6 66"72B 4 0.062 0.00357 0.375 1.4 26.3 11.2"3.6C 5 0.056 0.00208 0.043 1.7 6.6 1.3"2.8D 6 0.038 0.00117 0.153 2.8 30.7 4.5"1.7E 7 0.007 0.00094 0.171 4.3 38.3 5.2"1.1F 8 0.036 0.00073 0.126 5.5 36.9 3.9"0.8G 9 0.021 0.00042 0.138 6.1 52.7 4.1"0.7H 10 0.054 0.00030 0.145 4.2 62.8 4.3"1.9I 12 0.056 0.00039 0.101 3.7 47.3 3.0"1.7J 14 0.0290 0.00030 0.102 2.5 54.0 3.0"1.7K 18 0.044 0.00039 0.094 2.7 45.6 2.8"1.6L 25 0.015 0.00017 0.147 3.1 75.0 4.3"1.9Integrated ages 4.7"0.9Plateau age, steps C–Ls 3.9"0.4

Single-crystal total-fusion analyses1 t.f. 0.041 0.00392 0.227 2.7 16.4 6.9"1.12 t.f. 0.044 0.00694 0.050 1.8 2.4 1.5"1.73 t.f. 0.038 0.00687 0.165 2.5 7.5 4.9"1.34 t.f. 0.041 0.00398 0.170 2.4 12.6 5.2"1.25 t.f. 0.035 0.01308 0.321 1.7 7.7 9.7"2.16 t.f. 0.039 0.00038 0.166 1.1 59.6 4.9"2.47 t.f. 0.040 0.01361 0.120 1.9 2.9 3.7"1.88 t.f. 0.041 0.01563 0.180 1.7 3.8 5.4"2.29 t.f. 0.049 0.00635 0.195 2.0 9.4 5.8"1.510 t.f. 0.043 0.02469 0.203 0.5 2.7 6.0"6.311 t.f. 0.041 0.01346 0.209 1.5 5.0 6.2"2.2Weighted-mean ages 5.5"0.7MSWDs 1.1

Two identical dates of 3820"50 years BP oncharcoal from thin humified layers underlying theFossa Lupara and Astroni 3 tephras, represent lowerbounds for the AMST age, since those eruptionsoccurred after the AMST event. This limit is consis-

tent with several dates on small carbonized treetrunks or branches from the northern side of the

Ž .Astroni crater Alessio et al., 1973 . These Astroni-associated dates range from 3630 to 3830 years BP,and probably post-date the earlier AMST eruption.


Ž .Fig. 14. A Incremental-heating apparent age spectrum. Apparent-age uncertainties of the individual steps are shown at 2s , whereasŽ .uncertainties in the plateau age and integrated age are shown at 1s . B Age–probability density spectrum for the single-crystal analyses,

Ž 39 40 U .with compositional parameters mol Ar, % Ar , CarK, and values of individual analyses with 1s analytical uncertainties in rank order .Ž .The modal value in years BP of the age–probability density curve is also indicated at the top of the peak: the weighted-mean values of all

analyses with 1s S.E.M. is given near the bottom axis.


Table 3Previous dates on charcoal from the AMST

14Ž .Sample Locality Description Age C years BP References

R-682 Via Terracina, Napoli carbonized wood in pumice-flow 4000"50 Alessio et al., 1971R-785 Via Terracina, Napoli carbonized wood in pumice-flow 4070"50 Alessio et al., 1973ZR-1 La Starza terrace carbonized wood in pumice-flow 4660"80 Rosi and Sbrana, 1987ZR-5 E of Solfatara carbonized wood in pumice-flow 4375"160 Rosi and Sbrana, 1987ZR-51 Monte St. Angelo carbonized wood in pumice-flow 4205"107 Rosi and Sbrana, 1987

UUZR-52 Monte St. Angelo carbonized stump in upper pyroclastic-flow 4460"65 Rosi and Sbrana, 1987CFAP6 SE Quarto plain carbonized wood in vesiculated tuffs 4065"160 Rosi and Sbrana, 1987

UUMean of dates on two aliquots of a large carbonized stump.

Ž .Rosi and Sbrana 1987 quote four dates of 4490"

150, 3890"100, 4255"100, and 3650"95 BP forcharcoal collected from the lower part of the FossaLupara products in a quarry on the southern side ofthe Fossa Lupara volcano. However, these results areproblematic. The first sample appears older than oursix dates which provide upper limits for the AMST

Ž .which preceded Fossa Lupara Table 3 , and hence isprobably reworked, while the remaining three agesŽ .on separate aliquots of the same sample show verylarge scatter. In light of all the evidence, 3800 yearsBP probably represents the best available lower limitfor the AMST age.

