International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com
Erupsion Center Determination of Ancient Volcano Based on Geomagnetic Method around Gunungkidul Area, Daerah Istimewa Yogyakarta, Indonesia. Agus Santoso Ph. D. Student, Physics Department, Gadjah Mada University, Indonesia. Geophysical Engineering, UPN "Veteran" University, Yogyakarta, Indonesia.
Sismanto Physics Department, FMIPA Gadjah Mada University, Indonesia. Ary Setiawan Physics Department, FMIPA Gadjah Mada University, Indonesia. Subagyo Pramumijoyo Geology engineering Gadjah Mada University, Indonesia. Abstract The center eruption of an ancient volcano eruption in Gunungkidul can be known from the field data like material products of ancient volcanic produced in the past, it is important to understand the ancient volcanic material elements by studying the geology of the region concerned, to prove the existence the center eruption of ancient volcano by using geomagnetic method. Field data that are found are as follows: there are existence of 1. Rocks of Pyroclastic flow 2. Pyroclastic fallout ("Pyroclastic Fall") like Tuff and lapilli tuff 3. breccia, tuffaceous sandstone and tuff breccia. 4. Pillow Lava in Berbah area the area that shows the lava sedimentation in the sea. 5. igneous rocks with scoria structure which is an volcanic extrusion in Parang Kusumo. 6. Lava which is the result of the eruption / volcanic eruptions. 7. Parangwedang hot springs near Parangtritis.
Introduction
Geomagnetic method [1] is one of the methods in geophysics that utilize magnetic properties of the earth. By using this method the suceptibility distribution of rock will be obtained in the contour map with horizontal direction. From the susceptibiliti’s value then it can be further localized / separated rocks which containing magnetic properties and not. The equipment that used in this method is Precission Proton Magnetometer (PPM) to measure the total magnetic field strength value. Other equipment that supporting the magnetic survey is Global Positioning System (GPS). This equipment used to measure the position of the measurement point which includes the longitude, latitude, altitude, and time. In determining the position of a point location it is use satellite assistance. There are 1600 Numbers of measurement data in this research. Geomagnetic data interpretation [2] by using the magblox program is based on data and magnetic field anomali’s pattern contours that originating from the distribution of objects magnetized or geological structures below the earth's surface. From the interpretation of research there is found the caldera surrounding the center of the eruption of an ancient volcano that had been active under Miocene. The magma chamber’s depth is 14. 3 Km [3].
Geomagnetic method mapped local disturbances in the Earth's magnetic field caused by variations in rock magnetism. This method is the oldest geophysical method. This method is often used for monitoring (monitoring) volcano The Results of this research is come from the interpretation of "Magblock" program, it can be concluded that in the past there is an ancient volcano eruption in the waterfront that has several tectonic processes of under Miocene. The eruption shape caldera with magma chamber’s depth of 1 4. 3 km below the surface. Keywords: Ancient volcanik, central eruption, geomagnetic, magblock, magma chamber’s depth of 14. 7 km
Research area
Figure 1: Location of research areas in Indonesia
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com M = p l r1 = M r13 3 M is a unit vector in the direction of the vector r 1 from negative to positive pole.
General Geology Southern Mountain zone [4] can be divided into three subzona, namely Subzona Baturagung, Subzona Wonosari and Subzona Gunung Sewu [4]. Subzona Baturagung mainly located in the northern part, but it stretches from the west (heights of G. Sudimoro, ± 507 m, between Imogiri and Patuk), in the north (G. Baturagung, ± 828 m), to the east (G. Gajahmungkur, ± 737 m). In the east, Subzona Baturagung formed altitude slightly apart, namely G. Panggung (± 706 m) and G. Gajahmungkur (± 737 m). Baturagung Subzona form crudest relief with slope angles between 10-30 and difference heitght from 200 till 700 m and is almost entirely composed of rocks of volcanic origin. Subzona Wonosari is a plateau (± 190 m) located in the middle of South Mountains Zone, namely in Wonosari and the surrounding area. This plateau is restricted by Subzona Baturagung in west and north, while in the south and east restricted by Subzona Mount Sewu [5]. The major river flow in this area is Kali Oyo which is flow to the west and fused with Kali Opak. The surface sediment in this area is the black clay and ancient lake, while the bed rock is a limestone. Subzona Mount Sewu is a hils with karts landscape, that is landscape with limestones hills forming many cones with several tens of meters height. Among these hills lake and sink holes are found and in the subsurface there are limestone caves and underground streams. These karts landscape stretches from Parangtritis beach in the west to Pacitan in the east [6].
The Magnetism Intensity Magnetic objects can be seen as a set of magnetic moments. When the magnetic objects are placed in the external field, the object becomes magnetized due to induction. Therefore, the magnetization intensity (I) is the ability of magnetic moment to have the same direction an external magnetic field, or defined as magnetic moment per unit volume: I=M/V 4 Practically induced magnetization is mostly straighten dipoledipole magnetic material, so it is often referred to as a magnetic polarization. If the magnitude is constant with the same direction, then it is said to be uniform magnetized objects. Suceptibility of Magnetism The ability of a magnetic object [9] to be magnetized is determined by magnetic susceptibility or k, it is can be written as:
I kH
5 It is dimensionless quantity and basic parameters used in the magnetic method. K value in the rock is bigger when the rock has many magnetic minerals. Factors affecting k value of the rock are : 1. lithology 2. The mineral content of rocks
Basic Theory .
