MORPHOMETRY STUDY AND THEIR MECHANISM OF COLUMNAR JOINT AT MENGANTI BEACH, CENTRAL JAVA
Columnar joint which located at coastline of Menganti beach Central Java, is one of the ideal column morphology consisting of colonnade fasies and interpreted was located between hyaloclastite. Research area is included in the Gabon Fm., Andesite intrusion, Kalipucang Fm., and alluvium deposits. Sev...
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| Format: | Theses |
| Language: | Indonesia |
| Online Access: | https://digilib.itb.ac.id/gdl/view/48071 |
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| Institution: | Institut Teknologi Bandung |
| Language: | Indonesia |
| Summary: | Columnar joint which located at coastline of Menganti beach Central Java, is one of the ideal column morphology consisting of colonnade fasies and interpreted was located between hyaloclastite. Research area is included in the Gabon Fm., Andesite intrusion, Kalipucang Fm., and alluvium deposits. Several previous studies have discussed about petrogenesis and rocks chemistry. This research will examine about morphometry study of columnar joint related to their mechanism of columnar forming, maturity of columnar order, and columnar orientation. The various order and orientation of columnar joint are thought to be influenced by paleoslope and rapid cooling rate. Moreover, this study also examine the relation of rock chemistry, magma characteristic, and tectonic setting of lava to columnar joint forming.
Columnar joints of Menganti beach have been studied with measurement of column dimensions at six localities. The aim of measurement is to determine hexagonality index (Xn) of columnar joint. The analyzed 19 thin sections of sample to observe textures of thin section, mineral texture, and mineral composition. Hereafter, three samples were selected for X-Ray Fluuorescene (XRF) and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) to determine rocks classification, magma affinity, and tectonic setting of lava. The same samples were also measured the distribution of plagioclase size using crystal size distributions (CSDs) to obtain residence time and cooling rate.
Based on filed observation, the orientation of columnar joint is vertical, oblique, and horizontal. Based on the petrographic analysis, the lava composed of olivine, pyroxene, and plagioclase, within microlite groundmass of plagioclase, pyroxene, and volcanic glass. Plagioclase phenocryst textures consist of sieve, glomerocryst, and sinneusis. Based on CSD analysis indicates an increasing magma residence time. The crystallization time of ST4 is 2 months, 3 months for ST3, and 4 months for ST6. The crystallization time of magma is linear with the cooling rate of magma that is ST4 2.2x10?8 ????/????, ST3 1.9x10?8????/????, ST6 1.6x10?8????/????, the cooling rates of magma decreases when the crystallization time increases.
Based on analysis of morphometry, columnar joint at Menganti Beach have high hexagonality index (Xn) which is containing 4, 5,6, and 7–sided columns, and dominated by 5-sided columns. The high Xn indicates high cooling rate that reflects thinner lava flow and have narrow striae. The columnar joint orientation at Menganti Beach affected by different paleoslope that reflected by high internal angle. The maturity of columnar joint at Menganti Beach affected by crystallizing time of magma, cooling rate, and paleoslope that reflected on hexagonality index and relative standard deviations of columnar dimension. The geochemistry analysis represent basalt with tholeiitic affinity, and island arc tectonic setting.
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