Abstract
The response of a large geothermal system to magmatic resurgence was analyzed by a survey of soil gas radon and elemental mercury at 600 sites in the silicic Long Valley Caldera, California. The broad geochemical anomaly over the caldera has superimposed on it a small zone of pronounced radon enrichment and mercury depletion coincident with the surface projection of a postulated dike of rising magma. Soil gas geochemistry studies can complement traditional geophysical and geodetical methods in the evaluation of potential volcanic eruption hazards.