Laminar Viscous Flowage Structures in Ash-Flow Tuffs from Gran Canaria, Canary Islands

Abstract

At least $$100 km.^{3}$$ of trachytic and peralkaline soda rhyolitic ash flows were erupted during late Miocene to early Pliocene time from a source near the center of Gran Canaria, Canary Islands. Their deposits cover more than $$350 km.^{2}$$ and are typical of ash-flow deposits elsewhere in that they show vertical changes in color, degree of crystallization, and denseness of welding. They underwent unusually intense vapor-phase crystallization, chiefly to aegirine-augite, Na-amphibole(?), and alkali feldspar. Several structures unusual for ash-flow deposits typify those of Gran Canaria. The structures are: (1) stretched pumice fragments; (2) broken and rotated pumice fragments; (3) tension cracks in the matrix; (4) hollows around rotated inclusions; (5) folds; (6) imbricated pumice fragments; (7) ramp structures. The structures are found throughout the $$350-km.^{2}$$ area, and those with directional significance show a uniform trend away from the source of the ash flows. The structures indicate that the ash flows probably advanced as viscous, laminar-flowing masses for distances on the order of several meters just before halt. We suggest that the ash flows-probably never greatly inflated-gradually but continuously degassed, grew denser, and consequently slowed as they advanced. Compaction welding began when the density became sufficient, and further flowage as a non-homogeneous viscous fluid proceeded until viscosity overcame momentum, and movement ceased. The alkali trachytic and peralkaline soda rhyolitic ash flows have high Fe/(Si + Al) and Na/K ratios which seem to lower viscosity, and they were so hot that deposits a few meters thick could weld. Moreover, large quantities of volatiles remained dissolved in the glass shards and pumices until movement ceased. Viscosities of the glass particles were consequently low, thereby accounting for their laminar flowage.