Compressive phenomena of pyroclastic deposits and reliability of old documents

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Uniaxial compressive experiments were performed using pumice and scoria particles of the 1707 eruption of Fuji volcano and 1783 eruption of Asama volcano, Japan. Under dry condition, the decrease in thickness of the samples increases logalithmically linearly with time.The slope of the log-linear plottings, time versus displacement, of the Asama pumice, under uniaxial compression, is about 3.6 times greater that that of Fuji ejecta. When the particle size is uniform, the ratio of minimum to the maximum apparent density of the aggregates remains the same, at 0.9, irrespective of the kinds of particles.Compression experiments revealed that these pyroclastic particle deposits reaches a maximum value after a short time interval following deposition. Then, 200 to 300 years after the deposition, the apparent density stays more or less the same.Old historic documents were examined which were collected and compiled by the former researchers on the 1707 scoria fall deposit of Fuji volcano. When compared with the results by Aramaki et al. (1998) on the 1783 ejecta of Asama volcano, some significant discrepancies were found between the two. In Fuji ejecta, it is tentatively concluded that about 1.69 time of exaggeration is observable, while in the 1783 Asama pumice deposits, about 1.34 times of exaggeration for the volume-based thickness observation, and 2.01 time for the straight measurement of the thickness are observed.Filed work was made on the pumice fall deposits of the 1929 eruption of Hokkaido-Komagatake and the 1770 and 1814 eruption of Sakurajima volcano. In both cases, the preservation of the deposits are so poor, the collected data points are not good enough so that the detailed evaluation of the data are not yet completed.