Geological evolution of a complex basaltic stratovolcano: Mount Etna, Italy

ABSTRACT

An updated geological evolution model is presented for the com-posite basaltic stratovolcano of Mount Etna. It was developed on thebasis of the stratigraphic setting proposed in the new geological mapthat was constrained by 40Ar/39Ar age determinations. Unconformity-bounded stratigraphy allows highlighting four main evolutionaryphases of eruptive activity in the Etna region. The Basal TholeiiticSupersynthem corresponds to a period, from about 500 to 330 ka, ofscattered fissure-type eruptions occurring initially in the foredeepbasin and then in a subaerial environment. From about 220 ka, anincrease in the eruptive activity built a lava-shield during the TimpeSupersynthem. The central-type activity occurred at least 110 ka agothrough the Valle del Bove Supersynthem. The earliest volcanic cen-tres recognized are Tarderia, Rocche and Trifoglietto and later MonteCerasa, Giannicola, Salifizio and Cuvigghiuni. During the Stratovol-cano Supersynthem, from about 57 ka ago, the intense eruptive activ-ity of Ellittico volcano formed a roughly 3600 m-high stratocone thatexpanded laterally, filling the Alcantara and Simeto paleovalleys.Finally, effusive activity of the last 15 ka built the Mongibello vol-cano. Its eruptive activity is mainly concentrated in three weaknesszones in which the recurrent magma intrusion generates flank erup-tions down to low altitude. The four main evolutionary phases mayfurnish constraints to future models on the origin of Etna volcanoand help unravel the geodynamic puzzle of eastern Sicily.

KEY WORDS: Mount Etna, geological evolution, stratigraphy,UBU, eruptive history.

INTRODUCTION

The new stratigraphic setting of Etna volcano, recon-structed through recent geological studies (BRANCA et alii,2011), provides a modern evolutionary model of this com-plex stratovolcano where about 900,000 inhabitants live.The early geological investigations of Etna volcano wereperformed during the 19th century by several scientistsamong which DE BEAUMONT (1836), GEMMELLARO (1858),LYELL (1859), and WALTERSHAUSEN (1880) defined forthe first time the composite polygenetic origin of the vol-cano edifice, identifying two main stratocones superim-posed named Trifoglietto and Mongibello (for details seeBRANCA et alii, 2011). During the second half of 20th cen-tury, the modern studies on Etna geology mostly per-formed in the Valle del Bove area by KLERKX (1963;1968a and b) were synthesized by RITTMANN (1973), who

proposed a geological setting of Etna volcano divided intofive evolutionary stages (tab. 1). The first stage, namedPre-Etnean volcanic activity, corresponds to a long periodof submarine and subaerial fissure-type eruptions. Thefollowing stages are, instead, related to the constructionof the central volcano edifice formed by the superpositionof four main stratovolcanoes: Calanna, Trifoglietto I and Trifoglietto II located in the area of Val Calanna andValle del Bove, and then the present volcanic edifice, the so called Mongibello. The stratigraphic reconstructiondefined in the geological map of ROMANO et alii (1979)allowed ROMANO (1982) to propose an updated geologicalevolution of the volcano edifice into four main phases(tab. 1). The oldest phase (Basal Subalkaline Lavas)groups the tholeiitic submarine volcanic products locatedalong the Ionian coast from Aci Castello to Aci Trezza,and the later tholeiitic subaerial lava flows cropping outalong the left bank of the Simeto river valley on Etna’ssouth-western foot. The second phase (Ancient AlkalineCentres) corresponds to the transition from subalkaline toalkaline products, marking a change in style from fissureto central eruptions. According to ROMANO (1982), theemission of lava flows from monogenetic centres at thebeginning of this phase probably built a primitive shieldvolcano, extending from the Alcantara to Simeto rivers.Thereafter, three volcanic centres (Monte Po, Calanna andTrifoglietto I) developed in the Val Calanna and Valle delBove areas (tab. 1). The third phase marks the growth of acomplex stratovolcano, named Trifoglietto unit, throughthe superimposition of some small volcanic centreslocated on the south-western sector of the Valle del Bove(Serra Giannicola Piccola, Trifoglietto II, Zoccolaro,Vavalaci and Belvedere). The last phase comprises the for-mation of a large stratovolcano (Mongibello volcano) onthe north-western slope of the Trifoglietto volcano. TheMongibello volcano activity was divided in two stages: theAncient Mongibello formed by two distinct eruptive cen-tres (Ellittico and Leone) and the Recent Mongibello thatincludes all volcanics erupted in the last 3-5 ka.

During the three decades after the publication of thegeological map of ROMANO et alii (1979) some detailedgeological and structural studies, mainly focused on theValle del Bove area and the eastern flank, were per-formed. In the 1980s, English and French authors pro-posed different stratigraphic reconstructions of the vol-cano reported in MCGUIRE (1982), CHESTER et alii (1985)and KIEFFER & TANGUY (1993). In particular, the geologi-cal framework proposed by CHESTER et alii (1985), focus-ing on the stratigraphy of the southwestern wall of theValle del Bove, is slightly different from those proposedby ROMANO (1982) (see tab. 1 for comparison). Indeed,

(*) Istituto Nazionale di Geofisica e Vulcanologia, Osserva -torio Etneo, sezione Catania, Piazza Roma, 2 - 95123 Catania(Italy). Corresponding authors: phone: +390957165800; e_mail: [email protected]

(**) C.N.R - Istituto per la Dinamica dei Processi Ambientali,sezione di Milano, Via Mangiagalli, 34 - 20133 Milano (Italy).

