2:45 PM - 3:00 PM
[SVC30-05] Reexamination of the characteristics and eruption style of Haruna-Hakoda tephra found from the southern foot of Akagi Volcano to the eastern foot of Haruna Volcano in North Kanto, Northeast Japan
Keywords:Haruna Volcano, Haruna Hakoda tephra, volcanic product, distribution, refractive index
Haruna Volcano on the volcanic front at the Northeast Japan Arc is located in the center of Gunma Prefecture. Tephra such as Haruna-Hassaki (Hr-HP), Haruna-Futatsudake Shibukawa (Hr-FA), and Haruna-Ikaho (Hr-FP) that erupted from the Late Pleistocene to the Holocene from this Haruna Volcano have been studied in detail (Soda, 1989; Oishi et al. 2011, etc.). In addition, Haruna Miharada pumice deposited about 32ka (Takemoto, 2008) was called Miharada pumice firstly (Takemoto, 1985), and its source was presumably provided from Haruna Volcano (Takemoto, 1998). On the other hand, Soda (1996) called the same tephra bed Haruna-Hakoda tephra (Hr-HA). Its stratigraphic position shown by previous researches is above Hr-HP and Akagi Kanuma tephra (Ag-KP: Machida and Arai, 2003), and below Aira-Tn tephra (AT: Machida and Arai, 2003). However, the mineral assemblage differs depending on the literature, such as the composition of heavy minerals contained in this tephra. Furthermore, although it has been reported that this tephra has a distribution axis in the northeast direction from Haruna Volcano (Takemoto, 1998), but its distribution range has not been clearly determined. Therefore, the purpose of this study is to clarify the heavy mineral composition, refractive index of hornblende and orthopyroxene, the distribution and eruption style of this tephra. The name of this tephra is called Haruna-Hakoda tephra according to Soda (1996). However, the abbreviation Hr-HA called in the same thesis is derived from Haruna-Hassaki Ash (HA), which was the former name of this tephra, so we newly named it Hr-Hkd.
As a result of investigating from the southern foot of Akagi Volcano to the eastern foot of Haruna Volcano, outcrops exposing Hr-Hkd were found at eight locations. These outcrops are distributed in the range of 8 to 30 km in the east-northeast direction from Haruna Volcano. Hr-Hkd was divided into four units A to D from the bottom. Unit A is a light brown to gray falling ash layer containing accretionary lapilli. At some locations, white fall pumice is clustered at the bottom of unit A. Unit B is a white fall pumice layer composed of poorly sorted and fine-grained pumice. Unit C is a white fall pumice layer composed of poorly sorted and coarse-grained pumice. The matrix of unit C is gray to brown volcanic ash containing gray lapilli of about 1 mm in diameter. Unit D is a pyroclastic surge deposit containing white fall pumice. Furthermore, from the thickness of tephric soil deposits including Hr-Hkd, AT, and Ag-KP determined at several outcrops, it can be inferred that the eruption age of Hr-Hkd is around 37.4-40.0 ka.
Analyses of the heavy mineral composition of Hr-Hkd is shown below in descending order. Unit A, B, and D is hornblende, orthopyroxene, titanomagnetite, and cummingtonite, and unit C is hornblende, orthopyroxene, cummingtonite, and titanomagnetite. Furthermore, the measurement of the refractive index by RIMS2000 revealed that the refractive indices of hornblende and orthopyroxene contained in Hr-Hkd differed from unit to unit. Comparing the modes for each unit. For hornblende, unit A is 1.671-1.681 (1.677), unit B is 1.674-1.683 (1.680), unit C is 1.670-1.682 (1.675), and the unit D is 1.671-1.682 (1.676). Thus, the refractive index of hornblende decreases from unit A to D. For orthopyroxene, unit A is 1.706-1.714 (1.710), unit B is 1.705-1.713 (1.709), unit C is 1.703-1.713 (1.708), and the unit D is 1.701-1.712 (1.709). Thus, the refractive index of orthopyroxene tends to decrease from unit A to D. These indicates that the upper unit has less iron.
