Coupled textural and compositional characterization of basaltic scoria: Insights into the transition from Strombolian to fire fountain activity at Mount Etna, Italy Margherita Polacci Rosa Anna Corsaro Daniele Andronico Istituto Nazionale di Vulcanologia, sezione di Catania, Piazza Roma 2, 95123 Catania, Italy
ABSTRACT Strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions. However, the transition between these two eruptive styles and their source mechanisms are still debated. We use textural and compositional studies to characterize pyroclastic material from both the Strombolian and Hawaiian-style fire fountain phases of the January–June 2000 Etna activity. We find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes. Overall, magma that forms Strombolian scoria is far more crystallized, less vesicular, and more evolved, indicating strong volatile depletion and longer residence time before being erupted. Fire fountain scoria indicates a fast-rising magma with evidence of moderate syneruptive volatile exsolution. The new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains, and to frame the transition from Strombolian explosions to fire fountain activity into a model that may apply to future eruptions at Mount Etna as well as other active basaltic volcanoes. Keywords: Mt. Etna, explosive activity, scoria, textures, glass compositions, eruption dynamics.
INTRODUCTION Persistently degassing basaltic volcanoes are characterized by lava effusions and explosive activity of variable intensity. Episodes of Strombolian explosions and fire fountaining represent the most frequent expression of explosive activity for Hawaiian volcanoes (ongoing Pu ` u ` O ` o-Kupaianaha eruption of Kilauea volcano; see Heliker et al., 2003). At Mount Etna, Italy, explosive activity may span from mild to strong Strombolian explosions to the transition to violent lava fountain episodes consisting of vigorous continuously sustained jets of liquid magma and gas. These are frequently followed by a buoyant ash and lapilli column a few kilometers high, with fallout of juvenile material on both uninhabited upper areas and villages located on the volcano slopes as much as 20–30 km away from the summit craters. The volcanic risk associated with the Etna explosive activity may therefore vary greatly, according to the intensity and associated dispersal of the tephra fallout. Despite the frequency of occurrence, and in contrast to its silicic counterpart, this type of activity has not been the focus of much work. Conditions controlling the different styles of basaltic volcanic activity in Hawaii have been investigated and modeled by many (Wilson
and Head, 1981; Vergniolle and Jaupart, 1986; Parfitt and Wilson, 1995; Vergniolle and Mangan, 2000; Parfitt, 2004, among others), as have the roles of gas release and/or accumulation, volume flux, and magma recycling on the height, structure, and general behavior of Hawaiian-style lava fountains (Head and Wilson, 1987, 1989; Jaupart and Vergniolle, 1989; Wilson et al., 1995). These studies are limited in that they mostly focus on one specific basaltic eruption (Pu ` u ` O ` o-Kupaianaha eruption), and are not well constrained by
Figure 1. Fire fountain activity at Southeast Crater of Mount Etna in 2000. Horizontal edge of photo represents ~500 m.
results derived from integration of volcanological and geophysical studies. There are a few examples of textural investigation of volcanic products from basaltic explosive eruptions: Mangan and Cashman (1996) used the structure of basaltic scoria and reticulite to reconstruct vesiculation and fragmentation processes in Hawaiian lava fountains. Taddeucci et al. (2002, 2004) characterized the texture, components, and glass chemistry of ash emitted during the 2001 Etna eruption to monitor daily the evolution of the eruptive activity and to infer the eruption dynamics. Lautze and Houghton (2005) used vesicle textures in scoria to investigate the shallow conduit processes at Stromboli volcano. Here we use textural and compositional studies to characterize the eruptive period of 64 fire fountains that occurred at the Southeast Crater (SEC) from 26 January to 24 June 2000 (Fig. 1). The 64 fire fountains always initiated with increasing Strombolian explosions, usually accompanied by weak effusive activity, and were then followed by a paroxysmal phase during which powerful Hawaiian-style fire fountains and high-effusion-rate lava flows occurred. Each episode declined with minor Strombolian activity and lava flow out-
q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; March 2006; v. 34; no. 3; p. 201–204; doi: 10.1130/G22318.1; 5 figures; 1 table.
