Sub-surface dynamics and eruptive styles of maars in the Colli Albani Volcanic District, Central Italy

Journal of Volcanology and Geothermal Research 180 (2009) 189–202

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Sub-surface dynamics and eruptive styles of maars in the Colli Albani VolcanicDistrict, Central Italy

G. Sottili a,⁎, J. Taddeucci b, D.M. Palladino a, M. Gaeta a, P. Scarlato b, G. Ventura b

a Dipartimento di Scienze della Terra, Sapienza-Università di Roma, Rome, Italyb Istituto Nazionale di Geofisica e Vulcanologia, Dept. of Seismology and Tectonophysics, Rome, Italy

⁎ Corresponding author. Dipartimento di Scienze dedi Roma, P.le Aldo Moro 5, 00185, Rome, Italy. Tel.: +39

E-mail address: [email protected] (G. Sott

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.07.022

a b s t r a c t

Article history:

Keywords:maarColli Albanihydromagmatic eruptionlithic entrainmentcored scoria

ed with the possible reactivation of maar-forming events in the Quaternary,ultrapotassic Colli Albani Volcanic District (CAVD) provides implications for volcanic hazard assessment inthe densely populated area near Rome. Based on detailed stratigraphy, grain size, componentry, ashmorphoscopy and petro-chemical analyses of maar eruption products, along with textural analysis of coredjuvenile clasts, we attempt to reconstruct the eruptive dynamics of the Prata Porci and Albano maars, asrelated to pre- and syn-eruptive interactions between trachybasaltic to K-foiditic feeder magmas andcarbonate–silicoclastic and subvolcanic country rocks. Magma volumes in the order of 0.5–3.1×108 m3 wereerupted during the monogenetic Prata Porci maar activity and the three eruptive cycles of the Albanomultiple maar, originating loose to strongly lithified, wet and dry pyroclastic surge deposits, Strombolianscoria fall horizons and lithic-rich explosion breccias. These deposits contain a wide range of accessory andaccidental lithic clasts, with significant vertical stratigraphic variations in the lithic types and abundances.The two maar study cases hold a record of repeated transitions between magmatic (i.e, Strombolian fallout)and hydromagmatic (wet and dry pyroclastic surges) activity styles. Evidence of phreatic explosions, acommon precursor of explosive volcanic activity, is only found at the base of the Prata Porci eruptivesuccession. The quantitative evaluation of the proportions of the different eruptive styles in the stratigraphicrecord of the two maars, based on magma vs. lithic volume estimates, reveals a prevailing magmaticcharacter in terms of erupted magma volumes despite the hydromagmatic footprint. Different degrees ofexplosive magma–water interaction were apparently controlled by the different hydrogeological andgeological–structural settings. In the Prata Porci case, shifts in the depth of magma fragmentation areproposed to have accompanied eruption style changes. In the Albano case, a deeply dissected geothermalaquifer in peri-caldera setting and variable mass eruption rates were the main controlling factors of repeatedshifts in the eruptive style. Finally, textural evidence from cored juvenile clasts and analytical modeling ofmelt–solid heat transfer indicate that the interacting substrate in the Prata Porci case was at low, uniformtemperature (~ 100 °C) as compared to the highly variable temperatures (up to 700–800 °C) inferred for thegeothermal system beneath Albano.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Most maars are monogenetic volcanic edifices; however, poly-genetic maars may result frommultiple eruptive events separated bysignificant quiescence periods (Ollier, 1967; Lorenz, 1973). Both maartypes may show complex eruptive dynamics, including hydrother-mal, phreatic, hydromagmatic and magmatic (usually Strombolian)activities, with possible repeated style changes during individualeruptive events (e.g., Houghton et al., 1996; White and Houghton,2000; Carrasco-Núñez et al., 2007). Understanding modes and

lla Terra, Sapienza-Università0649914916.ili).

l rights reserved.

causes of shifting activity style during maar-forming events is thusa crucial point for related hazard assessment in nearby populatedareas.

In this paper we address the reconstruction of eruptive dynamicsand sub-surface controlling factors of representative maars in theColli Albani Volcanic District (hereafter CAVD), located SE of Rome(Fig. 1). We focus on two study cases: i.e., i) the monogenetic PrataPorci maar (crater 1 km across), with an undetermined age between308 and 70 ka (Marra et al., 2003), and ii) the Albano polygeneticmaar (crater area 4×3 km), which hosted at least seven eruptions inthe time span 70–36 ka, with intervening repose periods as long as104 years, including the most recent eruptive event at the CAVD(De Rita et al., 1995; Marra et al., 2003; Giaccio et al., 2007). Bothstudy cases exhibit the typical facies characteristics and associationsof CAVD maar successions, which point out an emplacement from

Fig. 1. a) Digital Elevation Model of the Colli Albani Volcanic District, showing the main morphological features and the locations of the Prata Porci monogenetic maar and the threemain craters (numbers 1–3) of the Albano polygenetic maar. The areal dispersal of fallout products from the most powerful Albano eruption (Alb-3) is also reported (from Giaccio etal., 2007). b and c) Aerial views of the Albano and Prata Porci maars, respectively.

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dry and wet pyroclastic surges, alternating with Strombolian scoriafallout and lithic-rich explosion breccias. Typically, the studieddeposits show highly variable degree of consolidation, from loose tostrongly lithified facies, and contain abundant lava and sedimentarylithic clasts, along with moderately vesicular to very poorly vesicularscoria clasts, trachybasaltic (Prata Porci) to K-foiditic (Albano) incomposition (see below). The two study cases also show differentgeological–structural settings: the Albano polygenetic maar devel-oped along a former caldera rim, in the central CAVD area, whereasthe Prata Porci maar formed in a relatively peripheral area of theCAVD (Fig. 1).

The main purposes of the present work are: i) to perform aquantitative evaluation of the proportions of the different eruptivestyles in the stratigraphic record of the two maars; ii) to assess thedepth and modes of interaction of potassic magmas with countryrocks (i.e., mechanisms and extent of entrainment of sub-surface rocktypes, as related to specific hydrogeological and geological–structuralsettings) as a possible controlling factor of activity style transitions,along with volume estimates of magma and lithic fragments involvedin individual eruptions. In this regard, we also focus on the analysis ofcored scoria clasts bearing information on the thermal state of theinteracting substrates in different maar settings.

