Miocene rhyolitic welded tuff infilling a funnel-shaped eruption conduit Shiotani, southeast of Matsue, SW Japan

摘要: At Shiotani, SW Japan, rhyolitic welded tuff forms a steep-sided funnel-shaped body, confined by Paleogene granitic rocks to an elliptical area 1–1.5 km across. The Shiotani is pervasively and foliated concordantly with the contact that dips inward at angles of 70–90°. In contrast, nearby contemporary volcaniclastic deposits are non-welded gently inclined. Near granite, plastically deformed shows lineations plunge 40–65°. Lithic crystal clasts in rheomorphic outer part rotated plane normal foliations parallel indicating downward flow tuff. geometry internal structures suggest was emplaced eruption conduit. Upon collapse plinian or phreatoplinian column, majority conduit-filling pyroclasts probably fell back en masse into Heat steam from underlying magma diffusion interstitial volatiles glass perhaps reduced viscosity juvenile facilitated welding conduit, especially deep levels. hot then flowed down conduit wall during compaction retreat magma. These processes resulted increased toward contacts concordant steep wall. Emplacement correlative mass-flow shelf upper bathyal environment suggests possibility that, when active, under sea. Welding would occur even sea provided generated fill prevented water access.