Economic and Environmental Geology

Fig. 6.

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Fig. 6. Depositional models of the welded lapilli tuff (a to b) and the normally graded lapilli tuff (c). (a) Sustained pyroclastic density currents can displace lake water from shoreline when entering into the lakes. As a consequence, the pyroclastic density currents do not interact with ambient water except for frontal part, and, thus, the resultant deposits retain heat, forming a welded texture. (b) Time-series (T1 to T3) depositional models of welded lapilli tuff at an inferred point X. Littoral eruptions at shoreline and/or fine-elutriation in the lake by water ingestion cause the formation of the coarse-grained frontal part of the pyroclastic density currents. Sedimentation from the frontal part results in coarse-grained, clast-supported lower part of the welded massive lapilli tuff (T1). Formation of coated ash pellets in the upper part of the fine-grained ash plume by condensation of water vapor derived from magma and evaporation of lake water. The coated ash pellets grow to accretionary lapilli by progressive accretion of fine-grained ash as a result of fluctuation of ash pellets in the ash plume by turbulence (T2). A growth of accretionary lapilli together with a decrease in intensity of turbulence during the final stage of deposition results in settling of accretionary lapilli in the topmost part of the welded lapilli tuff (T3). (c) Transformation of the unsteady, relatively short-lived, pyroclastic density currents into water-saturated turbidity currents as entering into the lakes.
Econ. Environ. Geol. 2022;55:295-307
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