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2. Geological background

The CAVD is part of the Quaternary ultrapotassic Roman Province(central Italy) and it is known for its peculiar magma evolution, leading toK2O-rich, SiO2-poor differentiated rock types, K-foiditic in composition. Inspite of relative uniform and remarkably SiO2-poor magma composition(as low as 41wt.% on a volatile-free basis), the CAVD activity encompassesa wide spectrum of eruption styles, intensities and magnitudes, rangingfrom effusive to mildly explosive, Strombolian and hydromagmatic,episodes to large pyroclastic-flow-forming events related to caldera col-lapses (e.g., A.A.V.V,1995;Fredaet al.,1997;Palladinoet al., 2001;Giordanoet al., 2006). The CAVD extends over an area of about 1500 km2 andcomprises a central volcanic edifice (i.e., the Tuscolano–Artemisio),truncated by a horseshoe-shaped caldera, an intra-caldera central edifice(i.e., the Faete) andanetworkof intra- andcircum-caldera scoria cones andeccentric tuff rings. According to available reconstruction of volcanichistory (e.g., De Rita et al., 1995; Karner et al., 2001; Marra et al., 2003;Giordano et al., 2006), the Tuscolano–Artemisio activity took place fromaround 560 ka to 355 ka. This period was by far the most important intermsof eruptedmagmavolumeandareal distributionofproducts, since itwas characterized by the emplacement of large pyroclastic flows (in theorder of some km3 to some tens of km3 of deposits for each event),resulting into multistage caldera formation. Subsequently, at about 300–250 ka, Strombolian and effusive activities prevailed, originating thecomposite intra-caldera Faete stratocone and several lava flows. Hydro-magmatism then dominated the most recent activity in the CAVD (about200–35 ka) and produced several eccentric, both monogenetic andpolygenetic, maars. In fact, the CAVD has to be regarded as a quiescentvolcanic area, considering that the time span elapsed from the last datederuptive event is shorter than average dormancy periods that character-ized its volcanic history (Marra et al., 2004; Giaccio et al., 2007).

A major structural–geological feature of the CAVD is the NW–SE-trending structural highbordering the Tuscolano–Artemisio caldera to thewest (Chiarabba et al., 1994), which mainly consists of Meso-Cenozoiclimestone and marly limestone and is characterized by present-day,

Fig. 2. Pictures of the study successions at the reference stratigraphic sections. Upper row, Albpicture is 0.5 m long); b) fine-ash bed at the base of Unit a and overlying coarse breccia dehorizon rich in whitish carbonate lithic clasts and a ballistic bomb sag. Lower row, Prata Ppaleosol, light grey fine-ash layer of possible phreatic origin, well-sorted, fine scoria lapillistrongly lithified, laminated ash deposits including meter-sized lava blocks; f) detail of (e) s

shallow, low magnitude seismicity (Amato et al., 1994). Funiciello andParotto (1978) investigated the lithologies of accidental lithic inclusions inthe CAVD volcanics and identified the sedimentary substrate as made upof Meso-Cenozoic carbonate successions (limestones and marls), transi-tional from basin to platform, which are covered by Neogenic, post-orogenetic silicoclastic successions (clays to sandy clays). Moreover, basedon lateral thickness variations of substrate successions, the same authorsconcluded that hydromagmatic activity involved preferentially horststructures of the Meso-Cenozoic carbonate successions. Recently, Gior-dano et al. (2006) discussed the location of CAVD maars as related tovertical displacements of the Tuscolano–Artemisio caldera floor.

The twomaars described in this paperdeveloped indifferent settings,as concerns the volcano-tectonic evolution of the CAVD and relatedsedimentary substrate (Fig. 1). The Albano feeder magmas erupted in aperi-caldera environment, characterized by a still active geothermalsystemwithin a carbonate substrate high (Ciampino high; Chiodini andFrondini, 2001) underlying the relatively thick volcanic cover of thecentral CAVDarea (400–900m;DeRita et al.,1995). Instead the eccentricsetting of the Prata Porci maar is characterized by the presence of arelatively undisturbed limestone substrate, corresponding to a periph-eral portion of the regional aquifer, overlain by thick, poorly permeable,marly–clay rocks beneath a relatively thin volcanic cover (ca. 200 m)(Funiciello and Parotto, 1978; Boni et al., 1995; De Rita et al., 1995).

A comprehensive stratigraphic data set is available for the Albanopolygeneticmaar, (e.g., Civitelli et al.,1975;DeRita et al.,1988; Freda et al.,2006; Giordano et al., 2006; Giaccio et al., 2007). In the present work, wemostly refer to the stratigraphic reconstruction and the geochronologicframework recently proposed by Freda et al. (2006), as representative ofproximal deposits, and Giaccio et al. (2007) for mid-distal settings.

Freda et al. (2006) and Giordano et al. (2006) recognised seveneruptive units separated by variably developed paleosols, which canbe grouped in twomain, geochronologically distinct eruptive cycles, atca. 70–68 ka and 41–36 ka, respectively. The first cycle produced aN60 m-thick pyroclastic succession at the Albano Lake proximalsection (Fig. 2). Two incipiently pedogenized ash layers allow

ano maar: a) low-angle, cross-laminated beds of Unit b (white ruler in the center of theposit; c) fine-grained, laminated deposits in Unit c interbedded with a coarser-grainedorci maar: d) base of the succession showing, from bottom to top, dark, pre-eruptivebed and lithified laminated ash deposits; e) middle part of the succession made up ofhowing dark grey, angular, lava blocks with impact sags and whitish carbonate clasts.

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distinction of three eruptive events (i.e. units a, bα and b; Fig. 3)that occurred in a relatively short time interval. A thick horizon ofaltered ash capped by a pedogenized layer on top of unit b marksthe ca. 30 kyr-long dormancy preceding the second eruptive

Fig. 3. Stratigraphy, grain size and component distributions of Albano and Prata Porci maars:DU3) and Alb-3 (Unit f and distal unit DU4) successions; c) Prata Porci (PP) succession. See

cycle. At near-vent sections, this is represented by the prod-ucts of four eruptive events (units c, d, e and f; Fig. 3), boundedby incipient paleosols and spanning a slightly longer temporalinterval.

a) Alb-1 succession (Unit a, Unit bα, Unit b); b) Alb-2 (Unit c, Unit d, Unit e and distal unitGiaccio et al. (2007) for Albano unit nomenclature.

