ANALISIS SIFAT OPTIS LAVA BASALT DAERAH KARANGKULON DAN SEKITARNYA KECAMATAN TEGALREJO KABUPATEN MAGELANG PROVINSI JAWA TENGAH

This study was conducted in the Karangkulon area and its surroundings, Tegalrejo District, Magelang Regency, Central Java Province, with coordinates 07°30'00" - 07°25'00" S and 110°15'00" - 110°20'30" E. The location was chosen due to the comprehensive exposure of basalt lava, which facilitates sample collection and analysis. This research aims to determine the mineral composition and identify and compare the types of minerals in basalt lava through petrological analysis. The methods employed include surface geological mapping through direct observation, supported by laboratory analysis and literature review. Laboratory analysis of three basalt lava samples revealed mineral structure and size variations. Sample 1 cooled rapidly and exhibited a scoria structure with small crystals. Sample 2 showed a vesicular structure and more uniform crystallization, while Sample 3 displayed larger crystals with an intergranular texture. These findings indicate different formation times among the three samples. Mineral composition analysis suggests a complex formation process at the study site, including rapid plagioclase crystallization during basaltic magma cooling. Magma differentiation separates minerals based on temperature and their mineral content. The presence of pyroxene and olivine indicates high temperature and pressure conditions during formation. The groundmass containing quartz microlites, feldspar, and volcanic glass shows rapid post-eruptive cooling, forming microcrystalline and amorphous structures in the basalt rock.