Geological evolution of a complex basaltic stratovolcano: Mount Etna, Italy

STEFANO BRANCA (*), MAURO COLTELLI (*) & GIANLUCA GROPPELLI (**)

35-R2 – BRANCA

Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 130, No. 3 (2011), pp. 306-317, 8 figs, 1 tab. (DOI: 10.3301/IJG.2011.13)© Società Geologica Italiana, Roma 2011

Queste bozze, cor rette deb bo no es serere sti tuite im med i at a mente alla Se g re te riadel la Società Geo log i ca Ital i a nac/o Di par ti men to di Scienze del la Ter raPi az zale Aldo Moro, 5 – 00185 ROMA

35-BRANCA 306-317_GEOLOGIA 28/11/11 12.46 Pagina 306

CHESTER et alii (1985) suggested a different evolution forthe eruptive centres taking place after Trifoglietto II, alsoon the basis of the geological investigations performed byMCGUIRE (1982). The stratigraphic reconstruction byKIEFFER & TANGUY (1993) was based mainly on the geo-logical studies realized by KIEFFER (1985). They inferredthe presence of a large stratovolcano (Ancien Etna) dur-ing the early stage of the alkaline activity that precededthe growth of the volcanic centres of Valle del Bove andVal Calanna areas (see tab. 1 for comparison).

Finally, during the 1990s, the continuous acquisition ofnew geological data on Etna region for the realization ofthe new geological maps of Italy at 1:50.000 scale (CARGProject, SERVIZIO GEOLOGICO D’ITALIA 2009a and b; 2010aand b), allowed achieving a reconstruction of the geologi-cal evolution of Etna volcano based entirely on modernstratigraphic concepts (BRANCA et alii, 2004a,b and 2008).

In this paper, we propose an updated reconstruction ofEtna’s geological evolution on the basis of the stratigraphicdata obtained from the new geological map of Etna vol-cano (BRANCA et alii, 2011) integrated with 40Ar/39Ar agedeterminations of DE BENI et alii (2011), that allow con-straining the age of the volcanic bodies, the temporal hia-tus and the significance of the main unconformities foundin the volcanic succession. The whole data set of the newgeological map expands and improves the knowledge ofthe eruptive history of Etna (fig. 1) and its evolutionarymodel with respect to the one proposed on the basis of thegeological map of ROMANO et alii (1979).

STRATIGRAPHY AND RADIOISOTOPIC DATINGS

Geological data collected during the realization of thenew geological map of Etna volcano (BRANCA et alii,2011) are based on modern stratigraphic principles that

enabled reconstructing the stratigraphic setting of the vol-cano edifice formed by the superposition of effusive andexplosive volcanic products, interbedded by reworkedmaterials. These products on the whole represent thelithologic, morphologic and volcanologic history of Etna(fig. 1).

The stratigraphic setting defined in the new geologi-cal map is the result of the combination of three differentcategories of stratigraphic units: synthemic, lithosomaticand lithostratigraphic ones (fig. 2) (for details see BRANCA

et alii, 2011). In particular, the application of Unconfor-mity-Bounded Units (UBU of SALVADOR, 1987 and 1994)furnishes an objective reading key for any stratigraphicreconstruction. The unconformity surfaces found into thevolcanic succession of Etna allowed to group lithostrati-graphic units into synthemic units. On the basis of therecognition of several unconformities within the volcanicsuccession we define synthems and supersynthems mark-ing the main steps of Etna geological evolution. Finally,we used a third stratigraphic category, the lithosome,with the aim of representing the spatial localization of thedifferent eruptive centres, named “volcano”, recognizedon the basis of their morphology.

The new stratigraphic studies of Etna volcanic succes-sion allow us to explain the complex spatial and temporalevolution of the volcanic system, the relationship betweenthe volcanic products and their lithogenetic and morpho-logic features and in addition to define the stratigraphicinterrelationship between the volcanic products and thesedimentary deposits along the periphery of the edifice.During the realization of the new geological map of Etnavolcano, a radioisotopic dating project was performed byDE BENI et alii (2011) applying the 40Ar/39Ar incrementalheating technique in order to chronologically constrainthe stratigraphic setting. In particular, forty samples werecollected from key outcrops selected on the basis of their

GEOLOGICAL EVOLUTION OF A COMPLEX BASALTIC STRATOVOLCANO: MOUNT ETNA, ITALY 307

TABLE 1

Comparative framework of Etna’s stratigraphy proposed since the 19th century by several authors (modified from BRANCA et alii, 2004a). Asterisks in CHESTER et alii (1985) column refer to volcanic centres identified by

MCGUIRE (1982).


stratigraphic position thereby obtaining consistent resultsfrom all analysed volcanics (fig. 2). These new age deter-minations allow assigning an age to most of the lithos-tratigraphic units, clarifying the uncertain stratigraphicposition of isolated volcanic units. In addition, theradioisotopic data constrains the temporal hiatus thatcharacterised the main unconformities, allowing us toinfer their evolutionary significance in the frame of thecomplex growth of Etna’s polygenetic volcano.

