Influence of aquifer lithology on priming conditions and eruptive mechanisms in large-scale hydrothermal eruptions

Abstract Large-scale hydrothermal eruptions, although rare, pose significant hazards in geothermal regions. While aquifer lithology is known to influence fluid dynamics and eruption initiation for smaller-scale events, systematic studies linking lithological properties to large-scale eruptions remain limited. Here, we investigate the role of lithological variability in priming and eruption mechanisms at Rotokawa, New Zealand. We measured petrophysical properties and conducted rapid decompression experiments on disrupted aquifer lithologies, including consolidated lacustrine silts, clay-altered vitric tuffs, silicified volcaniclastic sandstones, and silicic-argillic altered paleo-breccia. Lithologies cover a broad range of properties and fragmentation characteristics, namely, density (0.9–1.9 g/cm 3 for bulk, 2.5–3.2 g/cm 3 for skeletal), porosity (3.7–63.5%), permeability (1.2 × 10 −17 –1.2 × 10 11 m 2 ), compressional wave velocity (0.9–5.2 m/s), fragmentation threshold (2–22.5 MPa), energy density (1.3–12.8 MJ/m 3 ), and fragmentation velocity (1–160 m/s). We evaluate contrasting priming scenarios for large hydrothermal eruptions, from rapid lake drainage to influx of deep hot fluids with either localized or widespread mixing. Results indicate that widespread and efficient mixing of hot and cold fluids in clay-altered aquifer lithologies is essential to reach energy thresholds for large-scale eruptions. Lithologies that have undergone argillic alteration are mechanically weakened and promote fragmentation, whereas silicification increases fragmentation thresholds by a factor of 2–3. We propose a conceptual model for the Rotokawa eruption in which initiation and progression are governed by lithology-dependent fragmentation energies, ranging from 1.6 to 8.5 MJ m 3 in lacustrine silts and sands, 1.3–6 MJ m 3 in altered vitric tuffs, 1.6–6.5 MJ m 3 in silicified sandstones, and 1.3–12.8 MJ m 3 in paleo-breccias. These findings refine estimates of the energy required for hydrothermal eruptions and define conditions favoring large-scale events. By improving understanding of priming conditions, eruption mechanisms, and lithological susceptibility, this study enhances forecasting and hazard mitigation strategies in geothermal systems.