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Quaternary Volcanoes > Active Volcanoes > Hachijojima
Hachijojima Volcano   textGeologicMap3DgeologicMap
7: Petrological characteristics - 8: Characteristics of eruptive activities

7: Petrological characteristics
   The petrological characteristics of the Hachijojima Volcano have been summarized by Nakano et al. (1991, 1997) and Tsukui et al. (1993). Vocanics from the Higashiyama Volcano have a wide range of chemical compositions (SiO2 wt% = 47%–74%) ranging from basalt to rhyolite but mainly basalt to andesite ( Figure 5 and Table 3). Rhyolitic compositions are only found in pumice fall and pyroclastic deposits and not in lava. Basalt to andesite rocks generally contain phenocrysts of plagioclase, clinopyroxene, orthopyroxene, olivine, and ilmenite. The SiO2 content of the Higashiyama Volcano clearly changes during each stage—basalt and dacite during the Sueyoshi Stage, basalt to basaltic andesite during the Nakanogo Stage, and andesite to dacite during the Mitsune Stage (Tsukui et al., 1993).

   On the other hand, the Nishiyama Volcano has a narrow range of chemical compositions (SiO2 wt% = 49%–56%), where almost all effucives excluding those in the Mitsune Stage have SiO2 wt% ranging from 49% to 52% ( Figure 5). All samples are tholeiitic basalt to basaltic andesite with low potassium content. The phenocryst minerals are mainly plagioclase, clinopyroxene, and a small amount of olivine, but small amount of aphyric basalts are also present. Major element composition of the Nishiyama Volcano magma is strongly controlled by the accumulation of plagioclase phenocrysts with low SiO2 and MgO and high Al2O3 content.

   Scoria fall from the summit of the Nishiyama Volcano during the Okoshigahana Stage, Funatsukihana Scoria Cone deposit and lava flows during the Fujitozando Stage, and Fujitozando Scoria Cone deposit, scoria fall, and lava, which are the latest deposits from the Nishiyama Volcano, can be distinguished based on their chemical compositions. Fujitozando Scoria and lava are characterized by their SiO2 content and FeO*/MgO ratio, which are slightly higher than those in older deposits. The trace element compositions of the Nishiyama Volcano magma have an extremely narrow range from the Senjojiki Stage to the Fujitozando Satge, and there is no clear change over time. On the other hand, the Mitsune Stage has significantly different composition than the younger deposits.

   Volcanic products from the Kojima Volcano is basaltic andesite to andesite, and their chemical composition has a narrow range (SiO2 wt% = 54–56 wt%). The phenocrysts are plagioclase, clinopyroxene, and a small amount of olivine.


8: Characteristics of eruptive activities
   Observation of Hachijojima Volcano eruptions have been extremely limited. Therefore, it is difficult to estimate its activity based on past eruption record. Nevertheless, judging from the characteristics of the volcanic deposits and eruption history, recent activity has been determined at the Nishiyama Volcano. Evidenced by the well-preserved cone-shaped edifice, the volcano consists mainly of accumulation of effucives from the summit crater. The geological record clearly indicates that the Nishiyama Volcano developed above sealevel by 4000–3000 yr BP and grew rapidly between 3000 and 1000 yr BP by erupting a large amount of basalt. Summit eruption repeatedly occurred until recent. When summit eruption occur at Nishiyama, most probably, there will be repeated pyroclastic eruptions and lava flows from the summit crater, similar to the Okoshigahana Stage activity at 700 yr BP. If a large amount of pyroclastic materials rapidly deposit on steep slopes near the summit, pyroclastic flows might reach the foot of the volcano. On the other hand, similar to the activity during the youngest Fujitozando Stage, ffissure eruptions could occur at several places on the slope of the Nishiyama Volcano, such as in the Mitsune and Okago areas, which are the largest urban areas of Hachijo-machi. Scoria falls and lava flows are expected from the fissures during these eruptions. If the fissures reach the shore, phreatomagmatic eruption could easily develop. Furthermore, the shores and offshore areas of the Nishiyama Volcano have several satellite cones that erupted after 10,000 yr BP. Submarine eruptions in these areas must also be considered.


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