GEOLOGICAL EVOLUTION OF ETNA VOLCANO

In this section we describe the geological evolution ofEtna volcano as the result of: i) the new proposed recon-struction of the complex Etna volcanic succession, com-posed of 27 lithostratigraphic units; ii) the description ofthe morpho-structural features of the lithosomatic unitsthat allowed us to identify 9 volcanoes; and iii) the recog-nition of the main unconformities that subdivide the

308 S. BRANCA ET ALII

Fig. 1 - Scheme of the synthems recognized in the stratigraphy of Mt. Etna (modified from BRANCA et alii, 2011): 1) Present and recent covers; 2) Il Piano Synthem a) sedimentary deposits; 3) Concazze Synthem a) sedimentary deposits; 4) Zappini Synthem; 5) Croce MenzaSynthem; 6) S. Alfio Synthem; 7) Acireale Synthem a) sedimentary deposits; 8) Adrano Synthem; 9) Aci Trezza Synthem, a) sedimentary deposits; 10) Sedimentary and metamorphic basement. The arrow indicates the direction of strike movement in the fault symbol.


entire volcanic succession into 8 synthems and 4 super-synthems. The stratigraphic data collected for the realiza-tion of the new geological map of Etna volcano (BRANCA

et alii, 2011) integrated with the radiometric data of DE

BENI et alii (2011) allow defining the geological evolutionof Etna volcano entirely on objective and modern strati-graphic concepts (UBU), thereby updating the previousgeological reconstruction proposed in BRANCA et alii(2004a) and BRANCA et alii (2008). In the following discussion, we illustrate the main phases of the eruptiveactivity according to the proposed supersynthems classifi-cation (fig. 2) since they recorded the main hiatus and themajor changes in both position and eruption style of thesucceeding eruptive centres.

BASAL THOLEIITIC SUPERSYNTHEM

The Basal Tholeiitic Supersynthem groups the earliestvolcanic products erupted in the Etna region and thecoeval sedimentary deposits. The oldest ones belong toAci Trezza Synthem and were intruded in the shallowclay sediments or emplaced on the seafloor of the Gela-Catania Foredeep basin about 500 ka ago (DE BENI et alii,2011), whereas the earliest subsequent subaerial prod-ucts, belonging to the Adrano Synthem, were eruptedabout 330 ka ago (fig. 3). Shallow subvolcanic bodies fedthe submarine eruptions of Aci Trezza Synthem duringthe sedimentation of the Argille grigio-azzurre Formation.This volcanism produced the pillow lava and volcaniclas-tic breccia succession of Aci Castello formation. The lim-ited exposition of these submarine deposits allows only apartial reconstruction of their eruptive history, which isrelated to the occurrence of effusive fissure-type erup-tions with associated minor hydromagmatic activity dur-ing the Middle Pleistocene in the northern portion of theforedeep basin (fig. 3a). In this area, the regional uplift(DI STEFANO & BRANCA, 2002) caused the passage from amarine depositional environment to a transitional one,outlined by the sedimentation of the San Giorgio sands.The uplift process produced the definitive emersion of theLower-Middle Pleistocene sediments and the followingdevelopment of the Simeto river paleovalley. In this sub-aerial paleogeographic environment, about 330 ka ago(DE BENI et alii, 2011), scattered fissure-type eruptionsgenerated thin-lava flows succession (Adrano Synthem)that invaded the Simeto paleovalley floor, carved on boththe Apenninic-Maghrebian Chain terrains and the Pleisto -cene foredeep sediments. Therefore, the earliest subaerialvolcanism in the Etna region is represented by a widethin lava plateau (S. Maria di Licodia formation) dippinggently SSE, that crops out discontinuously betweenAdrano and Paternò towns. This lava plateau extended formore than 25 km along the paleovalley floor and reachedthe sea at Paternò-Valcorrente area (fig. 3b), where thelava flows show well-developed pillow structures alongthe paleo-shoreline (BRANCA et alii, 2008). Explosivephreatomagmatic deposits of two N-S oriented eruptivefissures recognized at Valcorrente and Motta S. Anastasiaindicate the interaction between magma and ground -water located in the sediments of the paleo-Simeto allu-vial plain. In this paleo-landscape, the possible presenceof lacustrine environments is indicated by the occurrenceof pillow-lava facies at the base of the lava plateau. Thefluvial erosion of the lava plateau fed the deposition of Mt Tiritì gravels, formed by sedimentary cobbles with

tholeiitic lava pebbles at the mouth of the paleo-Simetoriver.

After the emplacement of the Adrano Synthem lavas,a long period of erosion related to the entrenchment ofthe paleo-Simeto river affected this area. Both the sedi-


Fig. 2 - Scheme of the stratigraphic relationships reconstructed forMt. Etna volcanic district and radioisotopic ages of the volcanic suc-cession (modified from BRANCA et alii, 2011) obtained by 40Ar/39Ardating (age error in 2σ) of DE BENI et alii (2011). (§) K\Ar age ofGILLOT et alii (1994); (*) C14 age of KIEFFER (1975); (°) C14 ages ofCOLTELLI et alii (2000), (^) inferred age of CALVARI et alii (2004).


mentary terrains and the lava plateau wore away from theriver banks, generating the alluvial conglomerates of theSan Placido formation. This period is recorded by theunconformity that separates the Basal Tholeiitic from theTimpe Supersynthem. Radioisotopic and stratigraphicdata indicate as about 100 ka the duration of the hiatus

between these supersynthems, although it should beshorter since the base of Acireale Synthem (Timpa di DonMasi formation, S. Caterina member) rests below the sealevel and, consequently, is still unknown.

Overall, the volcanism of the Basal Tholeiitic Super-synthem is characterized by scattered eruptions with sim-ilar chemical composition (tholeiitic affinity, TANGUY

1978, CORSARO & POMPILIO, 2004a) and eruptive style tothose of the volcanism of the northern border of theHyblean plateau occurring during the Pliocene-lowerPleistocene (SCHMINCKE et alii, 1997).

