CHARACTERISTICS AND MORPHOMETRIC RELATIVE AGE OF SCORIA CONES, AND RELEVANCE TO THE GEOCHEMICAL COMPOSITION OF LAMONGAN VOLCANIC COMPLEX, EAST JAVA PROVINCE

• Main Author: Febriana Triastiningsih, Eka
• Format: Final Project
• Language: Indonesia
• Subjects: Teknik (Rekayasa, enjinering dan kegiatan berkaitan)
• Online Access: https://digilib.itb.ac.id/gdl/view/55694
• Institution: Institut Teknologi Bandung

Mount Lamongan Complex is located between Lumajang Regency and Probolinggo Regency, East Java Province. It is one of the active volcanoes in Indonesia. The Mount Lamongan Complex has unique geomorphological features of dozens maars and scoria cones. This study aims to describe the characteristics of the scoria cone, divide the morphometric age groups, and relate it to the geochemical characteristics of rocks in the Mount Lamongan Complex. The primary data used is the result of morphometric calculations using the Digital Elevation Model. While the secondary data are geochemistry which are oxide compounds, 87Sr/86Sr isotopes and anorthite compositions from previous studies. Morphometric age determination has been widely used to obtain basic information about the relative age of volcanic cone by analyzing standard the morphometric characteristics, such as the ratio of cone height divided by basal diameter width or (Hco/Wco )ratio and slope angle (Save and Smean). Theoretically, both morphometric parameters decrease with age. The Mount Lamongan Complex cones in the study amounted to 60 pieces which are located on the northeast, north, northwest, west, southwest and south sides. The mean basal cone diameter (Wco) was 608.15 m with a median of 493.75 m, and the mean height (Hco) was 66.3 m with a median of 52.25 m. The mean angle (Save) relatively decreases with increasing age of cone morphometry. Based on the ratio of Hco/Wco values, the cones were divided into 3 morphometric age groups and compared with volcanostratigraphy, including age group 3 (Hco = 0.179 Wco), age group 2 (Hco = 0.124 Wco) and age group 1. Age group 1 was divided into two parts. based on SiO2 plot data and geochemical composition to 1a and 1b at (Hco = 0.1 Wco). Rock affinity in the cone of Mount Lamongan Complex is in (medium - high K) calc alkaline to (high K) calc alkaline. Based on the Hco/Wco and SiO2 plots, it shows that the age division is concentrated into two parts, SiO2 < 50 wt.% and SiO2 > 50 wt.%. The effect of SiO2 values is that high SiO2 values have a wider rock age range, while at low SiO2 values the age range is narrower. This can be influenced by differences in the level of rock resistance. In addition, there is a time gap between the cones of age group 1 (old) and cones of age group 2 (young) which can be interpreted as a change in the period between Mount Tarub and Mount Lamongan. Changes in the geochemical composition of the old cones and the time interval between eruption periods indicate a magma differentiation process which is crystal fractionation in the cones of age group 1. In addition, the isotopic plots of 87Sr/86Sr vs SiO2 and changes in the composition of anorthite indicate magma differentiation in the form of crystal fractionation which is dominant in the age group 1 and crystal fractionation + assimilation/magma mixing were dominant in age group 2. In the cones of age group 3, the cone distribution approaches the center of the eruption of Mount Tarub-Lamongan and has more primitive composition. It is estimated that the distance of formation between cones of ages 2 and 3 is not far when viewed from the composition of rock and magma through a special path. The location near the center of the eruption can affect the temperature rise and the speed of cone formation so that the assimilation process is not dominant.