The 79 AD eruption of Somma: The relationship between the date of the eruption and the southeast tephra dispersion

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Somma-Vesuvius is a composite volcano on the southern margin of the Campanian Plain which has been active during the last 39 ka BP and which poses a hazard and risk for the main population center situated around its base. The fieldwork and data
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  See discussions, stats, and author profiles for this publication at: The 79 AD eruption of Somma: The relationshipbetween the date of the eruption and theSoutheast Tephra dispersion  Article   in  Journal of Volcanology and Geothermal Research · January 2008 DOI: 10.1016/j.jvolgeores.2007.08.020 CITATIONS 14 READS 408 4 authors , including:Giuseppe RolandiUniversity of Naples Federico II 140   PUBLICATIONS   9,660   CITATIONS   SEE PROFILE Angelo PaonePusan National University 13   PUBLICATIONS   277   CITATIONS   SEE PROFILE All content following this page was uploaded by Angelo Paone on 07 June 2014. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.    This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and educational use, including for instruction at the author’s institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit:  The 79 AD eruption of Somma: The relationship between the date of the eruption and the southeast tephra dispersion G. Rolandi a, ⁎ , A. Paone a  , M. Di Lascio a  , G. Stefani  b a   Dipartimento di Scienze della Terra  ––  Università degli Studi di Napoli  “   Federico II  ”  , Via Mezzocannone 8, 80138 Napoli  ––   Italy  b Sopraintendenza Archeologica di Pompei, Italy Received 1 March 2007; accepted 24 August 2007Available online 18 September 2007 Abstract Somma-Vesuvius is a composite volcano on the southern margin of the Campanian Plain which has been active during the last 39 ka BP and which poses a hazard and risk for the main population center situated around its base. The fieldwork and data analysison which this report is based are related to the eight Plinian eruptions that have occurred in the last 25 ka. For six of these eruptions,the fallout products were dispersed to the east  – northeast, whereas deposits from the 25 ka Codola and AD 79 eruptions weredispersed in a south-easterly direction. During the AD 79 eruption, in particular, the dispersal axis migrated from the east  – southeast to south – southeast. New high level wind data collected at the weather stations of the Aereonautica Militare data centres at Praticadi Mare (Rome) and Brindisi have been compiled to characterize the prevailing wind condition in the Somma-Vesuvius region. Thecommon north-easterly dispersal directions of the Plinian eruptions are consistent with the distribution of ash by high-altitudewinds from October to June. In contrast, the south-easterly trend of the AD 79 products appears to be anomalous, because theeruption is conventionally believed to have occurred on the 24th of August, when its southeast dispersive trend falls in atransitional period from the Summer to Autumnal wind regimes. In fact, the AD 79 tephra dispersive direction towards thesoutheast is not in agreement with the June – August high-altitude wind directions that are toward the west. This poses serious doubt about the date of the eruption and the mismatch raises the hypothesis that the eruption occurred in the Autumnal climatic period,when high-altitude winds were also blowing towards the southeast. New archaeological findings presented in this study definitively place the date of eruption in the Autumn, in good agreement with the prevailing high-altitude wind directions above Somma-Vesuvius.Moreover, wind data and past eruptive behaviour indicate that a future subplinian – Plinian eruption at Somma-Vesuvius has agood chance to occur when winds are blowing toward the eastern sectors (northeast  – southeast), in the Autumnal – Winter period,and only a slightest chance in Summer, when winds are blowing toward the west, depositing ash fallout on the Neapolitancommunity.