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Enigmatic transition of maar eruptions: A petrology and textural studies at Ranu Grati Maar, East Java, Indonesia

Andriansyah Gurusinga1,2,3, Mirzam Abdurrachman2, Tsukasa Ohba1, Takashi Hoshide1, Wildan Nur Hamzah1, Idham Andri Kurniawan2

  • Affiliations: 1Department of  Earth Resource Science, Akita University, 1-1, Tegata Gakuenmachi, Akita 010-8502, Japan. 2Department of Geological Engineering, Bandung Institute of Technology, Bandung 40132, Indonesia. 3Center for Regional Revitalization in Research and Education, Akita University, 1-1, Tegata Gakuenmachi, Akita 010-8502, Japan. 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 1

  • Programme No: 3.7.8

  • Theme 3 > Session 7

Abstract

Maar volcanoes represent an important yet often poorly understood archive of phreatomagmatic eruption dynamics. This integrated field, textural, petrographic, and geochemical study aimed to reconstruct the eruptive evolution and drivers of transitions at Ranu Grati Maar in East Java, Indonesia. Detailed stratigraphic logging of a ~15 m thick tephra succession exposed around the southern crater rim defined four tephra units (Unit 1 -- 4) based on characteristic variations between brown-orange, brown-grayish, and black-grayish lithofacies. Grain size and component analyses revealed temporal shifts between dominantly juvenile black scoria phases (comprising 14-67% of the deposits), indicative of lower energy activity, and intervals of well-sorted tephra containing more evolved gray and golden (50-81%) scoria, suggestive of higher-intensity phreatomagmatic eruptions. Three distinct scoria types exhibit systematic mineralogical and compositional differences, with black scoria containing higher bulk crystallinities (average 44%) and mafic mineral assemblages (olivine ~2.4%), representing deeper and more mafic magmatic inputs. Geochemical trends between units suggest intermittent influxes of mafic and evolved magmas that modulate eruptive styles. Petrological and textural constraints indicate fluctuating pre-eruptive conditions in the shallow plumbing system, with recharge events triggering eruptive transitions between magmatic-dominant and phreatomagmatic activity over time. In summary, Ranu Grati's stratified deposits provide a record of cyclical maar behavior primarily driven by the coupling between recharging and evolving magma, with implications for volcano monitoring and hazard assessment.