Violent Explosions Yield New Insights into Dynamics of Stromboli Volcano

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1 Eos, Vol. 80, No. 5, December 8, EOS, T R A N S A C T I O N S, A M E R I C A N G E O P H Y S I C A L VOLUME 80 U N I O N NUMBER 5 DECEMBER 8, PAGES Violent Explosions Yield New Insights into Dynamics of Stromboli Volcano PAGES 6 3 3, Crystal-poor magma may b e a chief factor in the steadiness of activity at the Stromboli vol c a n o in Italy This was o n e of the findings to emerge from the documentation of a number of violent explosions there last year. The find ings are giving scientists new insights into the dynamics of the volcano. Stromboli, on an island of the s a m e name in the Aeolian Archipelago, is known as "the lighthouse of the Mediterranean" b e c a u s e of its persistent explosive activity, which is visible from great distances.the well-documented explosions, however, were more violent than normal "strombolian" activity, and we call them "paroxysmal" explosions. Stromboli has long puzzled volcanologists because of the persistence of its normal explo sive activity over many centuries. Such a state reflects an unusual steadiness of the volcanic and magmatic plumbing system in which con tinuous cooling and degassing is somehow compensated by a supply of heat and gas from below. The fact that the lava level inside con duits remains basically unchanged through time also means that the mass of lava ejected by explosions is replaced by a corresponding supply of magma from below. But we believe the study of paroxisms, the more violent explosions, is the key to under standing the characteristics and dynamics of the feeding system. A full understanding of how Stromboli works is also of paramount importance in identifying the trigger mecha nism of paroxysmal explosions. Achieving this goal would improve the short-term forecast ing capability of Stromboli's surveillance sys tem. This has important implications in the mitigation of volcanic hazard b e c a u s e parox ysmal explosions represent the main threat for the numerous visitors to the volcano. Documentation last year [Bonaccorso et al., 1996] included evidence of paroxysmal emis sions of small volumes of crystal-poor, proba bly gas-rich, relatively primitive magma.the amounts were 1 0 to 1 0 m.this primitive magma may have risen rapidly through crys tal-rich, more viscous magma in the conduits. The rise of crystal-poor magma has b e e n a c o m m o n feature of historic paroxysmal explosions. The petrographic characteristics and the composition of products emitted dur ing the last 1800 years are relatively constant and, what is more important, compositional relationships between crystal-rich and crystalpoor magmas remained virtually constant with time (M. Rosi et al., manuscript in prepa ration, 1999).This suggests that, in addition to the highly porphyritic magma feeding normal strombolian activity the crystal-poor magma is an important constituent of Stromboli's sys tem and possibly a chief factor in the mainte n a n c e of the steadiness of this system. "Crater Terrace" Stromboli is a stratovolcano that rises from a depth of about 1500 m b e l o w s e a level to an elevation of 9 4 m a b o v e s e a lev el ( a s l ). T h e activity takes p l a c e at vents about 750 m asl in a flat area known as the "crater terrace" within the Sciara del F u o c o (Figure l ), a horseshoe-shaped depression o p e n to the northwest.three funnel-shaped craters up to o n e hundred meters in diame ter are conventionally n a m e d 1 to 3 from northeast to southwest and o c c u p y the cen tral part of the crater terrace. Typical Activity The typical activity at Stromboli consists of intermittent mild explosions ejecting scoria- Fig. 1. Aerial view of Stromboli volcano's summit from southwest. Pizzo Sopra la Fossa is where people usually stand to watch the activity. The distance between the crater terrace and Pizzo Sopra la Fossa is about 50 m. Original color image appears at the back of this volume.

