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Changes in permeability can impact geological processes, geohazards, and geothermal energy production. In hydrothermal systems, high‐temperature heat sources drive fluid convection through the pore network of reservoir rocks. Additionally, thermal fluctuations may induce microfracturing and affect the mineralogical stability of the reservoir rock, thus modifying the fluid pathways and affecting permeability and strength. This study describes the results of thermal heating events lasting several hours on a “moderately altered” plagioclase‐clinochlore‐calcite‐quartz andesite and a “highly altered” plagioclase‐clinozoisite‐quartz‐clinochlore andesite from the Rotokawa Geothermal Field, New Zealand. We use a low thermal gradient (~1.2 °C/min) in an H2O‐saturated, 20‐MPa pressure environment to constrain changes in petrophysical properties associated with transitory thermal phenomena between 350 and 739 °C. As the treatment temperature increases, the mass reduces, while porosity and permeability increase. These effects were greater in the “moderately altered” andesite than in the “highly altered” andesite. Microfracturing is responsible for these changes at lower temperatures (e.g., ≤400 °C). At higher temperatures (e.g., >400 °C), microfracturing remains partially responsible for these rock property changes (e.g., higher permeability); however, these changes are also a product of clinochlore, quartz, and (when present) calcite reacting out of the altered andesite, and increasing porosity. We propose that at temperatures >400 °C, volumetric phase changes associated with heat‐driven reactions in a wet environment can contribute to microcracking and porosity/permeability changes. Our data support observations where high‐temperature conditions at the margins of magma bodies can be associated with substantial increased permeability and decreased strength.

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