On magnetization of Mt. Fuji (Huzi) and other volcanoes in Japan

8. Conclusion

In this paper, the writers have described the outline of geomagnetic phenomena of volcanoes in Japan and given the interpretation from the volcanological stand-point.

On the basis of geomagnetic surveys of Mt. Huzi and other volcanoes 29), we have made clear that active and dormant volcanoes in Japan are magnetized nearly towards the north.

Although the intensity of magnetization of these volcanoes depends markedly on the chemical composition of lavas, namely, whether they are basaltic or andesitic, it depends also on the structures of these volcanoes. It is common to volcanoes that their magnetizations are caused partly by the remanent magnetization and partly by the induced magnetism. According toNagata’s experiments of lava-specimens collected from various volcanoes, the intensity of thermo-remanent magnetism of rocks is generally several or ten times that of induced one. But in the actual geomagnetic phenomena of volcanoes, it is found that anomalous distributions of geomagnetic force near the andesitic volcanoes such as Kusatu-Sirane are seriously affected by induced magnetism due to the present geomagnetic field, and not by the remanent one. On the contrary, the intense magnetization of new lava-flows such as that of Miyake-sima in 1940 or Sakura-sima in 1946 is almost perfectly explained by the thermo-remanent magnetism, and not by the induced magnetism only.

From the geomagnetic phenomena mentioned above, it may be concluded reasonably that the geomagnetic anomaly of volcanoes should be explained not only on the foundation of magnetic properties of lava-specimens but also by taking the structures of volcanoes into account. We hope that numerous problems with respect to subterranean structures of volcanoes and their history will be brought to light in the near future, on the basis of future observations of geomagnetic phenomena and experiments on magnetism of rocks.

In conclusion, the writers wish to express their sincere thanks to Mr.K. Matsushita, Mr.K. Iwama, Mr.S. Uchibori and MissT. Takahashi for their cooperation in their field investigations and in preparation of this paper.