In addition, the series of eruptive event which provided Hr-Hkd can be estimated as below. Unit A contained abundant accretionary lapilli, and pumice was contained at the bottom of unit A at 8 to 19 km from the Haruna Volcano, these are consistent with the characteristics of the phreatomagmatic eruption ejecta mentioned in Hiroi et al. (2015). Therefore, it is considered that the first Hr-Hkd eruption was a phreatomagmatic eruption. Then, in transition to the Plinian eruption, white fall pumice of unit B and unit C erupted. Of these, unit C had multiple fall units at two locations about 8 km east and about 15 km northeast of the Haruna Volcano. Therefore, the eruption of unit C is considered to be multiple eruptions with almost no time-gap. Finally, it is probable that the pyroclastic flow flowed down unit D was deposited.
In the future, if the outcrops of Hr-Hkd are found in a wider area, more detailed distribution and eruption style can be considered. It will be important basic data for studying the history of eruptive activity of the Haruna Volcano.
As a result of investigating from the southern foot of Akagi Volcano to the eastern foot of Haruna Volcano, outcrops exposing Hr-Hkd were found at eight locations. These outcrops are distributed in the range of 8 to 30 km in the east-northeast direction from Haruna Volcano. Hr-Hkd was divided into four units A to D from the bottom. Unit A is a light brown to gray falling ash layer containing accretionary lapilli. At some locations, white fall pumice is clustered at the bottom of unit A. Unit B is a white fall pumice layer composed of poorly sorted and fine-grained pumice. Unit C is a white fall pumice layer composed of poorly sorted and coarse-grained pumice. The matrix of unit C is gray to brown volcanic ash containing gray lapilli of about 1 mm in diameter. Unit D is a pyroclastic surge deposit containing white fall pumice. Furthermore, from the thickness of tephric soil deposits including Hr-Hkd, AT, and Ag-KP determined at several outcrops, it can be inferred that the eruption age of Hr-Hkd is around 37.4-40.0 ka.
Analyses of the heavy mineral composition of Hr-Hkd is shown below in descending order. Unit A, B, and D is hornblende, orthopyroxene, titanomagnetite, and cummingtonite, and unit C is hornblende, orthopyroxene, cummingtonite, and titanomagnetite. Furthermore, the measurement of the refractive index by RIMS2000 revealed that the refractive indices of hornblende and orthopyroxene contained in Hr-Hkd differed from unit to unit. Comparing the modes for each unit. For hornblende, unit A is 1.671-1.681 (1.677), unit B is 1.674-1.683 (1.680), unit C is 1.670-1.682 (1.675), and the unit D is 1.671-1.682 (1.676). Thus, the refractive index of hornblende decreases from unit A to D. For orthopyroxene, unit A is 1.706-1.714 (1.710), unit B is 1.705-1.713 (1.709), unit C is 1.703-1.713 (1.708), and the unit D is 1.701-1.712 (1.709). Thus, the refractive index of orthopyroxene tends to decrease from unit A to D. These indicates that the upper unit has less iron.
In addition, the series of eruptive event which provided Hr-Hkd can be estimated as below. Unit A contained abundant accretionary lapilli, and pumice was contained at the bottom of unit A at 8 to 19 km from the Haruna Volcano, these are consistent with the characteristics of the phreatomagmatic eruption ejecta mentioned in Hiroi et al. (2015). Therefore, it is considered that the first Hr-Hkd eruption was a phreatomagmatic eruption. Then, in transition to the Plinian eruption, white fall pumice of unit B and unit C erupted. Of these, unit C had multiple fall units at two locations about 8 km east and about 15 km northeast of the Haruna Volcano. Therefore, the eruption of unit C is considered to be multiple eruptions with almost no time-gap. Finally, it is probable that the pyroclastic flow flowed down unit D was deposited.
In the future, if the outcrops of Hr-Hkd are found in a wider area, more detailed distribution and eruption style can be considered. It will be important basic data for studying the history of eruptive activity of the Haruna Volcano.