Maximum size, median diameter, and sorting of tephra

Abstract

The many variables involved in the production, ejection, transportation, and deposition of pyroclastic fragments (tephra) with wide ranges in size, density, and shape are complex, but amenable to sedimentary statistical analysis. The parameters discussed in this report are the variations—with respect to distance—of maximum size, Wentworth grade sizes, median diameters, and sorting coefficients. Plots of these data give qualitative information on ancient wind direction and strength and on the strength of volcanic eruptions. Maximum sizes of fragments ejected from a volcano decrease as the distance from the source increases. Log-log plots of maximum size versus distance for tephra from eruptions at Crater Lake, Oregon, and Mount Asama, Japan, indicate that the tephra deposited in the northeast direction from Crater Lake was from the more powerful eruption. Fragment sizes decrease with distance from Crater Lake and Mount Asama at similar rates, which suggests that wind velocities may have been similar during their deposition. Maximum size measurements of pumice fragments from a small tephra sheet near Medicine Lake in northern California demonstrate that the wind blew northeastward during the eruption of Little Glass Mountain. Contour maps of Wentworth grade sizes of the Crater Lake pumice fall and a contour map of median diameters from the same data show three size lobes instead of the two shown by the isopach map. The differences between the contour configuration of successive gradesize maps, which result from differing distributions of fragments, are interpreted as being caused by differing wind strengths and directions at various elevations to which the fragments were ejected. Median diameter values for 114 tephra samples suggest that the minimum limit for pyroclastic comminution approaches a value near 2 μ. Log-log plots of median diameter versus distance show a complex mathematical relationship between these two parameters. The sorting coefficient of tephra generally decreases with increasing distance from source, and, as with median diameter, the mathematical variation of sorting coefficients with distance is complex. It is of interest that the mode of sorting coefficients for the 114 samples on which data are available is between 1.2ϕ and 1.4ϕ, even though median diameters range from −4.74ϕ to 6.38ϕ for distances as great as 780 km.