In summary, our 14C results suggest that AMSTeruption occurred some time between limiting agesof 3800 and 4100 years BP. Three of the seven agesin Table 3 for the AMST are significantly older thanour upper limit, but we believe they can reasonably

be explained by a combination of large dating uncer-tainties and reworking or old wood effects. Ourresults are consistent with the assignment by Scan-

Ž .done et al. 1991 of 4000"50 years BP as the‘best’ AMST age, based on the age of youngest ofthe remaining four samples in Table 3, and perfectlyagree within the error with the 40Arr39Ar isochronages of 4100"400 years BP for the single-crystaland incremental-heating techniques previously pre-sented.

6. Petrology

The rocks of the AMST have been sampled andanalyzed for petrological studies. Samples consist ofpumice and scoria fragments collected at variablestratigraphic height along the section numbers 106,

Table 4Ž .Radiocarbon date for this study. Stratigraphic units nomenclature is from Di Vito et al. 1999-this issue

14 14a Ž .Sample Locality Stratigraphic unit Age C years BP C lab a

bOCF-MS 103 Trefola basal AMS ash – –OCF-MS 130 Masseria Merolla paleosol underlying AMS tephra 4130"50 CAMS-35681OCF-157 Pa Torre Caracciolo paleosol underlying Paleoastroni 2 tephra 4170"40 CAMS-35682OCF-US23 Orta di Atella paleosol underlying Paleoastroni 1 tephra 4140"50 CAMS-36585

cOCF-US21a Orta di Atella fireplace buried by Paleoastroni 2 tephra 4170"100 CAMS-35686cOCF-US21b Orta di Atella fireplace buried by Paleoastroni 2 tephra 4280"90 CAMS-35715

OCF-97 110E Pignatiello thin paleosol underlying Cigliano tephra 4160"50 CAMS-38520OCF-97 125AS Torre Poerio thin paleosol underlying Astroni 3 tephra 3820"50 CAMS-38439OCF-97 125 SE Torre Poerio thin paleosol underlying Fossa Lupara tephra 3820"50 CAMS-38522

aSample received acid–base–acid treatment to remove contaminants, and were combusted, converted to graphite, and analyzed at LawrenceLivermore National Laboratory, USA, using standard techniques of accelerator mass spectrometry.b No date-insufficient carbon was present after pretreatment.c Ž .Two separate charcoal aliquots were dated both very small after pretreatment, resulting in large uncertainties . Both dates are older thanthe result for OCF-US23 which underlies the earlier Paleoastroni 1 tephra, but given the uncertainties, we do not regard this inversion asserious.


Ž .108, 152 and 134 Fig. 1B . Each sample was com-posed of many fragments similar in color, textureand crystal content. Major and trace element concen-

Žtrations with CIPW norms calculated assumingFe O rFeOs0.5, see D’Antonio et al., 1999-this2 3

.issue , and Sr-isotopic compositions of whole-rocksand separated minerals are reported in Table 5.

6.1. Classification

The AMST rocks are potassic alkaline productsŽ .according to Middlemost 1975 . They are trachyte

Žin the total alkali–silica classification grid TAS, Le.Bas et al., 1986; Fig. 15A , and range in composition

from trachyte to alkali–trachyte in the normative NeŽ . Žvs. differentiation index DI snormative AbqOr

. ŽqNe classification grid Armienti et al., 1983; Fig..15B , here preferred since it is commonly used in the

literature for classification of the volcanic rocks fromthe Campanian region. Glass in pumice and scoriafragments is trachytic according to the TAS, andalkali–trachytic and subordinately trachytic accord-

Ž .ing to the Ne–DI classification grids Fig. 15A, B .

6.2. Petrography and mineral chemistry

Pumice and scoria fragments are porphyritic, withphenocrysts of plagioclase and alkali–feldspar,clinopyroxene, black mica, apatite and opaques inorder of decreasing abundance. Feldspar, clinopyrox-ene and black mica phenocrysts occur as singlecrystals or sometimes as aggregates. The groundmassis glassy and contains rare microlites of clinopyrox-ene, feldspar and black mica. Olivine is present inonly few samples as an occasional phase. Feldspar,pyroxene and olivine are often rounded and some-times show resorbed edges.

