"On 1 April, an eruption of Rincón de la Vieja was heard and seen by residents of the town of Buenos Aires de Upala, 7 km NE of the crater. This phreatic eruption originated from the active crater.

"Various rivers have headwaters on the N flank and flow NE (among them the Río Azul, Quebrada Azufrada, and Río Pénjamo) and lahars formed in some of them. At about 500 m above sea level, the Río Pénjamo and the Quebrada Azufrada left their channels, flooding agricultural land. No lahar formed in the Río Azul, although on 9 April it still had a gray color because of the quantity of suspended ash that it was carrying.

"According to a resident of the area, the eruption occurred at 0940. Twenty minutes later the lahar passed near the town of Buenos Aires. The seismic station of the Observatorio Vulcanológico y Sismológico registered an event at 0932.55 with a duration of 3 minutes."

Geological Summary. Rincón de la Vieja is a volcanic complex in the Guanacaste Range of NW Costa Rica. Sometimes referred to as the Rincon de la Vieja-Santa María Volcanic Complex, it consists of a slightly arcuate 20-km-long ridge of 12 craters and pyroclastic cones 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. The Santa María cone, the highest peak of the complex, is located on the E side of the ridge and has a lake within the 400-m-diameter 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 reported eruptions possibly dating back to the 16th century, have been from the active crater, near the center of the complex, with an acidic 300-m-diameter lake.

Information Contacts: J. Barquero, OVSICORI.