THE VOLCANIC ACTIVITY OF RINJANI, LOMBOK ISLAND, INDONESIA DURING THE LAST TEN THOUSAND YEARS, VIEWD FROM 14C AGE DATINGS

Asnawir Nasution1), Akira Takada2), Rosgandika Mulyana1)

1)Directorate of Volcanology and Geological Hazard Mitigation, Indonesia

2)Geological Survey of Japan

ABSTRACT

The eruptive history of Rinjani voclano during the last 10 ky before the caldera formation was studied. The eruption rate of Rinjani volcano was kept constant (0.6 km3/ky) during a stratocone building stage between 12-6 ka. It decreases becoming 0.15 km3/ky for the last period of 5.2 ky before the caldera forming eruption (6-8 ka). During the low activity stage, three eruptions occured; Propok Pumice and Lembar Lava Flow (DRE: 0.1 km3 and 0.4 km3, respectively), Rinjani Ash and Rinjani Pumice (DRE: 0.3 km3). The magma path shifted 5 km toward the eastern flank to grow Rinjani volcano. The volcanic activity migrated more 5 km eastward for low activity stage, erupting Propok Pumice and Lembar Lava Flow. The activity migrated back to Rinjani summit yielded ash and pumice. Syncaldera stage started with plinian pumice (DRE: 3 km3) and a huge pyroclastic flow (DRE: >7 km3), and, finally, Segara Anak caldera of 6 km x7 km was formed at the center of Central Lombok Volcanic Complex. 14C datings indicates that the climax of caldera forming eruption was the periode of AD 1210-1300 years B.P., which is the third youngest caldera in Indonesia.

Keywords: caldera, pyroclastic flow, Plinian eruption, eruptiion rate, Rinjani volcano, Lombok island