© 2007 Elsevier B.V. All rights reserved.  Keywords:  AD 79 eruption; stratospheric wind; archaeological data; tephra fall 1. Introduction Somma-Vesuvius is one of the three active volcanicdistricts of the Campanian Plain located near thesouthern border of the Plain. Other active volcanic  Available online at Journal of Volcanology and Geothermal Research 169 (2007) 87 – ⁎  Corresponding author.  E-mail address:  (G. Rolandi).0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2007.08.020  districts in the area are Campi Flegrei and Ischia Island.In the last decade, new volcanological studies have beencarried out at Somma-Vesuvius volcano increasing theknowledge of past explosive and effusive activity (Arnoet al., 1987; Barberi et al., 1990; Cioni et al., 1992; Lirer  et al., 1993; Mastrolorenzo et al., 1993; Rosi et al., 1993;Rolandi et al., 1993a,b,c; Rolandi et al., 1998; Cioni,2000; Arrighi et al., 2001; Lirer et al., 2001; Cioni et al., 2003a,b; Luongo et al., 2003; Rolandi et al., 2004). Somma-Vesuvius volcano has erupted frequently duringthe last 25,000 years, producing 8 Plinian eruptionswhose effects have been recognized as far as 100 kmfrom the volcano. Starting from the Avellino Plinianeruption (3.55 ka BP; Rolandi et al., 1993b), the activityof Somma-Vesuvius shows a regular trend which can bedefined as a sequence of Plinian events alternating withinterplinian activity characterized by vulcanian – strom- bolian events and some very intense effusive phases(Rolandi et al., 1998). This sequence of activity has occurred many times, and pyroclastic products inter- bedded with paleosoils commonly outcrop in the areasurrounding the volcano. This sequence of activity provides the basis for reconstructing the volcanologicalhistory of Somma-Vesuvius and for assessing thevolcanic hazard associated with future eruptions. Therange of past eruptive behaviour at Somma-Vesuvius, asindicated by the age and distribution of Plinian deposits,constitutes a specific analogue for future explosive behaviour of Somma-Vesuvius. In particular, important factors that affect the distribution of the tephra are thespeed and direction of modern winds at the samealtitudes reached by the Plinian columns in the past. Inorder to establish a correlation with the dispersion of the products from the Plinian eruptions, high level winddata from weather stations of the Aereonautica Militaredata centres at Pratica di Mare (Rome) and Brindisi have been compiled to characterize the prevailing windcondition in the Somma-Vesuvius region. The correla-tion includes the AD 79 eruption, for which the south-easterly dispersal trend has been considered to beanomalous for Somma-Vesuvius (Sigurdsson et al.,1985). This paper uses the distribution of past  pyroclastic fall products and high level wind data todemonstrate that the AD 79 distribution is not anomalous, because the southeast dispersive trend is inagreement with the direction of the October  – June high-altitude winds. This raises the hypothesis that theeruption in fact occurred in an Autumnal climatic period. In support of this hypothesis, we present newdata from two Roman epigraphs that emphasize therecording of the fifteenth imperial acclamation of theemperor Titus on the silver coin found in Pompeii,which strongly supports an Autumnal date for the AD79 eruption. 2. The Somma-Vesuvius volcano history and Pliniantephra dispersal The sequence of eruptive events of Somma-Vesuviusinthelast 25kaBPisknownindetail(Santacroce,1987;Rolandi et al., 1993a,b,c; Rolandi, 1998). About 25,000 years ago a Plinian explosive eruption knownas the  “ Codola eruption ”  deposited an eastern – south-eastern widespread layer above the 39 ka BP CampanianIgnimbrite (Rolandi et al., 2003). Most remarkable arethe next explosive Plinian eruptions which occurred between 18 ka BP and 8 ka BP, regarded as marking a partial destruction of Mt. Somma, the older volcano of the composite volcanic complex. The Avellino eruption(3.55 ka BP) is the first Plinian event characterized by predominantly phreatomagmatic activity (Rolandi et al.,1993b) indicating that excess water vapour pressure inthe feeding system was responsible for an important failure of the western and south-western sectors of Mt.Somma (Rolandi et al., 2004). Between 3.55 ka BP and AD 472 eruptive activity at Mt. Somma was dominated by the two Plinian eruptions of Pompeii (AD 79) (Lirer et al.,1973; Sheridanetal.,1981;Sigurdssonetal.,1985;Gurioli et al., 2002) and AD 472 (Santacroce, 1987;Rolandi et al., 2004), and by the so-called  “ interplinianactivity ”  (Rolandi et al., 1998) a weak explosive – effusive activity identified as: Protohistorical, Ancient historical, Medieval, and Modern historical periods.According to Rolandi et al. (1998), prior to the AD472 eruption there was a single peaked Somma calderastructure, and the Vesuvius cone was constructed duringthe Medieval effusive – weak explosive interplinianactivity, which occurred between AD 472 and AD1139. At this time, Somma-Vesuvius was a compositevolcano consisting of the older Mt. Somma with acaldera-like structure, in which the modern cone of Vesuvius is nested. The last eruption with a Pliniancharacter occurred in 1631 (Rolandi et al., 1993c; Rosiet al., 1993) from Vesuvius; afterwards the modernhistoric interplinian activity occurred until 1944.Fig. 1 represents the orientation and width of Pliniantephra lobes from Mt Somma and Vesuvius cone. Eachlobe consists of air fall tephra 10 cm thick from a singleevent. In Fig. 2, dispersal data are arranged to highlight themaximumpercentageofthesectorscoveredby10cmisopach.SixPlinian eruptions(2 – 18kaBP;3 – 16kaBP;4 – 8 ka BP; 5 – 3.55 ka BP; 7 – AD 472; 8 – AD 1631) areshown tohaveoccurredduring periodsof northeast  – east winds producing thick bands of tephra extending far  88  G. Rolandi et al. / Journal of Volcanology and Geothermal Research 169 (2007) 87   –  98  downwind from the volcano. The 25 ka BP (1) and theAD 79 (6) Plinian events (Fig. 1) show tephra lobes withthe most southerly dispersion from Mt. Somma. The AD79 Plinian event, in particular, produced air fall tephraconsisting of a lower part of white pumice, and an upper  part of grey pumice (Lirer et al., 1973; Sigurdsson et al.,1985). This abrupt change in the fall products has beenattributed to complex changes in the eruption dynamics(Gurioli et al., 2005). During the last six hours of the AD 79 gray pumice eruption, the Plinian column wasinterrupted several times by pumiceous and lithic pyroclastic flows and surges, attributed to the collapseof the eruption column (Sigurdsson et al., 1985). Faciesanalysis and AMS data of these pyroclastic densitycurrents indicate complex transport conditions due toeffects of flow dynamics, local topographic irregulari-ties, and presence of buildings (Gurioli et al., 2002; Nunziante et al., 2003).The south – southeast dispersion of the AD 79 eruptionis conventionally considered to be anomalous (Sigurds-son et al., 1985). To investigate the apparent anomaly, thenext section presentsthe results of a comparative analysisof wind-direction frequencies associated with the disper-siondirectionsfromthepast(Fig.2)andthedirectionandspeed of modern wind records. 3. Average modern wind direction and speed inatmosphere above Somma-Vesuvius The meteorological data collected in the weather stations of Aereonautica Militare data center at Pratica diMare (Rome) and Brindisi, in the period 1986 – 2004, Fig. 1. Distribution map of pyroclastic fall deposits of the Somma-Vesuvius deposited in the last 25 ka BP. Each lobe consists of air fall tephra 10 cmthickfroma singlePlinian eruption. Numbersarearrangedaccordingto thechronologicalsequenceof the eruption(see Fig.2). Fielddataare: (1),(2),(3) Rolandi, unpublished data; (4); (5) Rolandi et al. (1993a,b,c); Rolandi et al., 2004. Chronological data are from: (1) Alessio et al. (1971); (2) Bertagnini et al. (1998); (3) Cioni et al. (2003a,b); (4) Rolandi et al. (1993a); (5) Rolandi et al. (1993b). 89 G. Rolandi et al. / Journal of Volcanology and Geothermal Research 169 (2007) 87   –  98
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