TIMPE SUPERSYNTHEM

The Timpe Supersynthem groups the volcanic prod-ucts forming a primitive lava shield (Acireale Synthem)characterized by a sub-alkaline composition at the basepassing upward to a Na-alkaline affinity (TANGUY, 1978,CORSARO & POMPILIO, 2004a), and the coeval sedimen-tary deposits. Acireale Synthem consists of a lava flowssuccession cropping out discontinuously mainly on thelower eastern flank and secondarily along the southernmargin of Etna volcano. During the emplacement of thissynthem, from about 220 ka (GILLOT et alii, 1994) up to129 ka ago (DE BENI et alii, 2011), the eruptive activitywas strictly influenced by the extensional tectonics of theNNW-trending Timpe fault system (BRANCA et alii, 2008).The repetitive occurrence of effusive eruptions along N-Sfissures generated the superimposition of lava flows fromAcireale to the Ripa della Naca area (Timpa di Don Masiand Timpa formations) along the present Timpe faultsystem. The lava succession attitude shows a generalwestward dip on Acireale Timpa and Moscarello Timpafault-scarps, that turns toward northwest at Ripa dellaNaca, where the lava succession thickness reduces, andnear to S. Venera village, where it becomes very thin rest-ing on the sedimentary basement. The attitude and theshape of the lava succession indicate that the AcirealeSynthem comprises the products related to the first prim-itive composite volcano, interpreted as a lava shield. It iselongated for at least 22 km on a NNW-SSE axis (fig. 4a),thus larger than previously inferred by BRANCA et alii(2008). In fact, the southern extension of this shield vol-cano cannot be completely delimited because its baserests below the sea level down to 660 m of depth (CHIOCCI

et alii, 2011), whereas the northern margin is recognizedbetween Ripa della Naca and S. Venera.

During the growth of the shield volcano, eruptiveactivity also affected the lower southwestern and south-eastern sectors of Etna edifice (BRANCA et alii, 2008). Thinlava flows (Paternò member of Timpa formation) wereerupted from isolated vents overlapping with angularunconformities the remnants of the Adrano Synthem lavaplateau or resting directly on the Pleistocenic sedimentarybasement (fig. 4a). The absence of erosional unconformi-ties within the lava succession of Acireale Synthem indi-cates that the eruptive activity was almost continuous as a consequence of a more efficient magma ascent from themantle with respect to the volcanism of the Basal Tho -leiitic Supersynthem. The Acireale Synthem is bounded atthe top by an unconformity related both to the disman-tling of the lava-shield volcano that formed volcaniclasticdeposits (Timpa S. Tecla member of S. Maria Ammalatiformation) and by the entrenchment of the paleo-Simetodrainage system that resulted in the formation of alluvial


Fig. 3 - Schematic representation of the evolution of Basal TholeiiticSupersynthem: a) Aci Trezza Synthem; b) Adrano Synthem. The dotted line indicates the present boundary of Etna volcanics. SC) summit craters; VdB) Valle del Bove.


terraces (Piano Carrubba member of S. Maria Ammalatiformation).

The lava flow succession of S. Alfio Synthem coverswith angular unconformity the Acireale Synthem shieldvolcano. The intense effusive activity from fissure erup-tions (Calanna and Moscarello formations) marked thewestward shifting of the main feeder system toward thecentral portion of the present Etna edifice (Val Calanna-Moscarello area) between 129 and 126 ka ago (DE BENI etalii, 2011) (fig. 4b). In the meantime, scattered fissureeruptions occurred until about 112 ka ago (DE BENI et alii,2011) in the southeastern lower flank of Etna, forming asuccession of thin lava flows (Valverde formation) thatrest mainly on the Pleistocenic sedimentary basement.

VALLE DEL BOVE SUPERSYNTHEM

The Valle del Bove Supersynthem is characterized bythe construction of the earliest central-type polygeneticvolcanoes in the Etna region at least from about 110 kaago (DE BENI et alii, 2011) after the shield-forming erup-tive activity of the Timpe Supersynthem. Volcanics of thissupersynthem vary in composition from hawaiitic to ben-moreitic (CORSARO & POMPILIO, 2004a). During the timeinterval corresponding to the Croce Menza Synthem, theeruptions occurred from three centres: Tarderia, Roccheand Trifoglietto volcanoes, whose stratigraphic relation-ships are unknown due to the lack of overlapped out-crops (fig. 5a). Concerning Tarderia volcano, given itslimited products exposure, we have not sufficient data todefine its eruptive style but only to infer its broad mor-phology (BRANCA et alii, 2008). Tarderia volcano (Con-trada Passo Cannelli formation) is localized immediatelyto the south of the Valle del Bove slope where its south-ern flank is partially preserved. This volcano ended itseruptive activity 106 ka ago (DE BENI et alii, 2011). Roc-che volcano is a small eruptive centre, whose axis waslocated less than 1 km south of the Rocca Capra-RoccaPalombe reliefs on the northeast sector of the Valle delBove. Rocche volcano activity was mainly characterizedby lava effusion even if some thick pyroclastic flowdeposits indicate the occurrence of important explosiveeruptions (Rocche formation). Eruptive activity of thisvolcano ended about 102 ka ago (DE BENI et alii, 2011).The eruptive axis of Trifoglietto volcano was locatedabout 500 northward of Serra dell’Acqua and its exposedbase is more than 3 km wide, E-W elongated (fig. 5a).The Trifoglietto volcano was characterized by steep slopesand reached a maximum elevation of about 2600 m, representing the main centre belonging to this super -synthem. This volcano was characterised by the effusionof mainly autoclastic lava flows and deposition of breccialayers, whereas it underwent explosive activity during the final stage (Piano del Trifoglietto formation). Theexposed base of the Trifoglietto succession has an age ofabout 107 ka (DE BENI et alii, 2011), but the age of theonset of its eruptive activity is unknown since the bottomof the succession does not outcrop. Trifoglietto volcanoactivity ended 99 ka ago (DE BENI et alii, 2011) with abenmoreitic poly-phase plinian eruption that generated athick sequence of pumice fall and flow deposits (CavaGrande lithohorizon of BRANCA et alii, 2011).