2 Eos,Vol. 80, No. 5, December 8,1999 intensity of the phenomena between the two types of explosive activity, we use the term "paroxysmal" for all explosions more violent than normal strombolian activity Episodes of lava emissions have also occur red, on average every 4 years [Barberi et al., 1993]. Over the last 3 centuries, lavas have flowed down the Sciara del Fuoco, though creating no c o n c e r n for settled areas. For 1800 years, during most of the paroxysmal explosions, both crystal-rich black scoriae and significant volumes of crystal-poor golden pumice have been erupted [De Fiore, \9\4;Ritmann, 1931; M.Rosi et al., manuscript in prepa ration, 1999]. Petrological investigations carried out on the volcanics have shown that the pumiceous material is more primitive and less alkaline than the contemporaneous scori ae [Bonaccorso et al., 1996; Coltelli et al., 1999, Francalanci et al., 1999; M. Coltelli and M. Pompilio, manuscript in preparation, 1999]. The Paroxysmal Explosions ww imr Fig.. Dispersal of the products emitted during the August 3,1998, image appears at the back of this volume. c e o u s bombs, lapilli and ash from vents where glowing lava stands at high level in an open conduit.the explosions o c c u r when large bubbles of compressed gas, up to sever al meters in diameter, burst at the surface of the magma column, resulting in the formation of a jet of hot gas and incandescent lava frag ments. The explosions last a few s e c o n d s and take place at variable intervals, the most com mon time interval being 10-0 min (M. Coltelli and A. Cristaldi, manuscript in preparation, 1999). Explosive activity is associated with continuous streaming of gas with an estimat ed output of 6000 to 1,000 t/day and consist ing mainly of H 0 ( t/day), C 0 ( t/day), S 0 ( t/day), and minor HC1 and HF [Allard et al., 1994]. The persistence of gentle explosions, which c a n b e watched from a relatively close distance, and the easy a c c e s s to the volcano's summit, have m a d e Stromboli a tourist attraction, drawing thousands of peo ple every year. However, the routine activity c a n b e interrupted by more violent explo sions that eject meter-sized s c o r i a c e o u s b o m b s and blocks several hundreds of me ters from the craters, endangering those watching. Showers of ash and i n c a n d e s c e n t lapilli and b o m b s also c a n fall on the slopes of the volcano, setting vegetation on fire. ww explosion. Original color Violent Explosions These events consist either of one discrete explosion or of several explosions usually from different vents. On average, one to two explosions of this kind have occurred per year over the past 110 years [Barberi et al., 1993] but, b e c a u s e of the a b s e n c e of continuous monitoring, this may b e an underestimate. Fifteen violent explosions have b e e n record ed since 1993, when systematic monitoring of Stromboli by the Istituto Internazionale di Vulcanologia began. A surveillance c a m e r a that continuously records the eruptive activity was installed in the summer of 1994, further improving the continuous monitoring capabilities.the distribution in time of these explo sions is not h o m o g e n e o u s and peak frequen cies were reported in 1996 and Less frequently much more violent explo sions can occur.they represent the most pow erful eruptive manifestations of the volcano, and most in the past centuries have produc ed damage to the two villages, Stromboli and Ginostra, along the coast.these violent events have not occurred for more than 50 years. Based on their hazard implications, Barberi et al. [1993] named the most violent events "paroxysms" and the others "major explosions." Because there is not a real limit in terms of Products were mapped and sampled imme diately after four of the six 1998 paroxysmal explosions. On January 16, an explosion cleared an obstructed vent of Crater 1. On August 3, two consecutive strong explo sions interrupted the normal strombolian activity The first blast occurred within Crater 3, producing an ash column rising a few hun dred meters above the summit.three s e c o n d s later, tracks of large blocks moving radially out from the northern side of Crater 1 at a speed close to 100 m/s were recorded by the surveillance camera. An eruptive column, showing a sustained jet at the base and a convective top region, rose up to 1 km above the vent. Dispersal of the erupted products showed two lobate fallout areas.the lobe stretching north-northeast was formed of scat tered black spatters.the second, an eastsoutheast trending lobe, was formed of an almost continuous deposit of light-golden pumice, varying in size from b o m b s 1 m across to fibrous lapilli (Figure ). Another paroxysmal explosive sequence took place September 8. One main blast and three minor jets were produced by four vents in Craters 1 and 3 in less than 3 min.the main explosion threw large red bombs up to 150 m above the vent, followed by the rising of a 500 m high plume that lasted about 15 s (Figure 3). No pumice was found among the pyroclastic mate rial erupted during this paroxysmal episode. On November 4 a strong explosion took place at Crater l.a single large blast formed a 300 m high globe of red material followed by two pulses of 150 m high lava spattering, each lasting about 0 s. B o m b s ejected during the main explosion were mainly pumice. Two other paroxysmal explosions occurred, on D e c e m b e r 6 and D e c e m b e r 8. During

