Abstract
Abstract At Sakurajima volcano, strombolian eruptions forerun vulcanian explosions, and volcanian explosions are often followed by continuous ash eruptions. Change in the mode of its eruptive activity is drastic and release of volatiles from magma corresponding to its eruptive activity is examined by determination of fluorine and chlorine in volcanic ash. The F contents of all the ash samples are markedly higher than the arithmetic mean value of Japanese volcanic rocks (ca. 300 μg/g). Further, the water-soluble Cl contents of almost all the ash samples are also extremely higher than those of Japanese volcanic rocks (less than 50 μg/g). This is attributed to fixation of F and Cl in hot gas onto the volcanic ash particles in the crater and/or eruption plumes. The differences in those contents among the “strombolian eruption”-, “vulcanian explosion”- and “continuous ash eruption” groups are significant, which implies that hot gas rich in F and Cl are issued during strombolian eruptions, while those halogens are less concentrated in hot gas after strombolian eruptions. The water-insoluble Cl content of the “strombolian eruption” group is significantly higher than those of the other groups. This result implies that Cl and the other volatiles are intensely released from magma during the doming period before vulcanian eruptions. Yet, it is inferred that continuous ash eruption is the final phase of vulcanian one.
Highlights
Sakurajima is an andesitic stratovolcano and one of the most active volcanoes in the world
Sakurajima volcano by strombolian eruption, vulcanian explosion, and continuous ash eruption leads to the following conclusions
The F and water-soluble Cl contents of the ash samples are extremely higher than those of Japanese volcanic rocks, which can be attributed to fixation of F and Cl in hot gas onto the ash particles
Summary
Sakurajima is an andesitic stratovolcano and one of the most active volcanoes in the world. It lies about 5 km east of the city of Kagoshima at the southern end of Kyushu, Japan (Fig. 1). Typical vulcanian explosions have occurred repeatedly, yet strombolian and continuous ash eruptions have been frequently observed. Strombolian eruptions forerun vulcanian explosions, and vulcanian explosions are often followed by continuous ash eruptions, which is the ordinary sequence of the eruptive activity of Sakurajima volcano. Volcanic earthquakes observed at this volcano are classified as A-type, B-type, and explosion earthquakes. A-type earthquakes have clear P- and S-phases similar to ordinary tectonic earthquakes. A-type earthquakes often precede swarms of B-type earthquakes, and the swarms are usually followed by explosive activity (Kamo, 1978). Spectrum analysis discloses that A-type earthquakes occur in brittle regions, whereas B-type earthquakes swarm in ductile or molten regions (Ishihara and Iguchi, 1989)
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