The Ar36 content of densely welded glasses from ash-flow units provides a means by which the amount of air entrapped and subsequently resorbed by the glasses during compaction and welding may be calculated. The amount of air measured in glasses from nine upper Tertiary ash-flow sheets in the western United States ranges from 0.033 to 13 ppm; median is about 1 ppm. These values are very small compared with the total amounts of volatiles which probably were incorporated during welding. The data strongly suggest that large volumes of air are not incorporated by ash flows during their eruption and lateral movement.

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