Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 51 – 68
1Institute of Earth Sciences, Faculty of Science, Kagoshima University, Kagoshima, 890 Japan.
2Geochemical Laboratory, Volcanological Survey of Indonesia, Yogyakarta, Indonesia.
3Volcanological Survey of Indonesia, Bandung, Indonesia.
Abstract: Pumice flow of the 1883 Krakatau eruption occurs at Small Rakata, Rakata and Sertung Islands, those which roughly correspond to the wall of Krakatau caldera, Indonesia. It significantly differs in both mineral and chemical compositions from any other volcanic rock or ejecta of the Krakatau Group which belong to Miyashiro‘s (1974) tholeiitic series. Lithic fragments of granitic rock ranging in modal composition from quartz monzonite to quartz monzodiorite, which were found from the pumice flow are, in both mineral and chemical compositions, similar to west Malayan granitic rocks represented by biotite quartz monzonite which occurs as the dominant rock type in west Malay Peninsula (Hamilton, 1979).
No granitic rock occurs throughout the Krakatau Group, except the granitic fragments found from the pumice flow. Therefore, it should be considered that the granitic fragments came from the underlying complex at depths, where they were captured as foreign materials by the magma, and that genetically the pumice flow was closely related to the underlying granitic complex with regard to the production of its source magma. In the meantime, geochemical comparison between a suite of the 1883 Krakatau pumice flow, its related granitic fragment and selected Malayan granitic rocks and a suite of the Ata and Aira “Shirasu” pumice flows and their related granitic rocks shows that the respective pumice flows are well correlated in chemical character with the respective related granitic rocks in the locations of their plots on the AFM diagram. Such a fact suggests that the 1883 Krakatau pumice flow was genetically related to some granitic rocks nearby the Krakatau caldera.
Thus, it may be possible that sialic crustal materials, involving granitic rocks and sediments which occur in Sumatra are plunged into depths along a peculiar tectonic structure located at the Sunda Strait. This appears to be a shared portion caused by deformation of the Sunda arc due to differential movement owing to compression between the Indo-Australian oceanic plate and the Eurasian continental crust. These materials were partially melted and produced a magma of granitic composition, which was mixed with or assimilated by an ascending basaltic magma originating probably from the upper mantle resulting in a dacitic magma distinctly dominant in silica, alkalies and volatile components. As a result, the 1883 Krakatau eruption characterized by the pumice flow of dacitic composition took place. The ascending dacitic magma captured granitic fragments from the plunged sialic crustal materials at depths while passing through the peculiar tectonic structure along the shared portion. Thus the granitic fragments were erupted out together with the pumice flow.