Small-scale phreatic explosions from a low-enthalpy hydrothermal system caused the abandonment of Milos Island (Greece) in Roman times.

This study investigates the mechanisms underlying phreatic explosions within the geothermal system of eastern Milos Island. Field, laboratory and historical data suggest that a long-standing hydrothermal system, characterized by silica-rich crusts over altered rhyolitic lava domes and volcaniclastics, experienced multiple explosive events leading to its eventual disruption. The explosions, occurring at depths of 3-20 m, were likely triggered by rapid depressurization, possibly induced by seismic activity. Intermediate to large-magnitude local earthquakes or large-magnitude regional earthquakes, such as the AD 365 Crete event, could have generated dynamic stress sufficient to destabilize the system, leading to cavitation-driven explosions. Overlapping craters and deposits indicate repeated explosive activity over years, ultimately resulting in the exhaustion of the hydrothermal system. Archaeological evidence, including Roman-age pottery beneath phreatic deposits, suggests that the explosions were sudden and unanticipated. The concurrent decline in settlement activity on Milos during the fourth century AD may be linked to these disruptions, in combination with broader regional seismic and socio-economic factors. This study highlights the sensitivity of hydrothermal systems to external stressors and the potential role of seismic events in triggering phreatic explosions.