11:00 AM - 1:00 PM
[SVC32-P05] Reconstruction of the 1235 explosive basaltic eruption at Ohachi volcano, Kirishima, Japan, based on physical characterization of pyroclastic deposits
Keywords:Basaltic Volcano, Particle Morphology, Fragmentation
Basaltic volcanoes show various eruption styles even with low-viscosity magma: effusive eruptions, explosive eruptions, pyroclastic flows, and combination of these. Determining the mechanism and major factors of these changes and variation of these eruptive styles is important to understand the activities of basaltic volcanoes. Eruptive sequences of the past eruptions are reflected in stratigraphy of the tephra, rock textures and grain physical properties of the eruptive products, thus analyses of these features make it possible to reconstruct how the eruptive activities have changed. In this study, we focus on the 1235 eruption of Ohachi volcano, Kirishima Japan, and discuss the eruption sequence through analysis of tephra stratigraphy, physical properties of pyroclasts and observation of rock textures.
The tephra deposits (Takaharu tephra, ThT) of the 1235 eruption are divided into 3 major units (ThT-a, b, c). Each unit was formed by different explosive eruption. Whole rock chemical compositions show that there is a gap in SiO2 content (wt.%) between ThT-b and ThT-c. In this study, I divided each unit into 2 or 3 subunits, and in total I defined 7 subunits. Based on the tephra distribution for each unit, the total eruptive volume of this eruption was estimated to be 0.32 km3. This indicates that this eruption was much larger than previously thought. Seven subunits of tephra and one pyroclastic flow deposits were chosen for measurement of microscopy, shape analysis, apparent density, and connected bubble ratio analysis. The values of the shape parameters, Circularity, Roundness, and Solidity, increase from ThT-a to ThT-b and decrease slightly from ThT-b to ThT-c. The amount of middle size bubble (0.25-1 mm) increases from the bottom to the top in the unit ThT-a and b, but it shows the opposite trend in the unit ThT-c. The ratio of large-size bubble was the highest in unit ThT-a, which is followed by ThT-b, and c. Connected bubble ratio decreases around 5% in each unit. The amount of microlite was different for each subunit in qualitative, and was large in ThT-b and the upper part of ThT-c.
It is revealed that the values of the shape parameters represent some features observed in the field and their changes reflects the differences of the bubble structures. The amount of bubble and the connected bubble ratio correlate with eruption scale and intensity. The shapes of pyroclasts and rock textures (i.e., bubble structures) and their changes were used to reconstruct the sequence and intensity of the eruption. The eruption that formed ThT-a is estimated to be a relatively small, low-intensity event that became more explosive in the later of the eruption. ThT-b eruption is estimated to be largeer and higher-intensity than ThtT-a; however, it is similar to ThT-a in terms of the feature of more explosive in the later. ThT-c eruption was an event whose explosiveness decreases in the latter half of the eruption and seems to have followed a different path from the previous 2 eruptions.
The tephra deposits (Takaharu tephra, ThT) of the 1235 eruption are divided into 3 major units (ThT-a, b, c). Each unit was formed by different explosive eruption. Whole rock chemical compositions show that there is a gap in SiO2 content (wt.%) between ThT-b and ThT-c. In this study, I divided each unit into 2 or 3 subunits, and in total I defined 7 subunits. Based on the tephra distribution for each unit, the total eruptive volume of this eruption was estimated to be 0.32 km3. This indicates that this eruption was much larger than previously thought. Seven subunits of tephra and one pyroclastic flow deposits were chosen for measurement of microscopy, shape analysis, apparent density, and connected bubble ratio analysis. The values of the shape parameters, Circularity, Roundness, and Solidity, increase from ThT-a to ThT-b and decrease slightly from ThT-b to ThT-c. The amount of middle size bubble (0.25-1 mm) increases from the bottom to the top in the unit ThT-a and b, but it shows the opposite trend in the unit ThT-c. The ratio of large-size bubble was the highest in unit ThT-a, which is followed by ThT-b, and c. Connected bubble ratio decreases around 5% in each unit. The amount of microlite was different for each subunit in qualitative, and was large in ThT-b and the upper part of ThT-c.
It is revealed that the values of the shape parameters represent some features observed in the field and their changes reflects the differences of the bubble structures. The amount of bubble and the connected bubble ratio correlate with eruption scale and intensity. The shapes of pyroclasts and rock textures (i.e., bubble structures) and their changes were used to reconstruct the sequence and intensity of the eruption. The eruption that formed ThT-a is estimated to be a relatively small, low-intensity event that became more explosive in the later of the eruption. ThT-b eruption is estimated to be largeer and higher-intensity than ThtT-a; however, it is similar to ThT-a in terms of the feature of more explosive in the later. ThT-c eruption was an event whose explosiveness decreases in the latter half of the eruption and seems to have followed a different path from the previous 2 eruptions.