Origin of pumiceous and glassy textures in rhyolite flows and domes

Surface mapping and microscopic observation of textures in glassy and pumiceous rocks from several groups of silicic lava flows and domes, along with drill cores of the Inyo Scientific Drilling Project, indicate that development of the textural stratigraphy of the flows is controlled by a combination of cooling, microfracturing, and migration of gases released by crystallization. The Inyo cores have provided near-vent and distal views of the interior of the 550-yr-old Obsidian Dome rhyolite flow, as well as a profile through the unerupted portion of its feeder dike. The flow stratigraphy revealed in the drill core and in the fronts of several other Holocene-age silicic flows consists of a finely vesicular pumice carapace underlain successively by obsidian, coarsely vesicular pumice, obsidian with lithophysae, crystalline rhyolite, more obsidian with lithophysae, and basal breccia. The obsidian layers form where rapid cooling inhibits diffusion of ions and prevents crystallization. The transition from surface pumice to obsidian is controlled by the depth at which overburden pressure suppresses vesiculation. The thickness of the rigid crust is determined by the rapid decrease in the lava’s temperature-dependent yield strength with depth. The coarsely vesicular pumice layer forms as gases released by crystallization rise through microcracks and are trapped beneath the rigid pumiceous surface layer. Thickness and buoyancy of the coarsely vesicular pumice layer increase with flow length, eventually giving rise to diapirs that rise to the surface of the largest flows. Increasing gas content of the coarsely vesicular pumice layer in active flows can also lead to such volcanic hazards as explosive craters on distal flow surfaces or pyroclastic flows triggered by collapse of flow fronts.