392 PYROCLASTICS Table Major features of pyroclastic fall deposits and possible interpretations Fall deposit characteristic Geometry High aspect ratio cone Low aspect ratio sheet Circular isopachs/ isopleths Elongate Isopachs/ isopleths Changing isopach/ isopleth distribution with height Secondary thickening of fall deposit Structure Grading Inverse Normal Bedding Cross stratification, lenses Erosion surfaces (unconformities) Impact sags Compositional changes Chemical zoning of juvenile pyroclasts Layers rich in accidental clasts Vertical changes in accidental clast composition Layers rich in dense juvenile clasts (e.g., obsidian) Intercalated thin ash beds Pink ash coatings on pumice lapilli Accretionary lapilli; ash pellets; armoured lapilli Texture characteristics Poor sorting (presence of an ash matrix) Rounded pumice clasts Welding Interpretation Fallout from a low (2 >45 km); widespread dispersal Fallout during no wind conditions; fallout from a powerful eruption column undeformed by wind Strong distortion of eruption column by wind (elongation in downwind direction) Variable wind directions at different heights; changing wind directions with time Downwind aggregation of ash and premature fallout of large aggregates with greater settling velocities Fallout from a growing eruption column (waxing eruption intensity); gravitational (downslope) remobilization of deposits (e.g., Strombolian deposits) Fallout from a waning eruption column (waning eruption intensity) Fallout during unsteady eruption column dynamics (fluctuating eruption intensity) Reworking of falling or deposited clasts due to surface winds, PDCs or gravitational remobilization (rolling down steep slopes) Removal of deposit due to mass wasting (landslides), PDCs, surface processes (alluvial/slope erosion) Deformation by ballistic clasts Changes in the chemical composition of the erupted magma with time chamber eruption from a zoned magma Explosive fragmentation of vent walls following blocking, collapse or increase in erosive power Vertical or lateral (from fissure vents) shift in locus of vent erosion Explosive clearance of a cognate volcanic plug Short lived phreatomagmatic explosions which generate abundant fine ash; settling of fine ash from atmosphere during pauses in fallout activity Thermal alteration (oxidation) of pyroclasts Moisture driven aggregation of ash common during phreatomagmatic activity and during humid atmospheric conditions; electrostatic attraction of fine ash particles Partial phreatomagmatic activity (hybrid eruption); coeval deposition of fallout and fine ash from a dilute ash rich PDC or a co ignimbrite ash cloud Emplacement by lateral currents (PDC); reworking by alluvial processes Proximal high temperature agglutination of hot (fluidal) pyroclasts; contact fusing of pyroclasts by an overlying welded ignimbrite