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

Abstract

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 welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70-907. In contrast, nearby contemporary volcaniclastic deposits are non-welded and gently inclined. Near the contact with the granite, the tuff is plastically deformed and shows lineations that plunge inward at angles of 40-657. Lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff. The geometry and internal structures suggest that the Shiotani welded tuff was emplaced and welded in a funnel-shaped eruption conduit. Upon collapse of a plinian or phreatoplinian eruption column, the majority of the conduit-filling pyroclasts probably fell back en masse into the conduit. Heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit, especially at deep levels. The hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma. These processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall. Emplacement of nearby correlative volcaniclastic mass-flow deposits in a shelf to upper bathyal environment suggests a possibility that, when active, the Shiotani conduit was under the sea. Welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access.

An explosive–intrusive subglacial rhyolite eruption at Dalakvísl, Torfajökull, Iceland

“…The transition from explosive fragmentation to lava effusion is due to collapse and welding of permeable foam, with exsolved volatiles escaping through fragmental debris in the upper conduit (Eichelberger et al 1986) or through the conduit walls (Jaupart and Allegre 1991;Stasiuk et al 1996;Jaupart 1998). Despite the importance of shallow conduit processes in controlling eruption mechanisms, very few conduits and proximal deposits characterised by mixed explosive-effusive behaviour have been studied (Stasiuk et al 1996;Kano et al 1997). In particular, no study to date has considered what happens to rising, fragmenting foam when it encounters previously erupted tephra blocking a conduit.…”

The pyroclastic dyke and welded crystal tuff of the Morro dos Gatos alkaline intrusive complex, State of Rio de Janeiro, Brazil

“…Ekren, & Byer, 1976;Motoki, 1979;Reedman et al, 1987;Kano et al, 1996;Wolff et al, 1999). In Brazil, the examples have been reported in the felsic alkaline bodies, such as those of Mendanha (Mota & Geraldes, 2006;Motoki et al, 2007a;Alves et al, 2011;Mota et al, 2011), Itaúna (Motoki et al, 2008a), Cabo Frio Island , and Tunas (Vasconcellos & Gomes, 1992).…”

Alkaline Complexes as Potential Areas for Installation of Repositories of Nuclear Waste: Cretaceous Intrusions in Brazil

“…Intrusive pyroclastic rocks are little known in the world (e.g. Ekren and Byer, 1976;Motoki, 1979;Reedman et al, 1987;Kano et al, 1996;Wolff et al, 1999). Some of the pyroclastic bodies has been described as pyroclastic dyke and are very small, less than 4 m wide (Motoki et al, 2012).…”