Abstract. Pore-space topology. is studied in recent compact angular blocks and scoria fragments •¾om block-and-ash flow deposits of Merapi volcano, Indonesia. Connected porosity and electrical conductivity of cored clay4¾ee samples are measured in the laboratory at varying fluid salini,ty. The electrical •brmation fiactor, tortuosity and cementation index are derived and the pore-,space topology is •bund to be porosity-dependent. The electrical flow pattern is controlled by crack-like paths in compact angular block, and by vesicles geometry in more porous scoria •¾agments. Because Merapi lavas are highly viscous and crystal-rich, we infer that the development of pore connectedness and vesicle coalescence is promoted by shear strain. Along with ascent rate considerations we conclude that degassing of Merapi magma occurs mostly in the conduit during ascent.

S U M M A R YA palaeomagnetic study has been conducted from Indonesian lava flows and dykes. Sampling was concentrated to the Merapi and Merbabu volcanoes (central Java), the Bromo-Tengger volcanic complex, the Lurus volcano (both east of Java), and the Bawean Island. Numerical dating indicates that the rock units sampled have ages that are well distributed within the Brunhes chron. Only a few reverse polarity flows were probably emplaced during the late Matuyama chron. Alternating field (AF) and thermal demagnetization treatments were routinely used for all sites. Results from both methods were generally in good agreement, demonstrating the reliability and the reproducibility of the palaeomagnetic determinations obtained from 36 independent sites. The large dispersion of the directions and the age distribution indicate that a large spectrum of the secular variation has been sampled. Our best estimate of the mean Brunhes field direction in Java is characterized by a zero declination and an inclination anomaly of −4.7 • (with a 95 per cent confidence angle of 5.3 • ). This value agrees with an axial dipolar field model, or a model to which a persisting zonal quadrupolar term of a few per cent of the geocentric axial dipole is added. This study adds new constraints to recent results which suggested that a large zone of negative inclination anomalies may have persisted in the southwestern Pacific ocean over the past 500 Kyr, and possibly extended toward the eastern Indian ocean. The present results do not support such an extension. They also suggest that the inclination anomaly may have varied over 100 Kyr, and that it was more pronounced in recent times.

Sistem panas bumi daerah Candi Umbul-Telomoyo, Jawa Tengah merupakan salah satu lapangan panas bumi di Indonesia yang terbentuk pada lingkungan magma basaltik. Fluida panas satu fasa bertemperatur tinggi terbentuk pada zona resevoir yang memiliki permeabilitas tinggi sebagai fasa cair. Fluida ini dapat tersimpan dengan baik di reservoir dikarenakan ditutupi lapisan penudung berupa batuan ubahan yang bersifat kedap air. Zona upflow dari sistem panas bumi Telomoyo terbentuk di dalam kaldera komplek Telomoyo, sedangkan zona outflow terbentuk di daerah sekitar manifestasi Candi Dukuh, Candi Umbul dan Pakis Dadu. Karakteristik sistem panas bumi daerah Candi Umbul-Telomoyo menunjukkan bahwa daerah ini potensial untuk dikembangkan.

http://dx.doi.org/10.17014/ijog.vol2no1.20073Maninjau is a large collapsed caldera that was resulted from a large eruption of silicic pyroclastic material (220-250 km3), and was distributed to more than 75 km away from the source. Field observations have provided a good understanding to the eruptive mechanism of the Maninjau caldera formation, in respect to their stratigraphy, sedimentology and geochronology of the eruptive products. Two formations have been identifi ed, those are: the Maninjau Formation, comprising a sequence of ignimbrite and surge units erupted from the Maninjau Caldera, and the Malalak Formation as a prominent andesitic fall unit likely derived from the Singgalang - Tandikat Volcano that overlies the Maninjau Formation. The variable velocity of the fl ows suggests that the Maninjau eruption initiated by violent. The later phases of the eruption became more violent and associated with caldera collapse.

The purpose of this study was to determine the Characteristics of Geology, Geochemistry, Geophysics in Assessing the Geothermal Potential of the Bonjol Region, Pasaman, West Sumatra. The Bonjol geothermal area is included in the Bonjol District, Pasaman Regency, West Sumatra Province. The rocks in the study area can be divided into 13 rock units, namely two sedimentary rock units, 10 volcanic rock units, and one surface sediment unit, while geothermal manifestations in the research area are scattered in the central part which is dominated by the emergence of hot springs in Padang Baru, Takis River, Limau River, and Langkuik/Kambahan River. In addition to hot springs, alteration rocks are also found around Ds. Blimbing and in the northwestern part of the research area are both past alterations.