Fig. 3 (continued ).

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Giaccio et al. (2007) recognised a pyroclastic succession compris-ing four eruptive units widely spread in the northeastern sectors ofthe Colli Albani volcano, up to 15 km eastward from the Albano maar.Integrated tephrostratigraphic, morpho-pedostratigraphic, archaeo-logical, petrological and geochemical analyses enabled correlation tothe first, third, fifth and seventh Albano maar eruptions, thusenlarging significantly previously supposed dispersal areas. Moreover,the Albano maar distal products have been identified in LatePleistocene deposits of several intermountain basins of the centralApennine chain, as far as 100–120 km from the vent.

For the Prata Porci maar, the only available stratigraphy is reportedin Civitelli et al. (1975). New field investigations, photographic

documentation and level-by-level thickness measurements and faciesanalysis allowed us to obtain a detailed, representative stratigraphiclog for the whole Prata Porci eruptive succession. Here we focus onnear-vent exposures along the maar crater rim (Fig. 2).

3. Methodology

Methods used in this paper include the detailed definition of fieldcharacteristics of Albano and Prata Porci maar successions, thedetermination of relevant petro-chemical, grain size, component,and morphoscopic characteristics of the erupted products, along withestimates of the erupted juvenile and lithic volumes. In addition, a

Fig. 3 (continued ).

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textural analysis of cored scoria clasts was carried out, bearinginformation on pre- to syn-eruptive lithic entrainment and fragmen-tation mechanisms.

3.1. Field analyses

Field investigations include the detailed reconstruction of deposittextures and architecture of Albano and Prata Porci maar successions,by means of level-by-level thickness measurements and faciesanalysis, in the light of a critical revisitation of previous stratigraphicwork.

3.2. Grain size, component and FE-SEM analyses

Overall, thirty-seven grain size and component analyses wereperformed (twenty-three for Albano and fourteen for Prata Porci), asrepresentative of all the informal stratigraphic units identified and ofspecific layers showing quite distinctive componentry onmacroscopicobservation (Fig. 3). The dominant juvenile component consists ofpoorly to moderately vesicular fragments with microcrystallinegroundmass, and clinopyroxene, leucite, dark mica and olivine freecrystals. We also distinguished different accessory (i.e., granularcumulate, lava and pyroclastic fragments) and accidental lithic

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components. In particular, the distinction of sedimentary rock typesmostly bears on Funiciello and Parotto (1978). In addition, theproportions of interstitial carbonate phases (calcite) and post-depositional zeolite minerals were determined.

Conventional grain-size analyses by dry sieving techniques anddetermination of component abundances by weighting/counting (e.g.,Cas and Wright, 1987) were applied limitedly to loose pyroclasticdeposits. For the most part of the studied successions, being stronglylithified due to vapor phase zeolite crystallization, we developed amulti-scale approach from field to microscope observation, i.e. in thesize range from − 7 to 3 phi units. In the field, the maximum axislengths of all clasts coarser than 2 cm, exposed over a representativearea of at least 8 m2 (A1), were measured for the different clast types.Within each measured area, one or two hand specimens representa-tive of the particle population finer than 2 cm were sampled andobserved under binocular microscope. Then the long axes of all clastscoarser than 1 mm enclosed within an area of at least 8 dm2 (A2) weremeasured. Within the hand specimens, we identified further areas ofat least 8 cm2 (A3), representative of the particles b1mm, and the longaxes of all clasts coarser than 0.125mmweremeasured by thin sectiontechniques. Finally, we used a Field Emission Scanning ElectronMicroscope (FE-SEM, JEOL 6500, operating at 15 kV acceleratingvoltage and variable emission current at the HP-HT Laboratory, INGV,Rome) equipped with Energy Dispersion System (EDS) microanalysisto characterize fine-ash particles in the groundmass.

To obtain a homogeneous data set, size data were normalizedfollowing three steps: i) normalization of the measured areas. Fromthe first measured area (A1 N8 m2), we subtracted the area Ac occupiedby clasts larger than 2cm, obtaining the area Am=A1 − Ac occupied byclasts b2 cm. A correction factor C2 (C2=Am/A2) was applied tonormalize the abundance of each component determined within thesecondmeasured area (A2 N8 dm2). The same procedure was repeatedfor the third measured area (A3 N8 cm2); ii) stereological conversionfrom 2D to 3D data, using the simple “representative volume”stereological method described in the literature (e.g., Sahagian andProussevitch, 1998); iii) conversion of sieving data from weight tovolume percentages using literature density data for each component.

Finally, we quantified the abundances of each component over thetotal volume of products for each given eruptive event. To extrapolatecomponent distributions from the sample scale to the whole eruptionproducts, we estimated the deposit volume represented by eachsample.

3.3. Chemical analyses

For the Albano magma composition, we essentially refer to glasscompositions of the juvenile lapilli fraction from available literaturedata (Freda et al., 2006), while for the Prata Porci eruption products wereport chemical compositions of scoria clasts from new X-rayfluorescence (XRF) analyses. Whole rock analyses were performedusing a Philips PW 1400 XRF spectrometer. A correction for matrixeffects was applied following Franzini et al. (1972) and Kaye (1965).The precision is better than 5% for the analyzed elements. Theaccuracy was tested by international standards (DR-N, NIM-G, GSN,GA, BCR-1, NIM-l) and is better than 10%. FeO, Na2O and MgO contentsand loss on ignition (LOI) were determined through atomic absorptionspectrophotometry and wet chemical analyses, respectively.

3.4. Volume estimates of eruption products

We estimated the volume of each stratigraphic unit. For pyroclasticcurrent deposits, we assumed a fast decay law (e.g., Vespermann andSchmincke, 2000) that broadly fits thickness decay away from thevent, also taking into account local topographic control. Then, startingfrom the maximum observed thickness of the deposits, we calculatedthe total volume, adding 50% to account for fine-ash loss in the

atmosphere (German and Sparks, 1993; Wohletz and Raymond, 1993;Lane-Serff, 1995; Ernst et al., 1996). Moreover, we considered availablevolume estimates for specific units (Giordano et al., 2002a; Giaccioet al., 2007). For fallout units we used available isopach maps andliterature models (Pyle, 1989, 1995). Otherwise, as a first approxima-tion, we based on the volumes of fallout deposits reported in theliterature showing similar dispersal, thickness and grain-size dis-tributions (e.g., Andronico et al., 2005). Thus, the volumes obtainedhere are minimum estimates of the outcropping deposits, becausethey do not take into account intra-crater fills. Given the assumptionsinvolved in the volume calculations, we estimate an error of about± 50%.

