5:15 PM - 6:45 PM
[SVC30-P15] Preservation of tephra fallout deposits: An example of the about 10-year-old deposits of the 2011 eruption of Shinmoe-dake volcano, Japan
Keywords:Tephra fallout deposits , The 2011 eruption of Shinmoe-dake volcano, Preservation and disappearance of eruption records
Investigating pyroclastic deposits enables us to understand past eruptions, forecast volcanic eruption sequences, and risk assessment in the future. However, the number of geologically determined eruptions decreases very quickly with time (Kiyosugi et al., 2015). To determine the process and time scale of tephra-disappearance-mechanisms, erosion of the tephra fallout deposits of the 2011 eruption (VEI 3) of Shinmoe-dake volcano, Japan, was investigated. The deposits' current condition was investigated at the points where Miyabuchi et al. (2013) and White et al. (2017) investigated the deposits' initial depositional facies, thickness, and grain size immediately after the eruption.
In these points, the deposits were observed where artificial influence is unlikely, such as grasslands or forests. In these points, the deposits were observed in grasslands or forests, for example, where artificial influence is unlikely.
As a result, the following three areas were determined based on the appearance of the deposits.
(I) Area of the disappeared deposits: The deposits have disappeared in the distal area of more than 22 km from the vent along the deposition axis of the 2011 deposits. At most points in this region, the initial thickness and the median grain diameter were less than 1 cm and less than 1 mm, respectively.
(II) Area of the secondary deposits: Deposits are considered secondary deposits when they appear as uncontinuous beds in soil, deposits thicker than the initial thickness measured immediately after the eruption, or deposits containing accidental lithic particles or soil. The current distribution of these deposits is in an area of <22 km from the vent along the depositional axis of the deposits.
(III) Area of the primary deposits: Deposits are considered primary deposits when they are thick and continuous beds, concordant with the description of Miyabuchi et al. (2013), or found at points under which secondary deposition is unlikely. The current distribution of these deposits is in the proximal area from the vent (<18 km along the deposition axis of the 2011 deposits). An initial thickness of more than 1 - 2 cm and a median grain diameter of more than 1 mm is assumed to be necessary to preserve the primary deposits.
Comparisons with the area and mass of the original deposits suggest that those of the current primary deposits (the area and mass in the 1-cm isopach) decreased to about 5 % and 65 %, respectively. Although the mass of the primary deposits has decreased to this percentage of the original, the original mass of the deposits can be estimated by extrapolating the relationship between deposit thickness and distance from the vent. In that case, however, an empirical model of Hayakawa (1985) provides a better estimation than approximation models (e.g., Pyle, 1989) because of its insensitivity to the extrapolated thickness of the proximal hard-to-reach deposits and distal disappeared deposits.
This study suggests the condition (initial thickness and grain size) under which tephra fallout deposits can be preserved as geological records. It also provides insight into estimating the original amount of the deposits.
References
Hayakawa, 1985, Bull Earthq Res Inst Univ Tokyo, 60:507–592
Kiyosugi et al., 2015, Journal of Applied Volcanology, 4:17
Miyabuchi et al., 2013, J Volcanol Geotherm Res, 258:31–46
Pyle, 1989, Bull Volcanol, 51:1–15
White et al., 2017, J. Geophys. Res. Solid Earth, 122:281–294
In these points, the deposits were observed where artificial influence is unlikely, such as grasslands or forests. In these points, the deposits were observed in grasslands or forests, for example, where artificial influence is unlikely.
As a result, the following three areas were determined based on the appearance of the deposits.
(I) Area of the disappeared deposits: The deposits have disappeared in the distal area of more than 22 km from the vent along the deposition axis of the 2011 deposits. At most points in this region, the initial thickness and the median grain diameter were less than 1 cm and less than 1 mm, respectively.
(II) Area of the secondary deposits: Deposits are considered secondary deposits when they appear as uncontinuous beds in soil, deposits thicker than the initial thickness measured immediately after the eruption, or deposits containing accidental lithic particles or soil. The current distribution of these deposits is in an area of <22 km from the vent along the depositional axis of the deposits.
(III) Area of the primary deposits: Deposits are considered primary deposits when they are thick and continuous beds, concordant with the description of Miyabuchi et al. (2013), or found at points under which secondary deposition is unlikely. The current distribution of these deposits is in the proximal area from the vent (<18 km along the deposition axis of the 2011 deposits). An initial thickness of more than 1 - 2 cm and a median grain diameter of more than 1 mm is assumed to be necessary to preserve the primary deposits.
Comparisons with the area and mass of the original deposits suggest that those of the current primary deposits (the area and mass in the 1-cm isopach) decreased to about 5 % and 65 %, respectively. Although the mass of the primary deposits has decreased to this percentage of the original, the original mass of the deposits can be estimated by extrapolating the relationship between deposit thickness and distance from the vent. In that case, however, an empirical model of Hayakawa (1985) provides a better estimation than approximation models (e.g., Pyle, 1989) because of its insensitivity to the extrapolated thickness of the proximal hard-to-reach deposits and distal disappeared deposits.
This study suggests the condition (initial thickness and grain size) under which tephra fallout deposits can be preserved as geological records. It also provides insight into estimating the original amount of the deposits.
References
Hayakawa, 1985, Bull Earthq Res Inst Univ Tokyo, 60:507–592
Kiyosugi et al., 2015, Journal of Applied Volcanology, 4:17
Miyabuchi et al., 2013, J Volcanol Geotherm Res, 258:31–46
Pyle, 1989, Bull Volcanol, 51:1–15
White et al., 2017, J. Geophys. Res. Solid Earth, 122:281–294