The 1783 Activity of Asama Volcano Inferred from the Measurements of Bulk Density of Tephra (Pumice) and the Old Documents

Abstract

Pyroclastic material is considered to be a kind of “fossil” suggesting the pressure-tempe-rature condition when it was formed. From this point of view, as a case study, the tephra associated with the 1783 eruptive activity of Asama Volcano is investigated. The total thickness of the tephra at our outcrop is about 2 meters. The formation is composed of several pumice layers alternating with thin ash beds. The mean bulk density of each pumice layer is measured by using about 100 pieces from the corresponding layer. The resultant values obtained from the lowest pumice layer to the highest one change as 0.79, 0.70, 0.69, 0.76, 0.82, 0.73, 0.70, 0.73 and 0.75 gr/cm³. The error is within 0.03 gr/cm³. The values, therefore, range roughly from 0.70 to 0.85 gr/cm³. One of our most interesting findings is that there is a peak in the change of the value. Thus, according to the order from the lowest pumice layer to the highest one, the value decreases from 0.79 to 0.69 gr/cm³ and next, increases to 0.82 gr/cm³ which is the peak value, and the value again decreases to 0.7 gr/cm³ and slightly increases. The peak can be eventually explained by attributing not to the worse vesiculation throughout the magma but to the coexistence both vesiculated pumice and worse vesiculated one, of which bulk density ranges roughly from 0.85 gr/cm³. Since the eruptive activity was great, many old documents relative to it still remain. Much information obtained from them is compiled. The course of the eruptive activity can be inferred from the compiled information. The formation of pisolite is generally considered to be closely related to rain. Therefore, taking account of minglement of pisolite, as a key material, into the pumice fall deposit, we can correlate the result from the measurement of mean bulk density with the course of the eruptive activity inferred form the old documents, in which the information relative to rain is found out. Conclusively, we can determine the date when each pumice layer was formed and also, we might be able to say that the whole tephra was formed wihin about a week. Moreover, it might be able to say that the peak in the mean bulk density as mentioned above was attained just before the occurrence of the two pyroclastic flows termed as “Agatsuma” and “Kambara” and the occurrence of the lava flow termed as “Onioshi-dashi”.