Feldspar and clinopyroxene are the dominant phe-nocryst phases. Representative chemical composi-tions and formulae are listed in Table 6. Clinopyrox-ene generally occurs as green crystals typicallyshowing portions of variable color. Colorless crystalsare very rare. The green crystals occur both asphenocrysts and microlites with salitic compositionŽ U U .Ca Mg Fe –Ca Mg Fe . Salite phenocrysts46 44 10 46 37 17

are generally normally zoned and sometimes have aŽ U .rim of colorless diopside Ca Mg Fe , Table 6 .47 45 9

Feldspar phenocrysts are both plagioclase and al-Ž .kali–feldspar Table 6 . Plagioclase is bytownite with

Ž . Žeither normal An to An or reverse An to87 70 70.An zoning. Alkali–feldspar is a homogeneous77

Ž .sanidine Or . Coarse An-rich plagioclase and76 – 78

sanidine phenocrysts up to 1-cm long are often pre-sent. Black mica is a Mg–biotite with TiO content2

about 5 wt.% and BaO as high as 1 wt.%; opaque isa Ti–magnetite, with TiO content about 8 wt.%.2

Fluor–apatite occurs commonly as inclusions inMg–biotite and salite crystals.

6.3. Whole-rock and glass geochemistry

Whole-rock major- and trace-element contentsŽ .Table 5 are plotted in the diagrams of Fig. 16against DI, which ranges from 75 to 83. SiO ,2

Na O, Zr, Nb, Rb, and to a lesser extent Y contents2

show positive correlation with degree of chemicalevolution, whereas MgO, FeO , CaO, P O , TiO ,tot 2 5 2

Sr, Ni and Cr contents show negative correlation.K O content increases in trachytes, at D.I. values2

from 74 to 79, then decreases in alkali–trachytes;Al O and MnO contents are constant, although2 3

Ž .scattered Fig. 16 and Table 5 . Overall, the samplesdefine a single evolution trend in all the variationdiagrams, although the total range of variation forseveral trace element contents is large in spite of the

Žlimited variation of major oxide contents Srs700to 350 ppm, Nbs45 to 80 ppm, Zrs350 to 650

.ppm; Fig. 16 and Table 5 .Glass compositions are plotted as field in Fig. 16.

At the microscopic scale single pumice fragmentsare fairly homogeneous in composition with DI valueranging from 73 to 87; only few glass from layerBL1 has apparently anomalous high DI values of

Ž .about 90, low CaO -2% and Na O and high K O2 2

contents with respect to the general trend. Majorelement contents vs. DI describe a single evolution

Ž .trend in the variation diagrams Fig. 16 . SiO , K O2 2

and Cl contents increase with increasing degree ofchemical evolution, whereas MgO, TiO , CaO and2

FeO contents decrease. Na O content increases fortot 2

DI values from 73 to 87 then anomalously decreases,as previously described. P O , Al O and K O con-2 5 2 3 2

tents are fairly constant. Glass composition differswith respect to the whole-rock composition to someextent, likely because the latter depends upon thecrystal contents.

The chemostratigraphy of Fig. 17 shows that thelower portion of the typical stratigraphic sequence

https://www.researchgate.net/publication/239745028_A_Chemical_Classification_of_Volcanic_Rocks_Based_on_the_Total_Alkali-Silica_Diagram?el=1_x_8&enrichId=rgreq-098f22ea79a9efc5347600b0c753df77&enrichSource=Y292ZXJQYWdlOzIyMzc0MzU0MTtBUzo5ODU3NTUzNTA1MDc1NUAxNDAwNTEzNjQ3MzQ2


Table 5Major and trace-elements concentrations, Sr-isotope data on products of the Agnano–Monte Spina eruption. pspumice sample; vtsglasssample; ssscoria sample; wrswhole-rock; felds feldspar; cpxsclinopyroxene; btsbiotite. All samples were cleaned, washed indistilled water, crushed to lapilli-size particles, then ground and homogenized in an agate mortar. Powders were analyzed for major and traceelement concentrations by XRF at the Dipartimento di Scienze della Terra of the University of Trieste, by using the PW1404 XRF

Ž .spectrometer and the procedures of Philips 1994 for the correction of matrix effects. Analytical uncertainty is estimated to be less than 5%for major and 10% for trace elements, respectively. Major elements concentrations are normalized to 100% on anhydrous base. Sr-isotopiccompositions of whole-rock samples and separated feldspar, pyroxene and biotite phenocrysts were determined at the Dipartimento diGeofisica e Vulcanologia of the University ‘Federico II’ of Napoli. The whole-rock powders were leached with cold 2.5 N HCl for 10 minand with hot 2.5 N HCl for 10 min; crystals were leached with cold 40% HF for 10 min. Leached samples were rinsed thoroughly in puresub-boiling distilled water, and finally dissolved with high purity HF–HNO –HCl mixtures. Sr was extracted by conventional ion-exchange3

chromatographic techniques. Measurements were made using a VG 354 double-collector thermal ionization mass spectrometer running inŽ . Ž .jumping mode. The quoted error "1 is the standard deviation of the mean 2s and refers to the last digit. Repeated analyses ofm

