The Truth about Paroxysmal Merapi Volcano s Eruption from Geomorphology of Southwestern Merapi s Fluviovolcanic Plain

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1 Proceeding International Geomorphologist Seminar Yogyakarta, Indonesia, 2008 The Truth about Paroxysmal Merapi Volcano s Eruption from Geomorphology of Southwestern Merapi s Fluviovolcanic Plain Alva Kurniawan 1 Abstract The Bemmelen s statement about Merapi s volcano paroxysmal eruption at 1006 A.D. was a polemic till right now. Bemmelen stated that Merapi Volcano erupted explosively at 1006 A.D. There are many Indonesian scientists belief to Bemmelen s statement because it based on long geologic survey and geomorphologic analysis. The further study about this topic needed due to make a right conclusion about any study about Merapi Volcano and Geomorphology of southwestern Merapi Volcano s slope. The research objects are anticlines in southwestern of Merapi s fluviovolcanic plain such as Gendol hills, Gunungsari hill, and Gunungpring hill. The methods of the research are based on geomorphological and cultural approach. The objects located in Muntilan District, Salam District, Ngluwar District, Magelang Regency, Central Java, Indonesia. Analysis from geomorphological approach was done from map interpretation of RBI Map 1:25.000, Muntilan sheet, with number sheet ; map interpretation of Geologic Map 1: , Yogyakarta sheet, with number sheet and ; field geomorphologic survey, and also studying journals and reference books. Bemmelen s statement about Merapi s Volcano paroxysmal eruption in 1006 A.D. couldn t be proved. Based on the analysis result, the eruption never happened in 1006 A.D. even the Merapi s Volcano never erupts in paroxysmal level. The conclusion from geomorphologic analysis is also wrong, Bemmelen explained that the hills in southwestern Merapi Volcano are an anticline that formed by southwestern Merapi Volcano s slope slid but unfortunately the dip and strike of the hills are not match with the hill s forming process. Further study about this topic is still needed to calibrate the result of this research. Keywords : Bemmelen s statement, Merapi Volcano, paroxysmal, eruption, geomorphologic, southwestern slope of Merapi Volcano. 1 Department of Environmental Geography, Gadjah Mada University, alva_the_a@yahoo.co.id. 1

2 Proceeding International Geomorphologist Seminar Yogyakarta, Indonesia, Introduction The year of 2006 A.D. was commemorated as the first millennium of paroxysmal Merapi Volcano s eruption. In the year of 2006 A.D., Merapi Volcano showed its activities after hibernated for some years. Most of volcanologists worried about the Merapi Volcano s activity. They thought that Merapi Volcano could erupt explosively after hibernated for some years. Their way of think based on Bemmelen s statement about the paroxysmal Merapi eruption that wrote in a famous reference book, The Geology of Indonesia. Bemmelen stated that proves of the paroxysmal eruption was lied in the anticlines that formed at the southwestern of Merapi s fluviovulcanic plain. Bemmelen stated that the forming process of the anticlines had a strong relation with geomorphologic change of Merapi Volcano. Unfortunately the prove that Bemmelen told not match with geomorphologic survey Study Scope Regional study scope concentrates in southwestern anticlines of Merapi Volcano s fluviofulcanic plain such as Gendol hills, Gunungsari hill, and Gunungpring hill. Administratively the regional study scope is located at Central Java Province, Magelang Regency, Muntilan, and Salam. Object study scopes include Bemmelen s Statement about paroxysmal Merapi Volcano s Eruption in 1006 A.D., and southwestern anticlines physiographic of Merapi Volcano s fluviovulcanic plain Purposes The purposes of why this topic was chosen and why this paper was wrote such as: Figure 1. Gendols Hills from Gunungsari Anticline. a. it s important to correct a wrong statement and give the true fact about Merapi Volcano s eruption characteristics in order to make the next research about Merapi Volcano not wrong; b. this paper could give some recommendation ideas in disaster mitigation of the eruption Merapi Volcano; of c. this paper told facts about paleogeomorphic process that happen in southwestern Page 2

3 fluviovolcanic plain of Merapi Volcano; d. this paper could be a reference to the next study about Merapi Volcano s eruption characteristics; 2. Methods The methods in writing this paper are map interpretation of RBI Map 1:25.000, Muntilan sheet, with number sheet ; map interpretation of Geologic Map 1: , Yogyakarta sheet, with number sheet and ; field geomorphologic survey, and also studying journals and reference books about Merapi Volcano. Interpretation of both of Geologic and RBI maps were done to know about the geomorphologic condition of Gendol hills, Gunungsari hill, and Gunungpring hill. about geologic structural in each anticline. The surveys were done by strike measurement with Geologic Compass, and rocks description in each anticline. The surveys are important to know about the strike of the anticlines and to know about the anticlines composite materials which have been generalized in the geologic map. Studying references and journals that tell about Merapi Volcano is important to know about its eruption characteristic. Knowledge in Merapi volcano s eruption characteristic will be correlated with Bemmelen s statement about the paroxysmal Merapi Volcano s eruption. Figure 3. Gendols Hills in RBI Map Muntilan Sheet Figure 2. Gendols Hills in Geologic Map Yogyakarta Sheet Field geomorphologic surveys were done in all anticlines to extract data 3. Geomorphology of Southwestern of Merapi s Fluviovolcanic Plain Southwestern of Merapi s Fluviovolcanic Plain consist of some different landforms. Within the main landform of fluviovolcanic plain there are landforms that formed by different processes. There are Kaliputih River and Page 3

