The following reports are from the Observatorio Vulcanológico y Sismologico de Costa Rica (OVSICORI). "After several attempts to climb to the summit of Rincón de la Vieja were turned back by bad weather, we reached the active crater on 19 April with W. Melson of the Smithsonian Institution. During this visit, we were able to confirm that there had been a recent eruption, since we encountered recently erupted material and a devastated area, both SE of the crater.

"At the time of our previous ascent, in August 1985, this tephra had not been deposited. In a photograph taken 25 November 1985 by E. Valverde, it is possible to observe white tephra. In addition, the seismic station at the base of the volcano registered an increase in activity (harmonic tremor and A- and B-type events) between September and November, suggesting that the eruption occurred during that time.

"A fan-shaped area of about 0.25 km³ was affected. The ejecta reached a maximum distance of 500 m SE of the active crater. The erupted material is secondary, including ash, sand, and blocks as much as 20 cm across. On one rock, an ash deposit 6 cm thick was observed. The effect of the eruption on rain forest vegetation was marked about 500 m SE of the crater (in the E bank of the Quebrada Azufrosa) where trees had been knocked down in a radial pattern by the activity. This pattern is unusual in that the fallen trees appeared to radiate from a point near their center, not from the crater. In addition, various plant species in this area were affected by the acid in the pyroclastics and the associated water.

"On 19 April there was a strong and constant emission of gas that affected breathing because of its acidity, and made it difficult to observe the lake in the active crater."

Geological Summary. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: J. Barquero and E. Fernández Soto, OVSICORI.