Stratigraphy and Magmatic Processes of Tondano Caldera I: Domato Tuff, Minahasa, NE Sulawesi, Indonesia

Abstract:

Tondano Caldera is recognized as a remarkable eruption of non-single caldera formation in the NE Sulawesi region. Domato Tuff produced from Tondano Caldera I, on 2.0±0.4 Ma (⁴⁰Ar/³⁹Ar whole rock age), is the first event and the widest distribution of rhyolitic ignimbrites (~2000 km²). This study aims to characterize the stratigraphy and the physical properties as a reference to the magmatic processes.

We obtained the new findings of pyroclastic fall deposit below the flow deposits on western-eastern flank of Tondano Caldera. This stratigraphy gave new insight into the time development due to the shift of eruption style of large scale of caldera-forming event. This fact suggests that precursory Plinian occurred before large amount of magma was evacuated during PDCs (occupying ~70km³). Single vent as a pathway of pyroclastic fall was widened because the fresh volcanic rock abundance increases, occupying the interior of volcanic body, during ignimbrite formation.

Homogeneous whole rock composition of rhyolitic magma (73-75 wt.% in SiO₂ content) is characterized by <10% of crystal proportion and 71-76 wt.% in SiO₂ content of glass composition. Vesicle texture of pumice grain is typical of phenobubble-poor in the exponential trends form of BSDs (Bubble Size Distributions).

For the homogeneous area of smallest matrix-bubble (<0.1 mm of diameter of matrix-bubble) related to syn-eruptive decompression, we obtained BND (Bubble Number Density) values from 10¹⁶-10¹⁷ m^-3 for fall deposit. Range of BND values became higher in the ignimbrites deposit from 10¹⁷-10¹⁸ m^-3. These BND ranges of Domato Tuff indicate the highest one compared to reported data from other volcanoes (e.g. 1991 Pinatubo, 1980 St. Helens, and 184ka Santorini, etc). The increases of BND lead to a new idea of higher decompression rate in PDCs than fall. There are some aspects that control the change of eruption style (fall to PDCs). One of them is the increase of density to produce column collapse. On the other hand, in Tondano Caldera I formation, fall and ignimbrite have the same bulk density range (0.6-0.9 g/cm³). Using the bulk density parameter and combining the whole data, vent enlargement seems to be responsible for the eruption style change (Ignimbrite-forming) but it does not result in the decreases of explosivity level because of the increase of BND values.