Multicondition-driven mechanisms may produce pyroclastic deposits varying in fundamental properties such as dispersal, grain size, vesicularity and morphology of juvenile clasts, and the abundance of lithic or "wall rock" ejecta (xenoliths). Volcanic ash particles from different fragmentation mechanisms have different surface textures and morphologies. The analysis of the volcanic clast shape remained largely qualitative. A new method for ash particle characterization based on quadtree decomposition and surface gradient analysis is introduced. The approach is applied for assessing fragmentation mechanisms operating during eruptions. The surface descriptor variables like the number of quadtree blocks (nQT), the mean block size (mQT), the standard deviation of block sizes (sQT) and the surface descriptors derived from gradient analysis seem to be suitable for quantifying the structural changes of the ash surface due to variable explosion conditions. These parameters are presented in volcanology as distinctive key parameters for different eruption types. This may enrich our capabilities for effective prediction for the basis of planning to overcome the impending danger of eruptions. (c) 2005 Elsevier Ltd. All rights reserved.