4 Blongkeng River that formed natural levee, flood plain, depositional floor, back swamp, cut bank, point bar deposit, bar deposit, depositional terrace. Kaliputih and Blongkeng River are cooperating with Merapi s volcanic activity that provides avalanche materials, forming Merapi s Southwestern Fluviovolcanic Plain. There are strange landforms in the fluviovolcanic plain. A sequence of anticlines presents in the fluviovolcanic plain. Their genesis till now is still mysterious things. None of scientist can say for a sure, what kind of process that formed them. The anticlines formed an arc from the southeast to the northwest. The anticlines also formed a strange formation. At the most southeast antcline that locally named Gunungwukir, it separated from other anticlines. The anticlines near Gunungwukir are connected each other and they locally named Gendol hills, which consist of Gununglempong Anticline, Gununggendol Anticline, Gunungdepok Anticline, Gunungpuguhan Anticline, Gunungtegal Anticline, and Gunungtugel Anticline. On the northwest of them there are Gunungsari Anticline and Gunungpring Anticline that far separated each other by Blongkeng River and Kaliputih River. The famous scientist that explained about their genesis is Bemmelen. Bemmelen stated that the anticline formed by paroxysmal eruption of Merapi s Volcano or local scientist said a horseshoe eruption of Merapi Volcano. The eruption was greatly made a quake on the southwestern slope of Merapi s Volcano, then it result in a slid of the southwestern slope of Merapi Volcano. The materials that slid to the southwestern were hit the basement rock of Menoreh hills. It resulted in a fold in the contact zone between Merapi s slope and the Menoreh hills basement rock. Bemmelen also stated that it happen in around 1006 A.D., when ancient Mataram Kingdom s citizen still exist in there. Bemmelen also related that the movement of ancient Mataram Kingdom s citizen due to the paroxysmal eruption of Merapi Volcano. Figure 4. Folded Breccias in Gunungsari Anticlines. The composite materials of the landforms in the fluviovulcanic plain are almost same except the composite materials that form the anticlines. The materials that formed most of landforms in the fluviovolcanic plain mainly consist of Page 4

5 Quaternary alluvium sediment of young Merapi Volcano. The red soil can t be said as coaling soil because the rocks within the soil didn t appear to be morphed. The minerals of the rock sample within the soil seem like never heating since they froze firstly. In one of the outcrops there is a folded breccias formation. The measurement in folded breccias formation result that it has strike about N 60 E, and dip about 40. Figure 5. Tephra and lappilus deposit in Gendol Anticline. Differentially materials that formed the anticlines consist of breccias, agglomerate, tephra and lappilus deposit, lava avalanche, andesite, and olivine basalt. They are Quaternary older than the materials around. In some place there are some outcrops that shown the geologic structure of the anticlines and also the rocks that consisted anticlines. In one of the outcrops, the soil has red color like a soil from the result of coaling process. Figure 6. Red soil in Gunungsari Anticline. Figure 7. Dip and strike measurement in Gunungsari Anticline 4. Result and Discussion Merapi Volcano didn t erupt explosively in 1006 A.D. even it never erupts explosively since thousand years ago. That statement has one big evidence which clearly visible. The evidence is there no trace of Merapi Volcano s paroxysmal eruption left. Every great volcanic explosion in this world left a clear trace although it was happened thousand or even million years ago. There are some volcanoes in this world that ever erupted greatly like Toba Volcano, Krakatoa Page 5