4. Results

4.1. Field aspects

The Albano and Prata Porci pyroclastic deposits display all thelithofacies typical of maar volcanoes, including massive and plane-parallel to low-angle cross-laminated, loose to strongly lithified, ash andlapilli beds fromwet and dry surge episodes (Fig. 2). In addition,matrix-to clast-supported, coarse breccia layers, as well as millimeter- todecimeter-thick ash- and lapilli-fall horizons, are abundant throughoutthe successions. Millimeter- to centimeter-thick fine-ash beds are alsofound, usually toward thebase of individual subunits (Freda et al., 2006).Valley-ponded, matrix-supported, massive deposits, also related tomaar activity, possibly derived from pyroclastic currents and/orsecondary mass flows (Giordano et al., 2002a,b).

Details of the Albano maar stratigraphy are presented in Freda et al.(2006) and Giaccio et al. (2007). Here we recall the points relevant forthe purposes of the present paper. The multi-phase evolution of theAlbanomaar comprises seven eruptive events, clustered in two eruptivecycles dated at 70–68 ka and 41–36 ka, respectively; the second cycleincludes two sub-cycles at 41–38 ka and 36 ka. The 70–68 ka cycle(hereafter Alb-1, broadly corresponding to Units a, bα, and b of Fig. 3)and the41–38ka sub-cycle (Alb-2,Units c, d, e, andDU3; Fig. 3) comprisesix eruptions that produced most of the pyroclastic deposits exposedalong crater walls. These deposits are characterized by the typical maarfacies described above and intervening scoria fallout horizons frompurelymagmatic activity. The sub-cycle at 36 ka (Alb-3, Units f andDU4;Fig. 3) consists of a single eruption, which is notably different from theprevious ones for its subplinian intensity, larger dispersal area and quitesubordinate hydromagmatic surge activity. Following Freda et al. (2006),we correlated the Alb-1, Alb-2, and Alb-3 evolutionary stages andcorresponding successions to the threemain cratermorphologies of theAlbano polygenetic maar (Fig. 1).

The activity record of the Prata Porci maar (hereafter unit PP;uncertain age in the 308–70 ka time interval; Marra et al., 2003) doesnot evidence intervening time breaks, thus pointing out a singleeruption. However, the eruption style evolved with time, from earlydeposition of a thin ash layer, followed by the formation of an initiallypulsating and later stable small plume that originated a scoria falloutdeposit (from 0.3 to 1 m above PP base; Fig. 3), to pyroclastic surgeemplacement, and to the final resumption of a pulsating, low-levelcolumn activity (topmost, 1.5 m-thick sampled interval in Fig. 3).

4.2. Grain size and componentry

After data treatment, we split pyroclastic deposits into thefollowing components (Fig. 3; Table 1): i) juvenile, poorly to variablyvesicular scoria, with a microcrystalline groundmass; ii) volcanic lithicclasts (mostly lavas and quite subordinate pyroclastic rocks);iii) sedimentary lithic clasts (mostly limestones and subordinatelymarls and clays); iv) thermo-metamorphic lithic clasts (mostlymarbles) and/or cumulate and crystal mush rocks; v) loose crystals(clinopyroxene, leucite, dark mica, and olivine); vi) accretionary lapilli.

Table 1Variation ranges of component abundances in the Albano and Prata Porci maarsuccessions, as determined in the size fraction coarser than 0.250 mm (minimum–

maximum measured values, vol.%)

Juvenile Lithic

Scoria Volcanic Sedimentary Subvolcanic+thermometamorphic

Lava Pyroclastic Limestone Clay Cumulate Marble

Alb-1 1.9–48.5 0.0–39.2 0.0–7.9 0.1–20.8 0.0–0.9 0.0–2.2 0.0–23.1Alb-2 28.5–64.7 0.5–10.1 0.4–1.8 0.6–17.2 0.0–0.1 0.8–17.2 0.0–1.3Alb-3 67.8–72.3 0.0–7.4 0.0–0.1 0.0–12.4 0.0–0.0 0.0–9.3 0.0–0.1PP 0.0–92.7 0.3–14.9 0.4–16.3 0.0–14.4 0.1–13.3 0.0–0.4 0.0–0.0

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Generally, the studied samples display poorly sorted grain-sizedistributions and large variability of component abundances. Alb-1and Alb-2 show higher fractions of volcanic and sedimentary lithicclasts and a noticeable peak in glass-bearing cumulates, hydrother-mally altered, hauyna-bearing rocks, and thermo-metamorphicinclusions (skarns + marbles), while PP shows lower amounts ofloose crystals and volcanic lithic clasts, which almost lack cumulateand thermo-metamorphic ones, and higher proportions of fine ash,essentially turned to zeolite cement (Fig. 3, Table 1). In PP an upwardshift in the dominant sedimentary lithic type is observed fromcarbonate to clay and marly–clay. Juvenile components usually attainless than 50 vol.% of the deposits, with the marked exception of falloutunits. Particularly in the Albano successions, grain size and compo-nent proportions often vary abruptly within individual units; forexample, breccias rich in sedimentary lithic clasts directly underliepyroclastic surge deposits containing up to 40 vol.% of cumulateinclusions.

Component proportions have been reported in Fig. 4, taking intoaccount the volume fractions of corresponding deposit intervals.Clear trends in component proportions are not detected withinindividual eruptive units in Alb-1 and Alb-2 successions, except foran upward increase in the abundance of the juvenile componentsand a faint decrease in the proportion of sedimentary lithic clasts.Considering that Alb-3 includes a single eruptive unit that is largely

Fig. 4. Vertical variations in component proportions (volume %; legend as in Fig. 3) for the Althickness is proportional to the volume fraction of corresponding deposit intervals (for exproducts).

dominated by juvenile components, we thus note an overallincrease in the juvenile fraction through the whole Albano activityhistory.

Component variations along PP stratigraphy define an evidenttrend paralleling changing eruptive dynamics, i.e., the juvenilefraction dominates early and late magmatic phases (up to 93 vol.%),while the lithic fraction attains up to 90 vol.% during the intermediatehydromagmatic phase.