Ž .NBS-987 International Reference Standard yielded a mean value of 0.71024"1 Ns50 . The total blank was on the order of 6 ng duringthe period of measurements

Ž .from Abottom to Bbase is more evolved, alkali–trachytic in composition, whereas the upper portionŽ .from bBPF to E2 is less evolved, trachytic. The

Žproducts of the middle part of the sequence from.Bbase to bBPF have intermediate composition; fur-

thermore, levels with texturally and chemically dif-


Ž . Ž . Ž .Fig. 15. A TAS Le Bas et al., 1986 . B Ne–differentiationŽ . Ž .index DI Armienti et al., 1983 classification grids for the

products of the Agnano–Monte Spina eruption. Open circlesŽ . Ž .pumice fragments XRF ; solid circlesscoria fragments XRF ;

Ž .crossessglass fragments XRF ; diamondssglass in juvenileŽ .fragments WDS–EDS .

ferent pumice fragments also occur at different strati-graphic height, in the middle portion of the se-quence. Similarly, glass chemostratigraphy showsthat alkali–trachytic pumice fragments are slightly

Ž .less differentiated Fig. 17 , with a wider variabilityin composition in the middle portion of the sequence.

6.4. Sr-isotope data

Sr-isotopic composition of whole-rock samplesŽ .ranges from 0.70746"1 to 0.70756"1 Table 5

showing a slightly negative correlation with degreeŽ .of chemical evolution Fig. 17 . The alkali–trachytic

products of the lower and middle portions of thestratigraphic sequence have a Sr-isotopic ratio ofabout 0.70748. The trachytic products of the upperportion of the sequence have a larger variability with87Srr86Sr ratio ranging from 0.70750 to 0.70756.The alkali–trachytic products of the lower portion ofthe stratigraphic sequence contain mineral phases inisotopic equilibrium. Conversely the trachytic prod-ucts of the upper portion show evidence for isotopicdisequilibrium among whole-rock and some mineral

Ž .phases Fig. 17 . Generally, clinopyroxene and bi-otite show isotopic disequilibrium with respect tofeldspar and sometimes to one another and whole-rock. Clinopyroxene has the largest variability, withSr-isotopic ratio ranging from 0.70747 " 1 to0.70756 " 1. Biotite has the highest measured87Srr86 Sr ratios ranging from 0.70754 " 1 to0.70759"1. Feldspar is characterized by a narrowrange of Sr-isotopic composition at about 0.70748,

Ž .although a value of 0.70745 occurs Table 5 . Miner-alogical disequilibrium is also evidenced by the oc-currence of resorbed olivine, clinopyroxene andfeldspar phenocrysts, reverse and normal zoning ofplagioclase, and coexistence of salite and diopside,as previously discussed.

6.5. Discussion of petrological data

ŽThe chemical variability of the AMST rocks Fig..16 could be accounted for by a simple fractional

crystallization process involving the crystal phasesobserved in the rocks. A mass balance calculation

Žbased on major element contents Stormer and.Nicholls, 1978 indicate that it is possible to derive

an alkali–trachytic magma similar to sample MS101Abottom from a trachytic magma similar to sampleMS108D2base, by subtracting 34.1% of a solid as-

Žsemblage 56.0% sanq21.4% Ca-plq10.2% salq6.7% Mg-btq5.2% Ti-mtq0.8% F-ap, Ýr 2 s

.0.003 . However, modeling based on trace elementŽcontents partition coefficient from Lemarchand et

.al., 1987; Caroff et al., 1993; Pappalardo, 1994indicates that the subtracted solid assemblage does

Žnot support the Sr and Zr content variations thedifference between observed and calculated Sr and

.Zr contents is about 100 ppm and implies that thealkali–trachytic and trachytic magmas are not related

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Table 6Representative microprobe analyses of plagioclase, alkali–feldspar and clinopyroxene in volcanic rocks from the Agnano–Monte Spina eruption. Key for rock types:Trs trachyte; Alk–trsalkali–trachyte. Mineral formulae calculated on the basis of: 32 oxygens for plagioclase and alkali–feldspar; four cations for clinopyroxene, according to

