Time‐dependent (Creep) Compaction in Volcanic Rocks

Abstract Porous volcanic rocks form volcanic edifices and host key geothermal reservoirs worldwide. When subjected to constant stress above dilatancy in the brittle regime, these materials undergo time‐dependent failure known as brittle creep. In the ductile regime, where deformation is compactive, time‐dependent compaction—compaction creep—has been documented in sandstones but never in lavas. Here, we present the first experimental investigation of compaction creep in porous lava from Volvic, France. We performed triaxial constant strain rate and compaction creep experiments at an effective pressure of 100 MPa. Our results show that lava accumulates inelastic strain and loses porosity continuously with decreasing rate during creep deformation. Creep strain rates weakly depend on the applied stress, and microstructural observations reveal that time‐dependent compaction localizes into compaction bands. These findings demonstrate that compaction bands can form slowly over extended periods in lava with implications for the long‐term evolution of volcanic systems and geothermal reservoirs.