Abstract
Four major tephras and several minor ones have been produced by explosive eruptions of dacite pumice which occurred from Mt. Usu for a week from August 7 to 14, 1977. Apparent density measurements of the pumice and bombs from these tephras indicate that their vesiculation degree is highest at the beginning of the series of eruptions, then it gradually decreases with time toward the end, which may be interpreted in terms of a higher concentration of water in the upper part of the magma column. Within a single eruption the vesiculation degree changes also to some extent, decreasing generally toward the cessation. The relationship between the weather at the upper atmosphere at the time of eruption and the distribution pattern of the thickness and size composition of each tephra was examined. When the weather was stable with a wind direction almost the same throughout the upper atmosphere, the distribution axis shows a nearly straight line, the thickness pattern is symmetric to the axis, and the size composition is almost identical for the deposit at the same distance from the source. On the contarary, when the weather was unstable with a wind direction varying in clockwise with the height, the distribution axis curves in clockwise with increasing distance from the source, and the size composition of the deposit at the same distance from the source is coarser in clockwise. The size composition of these tephras shows a single mode and bimodal distributions to varying extent. It is indicated that the bimodality increases with abundancy of rain-drops in an ash cloud at the time of eruption. Accretionary lapilli is considered to have formed by a proper quantity of raindrops falling into a dense cloud of ash.