Magnetic Force In magnetism [7] there are two types of cargo, namely a positive charge and a negative magnetic pols. Both of them answer the Coloumb law. Magnetic poles which is different type will pull each others while similar will repel with force F. The basis of the method is the magnetic force between the two magnetic poles represented by Coloumb Law between two magnetic poles m 1 and m 2 that are separated by a distance the distance r (meters).
F
Start Geology information
Design survey
Data Acquisition
Daily and IGRF Correction
m1m2 rˆ r 2
Total Magnetic Anomaly map
1 Description : μ is the magnetic permeability. The magnetic permeability in a vacuum is 4 x 10-7 w / Am, F is the force of Coloumb (N), m 1 and m 2 are strong magnetic poles (A / m) and r is the distance between the two poles (m).
Magblox (3D) Program
no
yes Reduction to Pole
Magnetic Field Magnetic field [8] is the magnitude of the magnetic field at a point in space becase the existance of the magnetic poles with a certain distance from the point. Magnetic field H is defined as the force per unit time and can be written as
H
Interpretation
Finish .
F m 12 rˆ r m2
Figure 2. Flow diagram of data processing
2 Magnetic Moment When two opposite magnetic poles have strong magnetic poles + p and-p, and both of them separated with the distance l, the magnetic moment M can be written as:
Interpretation In general, the geomagnetic data interpretation is divided into two, namely the qualitative and quantitative interpretation.
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com Qualitative interpretation is based on the pattern of the magnetic field anomaly contour derived from the distribution of magnetized objects or geological structures below in the subsurface. Quantitative Interpretation aims to determine the form or model and experience anomalous object height or geological structure through mathematical modeling. To perform quantitative interpretation, there are several ways in which between one and another may be different, depend on the shape anomaly obtained, purpose achieved and the accuracy of measurement results. Quantitative Interpretation
Figure 4a: The depth of 0. 3 Km
2. At a depth of 1. 8 km, the intrusion and differentiation of igneous continues generate andesite, and susceptibility from 10000 to 30000 SI. Ocean depth is about 2-2. 5 km located in the southern coastal area stretching from west to east (Parangtritis-Baron-Sadeng). Figure 2: Magnetic intensity image map in Gunungkidul area
Figure 4: 3D magblox programming result in Gunungkidul area
Figure 4b: The depth of 1. 8 Km
3. At a depth of 3. 5 km, intrusion and continued differentiation produce basaltic andesite, and susceptibility 20000-30000 SI.
Magblox programming result is as follows [10] ; process that occurs in the base of magma chamber to the surface is: 1. At the depth of 0 to 0. 2 km, the morphology of this intrusion is the central circular area in Wonosari, the edge of this intrusion is Sudimoro mount, Nglanggran mount, Baturagung mount, Ponjong mount and Wediombo Beach, patterned circle.
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com
Figure 4c: The depth of 3. 5 Km
Figure 4e: The depth of 9, 7 Km
4. At a depth of 5. 6 km, intrusion and differentiation continued produce basaltic andesite, and suseptibility from 30000 to 40000 SI.
6. The maximum depth of the magma chamber is in 14. 1 km. It was apparent in the location indicated by suceptibility > SI 50000
Figure 4d: The depth of 5, 6 Km Figure 4f:The depth of 14. 1 Km 5. There was a process of differentiation of magma at the depth of 9. 7 km, resulting in crystallization and magma starts to rise to break through the surrounding rock, basaltic magma differentiation initial yield andesitic basalt rocks, and susceptibility from 40000 to 50000 SI
Qualitative Interpretation 1. At the under Miocene to Middle Miocene there was great eruption of the volcano which is located on the edge of the ocean [11]
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com 4.
Figure 5a: Early-Middle Meocene : Large Eruption volcano in Gunungkidul
Upper Miocene-Pliocene: Growing coral reefs forming Wonosari Formation
Figure 5d: Upper Meocene-Pleocene : Coral reef (limestone) was growing up formed Wonosari Formation
2. In the Middle Miocene: [12] eruption resulted subsidence and formed the caldera with circular pattern
5. Pliocene-Pleistocene: Occurs Regression or shrinkage of the sea so that the area of Gunungkidul outcrop
Figure 5e: Pliocene-Pleistocene : regression followed by intrusion around the edges of the caldera Figure 5b: Middle Meocene : Caldera formed by the eruption 6. Holocene-now: Erosion and denudation resulted in the formation of the present Wonosari Formation rocks or coral reef.
3. In the Upper Miocene: tectonics occurred which resulted transgression which produce sea water flood so that the area of Gunungkidul was sink.
Explanation Limestone/Reef (Wonosari Formation) Pyroclastic rocks : Breccia, Sandstone, Tuff (Semilir Formation & Nglanggran Formation) Magma
Figure 5c: Upper Meocene; Transgression down bellow sea level
Volcanic bedrock Oceanic floor
Figure 5f: Holocene-Erossion and denudation the formation of current karst topography now
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 4264-4269 © Research India Publications. http://www.ripublication.com
Conclusion 1. Morphology caldera circular formed with a radius of 1 3-1 8 km, with plains Wonosari which is a graben with limestone surrounded by igneous intrusions on the edge of the caldera. 2 Gunung Sudimoro, Mount Nglanggran is the edge of the caldera, form a circular until Baturagung and the mountain areas in the eastern part of the region of Ponjong and Wediombo. 3 From 3D programs successfully found ancient magma in the middle of the caldera with t depth of 14. 3 km 4. The existence of an ancient volcano which is an igneous intrusion of andesite which form straightness (lineament) between G. Sudimoro and G Nglanggeran intrusion interconnected to form an air gap direction northwestsoutheast, forming High morphology. 5. The low Susceptibility in the central area is limestone and estimated to be above of the Volcanic Breccia
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