Graph showing average cumulative weight percent of ash deposits for...

Graph showing average cumulative weight percent of ash deposits for different ash sizes (in phi units) at sites with increasing distance from Mount St. Helens volcano. The range of grain size for each site is shown by a different line.

Note that grain sizes become smaller with increasing distance from the volcano (the lines move progessively to the right with corresponding smaller grain size as the distance from the volcano increases). At a distance of 152 km, the ash deposit does not contain particles larger than 2 phi (0.5 mm, equivalent to coarse sand). Beyond about 300 km from vent, the overall grain size distribution of the deposits were similar (compare the right three lines). (Sarna-Wojcicki and others, 1981)