Integrated component and grain-size data from all samples allowreconstruction of the size distribution of sedimentary and volcanic lithicclasts through the whole successions (Fig. 5). Both lithic size distribu-tions are polymodal, with a coarse population centered at around − 6Ф(64 mm) and one or more finer modes. Lithic clasts from Albano areslightly coarser than those from Prata Porci. In both cases, volcanic lithicclasts are coarser and less sorted than sedimentary ones.

4.3. Magma compositions

The Albano juvenile clasts show a wide range of chemicalcompositions from tephrite to K-foidite following the TAS classifica-tion, which define a liquid line of descent typical of the CAVD,characterized by increasing alkali content without concomitant SiO2

increase (Gaeta et al., 2006). Most analyzed glasses (Freda et al., 2006)show K-foiditic compositions, with MgO b3 wt.% (Table 2; Fig. 6) andcan be considered evolved ultrapotassic magmas (Foley et al., 1987).Conversely, Prata Porci juvenile clasts exhibit a primitive ultrapotassiccomposition (MgO N4 wt.%; Table 2), which plots at the tephritic,phono-tephritic, shoshonitic and trachybasaltic field cross of the TASdiagram (Fig. 6) and thus represents one of most primitive composi-tions found in the CAVD.

4.4. Morphoscopic features

FE-SEM analyses of ash particles mostly focused on the fines-richlayers that occur at the very base of most eruption units and on thematrix of pyroclastic surge beds. All Albano samples comprise juvenileparticles characterized by rough and irregular morphologies, poorvesicularity, abundant microlites (leucite, clinopyroxene and dark

b-1 (left), Alb-2 (center), and PP (right) successions. For each representative sample, barample, the lowermost sample of Unit a represents 5% by volume of the whole Alb-1

Fig. 5. Total grain-size distributions of sedimentary (mostly carbonate) and volcanic (mostly lava) lithic clasts for Alb-1, Alb-2 and PP successions. Note better sorted, finer-graineddistributions of sedimentary relatively to volcanic clasts. Coarse tails result from ballistic populations.

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mica), presence of secondary minerals and occasional hydrationcracks; glass shards are absent (Fig. 7a and b). Samples from PrataPorci often resemble those of Albano, but hydration cracks are lessfrequent and microlites are largely dominated by leucite (Fig. 7c). Tonote, the thin ash layer at the very base of the Prata Porci successiondoes not contain juvenile particles (Figs. 2d and 7d) and is made up ofaltered volcanic clasts, diatoms fragments and broken, fairly roundedcrystals, pointing out a likely phreatic origin.

FE-SEM analyses of the matrix of lithoid units revealed two maincrystalline components: i.e., zeolite and calcite, thus supporting theabove component analyses. Zeolite occurs in form of clusters ofeuhedral, up to 50 µm-long, crystals mostly filling cavities (up tomillimeter-sized), or as diffuse aggregates of irregular submicrometriclamellae (b1 µm long and ~ 0.3 µm thick) (Fig. 7e, f). Calcite mostlyoccurs as sparse, anhedral to subeuhedral, individual crystals, a fewµm across, and is also found in form of thinly laminated (~ 1 µm), pore-filling masses, in places associated with zeolites (Fig. 7e, f).

Table 2XRF analyses of juvenile scoria lapilli from the Prata Porci maar succession. a: fallouthorizon from 0.3 to 1.0 m above the base (Fig. 3c); b: rim of cored scoria lapilli inpyroclastic surge deposit

a1 a2 b1 b2

SiO2 47.75 47.76 46.90 46.76TiO2 0.87 0.88 0.81 0.87Al2O3 15.70 15.80 15.10 14.73Fe2O3 9.32 8.52 7.99 9.30FeO 0.22 1.01 1.18 0.37MnO 0.17 0.16 0.16 0.19MgO 4.67 4.67 5.92 5.53CaO 9.80 9.53 10.74 10.80Na2O 1.84 2.15 2.09 2.45K2O 5.40 4.72 4.96 4.27P2O5 0.42 0.43 0.50 0.57LOI 3.84 4.37 3.65 4.16

100.00 100.00 100.00 100.00

4.5. Cored juvenile clasts

The studied deposits contain diffuse cored juvenile clasts,centimeter- to sub-millimeter-sized. Petrographic observation revealsthat they consist of a shell of juvenile material (poorly to moderatelyporphyritic, with a microcrystalline groundmass) enclosing one ormultiple lithic clasts (Fig. 8). In particular, well-rounded clasts madeup of a relatively thin coating of juvenilematerial around a single lithiccore, and irregular scoria clasts including one or multiple lithicfragments represent two broad end-members. The dominant type ofenclosed lithic clast varies from one to another succession. In theAlbano case, lithic cores mostly consist of either mafic granularcumulates or individual leucite, clinopyroxene, quartz and sillimanitebroken xenocrysts, while at Prata Porci enclosed lithic clasts aremostly sedimentary, shifting upsection from dominant limestone todominant marly–clay rock types.

For the first cored clast end-member we found a direct linear cor-relation between the thickness of the enclosing shell (Δr) vs. the ra-dius of the lithic core (a) (Fig. 9), which will be discussed in Section 5.2.

4.6. Volume of eruption products

The stratigraphic and component data sets complement eachother, allowing us to estimate the total volume of erupted products, aswell as the proportions of hydromagmatic surge and breccia vs.magmatic fallout products and of juvenile vs. lithic components forindividual maar events (Fig. 10). Overall, it appears that the eruptedvolumes decreased from Alb-1 (9.0 ×108 m3), through Alb-2(7.5×108 m3), to Alb-3 (5.0×108m3), while Prata Porci (PP) emplaced1.1×108 m3 of products. Calculations of related crater volumes, alsotaking into account the new bathymetry of the Albano lake (Anzideiet al., 2006), yieldminimumvalues of 2.3×108m3 (Alb-1), 7.0×108m3

(Alb-2), 0.2×108 m3 (Alb-3) and 0.1×108 m3 (PP). Excavated volumesbroadly match volume estimates for the lithic component in theerupted products, with the notable exception of Alb-3, where the

Fig. 6. Plot of juvenile chemical compositions from Albano glasses (data from Freda et al.,2006) and Prata Porci scoria clasts (XRFanalyses from thiswork, reported inTable 2) in theTotal Alkali Silica diagram. See Fig. 3c and Table 2 for Prata Porci sample locations.