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Figure 2. Binary images of scanned thin sections of Strombolian (A) and fire fountain scoria (B). Black features are vesicles. Horizontal view is ~35 mm.
put (Alparone et al., 2003). The eruptive sequence represents the most significant example of paroxysmal episodic activity within a short time in the known history of the volcano, and provided us with a unique opportunity to investigate the transition from Strombolian to fire fountain eruptive activity and to develop a combined methodology wherein the textural and compositional characteristics of basaltic scoria are used as complements to geophysical and geochemical techniques for the monitoring of volcanic eruptions. SAMPLING AND ANALYTICAL METHODS Sampling of Strombolian and fire fountain scoria during the 2000 Etna eruption was severely challenged by the high frequency of occurrence and intensity of most of the eruptive episodes, bad weather conditions, and fairly constant presence of snow on the flanks of the volcano. We therefore choose four fountain episodes in 2000 (4 March, 16 April, 15 May,
Figure 3. Backscattered electron images of vesicle textures. Vesicles are black, plagioclase and pyroxene phenocrysts are dark gray and gray, respectively. A: Strombolian scoria. B and C: Fire fountain scoria. Scale bar (for all images) is 1 mm.
and 17 May), well defined by volcanological and chronological observations, for which Strombolian scoria was successfully collected at sites opposite to downwind dispersal of fire fountain tephra fallout. Textural characterization of scoria clasts consisted of two steps. First, we performed a fast qualitative inspection of crystal, vesicle, and groundmass textures with an optical microscope on two to five scoria samples select-
TABLE 1. SUMMARY OF TEXTURAL MEASUREMENTS OF STROMBOLIAN AND FIRE FOUNTAIN SCORIA Sample 040300B 040300B1 160400B 160400C 160400C1 160400D 150500B 150500Ea 150500Eb 170500
Type of activity Strombolian Strombolian Strombolian Fire fountain Fire fountain Strombolian Fire fountain Fire fountain Fire fountain Fire fountain
Vesicularity* 0.56 0.52 0.56 0.53 0.57 0.56 0.74 0.66 0.61 0.73
(0.66) (0.64) (0.64) (0.58) (0.63) (0.63) (0.80) (0.72) (0.68) (0.78)
V n density* cm22
Vd mm
Pheno crystallinity§
1247 1662 720 2937 3216 1264 2561 3667 2793 3073
239 200 315 152 150 237 192 151 167 174
0.15 0.19 0.13 0.09 0.09 0.11 0.08 0.09 0.10 0.06
(1470) (1708) (823) (3203) (3388) (1645) (2790) (4070) (3110) (3265)
(0.34) (0.40) (0.29) (0.19) (0.20) (0.25) (0.31) (0.26) (0.26) (0.21)
Gm crystallinity
Gm n density mm22
0.36 0.30 0.44 0.74 0.60 0.40 0.12 0.17 0.23 0.12
3297 6880 52751 160700 122294 14416 2901 7253 11180 7278
Note: V stands for vesicle, n for number, d for diameter and gm for groundmass. Groundmass crystallinity and number density reported as vesicle-free. *Values in parentheses are crystal-free. § Values in parentheses are vesicle-free.