Local shifting of the shallow eruptive feeder systemand a short hiatus produced the angular unconformitythat separates the previous products of Croce Menza Syn-

them from those of Zappini Synthem. During ZappiniSynthem the eruptive activity occurred from four centres,producing the superposition of Monte Cerasa, Giannicola,Salifizio and Cuvigghiuni volcanoes (fig. 5b). Monte Ce -rasa volcano is a large composite stratocone whose axis


Fig. 4 - Schematic representation of the evolution of Timpe Super-synthem: a) Acireale Synthem; b) S. Alfio Synthem. Symbols: 2) AciTrezza Synthem; 3) Adrano Synthem; 4) Acireale Synthem; 5) S. AlfioSynthem. The dotted line indicates the present boundary of Etnavolcanics. SC) summit craters; VdB) Valle del Bove.


was located between Mt Centenari and Rocca Musarrarelief in the central portion of the present Valle del Bove.This volcano was mainly characterized by explosive activ-ity with the formation of thick pyroclastic flow depositsending with effusive eruptions that formed a thick lavasuccession 93 ka ago (DE BENI et alii, 2011) (Monte Scor-

sone and Monte Fior di Cosimo formations). Giannicolavolcano is a small centre made up of a 300 m large neck(Belvedere member) intruded in the northern flank of Tri-foglietto volcano about 85 ka ago (DE BENI et alii, 2011)followed by a succession of alternating pyroclasticdeposits and lava flows (Serra Giannicola Grande forma-tion). At the same time, the effusive activity of Salifiziovolcano produced a thick lava flow succession coveringthe eastern and southern flanks of Trifoglietto and MonteCerasa volcanoes. In this succession we recognise threedifferent and superimposed formations (Valle degli Zap-pini, Serra del Salifizio and Acqua della Rocca Forma-tions) based on their lithological properties associated tominor unconformities. The main vent of this volcano waslocated about 1 km eastward to the Trifoglietto eruptiveaxis. Finally, the most recent eruptive centre formed dur-ing the Valle el Bove Supersynthem was the Cuvigghiunivolcano whose activity began about 79 ka ago and contin-ued at least up to about 65 ka (DE BENI et alii, 2011). Theearly eruptive stage of this volcano was characterized bythe intrusion of subvolcanic bodies (Serra Cuvigghiuniformation, Laghetto member) in the western flank of Tri-foglietto volcano. Eruptive activity continued with theemission of thin lava flows and explosive activity that pro-duced spatter deposits (at the base) and a thick pyroclas-tic flow succession (at the top) forming a tuff cone(Canalone della Montagnola Formation). During the finalstage of Cuvigghiuni volcano the activity was mainly effu-sive with the emplacement of a lava flow succession.

STRATOVOLCANO SUPERSYNTHEM

After the construction of the volcanoes of the Valledel Bove Supersynthem, the volcano plumbing systemshifted by about 4 km NNW from the eruptive axis of theCuvigghiuni volcano. This shifting produced a change ofthe summit vent position and a strong angular unconfor-mity in the volcanic succession marking the boundarybetween the Valle del Bove and the Stratovolcano super-synthems. Stratovolcano Supersynthem was charac-terised by the definitive construction of the present bulkof Etna edifice that took place through the activity ofConcazze Synthem. In fact, Concazze Synthem comprisesthe products emitted by Ellittico volcano, the main erup-tive centre recognized in the Etna region (fig. 6), rangingin composition from alkali-basalts to trachytes withmugearites being the most abundant rocks (CORSARO &POMPILIO, 2004a). Ellittico volcano began its growthabout 57 ka ago (DE BENI et alii, 2011) on the northwestflank of the Valle del Bove Supersynthem volcanic edi-fices, reaching a maximum elevation of about 3600 maccording to our morphological reconstruction (fig. 6),which is lower than that estimated by KIEFFER (1985).This volcano was characterized by both explosive andeffusive activity, mostly from the summit vents and sub-ordinately from flank fissures. The occurrence of anintense explosive activity is testified by the presence ofthick pyroclastic flow and fall deposits interbedded in thelava succession of Serra delle Concazze and Pizzi DeneriFormations, that form the upper northeast flank of theEllittico volcano currently exposed along the northernwall of the Valle del Bove (fig. 7). Moreover, several distalpyroclastic fall deposits crop out on the lower easternflank of Etna belonging to the Tagliaborsa member ofPiano Provenzana formation. This indicates that a signifi-


Fig. 5 - Schematic representation of the evolution of Valle del BoveSupersynthem with the morphological reconstruction of the volcanicedifices: a) Croce Menza Synthem and b) Zappini Synthem. Symbols:1) Valle del Bove rim; 2) summit vent; 3) contour line (equidistance200 m). Symbols for the volcanoes: CV) Cuvigghiuni; GG) Giannico-la; MC) Monte Cerasa; SA) Salifizio; TD) Tarderia; TR) Trifoglietto;RC) Rocche.


