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[SVC34-P15] The eruptive history of Miyakono scoria-fall deposit of Kuju volcano
Keywords:Kuju volcano, Miyakono scoria, isopach map
Introduction
Kuju Volcano is an active volcano located in central Kyushu. It has been active for approximately 200,000 years (Kawanabe et al., 2015). The eruption area divided into the western(200-54 ka), central (160-0 ka) and eastern part (54-1.4 ka) (Kawanabe et al., 2015). The western and central parts primarily erupted andesitic and dacitic magma (Kawanabe et al., 2015), although part of the central region also erupted mafic magma (Fukuoka et al., 2023). In contrast, the eastern part exhibits abundant mafic magma activity (Kawanabe et al., 2015).
However, in a deposit older than 54 ka, Miyakono scoria-fall deposit (Kj-MS; Ohta, 1991) has been identified. The petrographic feature of Kj-MS (Nagaoka and Okuno, 2014) suggests that Kj-MS lower part is related to mafic magma eruption. It is crucial to clarify the eruption history, volume, source, and petrologic feature of Kj-MS for understanding the volcanic history and for forecasting future mafic magma eruptions of Kuju volcano. Although the stratigraphic face and thickness distribution of Kj-MS have been reported (Nagaoka & Okuno, 2014), many aspects remain unclear.
This study aims to clarify the eruption history, source, eruptive volume, and magma characteristics of Kj-MS. Here, we report the results of field survey and macroscopic and microscopic observation of Kj-MS scoria.
Study Area
The study area extends about 10 km to the northeast and 14 km to the east of Kuju Volcano (Figure; Locs. 1-5).
Eruption history of Kj-MS
Nagaoka and Okuno (2014) reported the Kj-MS in the soil beneath the Kuju-D ash fall layer (D). Based on the stratigraphic relationships, we examined Kj-MS at five locations.
Kj-MS comprises, from the bottom to the top, a whitish-gray ash layer (MS 1; 2-3cm thick), a dark gray ash layer (MS 2; 1cm thick) and an orange-brown scoria layer (MS 3 106cm thick). These are interpreted as products of a single eruptive episode. MS1 contains essential materials, suggesting that its eruption style may have been phreatomagmatic eruption. Therefore, we propose that Kj-MS initially formed during a phreatomagmatic eruption (MS1). Subsequently, a small-scale eruption occurred (MS2), and was then followed by a larger-scale eruption that generated the scoria(MS3).
Maximum Thickness of the MS 3
Based on the distribution of Kj-MS, we recognized the MS 3 reaches a maximum thickness of 106 cm at Loc. 2, which far exceeds the previously reported maximum of 25 cm (Nagaoka and Okuno, 2014). This finding suggests the need to reevaluate the eruptive volume.
Essential Material of the MS1 and MS 3
At Loc. 1 and 2, MS 3 is subdivided into lower and upper parts based on the proportion of lithic fragments and the grain size. In contrast, at Locs. 3-5, located farther from Kuju Volcano, the MS 3 appears as a single scoria layer.
Based on the macroscopic and microscopic observation, At Loc. 1, the lower scoria is brown to reddish-brown with low vesicularity, whereas the upper scoria is brown and relatively more vesicular. The mafic mineral assemblages differ: the lower scoria contains olivine and 2 pyroxenes, while the upper scoria contains two pyroxenes and brown amphibole. It is suggested a possible change in magma composition during the formation of Kj-MS.
In contrast, at Loc. 2, the lower and the upper parts of the MS 3 comprise highly vesicular brown to light-brown scoria and low-vesicular gray scoria. These two types exhibit banded structure. These scorias contain only two pyroxenes. The difference in mineral assemblages between the Loc. 1 and 2 suggests that the observed units may not be identical.
At Locs. 3-5, these scorias are mainly orange to reddish-brown, low-vesicular. The scoria at Loc. 4 contains clinopyroxene and olivine. The scoria at Loc. 5 contains olivine, clinopyroxene and orthopyroxene. Because these scorias contain olivine, these could be correlated with the lower part of MS3 at Loc. 1.
