1984 年 29 巻 TOKUBE 号 p. S184-S207
During the 1983 Miyake-jima eruption pyroclasts with the total weight of 1.5×107 ton were ejected, adding to lava flow with the total weight of 1.5×107 ton. Pyroclasts were distributed eastwards and divided stratigraphically into 83 units through field observations. On the basis of the tephra-stratigraphical study on the pyroclasts and the observations including photographs and video, the process of the eruption was clarified as follows. (1) The eruption began with a scoria fall of the E-1 Unit from the E craters on the southern flank of the Oyama-Volcano about 15 : 15, Oct. 3. The fissure extended toward NNE and SSW soon after the openning. Active lava fountains from A to H craters along the fissure gave birth to lava flowing down the western slope of the fissure. On the other hand active fountains and pyroclastic columns formed scoria cones and Miike Scoria Fall (Fig. 4) on the eastern side of the fissure. Soon after 16 : 00, the fissure extended to SSW and I, J and K craters opened before 16 : 30. Lava flowed down to south from the fountains of I and J craters. (2) About 16 : 40, phreatomagmatic eruption occurred from P explosion craters beside Lake Shinmyo. P・Q-2 Scoria Fall (Fig. 7) of poorly-vesiculated scoria, the most voluminous pyroclasts of the 1983 Eruption, was ejected from the craters. The explosive phreatomagmatic eruptions occurred frequently from K, P・Q and S craters until about 20 : 00. R craters whose eruptions were characterized by Strombolian type, began to build the cone near the sea coast about 17 : 10. S crater opened in the sea about 17 : 15 and began to build a ring-shaped cone (tuff ring) composed of surge deposits (S-2 Unit, Fig. 8). About 17 : 30, the eruptions of K craters were replaced by active lava fountain, built a cone and originated lava flows. It suggests the exhaustion of the groundwater. (3) The reopening of A to S craters occurred about 20 : 00-21 : 00 after the short pause. The activity was dominated by small-scale Strombolian eruptions. Among them, however, R-4 Scoria Fall composed of well-vesiculated scoria (Fig. 8) was dispersed far eastwards. S-3 surge deposits indicate the reactivation of S crater in the sea. Other ejecta of this stage were bomb, fine vesicular scoria and ash, showing the wanning stage. The eruption finished before early morning, Oct. 4. (4) Throughout the eruptions from each crater, similar sequential changes of two cycles in lithological facies of pyroclasts and deduced types of pyroclastic columns can be recognized : lower column of the opening stage (fine, excellently vesiculated scoria and ash), active lava fountains and higher column (coarse scoria building cones and generally well-vesiculated scoria dispersing to distal area), middle column (scoria and ash fall dispersing to rather distal area), and in final stage of the second cycle, small-scale eruption ejecting mainly bomb. Pyroclasts, however, resulted from the interaction of magma and external water are characterized by the dominance of non- or poorly-vesiculated scoria. (5) From P・Q and S craters, cock’s tail jets and base surge clouds were observed to be spouting out pulsatingly. The deposits equivalent to those were identified around those craters. Especially, P・Q-4 deposits originated from cock’s tail jets of P・Q craters are characterized by the unsorted deposits composed mainly of fine ash and lithic fragments in block size and by a number of tongue-shaped microtopography.