Viscosity, deformation and permeability of bubbly magma: Applications to flow and degassing in volcanic conduits.

摘要

Explosive eruptions are driven by the rapid expansion of bubbles in magma during flow up volcanic conduits. Consequently, the extent to which gases escape from magma into cracked rocks or out the top of the volcano will affect the style and destructive potential of an eruption. This dissertation explores relationships among magma flow, bubble deformation and permeability development, and considers implications for velocity profiles and degassing in conduits during silicic eruptions. The viscosity of bubbly magma, as well as the shapes and orientations of bubbles in flowing magma, are studied using an analog system: air bubbles in corn syrup. These results help with interpretations of textures in volcanic rocks to infer processes occurring inside volcanoes. The physical mechanisms by which magmas lose volatiles are assessed from trends in the volatile contents of obsidian, textures of pumice and obsidian, and permeability measurements of pumice and dome samples. The generation of distinctive precursory and syn-eruptive seismic signals by magmatic gases or magmas flowing through cracks in rocks is investigated theoretically.; Based on the results of analog experiments combined with chemical and textural analysis of natural samples, a physical picture of magma ascent during an explosive rhyolite eruption is developed. The shear-thinning rheology of bubbly magma causes "plug" flow with a large volume of relatively non-deformed material in the center of the conduit, and shear localization in magma closer to the walls. Thus most pumice has round bubbles but vesicular magma that was closer to the walls forms tube pumice with stretched bubbles due to higher shear stresses. Along the walls, shear stresses are so great that the shear strength of the magma is exceeded causing the magma to fracture and anneal multiple times. This brecciation incorporates lithic material into the magma, and creates a highly permeable fractured zone at the conduit margin that allows efficient degassing and fluxing of aqueous magmatic fluids.; This dissertation includes co-authored materials submitted for publication.