The Gravity Anomaly of Ungaran Volcano, Indonesia : Analysis and Interpretation

Abstract

Ungaran Volcano is located in the province of Central Java, Indonesia, and is a Quaternary volcano that consists of older and younger volcanoes. The older Ungaran volcano formed over 500,000 years ago, and the younger volcano was active until 300,000 years ago. The younger volcano seems to have been constructed inside a caldera formed by the older Ungaran activity. In this study, gravity data was used in an attempt to determine the exact location of the younger and older Ungaran volcanoes, and to investigate the relationship between fault structure and geothermal manifestations. A positive Bouguer anomaly was observed over the volcanic body. From detailed analysis of the gravity data, high anomalies were located over the northern part of the summit that correlate with the older Ungaran volcano. Various interpretation methods, such as horizontal gradient analysis, spectral analysis, and 2-D forward modeling, were applied to the gravity data. The younger Quaternary volcanic rocks, which consist of hornblende-augite-andesite (andesite lava) of the Gajahmungkur volcanics, have an average density of 2,390 ± 120 kg/m 3 , while the older Quaternary volcanic rocks, consisting of augite-olivine basalt flows (basaltic lava) from the Kaligesik formation, have an average density of 2,640 ± 100 kg/m 3 . The structural setting of the Ungaran volcano has characterized by circular structure where most geothermal manifestations are located. The result of gravity analysis shows that Ungaran volcano seems to have occurred in tectonic depression, and prominent caldera depression has not formed within Ungaran volcano without surficial caldera rim. The horizontal gradient analysis indicates that geothermal features at Ungaran volcano are structurally controlled and are located within the younger volcano.