Hydrothermal Sealing at Turrialba, Poás, and Rincón de la Vieja Volcanoes (Costa Rica) Promotes Phreatic and Phreatomagmatic Eruptions

Hydrothermal sealing is a mechanism thought to be responsible for challenging‐to‐forecast phreatic and phreatomagmatic eruptions. To better understand this process, we present a study in which we assessed the textural, mineralogical, and physical properties (porosity, permeability, and tensile strength) of blocks of hydrothermal seal that were explosively expelled as ballistics during recent activity at Turrialba, Poás, and Rincón de la Vieja (Costa Rica). The hydrothermal seals consist of altered lavas and hydrothermal breccias comprising angular fragments cemented by hydrothermal minerals. The seal‐forming materials have high porosity (typically >0.2), low tensile strength (typically <4 MPa), and low permeability (as low as 10 −18 m 2 ). Mineralogical and petrophysical similarities between hydrothermal seals from five volcanoes (including Whakaari, New Zealand, and Tokachidake volcano, Japan) speaks to the commonality of the process of hydrothermal sealing, and the importance of alunite group minerals as a cementing material. We first present equations that provide permeability and tensile strength estimations for a seal of known porosity. Hydromechanical modeling of fluid injection into a shallow volcanic system, guided by our laboratory data, highlights that: (a) overpressures are often within the range of measured tensile strengths; and (b) the likelihood of an eruption increases if fluid injection rate increases, permeability decreases, and/or tensile strength decreases. The modeling predicts small‐magnitude changes to vertical displacement, but suggests that tiltmeters close to the conduit could detect shallow hydrothermal sealing and pressurization. Understanding hydrothermal sealing and forecasting eruptions requires a multidisciplinary approach in which monitoring signals are coupled with laboratory and modeling insights.