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Topography and structural changes of Anak Krakatau due to the December 2018 catastrophic events Herlan Darmawan; Bachtiar Wahyu Mutaqin; Wahyudi Wahyudi; Agung Harijoko; Haryo Edi Wibowo; Nia Haerani; Mamay Surmayadi; Syarifudin Syarifudin; Raditya Jati; Suratman Suratman; Wikanti Asriningrum
Indonesian Journal of Geography Vol 52, No 3 (2020): Indonesian Journal of Geography
Publisher : Faculty of Geography, Universitas Gadjah Mada

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.22146/ijg.53740

The flank collapse of Anak Krakatau on 22 December 2018 caused massive topography losses that generated a devastating tsunami in Sunda Strait, which then followed by eruptions that progressively changed the topography and structure of Anak Krakatau. Here, we investigated topography and structural changes due to the December 2018 flank collapse and the following eruptions by using high resolution Digital Elevation Model (DEM) before and after the events and sentinel 1A satellite image post-flank collapsed. Results show that the volumetric losses due to the 22 December 2018 flank collapsed is ~127 x 106 m3, while the following eruptions caused ~0,8 x 106 m3 losses. Structural investigation suggests two structures that may act as failure planes. The first structure is located at the western part of volcanic edifice that associated with hydrothermal alteration and the second failure is an old crater rim which delineated an actively deform volcanic cone.

Magma Evolution of Lasem and Senjong Volcanic Complex: High-K Magmatism in Sunda Arc, Indonesia Haryo Edi Wibowo; Agung Harijoko; Mradipta Lintang Alifcanta Moktikanana; Mohammad Yazid Abdillah
Indonesian Journal on Geoscience Vol 9, No 1 (2022)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.17014/ijog.9.1.131-145

DOI:10.17014/ijog.9.1.131-145Lasem and Senjong Volcanic Complex (LSVC) is one of four Quaternary volcanic complexes with high-K magmatism distributed in the northern coast of Central Java. This research aims to understand the magmatic evolution of the volcanic complex. Morphostratigraphy analysis and field observation show twelve pyroclastic density flow units in Lasem Volcano, distributed mainly in the northern to eastern flanks, but minor occurrences in the southern and western flanks. Meanwhile, nine lava flow units of Lasem are concentrated on the south flank of the volcano. Lasem stratovolcano is attributed to four lava domes distributed on the northern and southern flanks. Senjong Volcano comprises one lava flow and four lava domes. Rocks of LSVC are composed of plagioclase, K-feldspar, hornblende, clinopyroxene, and opaque minerals embedded in the groundmass of volcanic glass and microlite. These calc-alkaline rocks range from basaltic trachyandesite to trachyte in composition. The magma of LSVC can further be grouped into High K/Rb and Low K/Rb types, which dominate the Lasem and Senjong Volcano products, respectively. These two magma types consistently show divergent trends in K/Rb and Rb/Nb plots against increasing silica, indicating distinct differentiation processes of similar magma source.

Emplacement Temperature of the Overbank and Dilute-Detached Pyroclastic Density Currents of Merapi 5 November 2010 Events using Reflectance Analysis of Associated Charcoal Haryo Edi Wibowo; Anggun Purnama Edra; Agung Harijoko; Ferian Anggara
Journal of Applied Geology Vol 3, No 1 (2018)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Merapi eruption in 2010 produced 17 km high column of ash and southward pyroclastic density current (PDC). Based on the deposits characteristics and distributions, the PDC is divided into channel and overbank facies (pyroclastic flow), and associated diluted PDC (pyroclastic surge). The hot overbank PDCs and the associated dilute-detached PDCs are the main cause of high casualty (367 fatalities) in medial-distal area (5–16 km), especially near main valley of Kali Gendol. We reported the emplacement temperature of these two deposits using reflectance analysis of charcoal. We used both entombed charcoals in the overbank PDC and charcoals in singed house nearby. Samples were collected on 6–13 km distance southward from summit. Charcoalification temperatures of the entombed charcoals represent deposition temperature of the overbank PDCs, whereas those of charcoals in the singed house resembles temperature of the associated dilute-detached PDCs. Results show mean random reflectance (Ro%) values of entombed charcoal mainly range 1.1–1.9 correspond to temperature range 328–444 °C, whereas charcoal in singed house range 0.61–1.12 with estimated temperature range 304–358 °C. The new temperature data of the dilute-detached PDCs in the medial-distal area is crucial for assessing impact scenarios for exposed populations as it affects them lethally and destructively

Cooling history (from magma ascent to lava extrusion) of the Watuadeg pillow lava, Berbah, Yogyakarta, Indonesia Indranova Suhendro; Agung Harijoko; Nugroho Imam Setiawan; Haryo Edi Wibowo
Journal of Applied Geology Vol 8, No 1 (2023)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.22146/jag.73942

The Watuadeg pillow lava (WPL) is known as one of the most famous pillow lava outcrops in Yogyakarta, Indonesia, and its origin has been attributed to rapid-cooling process of subaqueous lava extrusion. However, there is no quantitative evidence that implies such hypotheses. Therefore, this study aims to reduce such a gap by revealing the cooling times (t) of WPL on the basis of a quantitative approach. In particular, we measured the size distribution (CSD) and number density (MND) of plagioclase microlites from the core, medial, and marginal (rim) domains of WPL. We found that the CSD slope significantly increases towards marginal zones, namely 30.4ᵒ for the core, 53.4ᵒ for the medial, and 228.1ᵒ for the rim. Because CSD slope is inversely proportional to cooling time ( ), by assuming a typical plagioclase microlite growth rate (G) of 1×107 mm/s, it is therefore inferred that the rim experienced the fastest cooling time (±12.1 hours), followed by the medial and core ((±52.0 and 91.4 hours, respectively). The fact that MNDs value increases toward the marginal zones also does not deny this idea (0.3×1016 m-3 for the core, 1.4×1016 m-3 for the medial, and 2.4×1016 m-3 for the rim), as higher MND with the domination of acicular-spherulitic habit represents a higher degree of undercooling. Because microlite is syn-eruptive product, our estimation represents the cooling time of magma since it migrated from the reservoir to the surface.

