Rock properties within highly active hydrothermal systems, a case study of Poás volcano

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Understanding the physical properties of volcanic rocks is critical for assessing the stability and eruptive behaviour of hydrothermal systems. At Poás volcano, Costa Rica, rock physical and mechanical properties vary dramatically in response to hydrothermal alteration, exerting strong controls on fluid migration, pressurization, the initiation of phreatic and phreatomagmatic eruptions, and instability and collapse. Poás volcano provides an exceptional natural laboratory for examining these relationships due to its persistent unrest, dynamic crater lake and hydrothermal system, and well-documented eruptive activity. In this study, we characterize the physical and mechanical properties of altered rocks from the active crater of Poás, including porosity, P-wave velocity, permeability, thermal properties, and uniaxial compressive strength. Our results demonstrate that hydrothermal alteration at Poás produces highly heterogeneous rock frameworks characterized by contrasting physical properties. Alteration tends to reduce primary porosity while simultaneously generating secondary pore networks through mineral dissolution, resulting in complex changes to permeability. Uniaxial compressive strength is strongly diminished in highly altered rocks, particularly where weak secondary minerals replace the original mineral assemblage, increasing the susceptibility of shallow crustal materials to mechanical failure. The spatial distribution of permeability barriers and mechanically weakened zones thus influences both the location and style of eruptive behaviour and the location and size of failure and collapse. By linking measured rock properties to hydrothermal processes, eruptive mechanisms, and instability and collapse, this work provides a framework for evaluating how alteration modulates hazard at Poás and similar volcanic systems. Our findings underscore the importance of characterizing rock physical properties in active hydrothermal environments to better anticipate the conditions that engender volcanic hazards.