198 G. Sottili et al. / Journal of Volcanology and Geothermal Research 180 (2009) 189–202

erupted lithic volume is one order of magnitude larger than cratervolume, consistent with its peculiar eruptive style.

Within the Alb-1, Alb-2, and PP successions, hydromagmatic surgeand breccia deposits prevail, accounting for about 60 vol.% of thewholedeposits. Conversely, the Alb-3 succession has a dominant magmaticcharacter, with a volume ratio of magmatic fallout vs. hydromagmaticsurge and breccia products of 4:1. Hydromagmatic units in Alb-1, Alb-2and Alb-3 always contain less than 50 vol.% of juvenile clasts, whichinstead dominate in magmatic fallout products (at least 70 vol.%).

Fig. 7. Representative FE-SEM images of ash particles (left; a, b, c, d) and cemented matrix (riUnit b (Alb-1), showing irregular shapes; b) detail of an ash grain fromUnit c (Alb-2), showingshowing irregular shape, poor vesicularity and abundant leucite microlites; d) particles frodiatoms and angular to rounded crystal fragments; e) cemented portion of Albano Unit b,showing calcite lamellae (C) grown around euhedral zeolite crystals (Z).

Notably, in the PP succession, juvenile clasts attain up to 90 and 70 vol.%in Strombolian fallout and hydromagmatic surge and breccia products,respectively. In general, we note that the proportion of magma vs. lithiccomponent involvedduringbothmagmatic andhydromagmatic activityphases are always higher in the Prata Porci deposits than in the Albanoones. Using an average juvenile clast vesicularity of 0.5 and magmadensity of 2600 kg/m3, the volumes of erupted magma are 3.1, 2.6, 1.9,and 0.5×108 m3 (DRE) for Alb-1, Alb-2, Alb-3, and PP, respectively, andthe corresponding masses 8.1, 6.8, 4.9, and 1.3×1011 kg.

5. Discussion

Here we address the eruptive dynamics of the Albano and PrataPorci maars in terms of magmatic vs. hydromagmatic activity styleand themodes of their transitions, also related to the specific substratesettings. In this regard, we focus on the reconstruction of depth, mode,and timing of entrainment of country rocks by the erupting magmas.

5.1. Magmatic vs. hydromagmatic activity styles

The polygenetic Albano and monogenetic Prata Porci study caseswell document how complex eruptive behaviors may occur at maarvolcanoes on different magnitude and temporal scales.

Based on field textures (Figs. 2 and 3), componentry (Figs. 3 and 4,Table 1) and FE-SEM morphoscopy of juvenile ash particles (Fig. 7),three dominant activity styles are basically recognised in thestratigraphic records of the studied maar successions, as follows:i) magmatic activity, where magma fragmentation and eruption isessentially driven by exsolved magmatic volatiles, resulting in thegeneration of sustained plumes, Hawaiian to Subplinian in intensity.The related products, mostly consisting of fallout deposits rich inmoderately vesicular scoria and relatively lithic-poor, typically lackjuvenile ash particles bearing evidence of explosive magma–waterinteraction; ii) hydromagmatic activity, driven by the explosiveinteraction of magma with ground-water, generating pyroclastic

ght; e, f) from the Albano and Prata Porci deposits: a) poorly vesicular ash particles fromhydration skin and cracks; c) ash grain from a pyroclastic surge bed in the PP succession,m the thin ash layer at the very base of the PP succession including elongate brokenshowing a subhedral calcite crystal (C) and zeolite aggregates (Z); f) cemented Unit b

Fig. 8. a) Field appearance of a cored bomb from PP deposits enclosing a marly lithic clast; b) stereomicroscope image of a cored lapillus from Alb-1, enclosing a limestone lithic clast;c) microphotograph of a cored ash particle from Alb-1, showing a fractured and corroded clinopyroxene xenocryst core surrounded by a microcrystalline, non-vesicular matrixbearing leucite microlites (white, round areas); d) field appearance of an ovoidal cored bomb from PP deposits enclosing a limestone lithic clast; e) stereomicroscope image of a coredlapillus from Alb-1 enclosing a clinopyroxene xenocryst; f) microphotograph of a cored ash particle from Alb-1, showing a granular cumulate core.

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surge deposits with a juvenile component that displays typicalmicrotextural evidence of hydromagmatic fragmentation (e.g., poorlyvesicular particles with blocky shapes and hydration cracks, etc.; Fig. 7);iii) phreatic activity, without involvement of magma. In particular, theidentification of possible hydrothermal and phreatic episodes is a keypoint in short-term hazard assessment in the densely populated ColliAlbani area, these typesof activity representing a likelyeruptive scenariothat may lack the usual precursors associated with magma motion.

Fig. 9. Plot of core equivalent radius (a) vs. shell thickness (Δr) for 77 juvenile cored clasts fEq. (1) for different lithic types and initial lithic-magma contact temperatures, putting conmeasurement procedures: a) microphotograph of a cored clast (2.7 mm across), outlining thecore; b) previous image after thresholding and binarization; c) shell thickness (Δr) and core

Actually,we foundonlyone clear example of phreatic deposit, consistingof a thin ash layer lacking juvenile material, at the very base of the PrataPorci succession (Figs. 2 and 7). This phreatic episode probably occurreddue to heat transfer from the ascending magma to shallow lake water.

To quantitatively evaluate the relative contributions of thedifferent activity styles during the histories of the two maar-formingstudy cases, we bear on the amounts and volume proportions of thedifferent juvenile and lithic components (Fig. 10). Overall, we note that

rom the Albano and Prata Porci maar deposits. Straight lines represent the solutions ofstrains on the thermal state of interacting substrate rocks. Left insets depict a and Δrmargins of the microcrystalline, juvenile shell and the broken clinopyroxene xenocrystequivalent radius (a) as calculated from the binarized image.

Fig. 10. Bar diagrams of the volume percentages of juvenile and lithic components fromhydromagmatic and magmatic eruption phases (above) and estimated total volumes oferuption products (below) for the studied maar successions.

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in both cases magmatic eruptive style significantly prevailed overhydromagmatic one in terms of erupted magma volumes.

