Activity at Rincón de la Vieja, in Costa Rica, is typically characterized by weak phreatic explosions that generate gas-and-steam emissions and plumes. This report covers activity during March-August 2021, and is based primarily on weekly bulletins from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellites. Almost all of the reported explosions during March-August 2021 (table 6) were identified by OVSICORI-UNA as being phreatic; no explosions occurred after 13 December 2020 until 29 May 2021.

Table 6. Summary of activity at Rincón de la Vieja during March-August 2021 from weekly bulletins. Courtesy of OVSICORI-UNA.

The Alert Level was raised from 2 to 3 following the renewal of heightened activity on 29 May 2021, when two phreatic explosions were recorded. Additional minor phreatic explosions were detected on 1, 6, 15, and possibly 20 June. A more powerful explosion on 28 June generated a plume of steam, gas, and ash that rose 2 km above the crater rim and ejected material onto its flanks (figures 36 and 37). Part of the plume rose up to 5 km above the crater (7 km altitude), as measured from Turrialba volcano, more than 200 km SE. Residents in Gavilán and Dos Ríos (7 km NNW) and Bromelias (6 km NNE) reported volcanic gas odors and ashfall. Lahars descended multiple drainages on the N flank. OVSICORI-UNA officials were quoted by news organizations (The Costa Rica News and CNN) as stating that the 28 June activity was the most powerful explosion since at least 2011. Intermittent explosions continued through July and August, but weather conditions typically prevented visual observations.

Figure 36. Image of the eruption plume from the 28 June 2021 explosion at Rincón de la Vieja taken at 0542 local time. Courtesy of OVSICORI-UNA.

Figure 37. View of the ash-and-steam plume following the 28 June 2021 explosion at Rincón de la Vieja. Courtesy of OVSICORI-UNA, as published by Periódica Mensaje.

Seismicity was low during the reporting period, with occasional long-period (LP) earthquakes, tornillos, volcano tectonic events, and tremor. In March the frequency and amplitude of background tremor decreased, and remained low in April. After 24 April, short tremors of large amplitude and low frequency (below 1Hz) were recorded until early May, when they decreased in amplitude. During May and June, the number of LP earthquakes was 0-2 events per day. Before the 31 May explosion, tremor was banded (series of "tremor packets" occurring at almost regular intervals) with frequencies below 2 Hz. During the first week of June, tremor frequency was around 2.2 Hz, but decreased toward the end of the month. Tremor on 26 June had a very low amplitude and almost monochromatic frequency around 2.2 Hz. During the four days preceding the major explosion on 28 June, the number of LP earthquakes had increased from 3 to 9 daily events. Tremor had stopped by 3 August, but increased again on 30 August.

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: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); The Costa Rica News (URL: https://thecostaricanews.com/); CNN (URL: https://www.cnn.com/); Periódica Mensaje (URL: https://www.periodicomensaje.com/).