Abstract

Recent drilling of 550- and 140,000-yr-old rhyolite lava flows in the Inyo Dome chain, California, and Valles Caldera, New Mexico, and field inspection of 8 Ma flows in western Arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized. The upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian, coarsely vesicular pumice, and a second obsidian layer. Beneath these layers is the crystalline center of the flow, which in turn is underlain by a basal obsidian layer (lower vitrophyre). Differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence, devitrification, lava rheology, and migration of water vapor.

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