Forms and Structures of Lava Flows and Scoria Cones of the Oct. 1983 Eruption of Miyakejima Volcano

Abstract

On the eruption of 1983, Miyakejima volcano issued lava flows from the fissure, along which many scoria cones were formed at the opposite side of the flowing lava. The fissue is represented by an alignment of many small craters resulted from the drain back of lava to the vent. Some of them are a kind of pit crater formed by the marginal collapse of the primary craters. An explosion crater appearing at the skirt of the volcano and a tuff ring at the seashore were formed by the phreatomagmatic explosions. Difference in shape among the pyroclastic cones is mainly due to the difference in mode of eruptions, that is to say, the difference in ratio of water to magma. Only the explosion crater changed the mode of eruption from phreatomagmatic to magmatic one. Because most of the scoria cones are situated on the slope of the volcano and the volume of the ejecta from the vent is not so large, the volcanic edifice is not a typical, grown up scoria cone surrounded by the talus slope. Many cracks and fissures are opened on the surface of the scoria cones running parallel to the main fissure. Those are a kind of step-fault caused by the shock or the shaking of the earthquakes after the end of the main eruption. Some vents are filled up by the slide mass of the scoria cone. Viscosity of the lava seems to be rather low, and it thinly spreads over the wide area. Lava tree molds and natural levees are frequently found at the marginal part of the lava flow, and many kipukas are at the central part of the flow. The lava is mostly aa type. Only in the small area near the vent, the lava resembles slab pahoehoe, but the slabs themselves have spiny aa surface. The lava gradually changes into block type after a long travel through the lava channel. No lava flow formed by the secondary flowage of the agglutinate is observed. The writer discriminated a peculiar topography resulted from the secondary flowage of the already settled lava caused by earthquakes or collapse of scoria cones, and names it the re-mobilized part of lava. This type of movement may be common in the normal lava flow, but it is hard to distinguish the newly flowed part from the continuously moved lava. On the contrary, in the case of the eruption accompanied by the ejection of a large amount of tephra during or after the outpouring of lava, a peculiar lava topography formed in relation to the presence of tephra layers is usually recognized and it makes the discrimination of the re-mobilized part of lava very easy.