Linking variable explosion style and magma textures during 2002 at Stromboli volcano,Italy |
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| Authors: | Nicole C Lautze Bruce F Houghton |
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| Institution: | (1) Department of Geology and Geophysics, University of Hawai’i Manoa, 1680 East–West Rd. POST 606A, Honolulu, HI 96822, USA |
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| 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, irregularly-shaped 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. |
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| Keywords: | Stromboli Vesicle Conduit residence time Explosion intensity Viscosity |
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