Ž . U 2q 3qthe procedure proposed by Vieten and Hamm 1978 . psphenocryst; mpsmicrophenocryst; mlsmicrolite; F sFe qFe qMn. b.d.l.sbelow detection limit. Glass andminerals were analyzed by combined WDS–EDS techniques using the CAMECA SX50 electron microprobe at the Centro di Studi per il Quaternario e l’Evoluzione

Ž .Ambientale-CNR Rome . Data reduction was made using the ZAF4rFLS software by Link Analytical. An accelerating voltage of 15 kV, a beam current of 15 nA, and a spotdiameter of 5 mm were set to analyse both glass and crystal phases. Analytical uncertainty is on the order of 1%

Sample Plagioclase Alkali–feldspar Sample Clinopyroxene

Rock type 106Btop 106Btop 152BL1 152BL1 152BL1 152BL1 152BL1 152BL1 152BPFa 152BPFa Rock type 152BL1 152BL1 152BPFa 152BPFa 106D1bs 106D1bs

Spot Alk–tr Alk–tr Tr Tr Tr Tr Tr Tr Tr Tr Spot Tr Tr Tr Tr Tr Tr

M71 M72 M23 M24 M39 M40 M45 M46 M75 M76 M3 M4 M87 M88 M74 M73

p-core p-rim ml-core ml-core mp-core mp-rim ml ml p-core p-rim ml-core ml-rim p-core p-rim p-core p-rim

SiO 45.78 50.14 52.61 52.03 50.04 53.09 64.43 64.78 64.78 64.74 SiO 52.18 53.15 51.93 50.52 51.02 50.382 2

Al O 33.76 30.28 29.71 30.37 32.03 29.70 19.48 19.13 19.10 19.13 TiO 0.73 0.62 0.57 0.78 0.67 0.752 3 2

Fe O 0.62 0.54 0.61 0.77 0.64 0.58 b.d.l. b.d.l. b.d.l. b.d.l. Al O 2.93 2.48 2.13 3.71 2.38 3.282 3 2 3

CaO 16.98 13.54 12.19 12.82 14.72 11.96 0.95 0.66 0.63 0.51 FeO 6.85 5.46 7.46 7.81 9.30 8.24Na O 1.36 3.01 3.94 3.51 2.78 3.92 2.54 2.35 1.94 1.93 MnO 0.29 0.17 0.63 0.41 0.72 0.432

K O 0.12 0.49 0.82 0.66 0.45 0.96 12.38 12.88 13.53 13.56 MgO 14.78 16.00 14.34 13.34 12.62 13.142

SrO 0.36 0.21 b.d.l. 0.36 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. CaO 23.17 23.07 22.48 22.40 21.96 22.35BaO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.19 b.d.l. 0.29 Na O 0.25 0.16 0.36 0.35 0.49 0.422

Sum 98.98 98.21 99.88 100.52 100.67 100.21 99.78 99.99 99.99 100.14 Sum 101.18 101.11 99.90 99.32 99.16 98.99Formula Fe O 1.42 0.37 1.69 1.35 1.46 1.962 3

Si 8.533 9.315 9.574 9.444 9.089 9.621 11.803 11.867 11.878 11.870 FeO 5.57 5.12 5.94 6.60 7.98 6.48Al 7.417 6.630 6.373 6.496 6.857 6.345 4.207 4.130 4.128 4.134 Sum 101.32 101.15 100.07 99.45 99.31 99.19

3qFe 0.087 0.076 0.084 0.105 0.087 0.079 0.000 0.000 0.000 0.000 FormulaCa 3.391 2.695 2.377 2.492 2.863 2.322 0.186 0.129 0.125 0.099 Si 1.906 1.931 1.927 1.890 1.925 1.894Na 0.492 1.084 1.389 1.235 0.980 1.376 0.901 0.835 0.689 0.686 AlIV 0.094 0.069 0.073 0.110 0.075 0.106K 0.029 0.116 0.190 0.153 0.105 0.222 2.894 3.009 3.164 3.171 Sum T 2.000 2.000 2.000 2.000 2.000 2.000

VISr 0.053 0.031 0.000 0.051 0.000 0.000 0.000 0.000 0.000 0.000 Al 0.032 0.037 0.020 0.054 0.031 0.039Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.041 0.000 0.061 Ti 0.020 0.017 0.016 0.022 0.019 0.021