6 Volcano, Tambora Volcano, Vesuvius Volcano, Mount St. Hellens Volcano, Yellow Stone Volcano, etc. Toba Volcano left evidence for its great eruption in Toba Lake. Toba Lake was formed from collapsed batholiths after Toba Volcano throw up all of the magma inside its batholiths. Krakatoa Volcano and Tambora Volcano left evidence for their great eruption in a caldera. Unlike some volcano mentioned before, Mount St. Hellens Volcano and Vesuvius Volcano left evidence for their great eruption in very thick of volcanic material sediment that deposited around them. Yellow Stone Volcano is a volcano that supposed ever erupt in highest power eruption in the world that left a hot spring known as Yellow Stone National Park. Merapi Volcano didn t left any clear trace about its great eruption. There is no thick volcanic material deposit around Merapi Volcano. In some places around Merapi Volcano moderately thick volcanic materials buried some temple around Merapi Volcano s slope but from radio dating that was done by Mulyaningsih (2006), the materials did not originate from a single eruption but from multiple eruption which occurred in different years. Zen also stated that Indonesian Volcano s great eruption often left a clear trace like volcano tectonic depression that contains highly rhyolite ash flow and pumestonetuff (Zen, 2006; Kurniawan, 2007). Bemmelen s statement about anticlines genesis was so reasonable because the anticlines formation like an arc that curved to the southwestern slope of Merapi Volcano direction. Bemmelen s statement was also reasonable because mainly composite materials of the anticlines and the older volcanic cone of Merapi Volcano were same. There is a strange thing when looking to the anticlines formation. There are some anticlines that were connected each other and also there are some anticlines that were separated each other. If the Bemmelen s statement true, the anticlines should be connected each other and if there any separated anticline, it must not be separated far away. By looking for geomorphology of the southwestern slope of Merapi Volcano, it seems that Blongkeng River can t flow through Gunungsari Anticline and just flow around it. That thing can be concluded as the river flow can t flow through barrier. Gunungsari Anticline could become a water flow barrier if there are a massively rock structure that impermeable with water. Back to the Bemmelen s statement that the anticline formed by folding of Merapi Volcano s southwestern slope materials, Bemmelen indirectly said that the materials that formed the anticlines is Page 6

7 not a massive materials but a mixture of consolidated and unconsolidated materials. Logically those materials can be eroded by stream flow easily especially when the stream on flood condition, but as the fact Gunungsari Anticline can t be eroded by Blongkeng River flow. Figure 8. Bemmelen s strike direction estimation on the anticline. anticlines have strike about N 60 E, and dip about 40. Bemmelen explanation about their genesis should show strike about N 315 E not N 60 E. The strike about N 60 E gives an imagination that the anticline folded up by a power from N 150 E direction, and that direction is not in line with Merapi Volcano s southwestern slope that has direction from the anticline about N 45 E. Further research about this topic is needed in order to calibrate the result of this research. There are still many mysteries in this research that need to be discovered such as the power that formed the anticlines, what kind of processes that actually build the anticlines, and when the anticlines were built. Figure 9. True strike direction based on geological survey on the anticline. Field geomorphologic survey which was done on the anticlines; give different truth about their genesis. The 5. Conclusions Merapi Volcano never erupts explosively because there is no trace that proofs that event. Bemmelen s statement about the eruption related with anticlines on southwestern fluviovolcanic plain of Merapi Volcano is not match with the strike direction of the anticlines rock structure. The geomorphology of Merapi Volcano s southwestern fluviovolcanic plain can be correlated with Merapi Volcano s activity except the anticlines formation. The anticlines formation must be formed by a geologic event that happen Page 7

8 a long time ago and can t be correlated with Merapi Volcano s activity. 6. References Andreastuti, S.D Stratigraphy and Geochemistry of Merapi Volcano, Central Java, Indonesia. Implication for Assessment of Volcanic Hazards. Andreastuti, S.D Did A Large Eruption of Merapi Occure in 1006 AD?. Volcano International Gathering, Yogyakarta. Lipman, Peter W The 1980 Eruption of Mount St. Hellens, Washington. Washington DC : United States Government Printing Office. Mason, Ben G The Size and Frequency of The Largest Explosive Eruptions on Earth. Bulletin of Volcanology. Mulyaningsih, Sri Very Old and Younger Temple Discoveries in Yogyakarta Area: Based on Volcano- Stratigraphic Study. Volcano International Gathering, Yogyakarta. Newhall, Christopher G The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism. Journal of Geophysical Research. Brotopuspito, Kurbani Sri Merapi Volcano Inspires Scientific Curiosity. Volcano International Gathering, Yogyakarta. Kurniawan, Alva Erupsi Paroxysmal Gunungapi Merapi Tahun 1006 Masehi. Yogyakarta : Research and Development of Geoscience and Environmental Matter. Kusumadinata, K Data Dasar Gunungapi Indonesia. Jakarta : Direktorat Vulkanologi. Ollier, Cliff Volanoes. Massachusetts : The MIT Press. Ratmodipurbo, A Evolusi 100 Tahun Morfologi Gunung Merapi. BPPTK Siefferman, R.G An Ecosystem Under Acid Rain at Merapi Volcano in Central Java, Indonesia. Van Bemmelen, R.W The Geology of Indonesia (Second Edition). The Hague : Martinus Nijhoff. Voight, B Journal of Vulcanology and Geothermal Research, Special Issue Merapi Volcano. Zen, M.T Merapi : Dishtung und Wahreit. Volcano International Gathering, Yogyakarta. Page 8

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