On the time scale of the whole life of the polygenetic Albano center(in the order of tens of kyr), we document a general decrease of thehydromagmatic character with time through the whole evolutionaryhistory, culminating with the occurrence of a relatively strongmagmatic event (Alb-3; Fig. 10, Table 1), consistent with evidencefrom previous studies in mid-distal settings (Giaccio et al., 2007). Onthe time scale of the main maar-forming eruptive cycles (in the orderof a few kyr), we observe that both the Alb-1 and Alb-2 eruptive cyclesshow an upward increase in the proportion of magma involved inhydromagmatic activity, as testified by componentry (Fig. 4) anddeposit facies (Freda et al., 2006). Finally, on the time scale of a singleeruption (order of months), repeated activity style changes at Albanoemplaced pyroclastic deposits in the order of 0.1–1 km3 of volume.Eruptive phases with significant magmatic component (i.e., Strombo-lian fallout) attained up to 60% of magma volume fraction, whereaspyroclastic surge-forming, dominant hydromagmatic activity phasesinvolved magma volume fractions as low as 5%. Moreover, the magmafraction involved changed abruptly also within individual hydromag-matic phases (e.g., Unit b, Fig. 4).

In the Prata Porci case, eruptive dynamics shifted from Strombolianfountaining to hydromagmatic pyroclastic surges, and then again tofountain activity during a single, small scale eruptive event (ca. 0.1 km3

of deposits; Fig. 10).

5.2. Reconstruction of sub-surface dynamics

Three interdependent factors are generally thought to controlfluctuating magmatic–hydromagmatic activity, i.e.: i) magma flowconditions (e.g., pulsating mass eruption rates); ii) aquifer yield, andiii) depth of fragmentation (e.g., Sheridan andWohletz,1983;White andHoughton, 2000; Carrasco-Núñez et al., 2007). The evaluation of depth,amount andmodes of entrainmentof country rocks through the analysisof lithic types and abundances in pyroclastic deposits yield insights into

the definition of the magma feeder system and into sub-surfaceprocesses that control magma fragmentation and ascent, efficiency ofinteractionwith external fluids and eruption trigger (e.g., Valentine andGroves, 1996; Barberi et al., 1988; Carrasco-Núñez et al., 2007).

The Albano and Prata Porci maar successions display highlyvariable lithic populations (Figs. 3 and 4). Besides volcanic andsedimentary lithic clasts, which commonly occur in both cases, Albanoshows a broader variety of rock types, also including abundant glass-bearing cumulates, skarns and hydrothermally altered, hauyna-bearing rocks, and xenocrysts (Section 4.2), which indicates that theascending magmas interacted with thick successions of hypoabyssaligneous and thermometamorphosed country rocks, as a possibleevidence of an existing geothermal system. Conversely, the Prata Porcimagma seems to have crossed thermally unaffected sedimentary andvolcanic terrains, including terrigenous successions, scarcely repre-sented at Albano. These differences are consistent with the differentlocations of the two maars: i.e., Albano developed along the rim of theTuscolano–Artemisio caldera, and thus closer to the main magmachamber system of the CAVD, while Prata Porci was active in arelatively peripheral zone of the district (Fig. 1). Moreover, theobserved greater proportion of volcanic lithic clasts in the Albanosuccessions can be explained by the significantly thicker volcanic pilein the central CAVD zone beneath Albano (400–900 m; De Rita et al.,1995) relatively to Prata Porci (ca 200 m).

Overall grain-size distributions reveal different histories forsedimentary and volcanic lithic clasts. For instance, sedimentaryinclusions (mostly carbonate rock types) are finer and better sortedrelatively to volcanic ones (mostly lavas) (Fig. 5), which cannot beattributed to transport-related sorting mechanisms, yet to possiblesub-surface processes, like: i) a higher degree of fragmentation, and/orii) a more prolonged phase of post-fragmentation transport andabrasion in the volcanic conduit, consistent with a deeper entrain-ment level for sedimentary rocks, and/or iii) a higher degree offracturation of sedimentary country rocks ab-initio.

The observed lithic variability in both maar stratigraphies suggestsa temporal migration of the locus of lithic clast entrainment. In thisregard, sedimentary lithic clasts in the Prata Porci succession show aclear trend, as they lack in the early deposits from Strombolian falloutand hydromagmatic surges, then increase upward in the pyroclasticsurge deposits in the middle part of the succession, and eventuallydecrease and disappear in the uppermost fallout deposits. Concomi-tantly, sedimentary lithic clasts shift from dominantly limestone todominantly marly–clay upsection, as also observed within coredscoria clasts. In the light of the CAVD substrate stratigraphy (Funicielloand Parotto, 1978), the above lithic pattern evidences changes in thedepth of entrainment, paralleling the observed changes in eruptivestyles. In particular, shallow magma fragmentation characterized theearly Strombolian phase, followed by deepening of the fragmentationlevel during explosive magma–water interaction, and uprise of thefragmentation level during resumed magmatic activity toward theend of the eruption. Thus it appears that explosive magma–waterinteraction took place as the magma fragmentation level reached thelimestone main aquifer.

In the Albano case, instead, where a clear trend is lacking, repeated,abrupt shifts in the dominant lithic clast types point out concomitantshifts in the locus of lithic entrainment as a possible consequence oflargely variable magma withdrawal conditions, including masseruption rate, and local substrate factors, such as laterally variablelithologies in a deeply dissected peri-caldera setting. Overall, despitehigher eruption intensities, Albano displays a stronger hydromag-matic character in respect to Prata Porci (see the above reported lithic/magma proportions; Fig. 10), which is possibly explained by theAlbano substrate characteristics, i.e. disrupted country rocks hosting amajor hydrothermal aquifer (Boni et al., 1995; Chiodini and Frondini,2001). Volcano-tectonically disarticulated substrate rocks and diffuseself-sealing processes due to the presence of a geothermal systemmay

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have favored pressurization of the aquifer interacting with the magmafeeder system, leading to efficient fragmentation of magma andcountry rocks and a dominant hydromagmatic activity style. Extensivewater availability from the regional aquifer may have assuredprolonged, though impulsive, hydromagmatic activity. Mass eruptionrate, however, became the main controlling factor during the mostrecent Albano event (Alb-3), which showed a dominant magmaticcharacter and attained the maximum intensity among CAVD maar-related events, reaching a subplinian scale.

