IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3C-P17

Date/Time: July 23 Poster

Long-term volcanism around active calderas in Bali and Tengger-Bromo region, EastJava, Sunda arc
Kiyoshi Toshida1 , Shingo Takeuchi1 , Ryuta Furukawa2 , Akira Takada2 , Supriyati Andreastuti3 , Nugraha
Kartadinata3 , Anjar Heriwaseso3 , Rosgandika Mulyana3 , Asep Nursalim3 , Oktory Prambada3 , Yudi
Wahyudi31CRIEPI, Japan, 2 AIST, GSJ, Japan, 3 CVGHM, Indonesia
E-mail: toshida@criepi.denken.or.jp

We study the long-term volcanism around the active calderas in Bali and Tengger-Bromo region, East Java.
Mass fractionation correction method is utilised for the mass spectrometry of K-Ar dating, which accounts for thefractionation of initial argon ratios. Lava samples having pilotaxitic or intergranular groundmass texture are selected
for dating in order to obtain accurate and precise ages. Some of the samples dated are estimated to contain initialargon ratios that are fractionated from atmospheric values.
We have identified three active periods of volcanism in Bali. They are 1.6-1.5 m.y. BP, 0.7-0.5 m.y. BP, and 0.2
m.y. BP to present. Volcanic rocks to the west of Bratan caldera were formed by the 1.6-1.5Ma activity. Volcanoes
consisting the northern aprons of caldera sommas were formed by the 0.7-0.5 Ma activity. Between 0.2-0.1 Ma,volcanism occured extensively around present Batur and Bratan calderas. The shield volcanoes consisting thesomma of Batur and Bratan have started to form by covering the 0.5 Ma volcanoes. Batukau, EL 706m volcano
near Pasek, and Cemara volcanoes were also formed in this period. From 0.1Ma to present, the activity continued
at Batur somma and formed Abang peak. Agung volcano started to form by 0.05 m.y. BP. Both Batur and Bratansystems have produced caldera-forming eruptions multiple times in the past 0.03 m.y., and their intra-caldera activity
has continued along with the activity of Agung.
Caldera-forming eruptions of Tengger-Bromo system are older than Bali calderas, yet the volcano is still very much
active. The two calderas are Ngadisari and Sand Sea. The activity of Tengger volcano started with the formation ofnorthern somma (Pananjakan) in 0.5-0.45 Ma, after the 1.2 m.y. hiatus following the formation of 1.7 Ma Kukusan
volcano. The aprons were formed in the subsequent active periods. More than half of the edifice volume erupted
between 0.35-0.2 Ma and have formed much of the aprons, with average eruption rate greater than 2km3/ky. The
northeastern to southern somma, and the northern apron, were formed in 0.35-0.3 Ma. The eastern apron andthe southwestern apron were formed in 0.25-0.2 Ma. Lavas and cones in the northwestern apron, and Ngadas
basalt and andesite lavas (which fill the Ngadisari caldera), have formed in 0.08-0.06 Ma. The age of Sand Sea
eruption is determined to be about 0.05 Ma from stratigraphy relations. Therefore, the central cones are found tobe younger than 0.05 Ma, which is consistent to 14C ages of tephra at the caldera rim from previous study. Theaverage eruption rate of the central cone activity is calculated to be about 0.1 km3/ky. Petrography and whole-rockchemistry of ejecta from the central cones, including the active vent (Bromo), are similar to those of Ngadas lavas
and Sand Sea eruption PDC deposits. These observations imply that the present magma supply and accumulation
system is similar to the one prior to the Sand Sea eruption.

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