Grain-Size Characteristics of Pyroclastic Deposits

Pyroclastic fall and flow deposits occupy two distinct fields on an $$Md_{\phi}/\sigma_{\phi}$$ plot (Inman parameters), and a contoured diagram is given based on 1,600 samples to facilitate comparison of mechanical analyses. Analyses which plot where the fields overlap include rain-flushed ashes and thin flow deposits. Among factors influencing $$\sigma_{\phi}$$ of fall deposits is the wind: a strong wind will reduce its value. Another is the characteristics of the initial population-the entire assemblage of fragments coming from the vent-which is quite different for crystals than for pumice or lithic components. Each component in a polycomponent deposit has a different grain-size distribution due to this and subsequent air sorting. Histograms or cumulative curves where the weight percentages are plotted against the fall velocity are shown to be more meaningful than those against the grain size, and a quantity V is defined analogous to $$\phi$$. Ignimbrites are remarkably homogeneous, but two departures are here described, namely, the relatively well-sorted and sometimes cross-bedded basal layer which is generally (perhaps always) present and the relatively well-sorted pipes which cut some ignimbrites. Finer and lighter components have been partly removed, leaving a concentration of coarser and heavier components. These features help to distinguish nonwelded ignimbrites from mudflows.