Yahdi Zaim
Geology Department, Bandung Institute of Technology

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Dinamika pengendapan lahar permukaan pada alur-alur lembah di bagian selatan Gunung Api Merapi, Yogyakarta Mulyaningsih, Sri; Sampurno, Sampurno; Zaim, Yahdi; Puradimaja, Deny Juanda; Bronto, Sutikno
Indonesian Journal on Geoscience Vol 1, No 3 (2006)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | Full PDF (907.621 KB) | DOI: 10.17014/ijog.v1i3.15

Abstract

http://dx.doi.org/10.17014/ijog.vol1no3.20062Endapan aliran rombakan Gunung Api Merapi, yang lebih dikenal sebagai lahar, terbentuk dari hasil longsoran endapan awan panas yang dipicu oleh curah hujan yang sangat tinggi. Pada saat ini, endapan awan panas tersebut berasal dari guguran kubah lava. Material suspensi tersebut selanjutnya menuruni lereng dengan kecepatan yang tinggi, menghasilkan aliran turbulen. Aliran tersebut biasanya berkembang pada daerah dengan perbedaan morfologi berkemiringan lereng tinggi ke landai, atau yang sering dikenal sebagai daerah tekuk lereng. Studi ini didasarkan pada pengamatan dan pengukuran fragmen lahar yang berukuran besar di permukaan. Analisis meliputi arah penyirapan, bentuk, dan besar butir fragmen. Hasil penelitian mendapatkan model arah aliran fragmen besar lahar dari bagian atas aliran rombakan, yang membentuk “model punggung katak” atau “model punggung gajah”. Bagian depan katak atau gajah (kepala) yaitu arah aliran atau bagian depan aliran. Hasil penelitian juga menunjukkan bahwa model tersebut berlaku pada fragmen dengan diameter 90 cm atau lebih besar. Di daerah penelitian, fragmen dengan diameter 90 cm mencapai jarak hingga 22 km dari sumbernya. Hasil penelitian ini dapat digunakan sebagai model untuk menentukan arah aliran lahar (aliran rombakan) purba yang sumbernya belum diketahui.  
Perkembangan Geologi pada Kuarter Awal sampai Masa Sejarah di Dataran Yogyakarta Mulyaningsih, Sri; Sampurno, Sampurno; Zaim, Yahdi; Puradimaja, Deny Juanda; Bronto, Sutikno; Siregar, Darwin Alijasa
Indonesian Journal on Geoscience Vol 1, No 2 (2006)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | Full PDF (1562.85 KB) | DOI: 10.17014/ijog.v1i2.13

Abstract

http://dx.doi.org/10.17014/ijog.vol1no2.20065The 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.  
The Impact Of Coastal Line Development Of The Jambi Area During Late Pleistocene - Recent Time On Decline Of The Srivijaya Kingdom Prosperity Zaim, Yahdi; _, Aswan
AMERTA Vol 30, No 2 (2012)
Publisher : Pusat Penelitian Arkeologi Nasional

Show Abstract | Download Original | Original Source | Check in Google Scholar | Full PDF (3476.507 KB) | DOI: 10.24832/amt.v30i2.388

Abstract

Paleocoastal line analysis around Jawa indicates similar development to Sumatra, at least there are five paleocoastal lines that could be reconstructed due to periodically sea level raised periods along Late Pleistocene - Recent sea level fluctuations. Field studies both in Jawa and Sumatra which supported by laboratory analysis also show that coastal line evolution was shifted to the present seaward, which reveals the dropped sea level globally at the same time. In terms of Srivijaya Kingdom’s trading activity in Jambi area that predicted as river ports around the mouth of paleo-Batanghari River and as beach ports surrounding paleocoastal of Jambi, the paleocoastal lines evolution as mentioned above would gave a shallow effect in the upper reaches of the river. This shallowing up process due to the sedimentary deposition and dropped sea level would restrict the shipping activity for trading process. It means a new port, both beach and river ones as central tradingplaces have to be built and shifted also to the north-eastern part where a new coastal line formed and previous coastal lines became terrestrial. Movement of central trading activity in Srivijaya period to the northeast-outer area of Jambi will decrease or even put an end to economic activity in the new hinterland part of Srivijaya. Field geological investigation in Jambi area revealed that dropped sealevel, denudation, tectonic and sedimentation were most probably responsible to the inflation and declining of Srivijaya Kingdom until its last prosperity.