著者

山本 裕朗 谷口 宏充

出版者

東北大学

雑誌

東北アジア研究 (ISSN:13439332)

巻号頁・発行日

vol.3, pp.201-232, 1999-03-31

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Numerous cinder cones from Ojikajima monogenetic volcanic group are well-exposed by marine erosion that allows a detailed investigation of the proximal products. Observation of the exposures at sea cliffs and petrological study of the volcanic rocks revealed the formation process and the internal structure of cinder cones. These cones are divided into three types in terms of evolution: 1) scoria cone+lava flow, 2) spatter cone+lava flow, 3) maar,tuff-ring (phreato-magmatic phases)+scoria cones+lava flow. Four eruption styles of lava flow are found: 1) over flow from crater edge, 2) outflow from middle part of scoria cone with horseshoe-shaped collapse (intrusion of dike oblige from fissure), 3) without collapse, 4) outflow from middle part or basement (intrusion of dike parallel to fissure).論文Article

著者

山本 裕朗

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.48, no.1, pp.11-25, 2003-03-18

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The lava effusion process from a cinder cone and its mechanism are discussed based on the field observation of Ojika-Jima Monogenetic Volcano Group (OMVG). The cinder cones of OMVG are classified into two types, C-type and D-type cones, based on the mode of lava effusion from the cone. In the C-type cone, lava overflowed from the central crater, whereas in the D-type cone, lava flowed out from the flank. These types are related to the morphology and internal structure of the cone. The ratio of cone height (H_<co>) to width (W_<co>) of the C-type is smaller than that of the D-type, and the part of the dense welding is widespread around the cone. On the other hand, the welding area of the D-type is within the limits to the central part of the cone. The D-type is further divided into two types; Dc-type is accompanied by a mountain body collapse with lava effusion and Dp-type does not have this collapse. The majority of Dc-type cones are larger than Dp-type cones, although the ratios of H_<co>/W_<co> are similar. In the OMVG, a thin dike (less than 1 m thick) is generally observed inside the cone. However, if a dike intrusion does not have enough stress to collapse a mature cone, a branched dike system could cause a much larger load to the slope of cone and push a sector of the cone outward. Therefore, a branched dike system seems to control in cone breaching. The dike system is always observed inside Dc-type cones, while it is rare inside Dp-type cones. Considering the concept of crack propagation in an elastic body, the dike branches off under the condition that the breaking strength of the deposit around the tip of a feeder dike is low. Accordingly, the collapse of a cinder cone caused by a branched dike system is incident in the larger-scaled cinder cone, especially when the welded area is restricted to the central part of the cone and altitude difference between the lava lake in the crater and the top of the dike is large. It has been assumed in previous works that the density difference between the lava and cinder cone is the main controlling factor for the mode of lava effusion from the cinder cone. In this paper, the author concluded that the degree of welding around the feeder dike and total volume of the cinder cone are the major controlling factors in the dike propagation process.