Volcanic Glass in Soils of the Columbia Plateau, Pacific Northwest, USA

Eolian soils on the Columbia Plateau are downwind of volcanoes of the Cascade Range, but content of volcanic glass has not been known. Because of the need for information on soil properties that affect wind erosion, we collected 174 surface samples (0–15 cm) of Columbia Plateau soils. We wanted to assess the influence of glass on these soils, to determine the source of glass, and to develop an interpretive model for the observed regional pattern of glass. We sonicated samples while wet sieving them to quantitatively separate 20‐ to 53‐ and 53‐ to 106‐mm fractions and point counted glass using a polarizing microscope. Content of glass in the 20‐ to 53‐μm fraction averages 5.9% in samples from the southwestern plateau and increases to average 27.5% in the northeastern plateau. A kriged map of glass content confirmed the pattern of increasing glass content to the northeast, away from volcanoes of the Cascade Range. Microprobe analysis confirmed that glass in the samples is from eruptions both of Mount St. Helens (1980) and Mount Mazama (6850 before present [BP]). The spatial pattern of glass content, however, did not correspond with the fallout pattern from the 1980 eruption. The spatial pattern resulted instead from bioturbation of a relatively uniform quantity of 6850 yr BP Mazama tephra into a northeastern‐thinning sheet of post‐6850 BP loess. Soil erodibility is highest in Columbia Plateau soils with very low glass contents and is lowest in soils with high glass content, so it would be premature to attribute high wind‐erosion susceptibility of these soils to volcanic glass content alone. The average content of glass in surface samples is 12.1%, thus the soils may be termed ash influenced or volcanic influenced but are not volcanic per se.