Exploratory drilling has confirmed the existence of a geothermal system in the Los Humeros volcanic center, located 180 km east of Mexico City. Volcanic activity in the area began uith the eruption of andesites, folloued by two major caldera-forming pyroclastic eruptions. The younger Los Potreros caldera is nested inside the older Los Humeros caldera. At later stages, basaltic andesite, dacite, and olivine basalt lavas erupted along the ring-fracture zones of both calderas.

Geologic interpretation of structural, geophysical, and drilling data suggests that8 (1) The water-dominated geothermal reservoir is hosted by the earliest andesitic volcanic pile, is bounded by the ring-fracture zone of the Los Potreros caldera, and is capped by the products of the oldest caldera-forming eruption. (2) Permeability uithin the andesitic pile is provided by faults and fractures related to intra caldera uplift. (3) The geothermal system has potential for a large influx of meteoric uater through portions of the ring-fracture zones of both calderas. (4) Volcanic centers uith similar magmatic and structural conditions can be found in the eastern Cascades, U.S.A.

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