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Eruption mechanisms of mafic magma are strongly debated, especially the mafic Plinian eruption. Few examples of mafic Plinian eruptions prevent us from knowing what condition causes rapid magma ascent... and degassing processes. Kukusan and Genteng volcanoes, which are monogenetic volcanoes in the Ijen caldera complex in Indonesia, produced explosive eruptions of mafic magma. Their chemical compositions are similar, but the volcanoes are believed to have different eruption types. Therefore, these volcanoes were chosen to reveal the factors affecting the eruption types of mafic magma. Outcrops were observed at 31 locations in fieldwork. Geochemistry, petrography, and microtextural analyses of scoria were conducted. Based on the dispersal area and fragmentation degree of the volcanic deposits, the eruption type of Kukusan is considered as sub-Plinian, while that of Genteng is considered as Strombolian. The microtextural analysis showed that the Kukusan scoria had a higher bubble number density (BND), microlite number density (MND), and microlite crystallinity than the Genteng scoria. High BND and MND values suggest rapid decompression, i.e. rapid magma ascent in the volcanic conduit. High microlite crystallinity indicates high magma viscosity. The higher ascent rate and higher viscosity of the Kukusan magma suggest coupled degassing and fragmentation, leading to a more explosive sub-Plinian eruption. In contrast, the lower ascent rate and lower viscosity of the Genteng magma suggests decoupled degassing, which leads to a Strombolian eruption.show more
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