Indonesian Journal on Geoscience

9

Authors: M. Ulum A. Gani and Widodo

Activated carbon made from Tanjung Tabalong coal was investigated its absorption capability to organic and inorganic elements in industrial waste. Coal was carbonized at low temperature of 600oC to produce semicoke, and then was activated at temperature of 700oC with activation time of 120 minutes with water steam flow. The absorption capability of activated carbon to chemical oxygen demand (COD) was performed using 2.5 and 9.0 g activated carbon for 250 ml and 300 ml COD waste respectively. The agitation time of each experiment were 30, 60, and 90 minutes. Atomic absorption spectrophotometer (AAS) was used to analyze the COD waste. The result shows that 2.5 g activated carbon could absorb COD waste ranging from 6.9-67.5 %, while the utilization of 9 g could absorb COD waste ranging from 88.9 - 100 %. The more activated carbon and the longer time of agitation used in this experiment, the more the absorption of COD waste. Keywords: carbonization, activation, activated carbon, COD waste

10

Authors: H. Humaida, K. S. Brotopuspito, H. D. Pranowo, and narsito

The effusive eruption of Kelud Volcano in 2007 was different from the previous ones, which in general were more explosive. Among others, density and viscosity are factors that determine the type of eruption. Therefore, the study on the difference of the recent eruption style based on the density and viscosity of magma was carried out. The method used in this study was based on geochemical analysis of the rock and then a modeling was established by using the above parameter. The study on the explosive eruption was emphasized on the data of 1990 eruption, whereas the effusive eruption was based on the data of 2007 eruption. The result shows that the magma viscosity of Kelud Volcano depend on the H O concentration as one of the volatile compound in magma, and temperature which gives the exponential equation. The higher the increase of H O content the smaller the value of its viscosity as well as the higher the temperature. The H2O content in silica fluid can break the polymer bond of the silica fluid, because a shorter polymer will produce a lower viscosity. The density of the silica content of Kelud Volcano ranges between andesitic and basaltic types, but andesite is more likely. The fluid density of the material of 1990 eruption is different from 2007 eruption. Compared to the 2007, the 1990 eruption material gave a lower density value in its silica fluid than that of the 2007 one. The low density value of the silica fluid of the 1990 eruption material was reflecting a more acid magma. The level of density value of silica fluid depends on its temperature. At the temperature of 1073 K the density of the 1990 Kelud magma is 2810 kg/m3 and the 2007 magma is 2818 kg/m3, whereas at a temperature of 1673 K, the density is 2672 kg/m3 and 2682 kg/m3 of the 1990 and 2007 eruptions respectively. A modeling by using an ideal gas law of Henry’s Law illustrated that the ascent of Kelud’s magma to the surface may cause changes in it’s physical properties. The evolution of the flow pressure in the conduit is characterized by three different areas; based of the conduit until the pressure is saturated, then at the level between release and fragmentation, and then the level above the fragmentation, that implicates the decrease in the wall friction. Keywords: Kelud Volcano, viscosity, density, magma, modeling

9

Authors: Y. Suparman, E. Kriswati, dan Y. E. Pamitro

Moment tensor analysis had been conducted to understand the source mechanism of earthquakes in Soputan Volcano during October - November 2010 period. The record shows shallow earthquakes with frequency about 5 - 9 Hz. Polarity distribution of P-wave first onset indicates that the recorded earthquakes are predominated by earthquakes where almost at all stations have the same direction of P-wave first motions, and earthquakes with upward first motions.In this article, the source mechanism is described as the second derivative of moment tensor, approached with first motion amplitude inversion of P-wave at some seismic stations. The result of moment tensor decomposition are predominated by earthquakes with big percentage in ISO and CLVD component. Focal mechanism shows that the recorded earthquakes have the same strike in northeast-southwest direction with dip about 400 - 600. The sources of the high frequency shallow earthquakes are in the form of tensile-shear cracks or a combination between crack and tensile faulting. Keywords: moment tensor, source mechanism, frequency, P-wave, polarity, first motion

