Izu-Oshima Volcano   

1: Introduction / Topography of Izu-Oshima
2: Geology of Izu-Oshima Volcano
3: Rocks of Izu-Oshima Volcano
4: Historical outline of Izu-Oshima Volcano after the formation of caldera
5: Activities after 19th century
6: Monitoring / Points to be watched for disaster prevention
7: References

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Table 2 Mode of the large eruptions of the caldera-forming and post-caldera stages.

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Fig.3 FeO*/MgO-Al203 diagram of rocks of the Izu-Oshima volcano.

Rocks of Izu-Oshima Volcano

 The rocks of Izu-Oshima are basalt-andesite of tholeiitic rock series ( Table 2 ). Phenocrysts are plagioclase, olivine, clinopyroxene ( augite and rarely pigeonite ), orthopyroxene, and iron-titanate mineral. The proportion of plagioclase ( maximum 25 vol. % ) controls chemical composition of the rock. However, FeO*/MgO ratios ( Fig. 3 ) show that early stage of the activity is characterized by less differentiated magmas.

   Scoria erupted from central cone tends to contain more plagioclase phenocrysts than later lava flows. This is interpreted that within magma of comparatively high density, which contain less H2O and more iron, plagioclase crystals tend to float and concentrate to the top of the magma chamber and ejected first at an eruption.

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