Faktor Pembentukan dan Karakteristik Kekar Tiang di Desa Sembowo, Kecamatan Klepu, Kabupaten Pacitan, Jawa Timur, Indonesia

Abstrak

The columnar joints structure in Sembowo Village, Pacitan District, East Java is composed of andesite lithology with a mineralogical composition dominated by pyroxene and plagioclase phenocrysts with a cryptocrystalline and glass matrix, but olivine is not found. On the other hand, there are altered minerals in the form of chlorite in lithological samples which show alterations in the lithology. The presence of a very fine matrix (<1mm) indicates the rapid freezing of magma. The percentage of phenocryst presence in the lithology sample is about <40% and the pilotaxitic texture also indicates that the columnar joint is composed of extrusive lava flows. In terms of outcrops, the columnar joints in this research area have characteristics such as a relatively small diameter of about 10-25 cm, dominantly having an irregular fracture shape, and the height of columnar joints reaches ±45 meters. The thickness of the lava, which is quite thick and consists of only one unit of colonnade, is classified as having a lava flow of the single unit flow (SUF) type. Therefore, the formation of columnar joints at the research site is caused by two main factors: the geometry of the magma body in the form of a lava flow and the chemical composition of the constituent lithology, which is intermediate magma.