The upper portion of the Huichapan Ignimbrite (UHI), central Mexico, is a non-welded massive ignimbrite characterized by large and well-developed fines-poor elutriation pipes. The relevance of the source of fluidizing gas is discussed in relation to the zoning of flow types within a single flow unit and it is shown how this, and its attendant structures, can be used to estimate the relative importance of each gas source during and after flow emplacement. The implications of this classification are discussed with reference to the flow regimes, deposition slope angles, crystal concentration and “fossil fumaroles” variously developed in the flows. Type 2 and type 3 flows are mainly pumiceous ignimbrites and are distinguished by expansion induced coarse-tail grading, coupled with segregation structures in the latter. Type 1 flows are ungraded and mostly result from hot-avalanche flows and pyroclastic flows formed of relatively dense material. These sections are used to define a threefold genetic classification of pyroclastic flows which can be directly linked to conditions within a semifluidized parent flow.

The up (increasing gas velocity) curve on a gas velocity/bed-pressure drop plot for a poorly sorted mixture of irregularly shaped particles is divided into three sections non-expanded, expanded, and segregating. A series of experiments was carried out to review the process of fluidization for a number of particulate materials having various sorting and grain shape characteristics.

Pyroclastic flow types