12

Authors: U. Sudarsono and G. Hasibuan

Only few engineering geological researches on residual soil have been conducted in Indonesia, whereas many infrastructure projects take place on undulating area and mountainous area which are composed of residual soils. The paper discusses residual soil characteristics of the Lower Quaternary sediment (Qos) in Kertajati area, Majalengka. The methodology of the research consists of engineering geological mapping, in situ test using 2.5 ton penetrometer, sampling and laboratory test for index properties, Atterberg limits, grain size analysis, and strength tests. Field test analysis data show that the thickness of residual soil resting on the basement rock in the study area varies from 2.40 - 14.20 m. Upwards, it comprises sands, silts, and clays, respectively. Laboratory analysis data indicate that the residual soil is dominated by fine-grained fraction, composed of high plastic clay (CH). Relationship among engineering geological parameters was analysed using linear regression and tested using statistic method t-test to find real correlation among variables within 95 - 90% significance level.. Based on the test, there are real positive correlation with 19 degree of freedom and 5% and 10% level of significance are found in between plasticity indexes (IP) and liquid limits (LL), between plasticity indexes (IP) and clay contents (C), between specific gravities (G ) and clay contents (C). The real negative correlation with nine degree of freedom and 5% and 10% level of significance occurs in between cohesion (c) dengan friction angle (φ). Corelation between degree of saturation (G ) and clay contents (C) with 19 degree of freedom is found in 10% level of significance. Keywords: Lower Quaternary sedimentary rocks, residual soil, correlation, linear regression

9

Authors: D. Z. Herman

Either fossils of the hominid or vertebrate have long been known from terraces along the Solo River in Central and East Java. Most terraces consist of andesitic sand to andesitic tuffaceous sand with either gravelpebble or conglomerate and some of them contain vertebrate fossils. It is in this place, an ancient elephant fossil named Elephas hysudrindicus was discovered in 2009. This fossil was discovered at an abandoned sand quarry of Sunggun area, Medalem Village, Kradenan Subregency, Blora Regency which and can be mentioned as a great event for the Geological Museum. It was said as a phenomenal discovery, because the fossil was found within the terrace with condition of nearly complete skeleton of an individual elephant. Some bone fragments of Elephas hysudrindicus fossil is treated as rock specimens because a number of minerals fill in either pore spaces or cavities or cracks within bones, and such infilling minerals can be observed in cut sections of the bones. Main goals of the study are to determine the distribution and type of minerals within the bones, interpret environment of deposition, and identify fossilization type. The methodology used in this study consists of petrographic and Scanning Electron Microscopic (SEM) analyses. Based on the petrographical observation, some bone specimens of Elephas hysudrindicus fossil are characterized by fibrous and porous feature with cracks occuring locally. Whilst, examination with SEM shows that the bone specimens are apparently composed of collophane or massive cryptocrystalline variety of apatite as the principal component of fossil bone, having physical characteristic of spheroidal structure and cavities of 100 to 1500 micron (μ)in diameter. Most cavities and pore spaces are predominantly filled in by either authigenic crystals of rhombohedral calcite and lesser pseudohexagonal kaolinite with either slightly minerals of manganese oxide or iron oxide or ilmenite, including oxidized kaolinite and calcium iron silicate. Impregnation during diagenesis may be the most appropriate expression for fossilization process of the Elephas hysudrindicus. It is indicated by the existing authigenic minerals within the bones cavities, pore spaces, and cracks which are possibly due to precipitation of mineralized fluids originated from groundwater within the terrace. Keywords: Sunggun terrace, Elephas hysudrindicus, authigenic minerals, fossilization