3 Eos, Vol. 80, No. 5, December 8,1999 Fig. 4. Bulk rock and glassy matrix composi tion of the products erupted from Stromboli during the paroxysmal explosions of 1998 plotted in Si0 -K 0 classification diagram. Yellow = January 16; red = August 3; green = September 8; blue = November 4. Open circle = black scoria; open square = golden pumice; filled diamond = glassy matrix of golden pumice; filled triangle = glassy ma trix of black scoria. Original color image appears at the back of this volume. c o m p o s e d of s c o r i a and p u m i c e with lobate, sharp c o n t a c t s. l t also included the oc c u r r e n c e of p u m i c e clasts with a fairly high c o n t e n t of phenocrysts (up to 0-30 vol % ), most of which were wetted by brown glass with the composition of the glassy matrix of the black scoriae. Authors Fig. 3. Paroxysmal episode of September 8,1998, as recorded by a surveillance camera at Pizzo Sopra la Fossa, a) Strombolian explosion from E vent of Crater 1 (CR1). b-cj Main blast that par tially destroyed northwest rim of Crater 1. d) Bombs landing near the Pizzo while spattering con tinues from west vent of Crater 1. e) Ash emission from west vent of Crater 3 (CR3). f) Lava jets from east vent of Crater 3. Original color image appears at the back of this volume. the first, a 150 m high lava fountain rose from Crater 3 for 40 s; a 300 m high ash plume formed above the fountain, and lapilli fell on the village of Ginostra. During the second, a single blast from Crater 1 was followed by two lava fountains, one from Crater 1 and o n e from Crater 3. No pumice was erupted. The black scoriae and light-golden pumice had a vesicularity of vol % and vol %, respectively. Both materials ranged in com position from shoshonitic basalt to HK-basalt and were close to the composition of the pro ducts erupted during the past 6 years (Figure 4).The pumices were slightly less evolved than the scoriae, as was shown by their higher MgO content (7 wt % versus 6 wt % in the August 3 products) and lower abundance of incompatible trace elements such as Ba (830 ppm versus 980 p p m ), R b (60 ppm versus 75 p p m ), a n d Zr (150 ppm versus 170 ppm). Petrographic characteristics of the scoriae emitted during the paroxysmal explosions were virtually identical to those of the scoriae emitted during normal strombolian activity They exhibited a very high content of pheno crysts (47-56 vol % ) of plagioclase, clinopyroxe n e, a n d olivine set in a homogeneous, brown, glassy matrix, enriched in incompati ble elements and depleted in MgO with re spect to the bulk rock. Pumice erupted in the August 3 paroxys mal explosion were distinguished from the scoriae by a lower crystal content (1 vol % ) and a honey-colored, glassy matrix. Pheno crysts were of the s a m e mineral phases pres ent in the scoriae, but with a less evolved composition.the composition of the glassy matrix was markedly less evolved than the matrix of the scoriae and close to that of the bulk rock (Figure 4 ). Evidence of syneruptive mingling between the two magmas was ubiquitous when both p u m i c e and s c o r i a e were emitted. Such evi d e n c e included the o c c u r r e n c e of clasts A. Bertagnini, M. Coltelli, PLandi, M. Pompilio, and M. Rosi For more information, contact A. Bertagnini, Istituto Nazionale di Geofisica, c / o Dipartimento di Scienza della Terra, Universita di Pisa,Via S. Maria 53,5616 Pisa, Italy; bertagnini@dst.unipi.it or M. Coltelli, Istituto Internazionale di Vulcanologia, Piazza R o m a,9513 Catania, Italy; colt@iiv.ct.cnr.it References Allard, P, J. Carbonnelle, N. Metrich, H. Loyer, and P Zettwoog, Sulphur output and magma degassing budget of Stromboli volcano, Nature, 368,36-330, Barberi, F, M. Rosi, and A. Sodi,Volcanic hazard assessment at Stromboli based on review of his torical data,^cto Vulcanol, 3, ,1993. Bonaccorso,A., C. Cardaci, M. Coltelli, PDel Carlo,S. Falsaperla, S. Panucci, M. Pompilio, and L.Villari,An nual report of the world volcanic eruptions in 1993, Stromboli, Bull. Volcanic Eruptions, 33,7-13,1996. Coltelli, M., S. Falsaperla, P Del Carlo, M. Pompilio, and A. Bonaccorso,Volcanic, seismic, and ground defor mation data concerning the Stromboli volcano in \99S,Bull. Volcanic Eruption, 35,8-14,1999. De Fiore, 0.,I fenomeni eruttivi awenuti alio Strom boli dal 1909 al 1914 ed il loro meccanismo,z. Vulkanol, 7,5-45,1914. Francalanci, L, S.Tommasini, S. Conticelli, and G. R. Davies, Sr isotope evidence for short magma resi dence time for the 0th century activity at Stromboli volcano, Italy Earth Planet. Sci. Lett., 167,61-69,1999. Rittman,A., Der ausbruch des Stromboli a m 11 Sep tember \930,Z.Vulkanol., 14,47-77,1931.

4 Eos, Vol. 80, No. 5, December 8, 1999 Fig. 1.Aerial view of Stromboli volcano's summit from southwest. Pizzo Sopra la Fossa is where people usually stand to watch the activity. The distance between the crater terrace and Pizzo Sopra la Fossa is about 50 m.

5 Eos, Vol. 80, No. 5, December 8, 1999 T T T 15.' 15 13' 15 14' Fig.. Dispersal of the products emitted during the August 3,1998, explosion. Page 636

6 Eos, Vol. 80, No. 5, December 8, 1999 Fig. 3. Paroxysmal episode of September 8,1998, as recorded by surveillance camera at Pizzo Sopra la Fossa, a) Strombolian explosion from E vent of Crater 1 (CR1). b-c) Main blast that partially destroyed northwest rim of Crater 1. d) Bombs landing near the Pizzo while spattering continues from west vent of Crater 1. e) Ash emission from west vent of Crater 3 (CR3). f) Lava jets from east vent of Crater 3.

7 Eos, Vol. 80, No. 5, December 8, r SiO, wt% FM>. 4. Bu//e rock and glassy matrix composition of the products erupted from Stromboli during the paroxysmal explosions of 1998 plotted in Si0 -K 0 classification diagram. Yellow = January 16; red = August 3; green = September 8; blue = November 4. Open circle = black scoria; open square = golden pumice; filled diamond = glassy matrix of golden pumice; filled triangle = glassy matrix of black scoria.