On 30 September, OVSICORI-UNA reported phreatic eruptions from Rincón de la Vieja's active crater during the previous six weeks. A well-documented event on 16 September ejected sediment that covered sections of the upper N walls. Some of the material was washed down the flanks and caused changes in water quality along the main creeks and major rivers up to 18 km away from the source. The death of several fish species was noted the next day and sediment deposits 10-15 cm deep were sampled 2 km N of the active crater. Scientists conducting fieldwork during 27-29 September observed that the hot lake was convecting and grayish in color due to the suspended sediments. Preliminary deformation and temperature measurements did not indicate any significant changes.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)