Indonesian Journal on Geoscience

5

Authors: RUDY DALIMIN HADISANTONO

Papandayan is an A-type active strato volcano located at some 20 km SW of Garut or about 70 km SE of Bandung the capital city of West Java Province. Geographically, the summit of this volcano lies at the intersection between 07º 19’ 42” S and 107º 44” E. The 2002 Papandayan eruption was preceded by two felt earthquakes, 8 times of A-type volcanic earthquakes and 150 times of Btype volcanic earthquake. These events were followed by a phreatic eruption that took place on 11 November at 16.02 local time. Field observation shows that the summit region, mainly around the craters consists of rocks that have hydrothermally altered to yield clay rich material. This clay rich material covers most of the crater fl oors and the crater rim. Mount Nangklak that forms part of the rim also contains a lava plug from the Old Papandayan volcano. This mountain is covered by fi ne grained, unconsolidated material, and altered rocks. Much of this altered rocks coincides with solfataric and fumarolic activities of 80 to > 300°C. The summit area also contains high discharge of water either originating from the springs or surface water. The increase in seismicity, the fi ne-grained hydrothermal altered rocks, and the existence of some faults that pass through the summit region might have weaken the stability of the summit area. As the result, a landslide occurred on the north fl ank of Mount Nangklak where the landslide material blocked the upper course of Cibeureum Gede River. This landslide material had formed big mudfl ows that caused several houses of fi ve villages were partly burried, some bridges were devastated and several hectares of cultivated land were damaged. Keywords: Papandayan, earthquakes, eruption, hydrothermal, alteration, landslides

10

Authors: SRI MULYANINGSIH1, SAMPURNO2, YAHDI ZAIM2, DENY JUANDA PURADIMAJA2, SUTIKNO BRONTO3, and DARWIN ALIJASA SIREGAR3

The uplift of Southern Mountains in Early Pleistocene has formed the Yogyakarta Basin. In this basin, the Merapi volcanic activity has been developing. Based on 14C dating in cinder deposits exposed at Cepogo, the volcanic activity took place since ±42 ka. While on the basis of K/Ar dating in andesitic lava at Bibi Volcano, the activity took place since 0.67 ma. The high in the south and the appearance of Merapi volcanic dome in the north had caused a fl at valley. The southern part of the valley is bounded by the Southern Mountains and the western part is bounded by the West Progo Mountains. In the present time, the lithology of the areas which are interpreted as a palaeo-valley is composed of black clay deposits. This black clay is a contact between the basement rocks and Merapi volcanic deposits. The black clay deposits exposed in the Progo River (Kasihan) has been developed since ±16.59 to 0.47 ka, while in the Opak River (Watuadeg) 6210 y BP. Younger black clay deposits intersecting with lahars are also exposed at the Winongo River and have an age of 310 y BP. The age data of volcanic stratigraphy shows that Merapi activities had taken place since ±6210 up to ±310 years ago. Keywords: development, geology, plain, Yogyakarta, Quarternary, 14C

12

Authors: SUTIKNO BRONTO, ACHNAN KOSWARA, and KASPAR LUMBANBATU

The aim of this volcano stratigraphic study is to understand the historic relationship of each volcanoes in the South Bandung area. Methods used in this study are landsat analyses, fi eldwork, petrographic studies, and radiometric dating. Physiographically, South Bandung is composed of mountaineous area, hilly area, and high plain of Pangalengan and Bandung itself. Based on volcanic stratigraphy, volcanic rocks there are divided into eleven rock units, nine of them are identifi ed their volcanic sources, having Pliocene to Quaternary ages. The presence of subsurface Miocene volcanic rocks supports the super imposed volcanisms from Tertiary to Quaternary in this area. Mineral resources of sulphide metals are found in the central facies of Soreang, Kuda, and Dogdog volcanoes. Whereas, geologic hazards covering tectonic earthquakes, volcanic eruptions, and landslides also threaten this area. Mineral explorations and hazard mitigations are necessary to the presence of mineral resources and geologic hazard potential. Keywords: stratigraphy, volcanoes, South Bandung, sulphide metals, geologic hazards