Kuju Volcano is an active volcano located in central Kyushu. It has been active for approximately 200,000 years (Kawanabe et al., 2015). The eruption area divided into the western(200-54 ka), central (160-0 ka) and eastern part (54-1.4 ka) (Kawanabe et al., 2015). The western and central parts primarily erupted andesitic and dacitic magma (Kawanabe et al., 2015), although part of the central region also erupted mafic magma (Fukuoka et al., 2023). In contrast, the eastern part exhibits abundant mafic magma activity (Kawanabe et al., 2015).
However, in a deposit older than 54 ka, Miyakono scoria-fall deposit (Kj-MS; Ohta, 1991) has been identified. The petrographic feature of Kj-MS (Nagaoka and Okuno, 2014) suggests that Kj-MS lower part is related to mafic magma eruption. It is crucial to clarify the eruption history, volume, source, and petrologic feature of Kj-MS for understanding the volcanic history and for forecasting future mafic magma eruptions of Kuju volcano. Although the stratigraphic face and thickness distribution of Kj-MS have been reported (Nagaoka & Okuno, 2014), many aspects remain unclear.
This study aims to clarify the eruption history, source, eruptive volume, and magma characteristics of Kj-MS. Here, we report the results of field survey and macroscopic and microscopic observation of Kj-MS scoria.
Study Area
The study area extends about 10 km to the northeast and 14 km to the east of Kuju Volcano (Figure; Locs. 1-5).
Eruption history of Kj-MS
Nagaoka and Okuno (2014) reported the Kj-MS in the soil beneath the Kuju-D ash fall layer (D). Based on the stratigraphic relationships, we examined Kj-MS at five locations.
Kj-MS comprises, from the bottom to the top, a whitish-gray ash layer (MS 1; 2-3cm thick), a dark gray ash layer (MS 2; 1cm thick) and an orange-brown scoria layer (MS 3 106cm thick). These are interpreted as products of a single eruptive episode. MS1 contains essential materials, suggesting that its eruption style may have been phreatomagmatic eruption. Therefore, we propose that Kj-MS initially formed during a phreatomagmatic eruption (MS1). Subsequently, a small-scale eruption occurred (MS2), and was then followed by a larger-scale eruption that generated the scoria(MS3).
Maximum Thickness of the MS 3
Based on the distribution of Kj-MS, we recognized the MS 3 reaches a maximum thickness of 106 cm at Loc. 2, which far exceeds the previously reported maximum of 25 cm (Nagaoka and Okuno, 2014). This finding suggests the need to reevaluate the eruptive volume.
Essential Material of the MS1 and MS 3
At Loc. 1 and 2, MS 3 is subdivided into lower and upper parts based on the proportion of lithic fragments and the grain size. In contrast, at Locs. 3-5, located farther from Kuju Volcano, the MS 3 appears as a single scoria layer.
Based on the macroscopic and microscopic observation, At Loc. 1, the lower scoria is brown to reddish-brown with low vesicularity, whereas the upper scoria is brown and relatively more vesicular. The mafic mineral assemblages differ: the lower scoria contains olivine and 2 pyroxenes, while the upper scoria contains two pyroxenes and brown amphibole. It is suggested a possible change in magma composition during the formation of Kj-MS.
In contrast, at Loc. 2, the lower and the upper parts of the MS 3 comprise highly vesicular brown to light-brown scoria and low-vesicular gray scoria. These two types exhibit banded structure. These scorias contain only two pyroxenes. The difference in mineral assemblages between the Loc. 1 and 2 suggests that the observed units may not be identical.
At Locs. 3-5, these scorias are mainly orange to reddish-brown, low-vesicular. The scoria at Loc. 4 contains clinopyroxene and olivine. The scoria at Loc. 5 contains olivine, clinopyroxene and orthopyroxene. Because these scorias contain olivine, these could be correlated with the lower part of MS3 at Loc. 1.