Silicic caldera volcanism and the 1883 eruption of Krakatau Volcano, Indonesia

Thesis

Silicic caldera-forming eruptions are major geohazards. The largest eruption with detailed written accounts occurred in 1883 at Krakatau; this allows information on the eruptive progression to be integrated with the chemostratigraphy. Freshly exposed sequences of the 1883 stratigraphy allow for its reassessment. Matrix glass from the base of the stratigraphy is more evolved than the overlying sequence, indicating a more silicic melt-rich region was evacuated during the precursory activity in May 1883. Phase equilibria experiments suggest that this precursory magma was stored deeper than the paroxysmal (August 1883) magma. These results challenge the hypothesis that the 1883 eruption emanated from a single, zoned magma reservoir. Thermodynamic models simulating fractional crystallisation show it is possible to evolve to 1883 rhyodacite from an Anak Krakatau basaltic andesite. Modelled melt chemistry for the paroxysm, using trace elements in plagioclase and partition coefficients, shows melt evolution trends for plagioclase of An<67, which overlap with experimental matrix glasses generated under shallow crustal conditions (<6 km). Modelled melt chemistry using higher anorthite contents does not overlap with these experimental glasses, or show simple trends of melt evolution, instead indicating greater thermochemical diversity within the deeper system.

This study also investigates other pre- and post-1883 explosive records at Krakatau. More distal deposits originally attributed to the 1883 Krakatau eruption are found to require further geochemical analysis to place them within the context of Krakatau’s eruptive history. The sequence of 2018-2019 ash deposits, associated with tsunamigenic flank collapse on Anak Krakatau, overlies a likely tsunami deposit and is consistent with observed post-collapse pulsatory Surtseyan activity. Thus, these deposits provide no evidence for a direct magmatic trigger for the collapse of Anak Krakatau in 2018. This work suggests that the structure of Krakatau’s magmatic system is not dissimilar to other volcanoes in areas undergoing active extension.

Authors: Madden-Nadeau, AL

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fieldwork; volcanology; experimental petrology; caldera; petrology; volcanic stratigraphy
• Subjects: Volcanology