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The Rattlesnake Tuff of eastern Oregon comprises >99% of high-INTRODUCTION silica rhyolite glass shards and pumices representing ~280 km 3 of The origin, structure, and differentiation processes in magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass zoned magma chambers have been approached by many shards cluster in five chemical groups that range in color from white workers through the study of ignimbrites since the to dark gray with increasing Fe concentration. Compositional clusters ground-breaking work of Smith (1960) and Smith & are defined by Fe,

The 7.05 Ma Rattlesnake Tuff covers ca. 9000 km 2, but the reconstructed original coverage was between 30000 and 40000 km 2. Thicknesses are remarkably uniform, ranging between 15 and 30 m for the most complete sections. Only 13% of the area is covered with tuff thicker than 30 m, to a maximum of 70 m. The present day estimated tuff volume is 130 km 3 and the reconstructed magma volume of the outflow is 280 km 3 DRE (dense rock equivalent). The source area of the tuff is inferred to be in the western Harney Basin, near the center of the tuff distribution, based mainly on a radial exponential decrease in average pumice size, and is consistent with a general radial decrease in welding and degree of post-emplacement crystallization. Rheomorphic tuff is found to a radius of 40-60 km from the inferred source.Four facies of welding and four of post-emplacement crystallization are distinguishable. They are: nonwelded, incipiently welded, partially welded and densely welded zones; and vapor phase, pervasively devitrifled, spherulite and lithophysae zones. The vapor phase, pervasively devitrifled and lithophysae zones are divided into macroscopically distinguishable subzones. At constant thickness (20 + 3 m), and over a distance of 1-3 km, nonrheomorphic sections can vary between two extremes: (a) entirely vitric sections grading from nonwelded to incipiently welded; and (b) highly zoned sections. Highly zoned sections have a basal non-to densely welded vitric tuff overlain by a spherulite zone Editorial responsibility: W. Hildreth

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