The Role of Tuffs in Sealing Volcanic Conduits

Acid-sulphate alteration commonly changes the physicochemical properties of volcanic conduits. Although several conduit pressurization models suggest hydrothermal sealing, little experimental evidence exists from conduit-filling rocks on the development of such a seal. Here we show that acid-sulphate alteration affects conduit-filling lavas and tuffs differently, with implications for their role in sealing the conduit. In lavas, alteration creates fluid pathways and decreases rock stiffness by dissolving primary minerals. In contrast, in the inherently porous tuffs, alteration reduces fluid pathways and increases rock stiffness by precipitating secondary minerals. Compaction of tuffs under subsurface pressures together with such alteration-related sealing can form low porosity and low permeability zones within the conduit. Such zones could promote fluid-pressure build-up and predispose the volcano to explosive eruptions. We discuss our results in the context of observed seismicity and ground deformation and suggest using our elastic properties data to constrain geophysical inversions of acid-sulphate altered volcanic conduits. Plain Language Summary In volcanoes, hot fluids circulate through the rocks and change their physical and chemical properties. Here we study how these changes occur in different rocks within the volcano's interior. We find that in lavas, rocks that generally have a low volume of pores, these fluids create new pathways to flow through by dissolving the rock-forming minerals. In contrast, fluid pathways in tuffs, which are inherently highly porous rocks, are blocked by the precipitation of new minerals in the pore spaces. Compaction under the pressure of overlying rocks in the volcano's interior can further reduce the fluid pathways through tuffs. Such reduction in fluid pathways within the volcano can aid the build-up of pressure, making them prone to explosive eruptions. Key Points In volcanic conduits, lavas undergo net dissolution and tuffs net secondary mineral precipitation The elasticity of conduit-filling lavas and tuffs also changes due to alteration Inherently porous and permeable tuffs, when compacted and highly altered, can form seals within volcanic conduits