Magma Evolution of Lasem and Senjong Volcanic Complex: High-K Magmatism in Sunda Arc, Indonesia Haryo Edi Wibowo; Agung Harijoko; Mradipta Lintang Alifcanta Moktikanana; Mohammad Yazid Abdillah
Indonesian Journal on Geoscience Vol. 9 No. 1 (2022)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.17014/ijog.9.1.131-145

DOI:10.17014/ijog.9.1.131-145Lasem and Senjong Volcanic Complex (LSVC) is one of four Quaternary volcanic complexes with high-K magmatism distributed in the northern coast of Central Java. This research aims to understand the magmatic evolution of the volcanic complex. Morphostratigraphy analysis and field observation show twelve pyroclastic density flow units in Lasem Volcano, distributed mainly in the northern to eastern flanks, but minor occurrences in the southern and western flanks. Meanwhile, nine lava flow units of Lasem are concentrated on the south flank of the volcano. Lasem stratovolcano is attributed to four lava domes distributed on the northern and southern flanks. Senjong Volcano comprises one lava flow and four lava domes. Rocks of LSVC are composed of plagioclase, K-feldspar, hornblende, clinopyroxene, and opaque minerals embedded in the groundmass of volcanic glass and microlite. These calc-alkaline rocks range from basaltic trachyandesite to trachyte in composition. The magma of LSVC can further be grouped into High K/Rb and Low K/Rb types, which dominate the Lasem and Senjong Volcano products, respectively. These two magma types consistently show divergent trends in K/Rb and Rb/Nb plots against increasing silica, indicating distinct differentiation processes of similar magma source.

Source Determination of Debris Avalanche Deposit based on the Morphology and Distribution of Hummocky Hills on the Northeastern Flank of G. Sundoro and G. Sumbing, Central Java, Indonesia Rahayu, Eti; Wibowo, Haryo Edi; Moktikanana, Mradipta Lintang Alifcanta; Setianto, Agung; Harijoko, Agung
Indonesian Journal of Geography Vol 55, No 3 (2023): Indonesian Journal of Geography
Publisher : Faculty of Geography, Universitas Gadjah Mada

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.22146/ijg.88213

The presence of hummocky hills as a typical product of debris avalanche deposits is prominently visible in the northeastern flank of G. Sundoro and G. Sumbing, Temanggung, Central Java. In an attempt to better understand the past behavior of both G. Sundoro and G. Sumbing, we identify the source of the debris avalanche deposit. Interpretation is performed on the basis of the assumption of two possible sector collapse sources, i.e., G. Sundoro and G. Sumbing. The Sumbing source scenario is assumed as freely spreading type considering 1) distribution of the hummocky hills are relatively on the northeastern flank of the volcano, and 2) the present crater structure on the summit of the volcano which is opening to the northeast. The Sundoro source scenario is assumed as valley-filling type considering the distribution of the hummocky hills are relatively on the eastern flank of Sundoro extended to the far distal area and bounded by older high topography of G. Sumbing and North Serayu Mountains. The source identification was done on the basis of field observation of the deposit lithological characteristics combined with image analysis, including hummocky hills morphometry, displacement angle, and spatial distribution. Image analysis identifies approximately 645 hummocky hills ranging from 1,851 mz to 623,828 mz and average of 23,482 m2. Petrographic analysis of 5 representative block lava samples shows variation of olivine basalt, pyroxene andesite, to hornblende andesite. The results show that big size hummocky hills dominate the western side, while small size on the east. Displacement angle varied following the valley orientation with typical downslope topography. These suggested that the hummocky hills were originated from G. Sundoro as a valley-filling debris avalanche deposit.

Volume Estimation of the Thickest Scoriaceous Tephra-Fall Deposits on the South-Southeastern Flank of Mt. Raung Wibowo, Haryo Edi; Harijoko, Agung; Cahyani, Sherinna Mega; Moktikanana, Mradipta Lintang Alifcanta; Prawira Sari, Shafa Hadaina
Journal of Applied Geology Vol 8, No 2 (2023)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.22146/jag.80866

Thick scoriaceous tephra-fall deposits are widely distributed in the south to the southeast flank of Mt. Raung, indicating the existence of past large explosive eruptions. The deposits are relatively young as the deposits are situated near the surface. Scoriaceous tephra-fall deposits can be divided into four layers from bottom to top, Scoria Fall 1, Scoria Fall 2, Scoria Fall 3, and Scoria Fall 4. There is no time gap between these layers, as evidenced by the deposits not being separated by any weathered layer or soil, suggesting that the deposits represent an eruptive product of a single active period. We estimated the volume of the deposits using isopach maps following Weibull method to identify the magnitude of the eruption. We limited the estimation only to Scoria Fall 2 and Scoria Fall 3 deposits which were consistently exposed on 13 and 9 observation points, respectively. The volume of Scoria Fall 2 is ~0.54 km3 and Scoria Fall 3 is ~0.26 km3 making the total volume of 0.8 km3 (VEI 4).

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