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[SVC34-08] Eruption history and magma plumbing system of Miyakejima volcano 4,000–2,300 years ago
Keywords:Eruptive stratigraphy, Eruption history, Magma plumbing system, Miyakejima
The volcanic activity of Miyakejima volcano has been divided into four periods: Ofunato (~10–4 ka: including a dormant period from ~7–4 ka), Tsubota (~4–2.3 ka), Oyama (~2.3 ka–15th century), and Shinmio (since the 15th century)1. Although the eruption history after 2.3 ka has been well established based on the detailed stratigraphy and the temporal variations in whole-rock chemical compositions2, the volcanic activity before 2.3 ka is still poorly understood. In this study, we focus on the period between 4 and 2.3 ka because the activity is crucial for understanding the evolution processes of Miyakejima volcano from 4 ka to present after the long dormancy of Ofunato. Eruptive products between 4 and 2.3 ka are exposed in the northwestern (NW) and southern (S) areas, and are also recognized in the Tsubota borehole in the southern flank1,3. To establish a comprehensive stratigraphy of this period, we carried out a geological survey, radiocarbon dating, and whole-rock chemical analysis in the S area, where the eruptive products are relatively well exposed. In addition, we conducted geological and petrological investigations of the products at the NW area and Tsubota borehole for stratigraphic correlations with the S area.
In the S area, based on eruption styles and petrologic features, the period between ~4.0 and 2.3 ka can be divided into three stages. Stage 1 (~4.0–3.5 ka) is dominated by thick andesitic lavas (>5 m), such as Tsurunemisaki lava 1 (>0.040 km3), containing gabbroic xenoliths and xenocrysts. Stage 2 (3.5–3.1 ka) is characterized by explosive eruptions producing Tsurune tephra followed by repeated scoria with basaltic andesite composition, which might have been derived from the southwestern flank (SiO2 = 53.7–55.1 wt.%). Stage 3 (3.1–2.3 ka) is characterized by temporally evolved magmas from basalt to andesite (SiO2 = 51.4–59.4 wt.%), initiated by relatively larger eruption of Mizutamari1 (0.062 km3). Based on radiocarbon ages and petrologic features, the eruptive products in the NW area and borehole can be correlated with the stratigraphic framework in the S area. In the NW area, the Igaya accretionary lapilli, Izushita lava (4.5–4.0 ka1) (0.091 km3) and Igaya east scoria (4.3–3.7 ka1) correspond to stage 1. The Nagane scoria1 and Kannon-shita-bashi lava1, underlying a thin paleosol and Hathodaira tephra (2.3 ka2), are classified as stage 3. In the borehole, three lavas below a paleosol (4.2–3.9 ka3) correspond to stage 1, whereas scoria and lava above the paleosol are classified as stage 3.
Based on the stratigraphy and temporal change of the magmatic compositions noted by previous studies and this work, the eruption history of Miyakejima volcano during the last 4,000 years can be reconstructed as follows. In the period after dormancy, the first activity was andesitic eruption taking gabbroic xenoliths from the ascending paths (stage 1: 4.0–3.5 ka). The activity was followed by a period of predominantly explosive eruptions that produced basaltic andesite magma (stage 2: 3.5–3.1 ka). The activity then shifted to stage 3 (3.1-2.3 ka), which is characterized by the injection of basaltic magma at the beginning of the stage, and the shift of magmatic composition from basalt to andesite. Similar magmatic change from basalt to andesite cycled three times during the last 2,300 years2. Therefore, it can be concluded that the magmatic evolutionary cycle has been repeated four times every ~500–1000 years during the last 3,100 years. A new basaltic magma injection was observed in the AD 2000 eruption4. Based on the above cycles, the current magmatic activity may continue for the next 500–1000 years.
References:
1. Tsukui and Suzuki, 1998, Bul. Volcanol. Soc. Japan.; 2. Geshi et al., 2022, EPS; 3. Niihori et al., 2003, Bul. Volcanol. Soc. Japan.; 4. Geshi et al., 2002, Bul. Volcanol. Soc. Japan.
In the S area, based on eruption styles and petrologic features, the period between ~4.0 and 2.3 ka can be divided into three stages. Stage 1 (~4.0–3.5 ka) is dominated by thick andesitic lavas (>5 m), such as Tsurunemisaki lava 1 (>0.040 km3), containing gabbroic xenoliths and xenocrysts. Stage 2 (3.5–3.1 ka) is characterized by explosive eruptions producing Tsurune tephra followed by repeated scoria with basaltic andesite composition, which might have been derived from the southwestern flank (SiO2 = 53.7–55.1 wt.%). Stage 3 (3.1–2.3 ka) is characterized by temporally evolved magmas from basalt to andesite (SiO2 = 51.4–59.4 wt.%), initiated by relatively larger eruption of Mizutamari1 (0.062 km3). Based on radiocarbon ages and petrologic features, the eruptive products in the NW area and borehole can be correlated with the stratigraphic framework in the S area. In the NW area, the Igaya accretionary lapilli, Izushita lava (4.5–4.0 ka1) (0.091 km3) and Igaya east scoria (4.3–3.7 ka1) correspond to stage 1. The Nagane scoria1 and Kannon-shita-bashi lava1, underlying a thin paleosol and Hathodaira tephra (2.3 ka2), are classified as stage 3. In the borehole, three lavas below a paleosol (4.2–3.9 ka3) correspond to stage 1, whereas scoria and lava above the paleosol are classified as stage 3.
Based on the stratigraphy and temporal change of the magmatic compositions noted by previous studies and this work, the eruption history of Miyakejima volcano during the last 4,000 years can be reconstructed as follows. In the period after dormancy, the first activity was andesitic eruption taking gabbroic xenoliths from the ascending paths (stage 1: 4.0–3.5 ka). The activity was followed by a period of predominantly explosive eruptions that produced basaltic andesite magma (stage 2: 3.5–3.1 ka). The activity then shifted to stage 3 (3.1-2.3 ka), which is characterized by the injection of basaltic magma at the beginning of the stage, and the shift of magmatic composition from basalt to andesite. Similar magmatic change from basalt to andesite cycled three times during the last 2,300 years2. Therefore, it can be concluded that the magmatic evolutionary cycle has been repeated four times every ~500–1000 years during the last 3,100 years. A new basaltic magma injection was observed in the AD 2000 eruption4. Based on the above cycles, the current magmatic activity may continue for the next 500–1000 years.
References:
1. Tsukui and Suzuki, 1998, Bul. Volcanol. Soc. Japan.; 2. Geshi et al., 2022, EPS; 3. Niihori et al., 2003, Bul. Volcanol. Soc. Japan.; 4. Geshi et al., 2002, Bul. Volcanol. Soc. Japan.