The Masaya Caldera Complex, NIC, with the active crater, Santiago (Fig. 1), is the site of present-day basaltic volcanism1,2. Some of the faults that bound the caldera and lie within it can be located geologically; others are postulated. A possible older caldera rim lies to the south of the present margin. To investigate whether one can identify faults in this environment of active volcanism, concentrations of radon and mercury in the regolith were determined at 232 selected points in and adjacent to the caldera. Higher concentrations of Hg0 or Rn or both occurred over 75% of the known structures studied. Some hitherto suspected structures were confirmed while others were not. Lack of a chemical signature may be ascribed to the absence of a structure, inappropriate sampling sites, post-fault lava cover or other sealing of the structure. We show here how the use of Rn and Hg0 determinations has helped in structural analysis at this active volcano.
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