Emplacement Process of the 1991-1995 dome at Unzen Volcano, Japan

Thirteen lava lobes and big endogenous dome had grown during the 1990-1995 eruption of Unzen Volcano. Although most of lava lobes were collapsed and the surfaces were brecciated, their initial structures remain in several locations. The vent site of Lobe 11 was occupied by 4 lava lobes like large petals (petal loves) which grew in November 1993. These petal lobes were extruded from a linear vent (axis of creese structure), turned sideway and flowed to downstream. Spine intruded as the solid state into at the center of the endogenous dome, which had grown for several months in late 1994. Platy lobes were considered to be ramps (imbricated lava plates), which were formed during lateral spreading of lava as the crust of the endogenous dome in 1994. Long axes of phenocrysts in petal lobes are aligned normal to flow direction, and also to the surface of the lobes, except in the distal part and near the vent. They are roughly parallel to the elongation (intrusion direction) of the spine. They in platy lobes are weakly parallel to shear planes. Crystal preferred orientaion reflects deformation processes which were experienced during movment of the lava before complete solidification. Foliation records pure shear caused by injection force of magma and lineation records simple shear by gravity injected as supported experimentally by Buisson and Merle (2002). Foliation observed in spine may record simple shear along the conduit wall. Foliation of platy lobes may record compressive force due to lateral spreading of lobe before ramp structure formed. Water contents are 0.26 wt.% for petal lobe, 0.92 wt.% for platy lobe, and 1.26 wt.% for spine. Assuming that the water contents represent the confining pressures, spine had solidified in levels deeper than other lobes (about 420m below the surface). Therefore, crystal orientation in spine reflects stress patterns operated on moving magma in the upper part of the conduit.