THE CALDERA RINJANI OF LOMBOK, INDONESIA DURING THE LAST TEN THOUSAND YEARS

Convention Bandung 2004 (CB2004)
The 33rd Annual Convention & Exhibition 2004

Indonesian Association of Geologist
Horizon Hotel, 29-30 Nov, 1 Oct 2004, Bandung

THE CALDERA RINJANI OF LOMBOK,
INDONESIA DURING THE LAST TEN
THOUSAND YEARS

Asnawir NASUTION*, AkiraTAKADA**, Ryuta FURUKAWA**
Rosgandika MULYANA* and Roni FATUROCHMAN*

Directorate of Volcanology and Geological Hazard Mitigation
Geological Survey of Japan, 1-1-1 Higashi, Tsukuba, 305-8567, Japan

Abstract

Remote sensing survey has been conducted in Rinjani complex of Lombok
Island. JERS-1 SAR has all weather observation capability and provide a
valuable image in the region. The interpreted geological information are shown
by drainage patterns, texture and lineaments. The volcanic rocks of West and
East Rinjani complexes are characterized by rough texture and high resistance.
The Central Rinjani complex is characterized by radial drainage patterns in low
density, smooth textures and low resistance. Lineament trends of N-S, NE-SW
and E-W are represented by old and young Rinjani complexes, probably
indicate an influence of subduction zone from the Indian – Australian plate to
the south. The eruptive histories and related to caldera formation during the last
10 ky are recognized by 14C dating. The eruption rate 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 occurred;
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. Syn-caldera 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 centre of Central Lombok Volcanic
Complex. The climax of caldera forming eruption was the period of AD 12101300
years B.P. The collapse caldera remnants represent few alteration dykes
and fault structures with dipping 15 to 40o, on the north, south and east caldera
walls, probably indicating a flowing up magma through the old vents and weaker
structures before occurring the caldera formation.

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