5:15 PM - 6:30 PM
[SVC31-P05] Eruptive style and sequence of the 1235 eruption of Ohachi volcano, Kirishima, Japan -constraints from distribution, shape, and texture of pyroclasts-
Keywords:eruptive style, scoria, Ohachi volcano
The diversity of the eruptive styles of basaltic magma is still poorly understood. In particular, it is necessary to understand the processes that determine eruptive events, such as explosive eruptions and pyroclastic flows. Ohachi volcano, Kirishima, is known to have had multiple explosive eruptions during the historical period due to the presence of tephra layers. Although the general framework of the time-series change of the eruptions have been described, the details of the style change and the factors of the change have not been fully understood. In this study, we investigate Takaharu tephra associated with the 1235 eruption in detail, and discuss the eruption transition and its driving process based on the re-examination of the eruption volume and changes in rock shape, density, and textural characteristics along the time series.
Takaharu tephra is divided into three units (A-B) based on the characteristics of the constituent pyroclasts. The total volume of the tephra is estimated to be 1.5 to 2 times larger than previously estimated, indicating that the 13th century eruption may have been larger than previously thought. Furthermore, field observations unveiled that each unit can be further divided into two or three layers. We collected samples of pyroclastic flow (PDC) deposits from the stage of Unit C and each subdivided layer, and measured the apparent density and grain density (density excluding connected voids). We also carried out quantitative evaluation of grain shape, textural observation by polarized light microscopy, and identification of phenocrysts.
The apparent density measurement results showed different peaks for below the lower of Unit C and for above the middle of unit C. The lower part showed low values (0.8-1.0 g/cm3) and the upper part showed high values (1.0-1.2 g/cm3). PDC clasts also showed high values. The results of grain density measurement showed low values (about 2.6 g/cm3) for unit B and below, and high values (about 2.9 g/cm3) for unit C and above, but PDC clasts showed low values as well as unit B and below. This suggests that the PDCs have smaller amount of connected porosity. The actual calculated connected porosity is 0.59 for PDCs compared to 0.62-0.69 for tephra layers.
The shape of scoria has different characteristics for each unit at the macroscopic observation level; A is flaky, B is relatively smooth, and C is intermediate. The three indices of circularity, roundness, and solidity (shape indices) were obtained from the projected images of the particle shapes, and were found to reflect the irregularities and other features in the observation.
There is no significant difference in the phenocrysts (plagioclase, pyroxene, and olivine) throughout the layers. However, the whole rock composition is slightly rich in SiO2 in the late stage of eruption (Unit C) (Miyamoto and Tsutsui, 1996). The groundmass crystallinity is high in the order PDC >B >C >A, and the lower the crystallinity, the larger the crystal size. Microscopic bubble texture shows that A is dominated by small isolated bubbles, B is dominated by large connected bubbles, C is dominated by large but wall-separated bubbles, and PDC is dominated by small bubbles connected in a complex manner. The textures vary with stratigraphy.
A comparison of shape index and rock texture shows that there is a relationship between them, with the exception of pyroclastic density currents. For example, in Unit A, the low crystallinity and low connected pore fraction are seemingly reflected in the roughness of the particle surface. In this case, it can be interpreted that the low viscosity magma has a high ascent velocity and it was strecthed and broken apart by the explosion to form the specific shape. The relationship we have found between particle shape/density and rock texture may be an important constraint for understanding the origin of pyroclastic particles in different units and the eruptive sequence.
Takaharu tephra is divided into three units (A-B) based on the characteristics of the constituent pyroclasts. The total volume of the tephra is estimated to be 1.5 to 2 times larger than previously estimated, indicating that the 13th century eruption may have been larger than previously thought. Furthermore, field observations unveiled that each unit can be further divided into two or three layers. We collected samples of pyroclastic flow (PDC) deposits from the stage of Unit C and each subdivided layer, and measured the apparent density and grain density (density excluding connected voids). We also carried out quantitative evaluation of grain shape, textural observation by polarized light microscopy, and identification of phenocrysts.
The apparent density measurement results showed different peaks for below the lower of Unit C and for above the middle of unit C. The lower part showed low values (0.8-1.0 g/cm3) and the upper part showed high values (1.0-1.2 g/cm3). PDC clasts also showed high values. The results of grain density measurement showed low values (about 2.6 g/cm3) for unit B and below, and high values (about 2.9 g/cm3) for unit C and above, but PDC clasts showed low values as well as unit B and below. This suggests that the PDCs have smaller amount of connected porosity. The actual calculated connected porosity is 0.59 for PDCs compared to 0.62-0.69 for tephra layers.
The shape of scoria has different characteristics for each unit at the macroscopic observation level; A is flaky, B is relatively smooth, and C is intermediate. The three indices of circularity, roundness, and solidity (shape indices) were obtained from the projected images of the particle shapes, and were found to reflect the irregularities and other features in the observation.
There is no significant difference in the phenocrysts (plagioclase, pyroxene, and olivine) throughout the layers. However, the whole rock composition is slightly rich in SiO2 in the late stage of eruption (Unit C) (Miyamoto and Tsutsui, 1996). The groundmass crystallinity is high in the order PDC >B >C >A, and the lower the crystallinity, the larger the crystal size. Microscopic bubble texture shows that A is dominated by small isolated bubbles, B is dominated by large connected bubbles, C is dominated by large but wall-separated bubbles, and PDC is dominated by small bubbles connected in a complex manner. The textures vary with stratigraphy.
A comparison of shape index and rock texture shows that there is a relationship between them, with the exception of pyroclastic density currents. For example, in Unit A, the low crystallinity and low connected pore fraction are seemingly reflected in the roughness of the particle surface. In this case, it can be interpreted that the low viscosity magma has a high ascent velocity and it was strecthed and broken apart by the explosion to form the specific shape. The relationship we have found between particle shape/density and rock texture may be an important constraint for understanding the origin of pyroclastic particles in different units and the eruptive sequence.