富士火山西麓に分布する玄武岩質火砕流の成因

抄録

Formation mechanism of basaltic pyroclastic flows has not been sufficiently clarified yet because basaltic pyroclastic flows do not occur as frequently as felsic ones. We studied the Osawa pyroclastic flow 3 deposit (OsPfl-3), which took place on the western flank of the Fuji volcano between 2.9 and 3.0 ka. OsPfl-3 has two flow units and one cooling unit, which have a combined volume of 6.2 × 10⁶m³. The flow overlies another unit composed of two scoria fallout deposits (YokSfa-2a and 2b) which sandwich a pyroclastic flow deposit (OtPfl). OsPfl-3 mainly consists of welded blocks and dense blocks with composition and petrographical characteristics of basaltic andesite. Some of the dense blocks have cracks on their surfaces and look like “cauliflower-shaped bomb”. They have a flat surface on one side with concentration of vesicles near the surface. The matrix of OsPfl-3 has dense fragments that are thought to have originated from dense lava blocks and poorly vesiculated scoria. The emplacement temperature of the blocks is estimated to be higher than 580℃ from thermoremanent magnetization measurements. These observations indicate that the blocks in the OsPfl-3 originated from welded pyroclasts, lava flow or lava lake at the summit crater. The sequence of the eruptions that formed OsPfl-3 and underlying deposits are summarized as follows: Stage 1: Deposition of fallout tephras (YokSfa-2a and 2b) and an intercalated pyroclastic flow (OtPfl) which are composed of fairly vesiculated scoria; Stage 2: Formation of lava flow or lava lake at the summit crater, and deposition of pyroclastics on the lava; Stage 3: Occurrence of the pyroclastic flow (OsPfl-3) caused by collapse of lava and pyroclastics. OsPfl-3 is prominently distributed on the western flank. This observation implies that the westward flow from the source lava that filled the summit crater could cross the lower part of the crater rim.