17:15 〜 19:15
[SVC30-P04] Explosive Dynamics of the 45 ka Sandsea Caldera-Forming Eruption, Tengger Volcanic Complex (TVC): Insight from Grain Size Distribution, Componentry, and Textural Analysis
キーワード：Bromo Tengger Volcano Complex, Basaltic Caldera Forming, Componentry, Grain Size Distribution, Textural Analysis
The Tengger Caldera Complex (TVC) in East Java is one of the largest caldera in Indonesia, measuring approximately 8.5 x 10 km in diameter. This expansive caldera was formed by two major explosive events: the Ngadisari (150 ka) and the Sandsea Caldera eruption (45 ka). As a basaltic-andesitic volcanic system, the TVC produced voluminous pyroclastic falls and flows that were distributed across the surrounding flanks. The violent behaviour of these caldera-forming eruptions exemplifies how mafic-intermediate magmas can erupt explosively. However, despite the significance of these events, their deposits have yet to be comprehensively documented.
The current study focuses on characterizing the deposits associated with the Sandsea caldera-forming eruption. Stratigraphic data from the inner caldera rim and more distal areas reveal a sequence comprising pre-caldera and syn-Sandsea caldera eruption products. The pre-caldera units consist of lava flows, which mark the lower boundary of the Sandsea sequence. In contrast, the syn-Sandsea caldera eruption is represented by six eruption units (SS-PF1, SS-PDC1, SS-PF2, SS-PF3, SS-PDC2, and SS-PDC3). These syn-Sandsea units are characterized as ash-lapilli-sized, scoria-dominated pyroclastic flow and fall deposits exhibiting overlapping reverse, normal, and complex grading patterns.
Grain size distribution analysis indicates predominantly unimodal characteristics within most stratigraphic units, with median values ranging from −3phi and 2phi and standard deviation values between 1.31phi and 2.92phi. This variability suggests complex fragmentation processes and transport regimes throughout the Sandsea event. Componentry analysis has identified five distinct juvenile types (black scoria, glassy scoria, grey scoria, brown scoria, and banded fragments) as well as three non-juvenile components (fresh lithic, altered lithic, and free crystal fragments). The juvenile scoria types display a range of textures and vesicle characteristics, with the black scoria being the densest (0.7-1.5 g/cm3) and the glassy, grey, and brown scoria exhibiting more moderate to low densities (0.3-1.2 g/cm3). The componentry of PDC's deposits contains a higher percentage of non-juvenile materials (>20%) compared to pyroclastic fall deposits, likely due to differences in fragmentation levels and conduit widening process.
Our data presented here provide new insights into the dynamic of explosive Sandsea caldera-forming eruptions within TVC. The complexity of eruption behaviour, including variable fragmentation processes and heterogeneous magma properties, likely contributed to the violence of the event, which distributed substantial pyroclastic flows and falls around the Tengger caldera. Furthermore, detailed investigations of the micro-textural properties and geochemical composition of the Sandsea deposits will further elucidate the pre- and syn-eruptive conditions for this significant caldera-forming event.
The current study focuses on characterizing the deposits associated with the Sandsea caldera-forming eruption. Stratigraphic data from the inner caldera rim and more distal areas reveal a sequence comprising pre-caldera and syn-Sandsea caldera eruption products. The pre-caldera units consist of lava flows, which mark the lower boundary of the Sandsea sequence. In contrast, the syn-Sandsea caldera eruption is represented by six eruption units (SS-PF1, SS-PDC1, SS-PF2, SS-PF3, SS-PDC2, and SS-PDC3). These syn-Sandsea units are characterized as ash-lapilli-sized, scoria-dominated pyroclastic flow and fall deposits exhibiting overlapping reverse, normal, and complex grading patterns.
Grain size distribution analysis indicates predominantly unimodal characteristics within most stratigraphic units, with median values ranging from −3phi and 2phi and standard deviation values between 1.31phi and 2.92phi. This variability suggests complex fragmentation processes and transport regimes throughout the Sandsea event. Componentry analysis has identified five distinct juvenile types (black scoria, glassy scoria, grey scoria, brown scoria, and banded fragments) as well as three non-juvenile components (fresh lithic, altered lithic, and free crystal fragments). The juvenile scoria types display a range of textures and vesicle characteristics, with the black scoria being the densest (0.7-1.5 g/cm3) and the glassy, grey, and brown scoria exhibiting more moderate to low densities (0.3-1.2 g/cm3). The componentry of PDC's deposits contains a higher percentage of non-juvenile materials (>20%) compared to pyroclastic fall deposits, likely due to differences in fragmentation levels and conduit widening process.
Our data presented here provide new insights into the dynamic of explosive Sandsea caldera-forming eruptions within TVC. The complexity of eruption behaviour, including variable fragmentation processes and heterogeneous magma properties, likely contributed to the violence of the event, which distributed substantial pyroclastic flows and falls around the Tengger caldera. Furthermore, detailed investigations of the micro-textural properties and geochemical composition of the Sandsea deposits will further elucidate the pre- and syn-eruptive conditions for this significant caldera-forming event.