Events of the March 31, 2000 phreatomagmatic eruption at Usu Volcano can be subdivided into two stages: (1) large-scale black plume stage having 6 major events from 13:07 to 15:28, and (2) relatively small-scale, black plume and jet-type plume stage having 5 major events from 15:28 to 17:07. The total thickness of the March 31 deposit are 100 cm at a site 90 m north from the vent, 9 cm at 550 m NNE from the vent, and 2.5 cm at 1,030 m NE from the vent. The March 31 deposit at the proximal site (90 m north) comprises 10-70 mm thick 20 layers, consisting of sandy coarse 11 layers and sandy to silty matrix-supported 9 layers. The 20 layers at the proximal site can be correlated with each eruption event observed from 13:07 to 17:25 on March 31. In the proximal deposit, essential white pumice ratio is high in the early stage (42%), then the ratio decreases toward the final stage (up to 21%). Essential pumice fragments are white and subrounded, and contain many 0.05-0.2 mm vesicles. The pumice is relatively poorly vesiculated. Grain-size distributions of all layers in the proximal deposit show bimodal distributions, with peaks at -0.5φ to 1φ and 6.5φ to 7.5φ. Poorly-sorted muddy layers are observed in post April 1 deposits. These layers formed during repeated phreatic explosions associated with abundant mud-clast spattering. The proximal area data show that the discharged mass of the March 31 deposit was 2.2×108 kg in total and 1.0×108 kg in the proximal area to 8 mm isopach.