The two end-member types of cored juvenile clasts recognisedabove (Section 4.5) provide inferences on the modes and timing oflithic entrainment, along with thermal constraints for country rocks.On the one hand, the irregular scoria clasts including multiple lithicfragments, which resemble those described by Lorenz et al. (2002),possibly originated by hydromagmatic fragmentation of a magma-debris mixture as magma raised into the root zone of a maar andintruded wall rock debris.

On the other hand, rounded cored scoria clasts originated whensingle wall rock fragments were entrained by a low-viscosity magmaand erupted with an adhering rim of melt (Rosseel et al., 2006). Wallrock entrainment may occur on condition that the melt is well abovethe glass transition temperature, and the formation of a quenched rimof melt around the lithic core suggests that: i) a significanttemperature difference exists between melt and lithic core at theinitial contact, and ii) the time span between lithic entrainment andcored clast eruption is shorter than that required for re-melting thequenched rim.

If the initial wall rock temperature Tl is lower than the “freezingtemperature” of the magma Tf (see Rosseel et al., 2006), then lithicfragment entrainment induces a rapid temperature decrease in thesurrounding magma and leads to the formation of a quenched rim.This process can be modelled following industrial-derived coatingmodels (Jiao and Themelis, 1993), where the ratio between themaximum thickness of quenched rim (Δrmax) and the radius of theentrained lithic clast (a) is given by the equation:

Δrmax

a¼ 1þ ρl

ρc

Cpl Tf −Tl� �ΔHc

� �1=3−1 ð1Þ

where ρl and ρc are the densities of the lithic clast and the quenchedrim, respectively, Cpl is the heat capacity of the lithic clast, and ΔHc isthe latent heat of solidification of the melt. We applied Eq. (1)) to therounded cored clast type from the Alb-1, Alb-2 and PP successions. Byusing appropriate parameters for our study cases (Tf =900°C,ΔHc=4×105 J kg−1), we calculated Δrmax/a as a function of Tl and Cplfor different lithic clast types (Table 3, Fig. 9). By comparing thecalculated Δrmax /a and the measured Δr /a, the model outputprovides inferences on the syn-eruptive thermal state of countryrocks beneath Albano and Prata Porci.

The good linearity of Δr /a data for the Prata Porci lithic cores(mostly sedimentary rocks) indicates a relatively homogeneousentrainment temperature: the model parameters that best fit Δr /adata yield an entrainment temperature lower than 100 °C, consistentwith a limited magma-country rock heat exchange. More scatteredΔr /a data for Albano suggest a broader variety of entrainmenttemperatures for lithic cores (mostly xenocrysts and cumulates).

Table 3Magma and wall rock parameters used to model juvenile cored clast formation (Eq.(1),Fig. 9)

Heat capacity (J kg−1 K−1) Density (kg m−3)

K-foiditic magma 1050 2600Limestone 840 2500Clinopyroxene (augite) 800 3400Leucite 750 2400

However, a prominent trend reveals temperatures as high as 800 °C,consistently indicating the interaction of the feeder magma with anactive, high-enthalpy geothermal system.

Overall, evidence from cored clasts supports the occurrence of along-living magma chamber beneath the polygenetic Albano maar,as suggested by previous petrological and geochronological infer-ences (Marra et al., 2003; Freda et al., 2006). By contrast, the mono-genetic Prata Porci maar, fed by a homogeneous magma batch(Fig. 6), seems to have originated by an episodic dyke intrusion in aperipheral area.

6. Conclusions

The evaluation of expected eruptive scenarios and possibleevolution of maar-forming events at the Colli Albani Volcanic Districtyield implications for volcanic hazard assessment in the denselypopulated area near Rome. The comprehensive data set reportedabove, integrated by thermodynamic inferences from cored juvenileclasts, cast new light on CAVD maar activity as related to modes ofinteraction of potassic magmas with country rocks. The mainoutcomes from the present study can be summarized as follows:

i. the monogenetic Prata Porci maar and the three maar-formingstages of the Albano polygenetic center erupted 0.5×108, 3.1×108,2.6×108 and 1.9×108 m3 of magma (DRE), respectively;ii. although explosive magma–water interaction controlled theevolution of Albano and Prata Porci centers, which share thetypical maar features, in both cases the most part of magma waserupted during magmatic eruptive phases;iii. different extents of magma–water interaction at Albano andPrata Porci reflect different geological settings: while the Albanomagmas erupted along the volcano-tectonically disrupted Tusco-lano–Artemisio caldera rim and interacted efficiently with carbo-nate country rocks hosting an active geothermal system, the PrataPorci magmas erupted in a peripheral area of the CAVD, through athick terrigenous succession on top of a relatively undisturbedcarbonate succession, resulting into reduced efficiency of magma–water interaction;iv. in the Prata Porci case, shifts in the depth of lithic entrainmentwere paralleled by eruptive style changes, i.e. hydromagmaticactivity dominated when magma–water interaction occurred atdeeper levels, involving the carbonate aquifer. Conversely, atAlbano, the eruptive style changed repeatedly without a clearrelationship with the depth of lithic entrainment, possibly due tolaterally heterogeneous substrate lithologies in a disarticulatedperi-caldera setting and changing mass eruption rates;v. as a whole, sedimentary lithic clasts, derived from deeperinteraction levels, underwent prolonged fragmentation and trans-portation processes, resulting in better sorted and finer grain-sizedistributions relatively to volcanic clasts derived from shallowerenvironments;vi. evidence from cored juvenile clasts in the light of theoreticalmodeling of melt–solid heat transfer put thermal constrains on themagma-country rock interaction process, indicating that theinteracting substrate at Prata Porci was at low, uniform tempera-ture (b100 °C) as compared to the highly variable temperatures(reaching up to several hundreds of °C) of the Albano substrate;vii. record of phreatic explosions, regarded as a common precursorof explosive volcanic activity, exists at the base of the Prata Porcieruptive succession, while it lackswithin the proximal stratigraphyof the Albano composite maar. This may be a crucial aspect forvolcanomonitoring and eruption forecast at CAVDmaar volcanoes.

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Acknowledgments

This work was funded by the Dipartimento della Protezione Civile,Italy, in the frame of the 2004–2006 Agreement with IstitutoNazionale di Geofisica e Vulcanologia— INGV, project V3_1 coordinatedby M. Gaeta and P. Scarlato. We thank G. Carrasco-Núñez and B. Giacciofor reviewing the manuscript.

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