Sundoro Sumbing Physical Volcanology
Description
Reconstruction of the eruptive history of Sundoro volcano is needed to forecast the probability of future eruptions and eruptive volumes. Sundoro volcano is located in Central Java (Indonesia), 65 km northwest of Yogyakarta, and in one of the most densely populated areas of Indonesia. On the basis of stratigraphy, radiocarbon dating, petrography, and whole-rock geochemistry, we recognize the following 12 eruptive groups: (1) Ngadirejo, (2) Bansari, (3) Arum, (4) Kembang, (5) Kekep, (6) Garung, (7) Kertek, (8) Watu, (9) Liyangan, (10) Kledung, (11) Summit, and (12) Sibajak. The Ngadirejo (34 ka BP) to Kledung (1 ka) eruptive groups are inferred to have been the stratovolcano building phase. Based on compositions of deposits, one or more magma reservoirs of intermediate chemical composition are inferred to have existed below the volcano during the periods of time represented by the eruptive groups. SiO2 of juvenile eruptive products ranges from 50 to 63 wt%, and K2O contents range from high K to medium K. The chemical composition and phenocryst content of eruptive products change with time. The lower SiO2 products contain mainly plagioclase, clinopyroxene, and olivine, whereas the more evolved rocks contain plagioclase, clinopyroxene, orthopyroxene, and rare hornblende and olivine. Our work has defined Sundoro’s eruptive history for the period 1–34 ka, and this history helps us to forecast future activity. We estimated that the total amount of magma discharged since 34 ka is approximately 4.4 km3. The average eruption rate over this group ranges from 0.14 to 0.17 km3/kyr. The eruption rate and the frequency of individual eruptions indicate that the volcano has been very active since 34 ka, and this activity in combination with our petrological data suggest the presence of one or more magma reservoirs that have been repeatedly filled and then discharged as eruptions have taken place. Our data further suggest that the volume of the crustal reservoir system has increased with time, such that explosive eruptions are more likely in the future and that they may be larger than the most recent small eruptions.