Gardner Pyroclastic density currents are the largest natural examples of flowing granular matter and display complex patterns of size and density segregation. This project will study their deposits to gain insight into processes controlling mixing and segregation patterns difficult to study in smaller-scale granular flows. Analysis of the topology and particle sizes and densities of both massive and stratified facies will be used to reconstruct the internal workings of the parent current. The site to be studied is an explosive eruption of Nevado de Toluca, Mexico. Facies changes that will be explored by the means of three subsets of a GIS database, which will integrate sampling data obtained by determining componentry and grain size analysis with stratigraphic columns, GPS coordinates, and photographs. The first subset will allow determining the depositional mechanism of the massive facies, en masse or aggradation. This step will unlock the meaning of the reverse coarse-tail grading present at Toluca, a rare sedimentary pattern of broad interest because it is the opposite of the normal grading found commonly in other volcanoes. The local variations of the stratified facies given by the second subset will be compared to predictions from various models in order to assess whether surges are quasi-steady currents with strong internal fluctuations in particle concentration, or unsteady currents controlled by fluctuations at the source vent.
This work will be extended to other hazardous geophysical flows where similar sedimentation processes occur, such as snow avalanches, debris flows, and landslides. The results will also be applicable to industrial, chemical, and pharmaceutical applications attempting to avoid segregation during powder production.