3qSum 19.948 19.917 19.987 19.925 19.982 19.966 19.991 19.969 19.985 19.961 Fe 0.039 0.010 0.047 0.038 0.042 0.0552qAn mol% 86.70 69.19 60.09 64.22 72.52 59.23 4.68 3.26 3.13 2.51 Fe 0.170 0.156 0.185 0.206 0.252 0.204

Ab mol% 12.57 27.83 35.11 31.83 24.82 35.09 22.64 21.01 17.33 17.33 Mn 0.009 0.005 0.020 0.013 0.023 0.014Or mol% 0.73 2.98 4.80 3.96 2.66 5.67 72.68 75.73 79.54 80.16 Mg 0.805 0.867 0.793 0.744 0.710 0.736

Ca 0.907 0.898 0.894 0.898 0.888 0.900Na 0.018 0.011 0.026 0.025 0.036 0.031Sum M1 2.000 2.000 2.000 2.000 2.000 2.000qM2Ca at.% 48.90 48.68 48.11 49.27 48.31 48.93Mg at.% 40.07 42.81 39.25 37.57 35.63 37.11

UFe at.% 11.03 8.50 12.64 13.16 16.06 13.97


Ž . Ž .Fig. 16. Harker diagrams of selected major- wt.% and trace- ppm elements and Sr-isotope compositions vs. DI. Symbols as in Fig. 15.The field defines the WDS–EDS analyses of glass in juvenile fragments.

by crystal fractionation processes, as also suggestedby the results of the mineralogical and isotopicalinvestigation.

The detected geochemical and Sr-isotopic varia-tions between trachytic and alkali–trachytic AMSTproducts can be explained by a mixing processes


Ž . Ž . 87 86Fig. 17. DI, Zr ppm , Sr ppm and Srr Sr variations vs. stratigraphic position of the analyzed samples. Symbols for XRF andWDS–EDS analyses as in Fig. 15; upper triangles feldspar, lower trianglesclinopyroxene, right trianglesbiotite.

between two isotopically and chemically distinctmagma batches: i.e., the first erupted alkali–trachytecharacterized by 87Srr86Sr of 0.70750 and the lasterupted trachyte characterized by 87Srr86 Sr of0.70755. The mixing process between these twomagmas occurred during the eruption, causing het-erogeneous glass compositions, and crystals of thealkali–trachytic magma to be incorporated as par-tially resorbed xenocrysts in the trachytic magma,giving the observed mineralogical and isotopical dis-equilibria between whole-rock and mineral phases.Furthermore, the occurrence of xenocrysts of olivine,An-rich plagioclase and diopsidic clinopyroxene sug-gests that either a more mafic magma might havebeen also involved in the eruption, or xenocrystsfrom mafic volcanics may have been incorporatedwhile the magmas were rising through the conduit.The alkali–trachytic and trachytic magmas wereerupted at variable temperatures. The temperature

Žcalculated on the feldspar microlites Fuhrman and.Lindsley, 1988 varies from 896"40 to 943"408C

in the alkali–trachytic products, and is 820"408Cin the trachyte.

The observed total range of whole-rock 87Srr86SrŽ .ratio of the AMST 0.70746–0.70756 is similar to

Ž .that of NYT 0.70749–0.70755; Orsi et al., 1995Ž .Fig. 18 . Furthermore, in the AMST two modesoccur at about 0.70750 and 0.70755, respectively,similar to the two modes of the NYT. Thus, wehypothesize that the two magmas that fed the Ag-nano–Monte Spina eruption could have been differ-entiated residues of the NYT magmas left in thesystem since 12,000 years ago.

7. Eruption scenario and magma withdrawal

All the presented data have allowed us to recon-struct the eruption scenario of the AMST and itsrelations with magma withdrawal dynamics and vol-cano-tectonic collapse. The eruption took place inthe sector of the resurgent block inside the NYTcaldera, characterized by northwest–southeast fault

Žsystems related to an extensional stress regime Orsi.et al., 1996; Di Vito et al., 1999-this issue . We

suggest that such a stress regime, active in the area


Fig. 18. Frequency histogram of 87Srr86 Sr ratio of the Agnano–ŽMonte Spina products and of the NYT modified from Orsi et al.,

.1995, all data renormalized to the present value of NBS-987 .

since the beginning of the third epoch of volcanicŽactivity 4800 years BP; Di Vito et al., 1999-this

.issue , favored intrusion of a dike into one of theŽ 3normal faults. A small volume of magma 1.2 km

.DRE was extruded during the AMST eruption. Theage of the eruption is well constrained by both40 39 14 Ž .Arr Ar and C AMS dating at 4100 years BP.Before the beginning of the eruption, the reservoircontained two isotopically distinct batches of magma.The upper batch was slightly less-radiogenic alkali–trachytic while the lower batch was more-radiogenicand trachytic in composition: they partially mixedduring the course of the eruption. The exposed strati-graphic sequence does not record any deposit whichcould be related to the initial vent-opening phase ofthe eruption. This likely could be due to thevolcano-tectonic collapse of the vent area.

