IJEN CALDERA ERUPTION MECHANISM BASED ON QUANTITATIVE TEXTURAL AND GEOCHEMICAL ANALYSIS OF PUMICE DEPOSITS

• Main Author: Amalia, Putri
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/86965
• Institution: Institut Teknologi Bandung

In the northern part of the Ijen caldera wall, there are many eruption caldera deposits dominated by the presence of pumice. The abundance of pumice deposits ranges from relatively small pumice (±2 cm) to relatively large pumice (±10 cm). However, unlike most caldera eruption products in Indonesia that produce pumice deposits that are harmonious in terms of color, the Ijen Caldera eruption produced two different types of pumice, namely dark pumice (PG) and light pumice (PT) which are spread across several locations in the northern part of the caldera. The presence of dark pumice (PG) and light pumice (PT) indicates a difference in eruption time. Therefore, quantitative and geochemical textural analysis is very important to determine the eruption mechanism in the Ijen Caldera. The methods used in this study are field data collection and laboratory analysis, consisting of quantitative and geochemical textural analysis. Based on textural and geochemical data, PT samples were obtained with PVND values ranging from 0.4 x 109 to 1.3 x 109 m-3 and MVND ranging from 0.9 x 1015 to 1.0 x 1015 m-3 (SiO2 54 - 58 wt.%) while PG samples had PVND values ranging from 0.7 x 109 to 5.7 x 109 m-3 and MVND ranging from 1.1 x 1015 to 3.1x1015 m-3 (SiO2 60 - 62 wt.%). These results have high MVND values (many small bubbles) caused by high or fast decompression rates (108 Pa/s). This high decompression results in increasingly intensive explosive eruptions. So that the mechanism of the formation of the Ijen Caldera begins with sudden decompression with a high decompression rate (6.00 x 108 Pa/s) then increases and occurs intensively which produces the PT layer (PT-1 to PT-3). Because the sudden decompression in the previous phase has disturbed the magmatic system, the mafic magma rises and fills the felsic magma chamber and this mafic magma product is observed as dark pumice (PG). After that, the eruption reaches its climax when it releases all the existing magma so that it freezes the magma chamber. After erupting, the empty volcanic magma chamber can collapse to form a large circular depression called the Ijen Caldera.