Indonesian Journal on Geoscience

6

Authors: SUPRIATI DWI ANDREASTUTI, CHRIS NEWHALL, and JOKO DWIYANTO

Until now, the large eruption of Merapi in 1006 is believed to take place although the truth is still debatable. Previous investigation proposed that the ”pralaya” of the Ancient Mataram Kingdom in 928 Saka (1006) was due to a volcanic activity. Bemmelen also inferred that impact of the eruption had destroyed and covered the Mendut and Borobudur Temples and dammed the Progo River. However, if the “pralaya” was caused by Merapi eruption, why the deposit that correlates to the the eruption is not recognized. If so, the eruption that covered the temples should have been very large, and left deposits around Merapi and of course easy to fi nd. Historically, the “pralaya“ mentioned in the Pucangan Inscription did not happen in 1006, but in 1016 or 1017. However the “pralaya“ was caused by the attack of King Wurawari, not by the Merapi eruption. According to the history of Merapi eruptions, 11 large eruptions have occurred since 3000 years ago. However, none of those fi t with 1006 eruption. Except the large eruption (VEI 3-4), that produced Selo tephra, dated 1112 ± 73 years BP (765-911). Keywords: Merapi eruption, 1006, pralaya, Pucangan Inscription

7

Authors: ISYA NURRAHMAT DANA, EKA KADARSETIA, SOFYAN PRIMULYANA, MUHAMMAD HENDRASTO, and ASNAWIR NASUTION

Tangkubanparahu is an active strato volcano located in West Java lying about 30 km to the north of Bandung City. Its crest is 2084 m above the sea level. In order to gain a better understanding on volcanism and magmatism of this volcano, various research and monitoring have been carried out, such as geochemistry and geophysics. Chemical composition of volcanic gases collected from the Ratu Crater (950 C in 1994, 1997, 1998 and 2005, shows that the gas is hydrous with the main component of H2O, CO2, H2S and small amount of SO2; where CO2 content is higher than (SO2 + H2S). The gas composition showing high of value ratio of CO2/H2S, CO2/H2, and H2/Ar, is suggested to indicate the presence of a fast fl uid movement from the depth before condensed at the shallow surface water area. Hotspring from the Domas Crater contains a high concentration of SO4, low of Cl and absence of H2CO3. The high sulphate content is suggested to be originated from the volcanic gases, especially hydrogen sulphide oxydated near the surface, that then the gas infl uenced chemical composition due to shallow water. Continuous seismic monitoring uses one permanent station, while the other methods like Electric Distance Measurement (EDM), Global Positioning System (GPS) and Seismometer have been installed temporary. From geophysical evidence on April 2005 activity, some valuable information can be obtained. Hypocenter is located at the depth less than 2 km beneath an area between the Ratu and Domas Craters, while pressure source of deformation is below Domas Crater. Some low frequency volcanoquakes is possibly caused by volcanic gases released from the reservoir. Keywords: increasing activity, Tangkubanparahu, geochemistry, geophysics, crater, volcanoquake

16

Authors: INDYO PRATOMO

Indonesia is well known as a volcanic country, where more than 30% out of all the world volcanoes occupied this region. Volcanic region is generally densely populated, because of their soil fertility and other land use. Based on their historical eruptions noted since and before 1600 A.D., the Indonesian active volcanoes are regrouped in to A type (79 volcanoes), which were defi ned as volcanoes erupted since 1600 A.D., B type (29 volcanoes) erupted before 1600 A.D., and C type (21 volcanoes) are solfatar fi elds (Bemmelen, 1949; van Padang 1951; Kusumadinata, 1979). Studies on parts of the Indonesian active volcanoes, show different eruptive characters, which are generally related to hazard potentials. A new classifi cation of Indonesian active volcanoes was proposed based on the combination of their physical properties, morphology, volcanic structure and eruptive styles to the eight differents types, those are Tambora (caldera formation), Merapi (lava dome), Agung (open crater), Papandayan (sector failure), Batur (post-caldera activities), Sangeangapi (lava fl ows) and Anak Krakatau types (volcano islands and submarine volcano). This classifi cation would be make a better understanding to different characteristics of Indonesian active volcanoes, for the volcanic hazard and mitigation and also for the applied volcanological researches. Keywords: Indonesian active volcanoes, characteristics, classifi cation, volcanic hazard mitigation