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ed, in all episodes, for each of the two main eruptive phases. Scanned images were then acquired to cover the maximum area available for quantitative measurements. To ensure characterization of the small vesicle population (,100 mm), as many as 6 images per thin section were acquired at 253 magnification via scanning electron microscope (SEM) in backscattered mode. All the images were made binary (either for vesicles or crystals), and results from the two investigations were integrated to obtain two-dimensional (2D) crystallinity and vesicularity (area fraction of crystals and vesicles), vesicle number density (number of vesicles per unit area), and vesicle size [in terms of the average diameter d 5 2(w/pNA)1/2, where w and NA are the vesicle area fraction and number density]. We chose to report the raw 2D data to avoid limitations imposed by conventional stereological techniques. Further backscattered scanning electron images were taken at 20003 magnification to measure the area fraction and number density of groundmass crystals (microphenocrysts ,100 mm, microlites ,30 mm). Major element content in glassy groundmass was measured with a LEO-1430 SEM equipped with an Oxford energy-dispersive spectrometry microanalytical system. To reduce alkali loss, glassy matrix was analyzed with squared rasters from 5 mm, in the more crystallized samples, to 10 mm. Analytical error was ,1% for SiO2, Al2O3, FeOt, MgO, and CaO, and ,3% for TiO2, MnO, Na2O, K2O, and P2O5. TEXTURES At the thin-section scale, Strombolian scoria displays a population of large vesicles (from hundreds of microns to .1 cm) (Fig. 2A) whose convoluted and irregular shapes denote extensive coalescence and readjustment around plagioclase, pyroxene, and olivine phenocrysts (Fig. 3A). Groundmass vesicles (,100 mm) are few and generally round (Fig. 3A). By contrast, fire fountain scoria presents more textural heterogeneities. In general, vesicles from a few hundred microns to .1 mm are rounded or slightly deformed and less interconnected (Fig. 2B), and groundmass vesicles are spherical to subspherical and ubiquitously distributed (Fig. 3B). However, in some clasts, where extensive groundmass crystallization is present, the shape of vesicles from both the larger and smaller populations may be irregular and constrained by the spatial distribution and growth of microphenocrysts and microlites (Fig. 3C). Average measurements of textural parameters (Table 1) are in good agreement with observational results. Scoria from Strombolian activity exhibits similar values of vesicularity (range 0.52–0.56), vesicle number densities as GEOLOGY, March 2006
Figure 4. Backscattered electron images of groundmass textures. Dark gray crystals are plagioclase, light gray crystals are pyroxene and olivine, and white crystals are oxides. Black areas are vesicles. A, B, and C: Scoria from Strombolian explosions. D, E, and F: Scoria from fire fountain activity. Scale bar (for all images) is 50 mm.
low as 0.7 3 103 cm22, and corresponding high values of vesicle sizes (average diameter to 315 mm). Vesicularities in clasts from fire fountain activity span a wider range (0.53– 0.74) that partially overlaps with values of Strombolian scoria, vesicle number densities are higher (2.5–3.6 3 103 cm22), and values of the average vesicle diameter are smaller (150–192 mm). Observation and quantification of crystal textures have shown that both Strombolian and fire fountain scoria are porphyritic rocks with vesicle-free phenocryst content 0.19– 0.40. Groundmass crystals, however, exhibit complex trends (Table 1; Fig. 4). Crystallinity and crystal number density of clasts from fire fountains display a high variability (0.12–0.74 and 2.9 3 103 to 1.6 3 105 mm22) that extends beyond the lower and upper limits of the range of Strombolian samples (0.30–0.44 and 3.2 3 103 to 5.2 3 104 mm22). Areas of sideromelane (microlite-free and/or microlitepoor glass) and tachylite (variably crystallized groundmass glass) occur in scoria from both Strombolian and fire fountain activity (Figs. 4A–4F). Furthermore, tachylite and sideromelane frequently coexist and are heterogeFigure 5. Glass compositions from Strombolian and fire fountain scoria. Compositional fields of tachylite measured in Strombolian (dashed line) and fire fountain (continuous line) scoria are plotted for comparison. Error bar is analytical uncertainty. Arrows match compositional variation for 5% mineral crystallization starting from 2000 average bulk rock. Pl is plagioclase, cpx is clinopyroxene, ol is olivine, and ox is Ti-magnetite. GEOLOGY, March 2006
