The Magmatic and Eruptive Evolution of the 1883 Explosive, Caldera-forming Eruption of Krakatau, Indonesia
Abstract
Explosive, silicic caldera-forming eruptions are among the most hazardous volcanic phenomena. The magmatic evolution of these systems is poorly understood, owing to the lack of opportunity to monitor such a volcano in its long build-up phase prior to an explosive eruption. Petrological tools can therefore help to constrain the conditions leading up to, and during, these cataclysmic events.
The 1883 eruption of Krakatau is the only explosive caldera-forming eruption with accompanying written accounts. This work focuses on using the eruptive progression, along with stratigraphic context and analytical petrology, to investigate the structure of the magma plumbing system, as well as the magmatic processes leading up to the 1883 eruption. Tephra samples were collected to assess whether the chemistry of the eruptive deposit varies stratigraphically. Matrix glass, as well as chemical zonation of phenocrysts, have been characterised using Electron Probe Microanalysis. No systematic stratigraphic variations in whole-rock chemistry were found, suggesting that the magma reservoir was not chemically zoned. However, matrix glass analyses show a more diverse range in composition in the upper stratigraphic section, following the first lithic lag breccia. This supports the notion that the caldera collapsed in stages and that, after the first stage, magma was tapped from a more diverse range of sources. ) is still limited, suggesting that these sources may have coalesced and mixed. This interpretation is supported by chemical zoning in plagioclase, which varies greatly across the phenocryst population, with rim compositions ranging from An40 to An85. In addition, approximately 15 % of crystals have cores ranging between An70 to An90, suggesting the existence of a more mafic reservoir. Crystal thermometry and hygrometry show no variation with stratigraphy, with average magmatic temperatures for Fe/Ti oxides estimated at 914°C, and water contents at 3.9 wt %. The proposed model of decoupling and subsequent amalgamation of magma bodies as part of caldera cycles shows the complex processes involved at caldera-forming volcanoes, and has significant implications for how we monitor Krakatau.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51H0142M
- Keywords:
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- 1115 Radioisotope geochronology;
- GEOCHRONOLOGY;
- 3625 Petrography;
- microstructures;
- and textures;
- MINERALOGY AND PETROLOGY;
- 3642 Intrusive structures and rocks;
- MINERALOGY AND PETROLOGY;
- 8032 Rheology: general;
- STRUCTURAL GEOLOGY