cant explosive activity took place, producing eruptionsfrom strong strombolian to subplinian in magnitude.Thin lava flows spilling from the summit craters gener-ated steep slopes above 1600-1700 m of elevation, form-ing the typical conical shape of many andesitic stratovol-canoes (fig. 6). Furthermore, flank eruptions producedthe emplacement of the large lava flow fields of PianoProvenzana formation, leading to the gradual expansionof Ellittico flanks on the peripheral sedimentary base-ment. In particular, between about 40 ka and 30 ka (DE

BENI et alii, 2011) many lava flows expanded down to 25 km from the summit area as a consequence of flankeruptions occurring on the Ellittico lower slopes. Duringthis time span the gradual growth of the lower north andnortheast flanks of Ellittico produced a radical modifica-tion of the hydrographic setting on the northern marginof the volcano edifice (BRANCA & FERRARA, 2001). In par-ticular, the lava flows coming from the north and north-east slopes of the volcano reached and completely filledup the Alcantara paleovalley producing a northwardstream diversion of the paleo-Alcantara river bed into thepresent valley about 30-25 ka ago (BRANCA & FERRARA,2001; BRANCA, 2003). At the same time, the occurrence offlank eruptions at low altitudes along the western flank ofEllittico volcano, as testified by Mt La Nave and Mt Barcaeruptive fissures, allowed lava flows to flood the Simetoriver valley, causing repeated dam phenomena of thepaleo-watercourse. Flank eruptions along the easternflank produced lava flows that reached the Ionian coastnear Fondachello and Acireale. The eruptive style of someflank eruptions was characterised by an intense explosiveactivity that produced thick pyroclastic fall deposits. Theyshow phreatomagmatic features like the case of Mt Barcaeruption occurring about 29 ka ago (BRANCA et alii, 2009)on the lower western flank and the C.da Ragalia eruptionabout 19 ka ago (ANDRONICO et alii, 2001) on the lowernortheastern flank. During the last glacial maximum,between 30 and 20 ka, the Ellittico volcano piled up thelava flows of Pizzi Deneri Formation, frequently autoclas-tic due to a strong interaction between the ice cap and thehot lava erupted from the volcano summit on the steepcone slopes above 2500 m elevation. About 20 ka ago, theEllittico volcano had already reached its maximum arealexpansion having a maximum diameter of about 45 kmthat corresponds to the present diameter of the Etna edi-fice. The final stage of Ellittico activity was characterisedby the emplacement of plagioclase-rich porphyritic lavaflows on the upper flanks (the upper member of PizziDeneri Formation) and lava breccia deposits related toautoclastic viscous lava flows of Monte Calvario forma-tion, cropping out on the lower south-western flank.Finally, four plinian eruptions, occurring in a rather shortperiod around 15.5-15 ka ago (COLTELLI et alii, 2000),ended the activity of the Ellittico volcano, producing thecollapse of the summit area and forming the Ellitticocaldera whose northern rim (fig. 8a) is still preserved atPunta Lucia (2931 m a.s.l.) and Pizzi Deneri (2847 ma.s.l.). These plinian eruptions generated the pumice falldeposits belonging to Osservatorio Etneo member ofPortella della Giumenta formation (fig. 8b), which widelycrops out from the upper to the lower eastern and south-eastern flanks of Etna down to the coast near Acireale(COLTELLI et alii, 2000). Agglutinate spatter deposits andpyroclastic flows of the Biancavilla-Montalto Ignimbritemember crop out near the caldera rim and on the lower

southwestern flank respectively (fig. 8c). Lastly, the pecu-liar rheomorphic lava flows of Ragabo member areexposed on the northeastern flank around the Piano Pro venzana locality.

Il Piano Synthem comprises volcanics erupted afterthe Ellittico caldera collapse up to the present (Torre del Filosofo and Pietracannone formations). Il Piano Synthem lavas range in composition from hawaiite tomugearite, even though after 1970, the erupted productsare different from all previous historical Etnean lavas inbeing enriched in potassium and other alkalis and aremainly classified as K-trachybasalts (CORSARO & CRISTO-


Fig. 6 - Schematic representation of the evolution of Concazze Synthem with the morphological reconstruction of Ellittico volcano:Symbols: 1) Valle del Bove rim; 2) main vent; 3) contour line every200 m. Symbols for the Volcanoes: CV) Cuvigghiuni; EL) Ellittico;MC) Monte Cerasa; SA) Salifizio.

Fig. 7 - Aerial view from east of the northern wall of the Valle delBove showing the volcanic succession forming the Ellittico volcano(Concazze Syntem).


FOLINI, 1996 and references therein; CORSARO & POM-PILIO, 2004a and references therein).

The emplacement of Il Piano Synthem lava flows pro-duced the present morphological setting of Etna edifice inthe past 15 ka, namely the Mongibello volcano. Effusivesummit eruptions generated several lava flows that gradu-ally filled the Ellittico caldera almost completely, whereasthe flank eruptions mantled the previous morphology thatgradually disappeared on the volcano slopes where fis-sures and vents opened more frequently. Since the firststage of Mongibello volcano activity, several lava flowsgenerated by lower fissures reached the Ionian coast, theAlcantara valley and the present Simeto river floor, caus-ing several dam phenomena. The explosive activity fromthe eruptive vents formed single or coalescent monogenicstrombolian scoria cones, or spatter ramparts extendedalong the fissures dotted also by hornitos. Sometimes, thefire fountain activity generated eruptive columns thatproduced fairly widespread pyroclastic fall deposits.Gradually, the eruptive fissures became spatially clus-tered evidencing the development of some main weaknesszones into the Mongibello volcano edifice, named NE, S and W rifts, in which the recurrent intrusion of feederdykes occurred. A strong explosive activity also took placeat the summit craters starting from about 12 ka ago (M1tephra layer of COLTELLI et alii, 2000) and producing sev-eral strombolian to subplinian events whose pyroclasticfall deposits are mainly distributed on the eastern flank(Cubania member of Torre del Filosofo formation). Themorpho-structural arrangement of the Mongibello vol-cano was radically modified about 10 ka ago by a cata-strophic flank collapse that involved the eastern flank of