We suggest that the eruption was characterized byfive phases with variable dynamics and dispersal ofthe pyroclastic products. A volcano-tectonic collapseaccompanied the eruption and determined shifting ofthe vent.

The first phase of the eruption began with effi-cient water–magma interaction and generation of ahighly expanded ash cloud which deposited the thin

and widely dispersed basal part of layer A1. As aconsequence of these explosions, water was drivenout of the conduit by the pressure wave. Therefore,the next explosion was magmatic and generated a

Ž .relatively low about 5 km eruption column, thatdeposited the upper fallout bed of layer A1. Theoccurrence of unaltered lithic fragments and the lim-ited dispersal of this fallout deposit are evidence of ashallow fragmentation surface. Collapse of the col-umn and phreatomagmatic explosions, driven by wa-ter reaccessing the conduit, generated the pyroclas-tic-flows and -surges of layer A2. These lastphreatomagmatic explosions again drove the wateraway from the conduit. During this phase of theeruption the upper alkali–trachytic batch of magmawas tapped from the reservoir.

The second phase began with the formation of apulsating eruption column which reached a maxi-mum eight of about 23 km and deposited the falloutlayer B1. Partial collapse of this column generatedpyroclastic-flows which deposited the basal beds oflayer B2. Waning of the column, likely after enlarge-ment of the conduit, gave rise to high particle con-centration pyroclastic-flows, which also depositedlag breccias. Also these explosions tapped the upperalkali–trachytic batch of magma. At this stage anetwork of fractures, foreshadowing a volcano-tectonic collapse, likely formed and became the siteof scattered vents. Opening of fractures allowedground water to access variable parts of the reser-voir. Therefore, the following explosions were trig-gered by variable magma–water interaction and pro-duced pyroclastic-flows and -surges, and strombo-lian-fallout. Fragmentation surface deepened as sug-gested by the occurrence of hydrothermally alteredclasts, which also indicate involvement of a geother-mal system. The tapped magma resulted from min-gling between the remaining part of the alkali–trachytic magma and the trachytic magma batch.Such a mingling was likely favored by pressurelowering on the reservoir.

After this eruption phase, a pause in the eruptionallowed the ash still suspended in the atmosphere todeposit member C, while heavy rains caused strongerosion along steep slopes.

Increase in fracturing of the roof rocks caused alowering of the lithostatic pressure on the reservoir,that at this stage contained only the trachytic magma


batch. Pressure lowering determined volatiles exsolu-tion in the trachytic magma and its migration to thesurface. This rising magma interacted with thegeothermal system, causing its flashing. The follow-ing phreatomagmatic explosions marked the renewalof the eruption and the beginning of the third phase.They produced the pyroclastic-surge deposits at thebase of member D. Again the pressure wave drovethe water out of the conduit. The following explo-sions were magmatic and generated an eruption col-umn about 27 km high, that deposited layer D1. Thelarge amount of hydrothermally altered lithic clastsof this layer proves that the geothermal system wasinvolved in the explosions. The height of the columnsuggests that the fragmentation surface again deep-ened. Activation of faults along the margins of thepresent Agnano plain generated the main episode inthe volcano-tectonic collapse. Related to this episodethe column collapsed and formed high particle con-centration pyroclastic-flows which were confined in-

Ž .side the CFc depression layer D2 . New ventsopened along the volcano-tectonic faults and eruptedpyroclastic-surges that overtopped the northern sec-tor of the morphological boundary of the CFc andflowed into the plain.

Variation of the structural setting of the Agnanoarea allowed a large amount of ground water andgeothermal fluids to access the reservoir. The follow-ing phreatomagmatic explosions marked the begin-ning of the fourth phase and generated a highlyexpanded ash cloud which deposited layer E1. TheAgnano plain continued to sink and the eruptionvents migrated toward the northwestern sector of itsstructural boundary. The following explosions weremainly magmatic and generated a pulsating columnlikely low and short-lived, as suggested by the lim-ited thickness and areal distribution of layer E2. Thiscolumn very likely waned without a final collapse.The following explosions took place from severalvents along the faults bordering the Agnano plain.They mostly generated very dilute and high-mobilitypyroclastic currents from continuously collapsingeruption columns that deposited the lower part oflayer E3. The latest explosions of this phase weretriggered by less efficient water–magma interactionand formed the upper part of layer E3.