12

Authors: B.H. Harahap

Volcanic rocks in Kabanjahe region, Karo Regency, North Sumatra Province, are products of old Toba Caldera, Sibayak Volcano, and Sipiso-piso Volcano. Rhyolitic tuff is the main lithology distributed over a large area in this region. Others are basaltic, basaltic andesitic, andesitic, dacitic, and rhyolitic lavas. Data show that the rock was originated from magma of a continental origin formed at a subduction zone environment. Petrogenetic modelling suggests that the range in composition was mainly controlled by a fractional crystallization of plagioclase, clinopyroxene, hornblende, and biotite. Harker’s variation diagram of major and trace elements show a continuous range that indicates they are cognate. The lava in this area belongs to a high-K, calc-alkaline series, with particular high Nb concentrations. The composition of these high-Nb lavas is more similar to those of intra plate basalts rather than those of calc-alkaline or arc-tholeiitic basalt. The high anomaly of Nb which is accompanied by high Th, Rb, and normative corundum suggests that the source may also be enriched in incompatible elements, a characteristic feature of alkali magmatism. The similarity of the trace element of volcanic rocks to the within-plate basalts indicates that the convecting mantle wedge above subducted slabs contains variable proportions of MORB-source and OIB-source components; fluids added were derived from the subducted slab. Hence, it is interpreted that the high Nb concentration of volcanic rocks from Kabanjahe region were generated from subduction modified OIB source components. Alternatively, a deep seated faulting conduit magma from the lower mantle resulted in the alkaline enrichment of the volcanics. This article performs a petrological aspect, especially based on geochemical analysis including major elements, trace elements, and rare earth elements. The results are plotted into a general and specific classification used in petrology. Keywords: petrology, geochemistry, Kabanjahe, continental margin arc, tectonic

10

Authors: S. Hidayati, Y. Suparman, and A. Loeqman

Guntur Volcano in West Java is one of the most active volcanoes in Indonesia. The last eruption took place in 1847 and the volcanic activity has been dormant since then, however its seismicity is active. During the period of July to October 2009, the hypocenter distribution of VT earthquakes is mostly located at western flank of the volcano, beneath Guntur - Gandapura craters at the depth of less than 5 km. The depth pattern shows deeper to the northwest. The VT earthquakes deeper than 5 km were not found in this period. The focal mechanism of VT earthquakes are oblique normal fault, strike-slip fault and oblique reverse fault types. The mechanism of those earthquakes is not uniquely determined probably due to complicated structures at Guntur volcano complex area, which is aligned in NW-SE direction. T-axis of the oblique normal fault is trending in northwest - southeast direction similar to the structures found in the summit area of Gunung Guntur Volcano. Similarly, one of the strike-slip fault nodal line and P-axis of oblique reverse fault are also trending in northwest - southeast. Ploting of the earthquake source parameters (seismic moment, corner frequency, and stress drop) made to hypocenter distance shows no significant difference on those parameters between earthquakes at close and far distances to Kabuyutan station. It is probably due to the hypocenters are not concentrated in one zone. Meanwhile, the relationship between seismic moment (Mo) and seismic source radius (r) shows that for earthquakes with moment of smaller than 1018 dyne cm, the radius of the hypocenter is constant which is namely 60 m. Keywords: Guntur volcano, VT earthquakes, hypocenter, focal mechanism, earthquake source parameter

14

Authors: Praptisih and Kamtono

An understanding of deepwater turbidite sediments is very important mainly dealing with the determination of a reservoir geometry. The problem arising in the turbidite sediment geometry is very complex and varied, and its existence depends on how, when, and type of its environment. In Central Java, the broad turbidite sediment distribution of the Halang Formation is needed to be observed its facies association. This paper will discuss the turbidite facies of Halang Formation in the Ajibarang area. The method used to analyze the turbidite facies is by measuring stratigraphic sections and observing the lithofacies characteristics to reconstruct its depositional environment. The Halang Formation outcrop in the Ajibarang area is dominantly composed of alternating sandstone and claystone or marl. Based on the observation, the turbidite facies of Halang Formation, in the Ajibarang area, was deposited on a submarine fan system at the middle fan of suprafan lobes. Compiling with the previous studies, the source of the Halang Formation is indicated to be derived from the south southwest. Keywords: Halang Formation, facies, turbidite, submarine fan system