neously distributed within the same clast (Figs. 4A, 4B). However, with the exception of samples 160400C and 160400C1 whose groundmass is almost holocrystalline (Fig. 4D), groundmass crystallinity is higher in scoria from Strombolian explosions, and groundmass crystals are twice as large in comparison to those of fire fountains (Figs. 4A, 4B, 4E, 4F). Plagioclase is always the most abundant mineral phase, and is the only one present in significant amounts in some fire fountain scoria clasts (Figs. 4E, 4F). GLASS COMPOSITIONS AND THERMOMETRY Major element compositions of sideromelane measured within each investigated sample are homogeneous. When plotted in the CaO/Al2O3 vs. FeOtot/MgO diagram (Fig. 5), sideromelane glass compositions measured in Strombolian scoria are distinct from and slightly more evolved than in fire fountain samples. Both compositions plot along the liquid line of descent from a common magma represented by the average bulk rock composition of the 2000 activity. Tachylite has a more evolved composition than sideromelane,
and exhibits a strongly scattered distribution within the same sample, suggesting that the nature and high content of groundmass microlites control the liquid composition and its ability to homogenize. In addition, tachylite does not allow discrimination between scoria clasts from either of the two eruptive styles, as the compositional fields mostly overlap (Fig. 5). Eruptive temperatures, calculated via the MgO glass geothermometer calibrated for Etnean lavas by Pompilio et al. (1998), exhibit values from 1090 to 1095 (610) 8C in sideromelane from Strombolian scoria. Significantly higher temperatures, ranging from 1113 to 1124 (610) 8C, characterize sideromelane from fire fountain samples. Application of the same geothermometer to tachylite obtains temperatures in the range 1064–1103 (610) 8C. ORIGIN OF STROMBOLIAN AND FIRE FOUNTAIN SCORIA The textural and compositional data set provides new constraints on how and where scoria formed during the 2000 SEC Etna eruption. Overall, scoria from the Strombolian phase of each paroxysmal episode has a higher groundmass crystal content, larger groundmass crystals, lower vesicularity, lower vesicle number density, larger vesicles, and strong evidence of vesicle coalescence. This information, together with the more evolved glass composition and the lower eruptive temperature, suggests that magma that formed clasts of Strombolian scoria was volatile depleted, and resided long enough to cool and crystallize, develop a network of interconnected vesicles, and increase magma permeability, possibly generating pathways of passive or mildly explosive gas separation. This idea is consistent with textures of scoria produced during explosions at Stromboli, which preserves evidence of prolonged coalescence and outgassing prior to fragmentation (Lautze and Houghton, 2005). The high vesicle volume fraction and number density of scoria from the Hawaiian-style fire fountain phase of activity, the ubiquitous occurrence of small (,100 mm) vesicles, together with the lower groundmass crystal content, the less evolved glass composition, and the higher eruptive temperature, point to the rise, expansion, and disruption of a magma that was sufficiently fluid and volatile rich to produce moderate syneruptive volatile exsolution during each eruptive episode, but fast enough to prevent significant vesicle coalescence and microlite nucleation and growth during ascent in the conduit. Similar high vesicle number densities, found in products from fire fountains of Hawaiian eruptions, were also ascribed to the fast ascent and expansion of a volatile-saturated magma (Mangan and Cashman, 1996). 203
The extremely high groundmass crystallinity and number density values exhibited by fire fountain samples 160400C and 160400C1 may be explained in terms of the en masse groundmass crystallization of clasts closer to the outer portions of the fire fountain upon cooling (Head and Wilson, 1989). In agreement with this hypothesis, the calculated eruptive temperature for the samples is 1090 8C, 23–34 8C lower than that calculated for the less-crystallized fire fountain groundmass. These observations also agree with findings from ash characterization of the 2001 Etna eruption (Taddeucci et al., 2004), in which tachylitic ash particles are thought to originate in the region close to the conduit walls owing to progressive cooling during the eruption. INSIGHTS INTO CONDUIT DYNAMICS Two end-member models have been developed in the past 20 yr to explain the dynamics of fire fountain activity (for a comprehensive review see Parfitt, 2004, and references therein). In the