5:15 PM - 6:45 PM
[SVC30-P06] Re-investigation of the historic phreatic eruption deposit (JP7) at Azuma-Jododaira volcano, Fukushima prefecture, Japan
Keywords:Azuma volcano, Jododaira volcano, Phreatic eruption, Charcoal, Volcanic glass
Azuma volcano located north of Fukushima prefecture is a composite volcano with a basal radius of c.a. 10 km x 20 km. The latest activity of Azuma volcano formed Azuma-Jododaira volcano (hereafter referred to as Jododaira volcano) which is certified as an active volcano in Japan [1]. Jododaira volcano experienced numerous phreatic eruptions from ca. 7 ka, that generated at least 7 phreatic eruption units JP1 (oldest) to JP7 (youngest) [1]. In order to reexamine the age, style, and eruption sequence of JP7, we conducted 14C dating, componentry, and SEM-EDS analysis.
JP7 was observed at Kamoshika-zaka outcrop located 2 km north of Jododaira. We subdivided JP7 into a lower gray silty ash layer (1.5 cm thick) and an upper dark-gray sandy ash layer (0.5 cm thick). Component analysis allowed to classify JP7 particles into 9 types based on their color and shape (Fig. 1). These particle types include 4 types of lithic fragments (red/yellow-colored, black, gray, and white lithic fragments), volcanic glass shard, 3 types of isolated crystals (pyroxene, plagioclase, and quartz), and charcoal. (Fig. 2). Lithic fragments, glass shards and charcoal are distributed (in count %) in the order 79.0, 1.7 and 2.5 in the lower, and 90.4, 0.70 and 0.70 in the upper layers.
Quantitative core-to-rim chemical traverses were carried out for glass shards to evaluate the degree of alteration (Fig. 3). We recognized fresh glass shards showing homogeneous chemical composition from the core to rim ((mean wt% SiO2=78.6% (stdev.=0.14); K2O=2.9% (stdev.=0.09)). In some altered particles, the SiO2 was 3% higher in the rim and Na2O, CaO and Al2O3 were 1~2% lower in the rim, indicating leaching caused by acidic hydrothermal fluids [2].
14C dating of charcoal fragments in JP7 gave calendar ages of 1445 to 1513AD (80.1%) and 1591 to 1620AD (15.3%) (2σ). These ages are slightly older than the estimated eruptive age of JP7 (1711 CE based on historic records and 14C dates of soils on the upper layer [1]).
The presence of charcoal in JP7 is one of the most important features. Studies of forest buried by Sanbe-Azukibara pyroclastic flow deposits suggest that temperatures of eruption deposits must be 275 °C or higher for wood to be effectively carbonization [3]. Such high temperatures conditions have not yet been observed for phreatic eruptions. Temperatures of pyroclastic flows deposited by phreatic eruptions are generally much lower, in the range of 30~100 °C [4]). We therefore suggest that the eruption that deposited JP7 involved some magma input.
Fresh glass shards with homogeneous chemical compositions are considered to be juvenile materials. Products of the 2008-2011 phreatic eruption of Mt. Shinmoedake are dominated (65-30 vol%) by altered materials, with less than 1 vol% glass shards [5]. On the other hand, the amount of juvenile materials is higher (7.7 vol%) in phreatomagmatic eruption deposits [5]. The fraction of the juvenile materials (fresh lithic fragments and volcanic glass) in JP7 is relatively higher (14.1%) in the upper layer than in the lower (6.0%) layer. This suggests that a magmatic phase was involved in the deposition of the upper layer. We conclude that the eruption that deposited JP7 started with a phreatomagmatic phase, and that magma ejection increased with time.
[1] Yamamoto (2005) [2] Nogami et al. (1993) [3] Sawada et al. (2000) [4] Oikawa et al. (2016)
[5] Suzuki et al. (2013)
JP7 was observed at Kamoshika-zaka outcrop located 2 km north of Jododaira. We subdivided JP7 into a lower gray silty ash layer (1.5 cm thick) and an upper dark-gray sandy ash layer (0.5 cm thick). Component analysis allowed to classify JP7 particles into 9 types based on their color and shape (Fig. 1). These particle types include 4 types of lithic fragments (red/yellow-colored, black, gray, and white lithic fragments), volcanic glass shard, 3 types of isolated crystals (pyroxene, plagioclase, and quartz), and charcoal. (Fig. 2). Lithic fragments, glass shards and charcoal are distributed (in count %) in the order 79.0, 1.7 and 2.5 in the lower, and 90.4, 0.70 and 0.70 in the upper layers.
Quantitative core-to-rim chemical traverses were carried out for glass shards to evaluate the degree of alteration (Fig. 3). We recognized fresh glass shards showing homogeneous chemical composition from the core to rim ((mean wt% SiO2=78.6% (stdev.=0.14); K2O=2.9% (stdev.=0.09)). In some altered particles, the SiO2 was 3% higher in the rim and Na2O, CaO and Al2O3 were 1~2% lower in the rim, indicating leaching caused by acidic hydrothermal fluids [2].
14C dating of charcoal fragments in JP7 gave calendar ages of 1445 to 1513AD (80.1%) and 1591 to 1620AD (15.3%) (2σ). These ages are slightly older than the estimated eruptive age of JP7 (1711 CE based on historic records and 14C dates of soils on the upper layer [1]).
The presence of charcoal in JP7 is one of the most important features. Studies of forest buried by Sanbe-Azukibara pyroclastic flow deposits suggest that temperatures of eruption deposits must be 275 °C or higher for wood to be effectively carbonization [3]. Such high temperatures conditions have not yet been observed for phreatic eruptions. Temperatures of pyroclastic flows deposited by phreatic eruptions are generally much lower, in the range of 30~100 °C [4]). We therefore suggest that the eruption that deposited JP7 involved some magma input.
Fresh glass shards with homogeneous chemical compositions are considered to be juvenile materials. Products of the 2008-2011 phreatic eruption of Mt. Shinmoedake are dominated (65-30 vol%) by altered materials, with less than 1 vol% glass shards [5]. On the other hand, the amount of juvenile materials is higher (7.7 vol%) in phreatomagmatic eruption deposits [5]. The fraction of the juvenile materials (fresh lithic fragments and volcanic glass) in JP7 is relatively higher (14.1%) in the upper layer than in the lower (6.0%) layer. This suggests that a magmatic phase was involved in the deposition of the upper layer. We conclude that the eruption that deposited JP7 started with a phreatomagmatic phase, and that magma ejection increased with time.
[1] Yamamoto (2005) [2] Nogami et al. (1993) [3] Sawada et al. (2000) [4] Oikawa et al. (2016)
[5] Suzuki et al. (2013)