Abstract
Sakurajima in Japan is one of the world’s most active volcanoes. This paper presents the results of a petrological study of the Showa Crater volcanic ash samples ejected from January 2011 to November 2012 from Sakurajima. The aim of this paper is to reconstruct the evolution in time of the conduit magma system, based on the compositions and physical properties of the studied volcanic ash. We analyzed the composition of interstitial glass and microcrystal of Black Volcanic Rock (BVR) and Black Vesicular Volcanic Rock (BVVR) in order to estimate the magma ascent rate. The results show that SiO2 content of interstitial glass and crystallinity of the BVR is generally higher than for BVVR. The different types of juvenile material likely resulted from cooling-induced crystallization and decompression-induced crystallization in the conduit. The conditions of magma transit within the conduit from 2011 to 2012 differ: in 2011, the decompression rate and magma ascent rate of BVR were higher than for BVVR, but, in 2012, the decompression rate and magma ascent rate between BVR and BVVR were similar. As such, monitoring the petrological features of dated eruptive materials could provide useful information for evaluating ongoing eruptive activity.
Highlights
The nature of magma ascent through a conduit is directly linked to magmatic processes including rheology, composition of magma, geometry of the plumbing system, and the variability in eruption style of active volcanoes
The differences between the Black Vesicular Volcanic Rock (BVVR) and Black Volcanic Rock (BVR) melt are less than 10% differences for several elements
In some of the eruptions, we find that the results vary by more than 4 wt. % for all major elements and the sum of the squared residuals is more than 10%, which differs for several elements, due to errors from the calculation of modal proportion
Summary
The nature of magma ascent through a conduit is directly linked to magmatic processes including rheology, composition of magma, geometry of the plumbing system, and the variability in eruption style of active volcanoes. Magmatic properties evolve in space and time, reflecting the complexity of underground magmatic plumbing systems and the processes by which those systems are recharged and emptied [3,4]. To understand these processes better, and to understand the changes that occur during volcanic eruptions, it is important to quantify the rates at which magma rises to the surface [5]. The studies have focused on tracking changes in interstitial glass composition of juvenile materials [12,14], calculation of diffusion timescales across compositional zones in crystals as Geosciences 2016, 6, 12; doi:10.3390/geosciences6010012 www.mdpi.com/journal/geosciences
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