Types of volcanic eruptions in Japan

7. Summary and conclusion

The eruptions of volcanoes are various in surface features as well as in magnitude, depending upon their geographical position, structure, crater condition, physical and petrochemical nature of the ejecta, etc. Referring principally to the recent eruptions which occurred within the last few scores of years in Japan and which have been described in detail by the investigators, these active volcanoes in Japan are classified as shown in the tables (Tables 1 and 2). First of all, the active volcanoes are divided into two groups according to the frequency of eruption. One includes those volcanoes which are literally active with eruptions every few years; they usually have a persistent open crater from which the eruptions are taking place. Another includes those volcanoes which are usually dormant and are awakened into an eruption once in more than several scores of years; they usually have a closed crater and their next eruptions are liable to occur at new craters more or less inconsistent with the craters of the preceding eruptions. Eruptions themselves are divided first into magmatic and phreatic eruptions. The magmatic eruptions are then divided into effusive, mixed, and explosive eruptions. The effusive eruptions are rather simple, while the mixed and explosive eruptions may be further divided into certain sub-types according to the mode of occurrence of the ejecta.

As a means of quantitatively specifying the magnitude of volcanic eruptions, these are classified into ten grades (O to IX), according to the volume of solid ejecta, on the assumption that the intensity of eruption is proportional to the total volume of ejecta. This scale (Table 3) is applied to the eruptions, the volume of whose ejecta have been estimated.