Poás, located in Costa Rica, contains three summit craters along a N-S line. Its northern lake, Laguna Caliente, has been the site of frequent phreatic and phreatomagmatic eruptions since 1828. Eruptions often include geyser-like ejections of crater-lake water. The previous eruption occurred in April 2022 and was characterized by a phreatic explosion and an eruptive plume (BGVN 47:06). This report covers a new eruption during late July through September 2023 using reports from Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

OVSICORI-UNA reported that on 7 July volcanic tremor increased and fumarolic activity also intensified. An underwater image of the center of the lake showed a jet of gas-and-water mixed with sulfur rising 5 m above the bottom of the lake. The lake level began to decrease on 19 July by 1.2 m compared to the previous measurement taken on 6 July. A notable decrease in seismicity was reported during the week of 28 July; high amplitude levels were reported until 20 July. Pulses of short and intense tremor were recorded on 20 and 21 July. At 1652, on either 20 or 21 July (the date was not specified), a small hydrothermal eruption was reported. Sulfur dioxide emissions were measured at 149 ± 84 tons/day (t/d) during the last week of July.

A small phreatic eruption was recorded at 0153 on 3 August in the W part of the crater lake that ejected material 20 m above the water. Frequent, low-frequency volcanic earthquakes continued. Sulfur dioxide measurements were 150 ± 48 tons per day (t/d) during the first week of August. Another phreatic eruption was reported at 1655 on 5 August from three areas of the lake and ejected sediment and steam 50-100 m above the lake. At least two eruptive pulses were seen. Fumarolic degassing and lake convection were visible during 2-15 August. On 11 August at 0842 a small phreatic eruption at the center of the lake ejected material 5 m above the lake. Sulfur dioxide measurements were 175 ± 26 t/d during the week of 11 August. During the week of 18 August sporadic volcano-tectonic earthquakes were recorded to the N and E of the volcano and sulfur dioxide measurements decreased slightly to 120 ± 37 t/d. A small hydrothermal eruption was reported during the early morning of 21 August. Sulfur dioxide measurements and the lake level continued to fluctuate; the former was 152 ± 112 t/d during the week of 25 August.

OVSICORI-UNA reported a small phreatic eruption on 20 September at 1340 in the center of the crater lake. Sulfur dioxide measurements were 181 ± 50 t/d.

Geological Summary. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

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/).