During the fifth and last phase of the eruptionmagma supply was discontinuous and progressively

decreasing. A series of discrete phreatomagmaticexplosions generated the ash-fallout beds of memberF. Settling of particles suspended in the atmosphere,likely favored by rainfall, marked the end of theeruption.

8. Conclusions

The AMST eruption took place in the Agnanoarea which is located in the northeastern sector of theresurgent block of the CFc, affected by a tensionalstress regime. It extruded a volume of magma ofabout 1.2 km3. The age of the eruption has beenconstrained at 4100"50 years BP by combining 14CŽ . 40 39AMS and Arr Ar dating techniques. The smallvolume of erupted magma and the location of thevent relative to the deformational history of thecaldera suggest that the eruption likely was fed by adike intruded along a normal fault.

Petrological data show that the eruption was fedby two isotopically and chemically distinct magmabatches that mechanically mixed during the eruption.The upper magma batch in the tapped reservoir wasalkali–trachytic, while the lower batch was trachytic.On the basis of their Sr-isotope composition, weinterpret the two magma batches as differentiatedresidues of the NYT magmas still present into thePhlegraean system.

Field data and interpretation of cores of boreholeshave permitted a detailed reconstruction of the strati-graphic sequence of the unit. The entire sequence hasbeen subdivided into members and layers. An ero-sional unconformity occurs in the central part of thesequence. Pyroclastic-flow and -surge beds exposedoutside the CFc, have been correlated with proximaldeposits within the lowland. Such a correlation hasbenefited from identification of widespread falloutdeposits which have been used as marker beds. Thetwo main pumice-fallout deposits have variable east-erly dispersal axes. Facies variation has been estab-lished for many layers or beds. Pyroclastic currentsof higher grain concentration were unable to sur-mount the morphological boundary of the CFc. Cur-rents of lower concentration overtopped this bound-ary, mostly in the sectors with gentle slopes. Theytraveled at least 15 km over the surrounding plain.The path followed by the flows along the outer


slopes of the caldera was strongly affected by localtopography.

The reconstructed stratigraphic and chemostrati-graphic sequences, and the collected structural datahave been used to infer the eruption scenario and itsrelations with both magma withdrawal dynamics andvolcano-tectonic collapse. The eruption, interruptedby a pause during which formed an erosional uncon-formity, was characterized by five phases with vari-able dynamics and dispersal of pyroclastic products.It was accompanied by a volcano-tectonic collapseof the Agnano area. This collapse was preceded byfracturing of the vent area and occurred throughvariable episodes of sinking. Both fracturing andsinking episodes determined vent migration and af-fected the dynamics of the magma reservoir. Thecollapsed area was delimited by NE–SW and NW–SE faults that likely resulted from partial reactivationof faults of the resurgent block. The net verticaldisplacement during collapse was of 35 m. TheAgnano plain continued to subside at least till the

Ž .Astroni eruption 3800 years BP at a rate of about 8cmryr.

The Agnano–Monte Spina is the highest-magni-tude eruption occurred over the past 5000 yearswithin the CFc. Despite the small volume of eruptedmagma, this event caused a significant environmen-tal impact over a very large area. An area of about1000 km2, at the present inhabited by about 2 mil-lion people, was covered by at least 10 cm of ash-and lapilli-fallout, while an area of about 200 km2,presently inhabited by about 600,000 people, wasinvaded by pyroclastic currents. The occurrence ofsuch an eruption in the restless CFc within thedensely inhabited Neapolitan–Phlegraean area, con-tribute to define a very high volcanic risk. Theresults of our study permit to improve the knowledgeof the AMST eruption and can be used in planningmitigation procedures to be adopted in case of re-newal of volcanism within the caldera.

Acknowledgements

The authors are grateful to R. Petrini and P.Antonini for XRF analyses of major and trace ele-ments. M. Sarracino is acknowledged for kind assis-tance during microprobe analyses. The Archaeologi-

cal Superintendency of Napoli and Caserta and thearchaeologists A. Marzocchella and D. Giampaolaare gratefully thanked for their willingness and forthe very stimulating discussions. Critical reviews byG. Valentine and an anonymous reviewer improvedthe quality of this work. The work was carried outwith the support of the CNR-Gruppo Nazionale perla Vulcanologia.

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