Il valore di X H2O m , inoltre, è stato ottenuto tramite esperimenti condotti con apparecchiature EHPV tipo TZM a quenching ultrarapido. Da questi esperimenti, effettuati alle temperature e pres- sioni di interesse e per X H2O m compreso fra 0 ed il livello di saturazione, è stato possibile costruire una retta di calibrazione nella quale la composizione del plagioclasio in equilibrio con il fuso [X(An) Plag ] risulta funzione del quantitativo di H 2 O disciolta nel fuso stesso. In questo modo sono stati ricavati i contenuti d’acqua dei magmi riolitico e trachitico, stimati rispettivamente a 2,4-2,9% e 1,7-2,7%. Questi dati suggeriscono infine come l’evoluzione magmatica del sistema di alimentazione del Vulcano La Fossa, possa essere spiegata sulla base della differenziazione da termini latitici e trachi- tici delle eruzioni iniziali (Punte Nere e Grotte dei Palizzi) a quelli riolitici delle eruzioni più recen- ti (Pietre Cotte, 1888-90), nei quali è tuttavia preservata la testimonianza dei magmi primitivi. Bibliografia Sugawara T.; 2000: Empirical relationships between temperature, pressure and MgO content in olivine, piroxene saturated liquid. J. Geophys. Res., 105 (B4), 8457-8472. Ariskin A.A., Frenkei M.Y., Barmina G., Nielsen R.L.;1993: COMAGMAT: a fortran program to model magma differentiation processes. Comput. & Geosci., 19, 1155-1170. Green N.L., Usdansky S.I.; 1986: Ternary-feldspar mixing relations and thermobarometry. Am. Mineral., 71, 1100-1108. Ghiorso M.S., Sack R.O.; 1995: Chemical transfer in magmatic processes IV. Contrib. Mineral. Petrol., 119, 197-212. ETNA VOLCANO ERUPTS AGAIN ON 13 MAY 2008: AND MAGNETIC FIELD MONITORING IS A KEY PART OF THE PICTURE R. Napoli 1 , G. Currenti, C. Del Negro 1 , F. Greco 1 , D. Scandura 1,2 , A. Sicali 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania 2 Dipartimento di Matematica e Informatica, Università di Catania During the onset of 2008 volcanic crisis at Mt Etna,the near-real time magnetic data provided a continuous updating of the volcano activity state on the northern flank. On the morning of 13 May 2008, significant local magnetic field changes, with amplitude ranging between 1.8 nT and -6.5 nT (Fig. 1), marked the resumption of the eruptive activity characterized by the opening of a fracture field on the northern flank, and an eruptive fissure in the Valle del Bove. In agreement with the northward propagation of seismic events, magnetic signals at 5 stations (Del Negro et al., 2002) in the summit area revealed a nearly NNW-SSE oriented magmatic intrusion, which started at about 9:00 GMT, propagated northward for about 2 km, and stopped at 14:00 GMT before reaching the North-East Rift. The spatio-temporal evolution of magnetic variations has enabled to understand the mechanism of the volcanic activity resumed on 13 May 2008. After differential magnetic fields were filtered from the external noise, we detected conspicuous short-time changes accompanying the magmatic intrusion that occurred on the northern flank. The variations are clearly observed at all the summit sites (PDG, PDN, PTL, BCN, BVD), and the simultaneity and proximity in space indicate a com- mon geophysical mechanism as source of the anomaly. In particular, a piezomagnetic origin relat- ed to the response of the Earth’s crust under stress due to changes in gas pressure or magma injec- tion or a combination of both was proposed. The position and the geometry of source, which could explain the total intensity changes, were estimated. We suppose that the volcanic crisis was trig- gered by magmatic overpressure of a magma batch, injected within the summit conduits, and when reached enough pressure to overcome the strength of the surrounding rocks a lateral propagation of a dike occurred along the eastern side of the summit craters. Lateral dike propagation produced an 800 m wide fracture field, which developed in a few hours in the northern flank of the volcano. The 75 GNGTS 2008 SESSIONE 1.2