Linking variable explosion style and magma textures during 2002 at Stromboli volcano, Italy

Abstract

Microtextural characteristics of fresh ejecta from Stromboli volcano were examined from three periods of differing eruption style and intensity in 2002. Activity shifted from relatively weak and infrequent ash-charged explosions during January through May into two broad cycles of waxing activity in June through late September, and late September through December, followed by the onset on 28 December of the 2002/2003 effusive eruption. Analyzed sets of lapilli from May, September/October, and 28 December show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity. Three distinct textures are observed among the analyzed pyroclasts: low density (LD) with an abundance of subspherical bubbles, the presence of large, irregularly shaped bubbles, and a light-to-transparent glass matrix; transitional texture (TT) with an intermediate number of subspherical bubbles, a high frequency of large, irregularlyshaped bubbles, and a honey colored glass matrix; and high density (HD) with sparse relatively small bubbles, conspicuous large irregular bubbles, and a dark glass matrix. Observational and quantitative data (density, vesicle size) indicate that these textures are linked through variable residence time in Stromboli's shallow conduit, with an ongoing evolution from LD to HD magma. Calculations suggest that residual LD magma will evolve to HD texture in a period of hours to days. Contrasting amounts of the LD, TT, and HD magmas are present in each sample, with the most TT in May, the most LD in September/October, and the most HD in December. This implies that the shallow magma had a different rheology at each collection period. The viscosity of LD and HD magmas are calculated to be in the range of 2,000 to 2,600 and 3,000 to 5,000 Pa s, respectively, which, with their changing proportions, must have implications for rates of bubble slug ascent and processes of fragmentation. This study suggests that an increasing maturity of magma in Stromboli's shallow conduit (with resultant increase in viscosity) feeds back to reduce the intensity of explosions, whereas a steady flux of LD magma favors more powerful explosions.

Recycled ejecta modulating Strombolian explosions

“…The presence of degassed and crystallized magma at the top of the Stromboli conduits is well established (e.g. Lautze and Houghton 2006;Gurioli et al 2014), along with the potential role of clast recycling on its formation (D'Oriano et al 2014). Indeed, there must be an Fig.…”

Relating vesicle shapes in pyroclasts to eruption styles

“…Vesicle shapes are analyzed using a fixed magnification to ensure the same pixel resolution throughout. Thus, vesicle shapes were analyzed on all the vesicles imaged at a scale of 1 μm/px (×100 magnification) in backscattered electron images from a given thin section, previously used for the study of vesicle size distributions (Tables 2 and 4; Adams et al 2006;Sable et al 2006Sable et al , 2009Lautze and Houghton 2007;Houghton et al 2010;Stovall et al 2011). In these studies, the original, grayscale, SEM images were transformed into binary images using Adobe Photoshop and Scion Image (Scion Corporation, USA) or ImageJ (Schneider et al 2012) software.…”

“…Vesicle shape is another manifestation of magma ascent conditions, in particular bubble growth, coalescence, and shearing (e.g., Klug and Cashman 1996;Mangan and Cashmann 1996;Polacci et al 2003;Rust et al 2003;Okumura et al 2008;Wright and Weinberg 2009), and can therefore provide a valuable complement to conventional studies of pyroclast textures. Volume fraction of crystals in the groundmass Regularity (as defined by Shea et al 2010) (1) Parfitt (2004), (2) Stovall et al (2011), (3) Wallace (1998), (4) Lautze and Houghton (2007), (5) Pistolesi et al (2011), (6) Burgisser et al (2010), (7) Giachetti et al (2010), (8) Druitt et al (2002), (9) Adams et al (2006), (10) Hildreth and Fierstein (2012), (11) Houghton et al (2010), (12) Walker (1980), (13) Dunbar et al (1989), (14) Sable et al (2009), (15) Coltelli et al (1998), (16) Sable et al (2006), (17) Giordano (2003) Here, we study vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions, comprising six different eruptive styles, including the enigmatic Plinian eruptions of basaltic magma, for which the cause for high explosive intensity has been controversial (e.g., Walker et al 1984;Coltelli et al 1998;Houghton et al 2004;Sable et al 2006Sable et al , 2009Costantini et al 2009;Goepfert and Gardner 2010). We are primarily interested in the relationship between bubble growth, as a consequence of magma decompression, and vesicle shape.…”

Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data

“…This suggests they are recycled by the dynamic processes of the Strombolian activity, which appear to be particularly efficient in recycling crystals considering the high percentage content of these high Sr-isotope antecrysts. On the other hand, whatever the conceptual model of the volcano is proposed, it is nowadays believed that the periodic mild Strombolian explosions leave in the conduit a degassed dense HP-magma descending downwards possibly along the conduit walls (Chouet et al, 2003;Colò et al, 2010;Lautze and Houghton, 2007;Burton et al, 2007;Fornaciai et al, 2009;Metrich et al, 2010). This mechanism leads minerals (with possibly degassed silicate melt) to be also recycled from the upper part of the plumbing system into the HP-magma reservoir.…”