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 SiO 2 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.

No abstract

No abstract

This PDF file is a scanned version of a dissertation written by Prof. Emmy Suparka. She graduated her PhD in 1988 from the Department of Geology, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung. She was supervised by Prof.Dr. R. Soeria Atmadja.

The Geunteut area is located 55 km south of Banda Aceh. The research was conducted to characterize the hydrothermal alteration and mineralization of the porphyry-skarn deposits. The alterations and mineralizations were described according to their association and zonation of the hydrothermal minerals, alteration intensity, texture and ore mineral paragenesis. The porphyry-skarn deposits in the Geunteut area are related to the magmatic activity in Middle Miocene by granodiorite and diorite intrusions (Intrusion Rock Units) that resulted in the hydrothermal alteration and mineralization. Hydrothermal alteration can be grouped into five zones: biotite-orthoclase-actinolite, epidote-chlorite-actinolite, garnetclinopyroxene-tremolite, quartz-sericite and chlorite-calcite-clinoptilolite. The forming temperature of the hydrothermal alteration is between 120 -360°C with the intensity ranging from very weak to totally altered. The porphyry-skarn deposits exhibit two episodes of mineralization. The early episode was related to the hypogene mineralization, as evidenced by the formation of magnetite, ilmenite, chalcopyrite and pyrite. The late episode was related to the supergene enrichment that allowed the formation of chalcocite, covellite, iron oxide and malachite.