Abstract

Multiple generations of ductile and brittle deformation recorded in the Obsidian Dome lava, California, inform on the rheological complexities of silicic lava emplacement. Understanding the non-linear rheological evolution of advancing lava is key to improving understanding of the durations, extents, and explosive hazards of future eruptions. Numerous studies of silicic lavas, at Obsidian Dome in particular, have focused on micro-scale textures and features interpreted to record flow within the conduit, primarily. How the lava flowed as a growing mass outside the conduit is largely unconstrained. Field observations at Obsidian Dome identify cycles of ductile flow followed by brittle fracturing, followed by relaxation and continued flow. Mode 1 vertical fractures often served as conduits for tephra venting, presumably following spontaneous exsolution-driven vesiculation and volume expansion deeper in the lava. Tephra, often tack-welded to the fracture surfaces, are preserved in many fractures that clearly closed-up on relaxation of the obsidian. Therefore, crossing the glass transition was likely strain rate driven, rather than cooling- or exsolution-driven that would have inhibited relaxation after the stress was released. The interplay between flow and fracturing is also evident in the ubiquity of crease structures at decimeter to decameter scales over all of Obsidian Dome. Unraveling the complexities of flow and fracturing in obsidian lavas will require and enable improved understanding of similarly non-linear rheological evolutions in ice, salt, the mid crust, and the asthenosphere.