"In April 1988 and again on 3 April 1990 we visited two hydrothermal springs [on Chiles] and collected samples. The first, La Calera, is a developed hot spring with baths just W of the town of Chiles and 8 km E of the crater of Chiles volcano, at 3,180 m elev. The maximum temperature was 40°C and pH was 6.2. No significant sulfur deposition was observed at the natural source of hot water, ~ 100 m uphill from the commercial baths. The second site, La Hedionda, was unsuccessfully developed as a tourist bath area, reputedly failing because of deadly levels of H₂S. It is 3.5 km E of the crater, at 3,470 m elevation. The uppermost hot spring, with a temperature of 54°C and pH of 5.1, was sampled. These acid sulfate springs were actively depositing native sulfur and had an almost overwhelming odor of H₂S. Fumarole samples were collected at both springs. No observations were made, on either visit, of the summit area, which was always covered by clouds. The observations at Chiles are consistent with a stable system, dominated by hydrothermal processes."

Geological Summary. The Chiles-Cerro Negro volcanic complex includes both the Pleistocene Chiles and the Cerro Negro de Mayasquer stratovolcanoes astride the Colombia-Ecuador border. Cerro Negro has a caldera open to the west, with andesitic and dacitic lava flows of possible Holocene age (Hall 1992, pers. comm.) and solfataras on the shore of a small crater lake. An eruption reported in 1936 may have been from Reventador (Catalog of Active Volcanoes of the World). The higher, glacier-covered summit of Chiles, about 4 km ESE of Cerro Negro, last erupted about 160,000 years ago, but it has a caldera open to the north with hot springs and an active hydrothermal system on its eastern flank.

Information Contacts: S. Williams, Louisiana State Univ; J. Stix and E. Fontaine, Univ de Montréal.