The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations

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Authors

  • A.l. Madden-nadeau
  • M. Cassidy
  • D.m. Pyle
  • T.A. Mather
  • S.l. Engwell
  • M. Abdurrachman
  • M.e.m. Nurshal
  • D.r. Tappin
  • T. Ismail

Colleges, School and Institutes

Abstract

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions.

Bibliographic note

Funding Information: This work was supported by the Natural Environment Research Council (NERC). ALM-N acknowledges NERC Studentship NE/L202612/1 ; MC acknowledges NERC fellowship NE/N014286/1 ; MC, SW, SLE, and MA acknowledge a NERC urgency grant NE/T002026/1 ; DMP and TM acknowledge support from NERC COMET; MA acknowledges Program Riset Unggulan ITB 2020 grant. Publisher Copyright: © 2021 Elsevier B.V.

Details

Original languageEnglish
Article number107176
Number of pages21
JournalJournal of Volcanology and Geothermal Research
Volume411
Early online date9 Jan 2021
Publication statusPublished - Mar 2021

Keywords

  • Caldera, Fieldwork, Geochemistry, Petrology, Stratigraphy

ASJC Scopus subject areas