the Etna edifice producing the wide depression of theValle del Bove (CALVARI et alii, 1998, 2004 and referencetherein). In particular, according to several authors aseries of coalescing landslides generated the Valle delBove formation (GUEST et alii, 1984; KIEFFER, 1985; CAL-VARI et alii, 2004 and reference therein). The debrisavalanche deposit (Milo member of Pietracannone forma-tion) caused by the flank collapse is still preserved onlydownslope from the open end of the valley on the hangingmorphological step of the Moscarello Timpa fault. Theerosion and reworking of the debris avalanche depositformed a wide detritic-alluvial fan named Chiancone (amember of Pietracannone formation) along the Ioniancoast (CALVARI & GROPPELLI, 1996). In the recent part ofHolocene an unusual picritic explosive eruption occurred3930±60 ka (FS tephra layer of COLTELLI et alii, 2005),afterward we note an increase in the number and themagnitude of both explosive (DEL CARLO et alii, 2004)and effusive eruptions. In particular, large effusive erup-tions formed wide compound lava flows erupted fromvents that opened on the volcano flanks. The clustering ofthe vents along the rifts zone became predominant below2000 m elevation. Conversely, the vents show a radial dis-tribution in the summit portion of the volcano. In partic-ular, during the past 4 ka the flank eruptions ofteninvolved the southeast flank of Etna edifice, which wasthe most populated in historical time. In 122 BC thelargest explosive eruption of Mongibello volcano inHolocene time occurred. This was a plinian eruption of abasaltic magma (COLTELLI et alii, 1998) that produced awidespread pyroclastic scoria fall deposit on the south-east flank of Etna causing huge damage to the ancient


Fig. 8 - (a) View of the northern flank of Etna where the Ellittico caldera rim is preserved. Pyroclastic deposits of Ellittico plinian eruptions(Portella Giumenta formation); (b) pumices lapilli fallout deposit about 50 cm thick (evidenced by the dot white line) that crops out close to Aci Catena town at about 200 m elevation (Osservatorio Etneo member) and (c) pyroclastic flow deposit in the lower south-west flank(Vallone S. Filippo) at about 550 m elevation (Biancavilla-Montalto Ignimbrite member).


Roman town of Catania. This eruption was associated tothe formation of a summit caldera, named Cratere delPiano, representing the boundary between Pietracannoneand Torre del Filosofo formations. The buried rim of theCratere del Piano caldera is morphologically recognizablebetween Torre del Filosofo and Punta Lucia at about 2900 m elevation. The formation of this caldera causedthe disruption of the Ellittico caldera southern rim.Finally, the eruptive activity of the past 2 ka produced thefilling of the Cratere del Piano caldera, building the pre-sent Etna summit cone. Flank eruptions mantled all theupper and middle volcano flanks with wide compoundlava flows, confirming the spatial distribution observed atleast from 4 ka ago. However, only three lava flowsreached the Ionian coast during the medieval age (Mt Iliceand Mt Arsi di S. Maria eruptions, BRANCA et alii, 2011)and in 1669 when the largest flank eruption of this periodoccurred at low altitude along the south flank, causingthe destruction of several villages and part of Catania.Since the second half of 17th century eruptive activity ofMongibello volcano was characterized by both periods ofexplosive activity at the summit craters, from strombolianto lava fountain with sporadic short-lived subplinianevents, and the occurrence of flank eruptions whichappear to have no systematic relationship with the centralactivity (BRANCA & DEL CARLO, 2005). The main style offlank eruptions is almost purely effusive even thougheruptive events with associated long-lasting ash-plume-forming eruptions rarely occurred (classes A and B erup-tions of BRANCA & DEL CARLO, 2005). During this timespan magma intrusion mainly affected the NE and S riftzone of Etna edifice and the eruptive fissures are largelydistributed between 1600 and 2800 m elevation. After the1669 eruptions, only three eruptive fissures opened below1600 m elevation down to 1100 and 1300 m elevation in1809, 1928 (the eruption that destroyed Mascali village)and 1981. Finally, concerning the most recent eruptivebehaviour of Mongibello volcano, from the second half of 1970s, both summit and flank eruption frequencyincreased markedly as well as a corresponding sharpincrease in lava-erupted output rate (BRANCA & DEL

CARLO, 2004).

COMPARISON WITH THE EVOLUTION OF OTHER BASALTIC VOLCANOES

On the basis of the geological history of Etna basalticvolcano, we can compare the evolutionary stages definedfor the basaltic volcanoes of Hawaii islands with the evo-lution in four phases according to the above describedsupersynthems classification. As a matter of fact, werefer to the scheme of evolutionary stages first proposedfor Hawaiian volcanoes by STEARNS (1946), revised byMACDONALD et alii (1983) and then by PETERSON &MOORE (1987).

Hawaiian volcanoes have their main and complexphase of construction (Shield-building stage), from sub-marine to subaerial, of the lava shield above the embry-onic deep-sea edifice (Initial stage), and then a senile“Capping stage” forming a stratovolcano-type edifice onthe top of the former lava shield.

