A “mushroom cloud” model describes the depositional features of volcanic ash (and resultant bentonite beds) from highly explosive volcanoes. The model is represented as an expanding disk-shaped cloud from which deposition occurs. The model contains an original particle-sized distribution, allows areal expansion of the distribution in a turbulent atmosphere, and aggregation of particles due to water and static electricity in the volcanic cloud. Evaluation of the model yields elongate tongue-like deposits whose lateral and downwind spread is a function mainly of wind velocity and cloud height, but its dimensions are affected significantly by particle aggregation. Histograms of particle sizes from several positions on an ash bed show change of modal sizes and skewness in hypothetical ash beds. The hypothetical distributions show close affinities to particle sizes in real ash beds.