4:15 PM - 4:30 PM
[SVC37-10] Total grain-size distribution of pumice fallout deposits (Kj-P1) from 54 ka plinian eruption at Kuju Volcano
Keywords:grain size distribution, Kuju Volcano, plinian eruption, pumice fall deposit
Based on grain-size distribution, maximum pumice size and components, Kj-P1 pumice fall deposit is divided into finer lower and coarser upper layer. We analyzed each grain-size distribution for the lower and upper layers.
Generally, sieving and laser-diffraction analysis have been used for measuring grain-size of tephra and we also used these methods. However, because the upper layer of Kj-P1 has been altered and lumped, it can be hardly separated. In addition, because the altered pumice particle could be broken into finer particles under water, we cannot measure accurate grain sizes. Then, we challenged a new method, image analysis, outlining pumice particles on photographs of outcrops and measure the sizes and area ratio of each pumice particles. We combined the results of the image analyses, sieving and laser-diffraction analyses at 15 and 20 localities for the lower and the upper layers. Using volonoi distribution, Md phi and sigma phi are calculated to be -1.5 and 2.5 for the upper layer and 0.5 and 2.6 for the lower layer. The result fit the observation of altered pumice particles at the outcrops better than the result without the image analyses. To verify this method, analytical methods such as numerical simulation are needed.