rise-speed–dependent model, it is assumed that syneruptive volatile exsolution accompanies magma as it rises in the conduit upon decompression, and that fragmentation of the bubbly mixture occurs when the volume fraction of bubbles exceeds that of maximum packing. The collapsing foam model, instead, states that gas bubbles separate from the magma as they form, and accumulate in a layer at the top of a storage area forming a foam, whose repeated partial or complete collapse generates, respectively, Strombolian explosions or fire fountain activity. Volcanological observations and seismic data demonstrate a cyclical behavior of the Etna 2000 activity, which was interpreted as due to the input of a primitive volatile-rich magma into a shallow reservoir, where the dynamics of gas bubbles drove all the fire fountain events (Allard et al., 2003). Results from a spectroscopic investigation of magmatic gases during the 14 June event of the 2000 eruption clearly revealed the separation and ascent of a bubble foam layer as source of the powering fountain phase (Allard et al., 2005), suggesting that syneruptive volatile exsolution was not the primary mechanism. On the basis of the previous information, we use implications from the new textural and compositional data to explain the transition between the two eruptive styles in the context of the known eruption dynamics. In early 2000, the volcanic system of SEC comprised a conduit or dike of magma progressively more degassed going upward from a storage region filled with magma left by the previous 1999 activity (Alparone et al., 2003). Volatile exsolution caused the buoyant rise of the first bubbles to the roof of the magma reservoir. This pushed the column of degassed magma up in the conduit and 204
originated a passive extrusion of lava, which was always observed at the inception of each paroxysmal event. As more and more volatiles exsolved and accumulated at the top of the chamber, bubbles progressively coalesced into larger bubbles and generated gas pockets or slugs that entered the conduit, reached the open magma surface, and burst into the typical Strombolian explosions, fragmenting a melt of prolonged shallow residence. The transition to fire fountain activity occurred when coalescence became so efficient to produce a foam of gas bubbles that eventually collapsed, rose in the conduit as a gas core surrounded by a moderately vesiculating liquid annulus, and erupted explosively as vigorous jets of gas and liquid magma. However, while geophysical and geochemical evidence points to foam collapse as the main mechanism driving fire fountain activity, our results highlight a previously unrecognized role played by syneruptive volatile exsolution during each single eruptive episode, which is preserved in the small groundmass vesicle population (Figs. 3B, 3C) detected in all the examined fire fountain scoria products. This implies that the dynamics of the fire fountains were determined by a combination of foam collapse at the roof of the magma reservoir and syneruptive nucleation of bubbles in the annulus of liquid surrounding the ascending gas core. We believe that this latter process likely operated at the peak of intensity in the paroxysmal phase of each fountain episode, perhaps providing the impetus for a more efficient fragmentation of fine particles generating the buoyant ash columns (Fig. 1). These demonstrations of two distinct degassing processes during the same event raise interesting issues that require development of combined compositional, textural, and gas studies for future research on the dynamics of fire fountains. ACKNOWLEDGMENTS We thank Luigi Lodato for sample collection, Lucia Miraglia for help with the scanning electron microscopy, and Mike Burton for fruitful discussions and helpful suggestions. We also thank B. Houghton, J. Sable, and an anonymous reviewer for their thorough reviews. This work was supported by the Istituto Nazionale di Geofisica e Vulcanologia and Dipartimento per la Protezione Civile. REFERENCES CITED Allard, P., Alparone, S., Andronico, D., Burton, M., Lodato, L., Mure`, F., and Sgroi, T., 2003, Source process of cyclic fire fountaining at Mt. Etna in 2000: A multidisciplinary study of the June 14 (63rd) event: Geophysical Research Abstracts, v. 5, p. 13,079–13,080. Allard, P., Burton, M., and Mure`, F., 2005, Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas: Nature, v. 433, p. 407–410. Alparone, S., Andronico, D., Lodato, L., and Sgroi, T., 2003, Relationship between tremor and volcanic activity during the Southeast Crater eruption on Mount Etna in early 2000:
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