Phreatic eruptions at Whakaari Volcano driven by hydrothermal mineralisation and magmatic gas input

Phreatic eruptions, hydrothermally-driven explosive events that occur without direct magma involvement, are one of the most common types of volcanic eruption globally. They pose significant hazards due to their sudden onset and notoriously unpredictable nature. Numerical models are a vital tool to understand the geological, thermal, and hydraulic conditions under which phreatic eruptions occur. Here we show using multiphase 3D hydrothermal models that the magnitudes and timescales of subsurface pressurisation are highly dependent on permeability-reducing hydrothermal mineralisation and increased magmatic gas input from depth. Overpressure can exceed rock strength, causing eruption, within days in a sealed hydrothermal system with elevated magmatic gas input. Two to three months are required if magmatic gas input is low, or if mineralisation cannot effectively reduce seal permeability. We find that vapour condensation is induced by increasing pressure as magmatic-hydrothermal fluids ascend into the shallow system, contrary to conventional wisdom that boiling promotes phreatic eruptions. Temperature and gas flux increase at secondary degassing zones shortly after the onset of hydrothermal pressurisation, highlighting their potential as indicators for tracking unrest and forecasting eruptive activity. Our models are an important first step toward the quantitative, integrated interpretation of hydrothermally influenced monitoring data for improved volcanic eruption forecasting.