有珠山 1977 年噴火の推移と降下火砕堆積物

抄録

Four major explosive eruptions of dacite pumice and more than ten moderate and small ones occurred from the summit of Mt. Usu during a week from August 7 to 14, 1977. The new ejecta amount to as much as 8.3×10¹³cm³ in total. The whole sequence of these paroxysmal eruptions is described here with detailed information on the eruption which was supplied from various sources. The distribution and stratigraphic sequence of each tephra were systematically surveyed on and around the volcano, the results of which are correlated with each corresponding eruption column. These tephras show various patterns of distribution because of the rise and fall of eruption column and the change of weather. Especially, it is clearly shown that the approach of a low pressure in the middle of this eruption period resulted in an anomalous, very complicated distribution pattern of tephra from a single eruption column, i.e. the distribution axes of the tephras produced by the second and third major eruptions are on the west of the volcano near the source, then gradually curve toward the north, and finally change to the northeast far from the volcano. It was also observed that a multiple fall unit of tephra was produced from a single eruption column, although a single fall unit has been regarded to be the product from a single or continuous eruption column. Accordingly, such a multiple fall unit of tephra should be treated as one eruption unit. A possible mechanism for its generation is given in relation to the rise and fall of an eruption column.