利尻火山・くつ形溶岩流の内部構造

Abstract

Three types of vesicular segregated bodies are recognized in the Kutsugata lava flows, Rishiri Volcano; namely, “segregation layer”, “segregation cylinder”, and “segregation stock”. In transverse section. the segregation layer is a flat-lying parallel banding from several mm to more than 10cm thick. The segregation cylinder is a slender vertical column which is connected with the segregation layer at the upper end of the column. The segregation stock is defined as a pulge or pulme segregated from the host lava, and is associated with the segregation layer. These dark-colored segregated bodies must have formed during consolidation of the lava flaws which had ceased to move on the flat ground, in the definite sequence with segregation layers first, then segregation cylinders and finally segregation stocks. During cooling of basaltic magma, crystal particles and residual 1iquid with bubbles must have composed a dispersed or framewarked salid-fluid system. The field evidence suggests that the segregation layer was derived from the extension fracture developed perpendicular to the 1east principa1 stress direction in the frameworked so1id-fluidcomp1ex. The segregation cy1inde and stock, however, must have originated in the f1ow by the thixotropic break dawn and the release of stress-hardening of the solid-fluid system. It is considered that there were three mechanisms of material transport in the Kutsugata 1ava f1ows, viz., f1ow of a viscous 1iquid in the extensioh fractures, vo1ati1e channel1ing in the three-phase f1uidized system, and diapiric uprise of low density plastic masses. Eight chemica-analyses of the segregated bodies are compared with six from their hostparts in a typical thick lava flow together with all published bulk rock analyses from the Rishiri Volcano in order to trace fractionation trends of the magma after extrusion.