In the case of Etna, we observe that, after an initialsimilitude with oceanic basaltic volcanoes (Basal Tholei-itic Supersynthem), the early shield volcano underwent

limited development (Timpe Supersynthem) and theneruptions tended to be more localized, forming smallcentral edifices (Valle del Bove Supersynthem) instead ofa single widespread lava shield. The different behaviouris probably due to some key differences between the pri-mary magmas and tectonic setting of Etna and those ofHawaiian volcanoes. Etna’s alkalic basalts are typicallyvolatiles-rich (METRICH et alii, 1993; CLOCCHIATTI et alii,2004; CORSARO & POMPILIO, 2004a) due to both differentmantle source and crustal density (CORSARO & POM-PILIO, 2004b). In addition, there is a more complex tec-tonic setting underneath Etna volcano with respect tothat beneath oceanic volcanoes. Indeed, the boundarybetween the thrust belt of the Apenninic-MaghrebianChain to the north and the foredeep basin to the southoccurs just below the volcano. The volcanic pile is but-tressed on the chain to the west but rests on the IonianSea continental margin to the east, which slopes towardthe bathyal plan in a few tens of kilometres. Therefore, arapid change from compressive to tensional regimeoccurred beneath Etna. In the last 130 ka, the exten-sional area expanded from the coast westward, leading tothe building of a large and complex stratovolcano insteadof the typical basaltic shield volcano of oceanic environ-ments.

The hypotheses on Etna’s genesis, widely debated inthe literature of the last decades, can be constrained bet-ter by considering the evolutionary scheme presentedhere. This shows considerable discrepancies betweenHawaiian and Etna volcanoes in the development of eachevolutionary phase and lacks of any evidence of a deepermantle plume below the volcano (MONTELLI et alii, 2006).Moreover, Etna’s evolution is not compatible with that ofactive continental margin volcanism due to its geody-namic location, magma composition and geological his-tory. Finally, the recently proposed “slab window” model(GVIRTZMAN & NUR, 1999; DOGLIONI et alii, 2001; SCHEL-LART, 2010) justifies the occurrence of an upper mantleplume for the origin of Etna, as evidenced by MONTELLI

et alii (2006), but does not take into account the north-ward shifting of the mantle source from the Plio-Plei -stocene volcanism of Hyblean plateau to the Etna region.

CONCLUSIVE REMARKS

Stratigraphical, morpho-structural and geochronolog-ical studies performed to compile the new geological mapof Etna volcano (BRANCA et alii, 2011; DE BENI et alii,2011) allowed us to reconstruct the evolution of the erup-tive activity in the Etnean region during the past 500 kain four main phases, corresponding to the Supersynthemsrecognized in the volcanic succession.

– The Basal Tholeiitic Supersynthem includes theoldest volcanics cropping out on Etna region. They corre-spond to a long period of scattered fissure-type eruptionsdistributed firstly in the Pleistocene foredeep basin about500 ka and then from about 330 ka ago in a subaerialenvironmental. This volcanism represents the northwardmigration of the magmatic source from the foreland(Plio-Pleistocene volcanism of Hyblean plateau) to theforedeep of Apenninic-Maghrebian Chain.

– The Timpe Supersynthem marks a significant changein the eruptive history of Etna region around 220 ka, when



Na-alkaline magma started raising more efficiently fromthe mantle and eruption sites concentrated on a narrowbelt along the Ionian coast, where the Timpe fault systemis currently located. The eruptive activity formed an ear-lier polygenetic volcano corresponding to an elliptic lava-shield, elongated in a NNW-SSE direction. Between 129and 126 ka, the shallow feeder system gradually migratedwestward to the central part of Etna region.

– During the Valle del Bove Supersynthem the magmaascent path through the upper crust became more local-ized, thereby developing an efficient plumbing system thatpermitted the growth of the earlier stratovolcano structurein the Etnean region that was characterized by the super-position of seven small central volcanic edifices.

– The final Stratovolcano Supersynthem marks thedefinitive stabilization in the present position of Etna’splumbing system from about 57 ka. The huge amount ofvolcanic products erupted mainly from a steady centralvent determined the growth of two large superimposedvolcanoes (Ellittico and Mongibello), which constitute thebulk of Etna edifice.

The geological reconstruction of Etna volcanoobtained from the new stratigraphic data and radio -chronologic constraints shows a different evolution whencompared to other basaltic composite volcanoes as theHawaii islands. For instance, in the Etna region the devel-opment of a shield volcano has been replaced by thegrowth of a large stratovolcano as consequence of thecomplex tectonic framework of eastern Sicily. Conse-quentially, the new geological map of Etna volcano fromwhich we derived the four evolutionary phases proposed,may furnish reliable constraints to future models on theorigin of Etna and help unravel the geodynamic puzzle ofthe eastern Sicily that permitted the growth of a largebasaltic volcano in an apparently anomalous position onthe continental crust in front of the Sicilian thrust belt.

ELECTRONIC SUPPLEMENTARY MATERIAL

This article contains supplementary material, which is availableonline to authorized users (DOI: 10.3301/IJG.2011.13).

ACKNOWLEDGMENTS

The authors are grateful to R. Cioni, F. Lucchi and J.C. Tanguyfor their helpful reviews that greatly improved the manuscript and S. Conticelli for its careful advice concerning manuscript publishing.

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Manuscript received 20 December 2010; accepted 28 June 2011; editorial responsability and handling by R. Cioni.


Geological evolution of a complex basaltic stratovolcano: Mount Etna, Italy

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