Poás is characterized by intermittent explosions from its hot crater lake. Several occurred in 2014 (BGVN 40:11). This report covers activity from 1 January 2015 through February 2017. There were no reports of activity during 2015 through May 2016. Phreatic eruptions were recorded between 5 June and 16 August 2016.

According to news articles (La Prensa Libre, Prensa Latina), phreatic explosions from the hot crater lake occurred multiple times in June 2016. Explosions at 0900 on 5 June, at 1854 on 13 June, and at 1952 on 14 June ejected water and steam many meters above the lake's surface. Three small explosions, lasting about five seconds each based on the seismic signals, occurred during 0600-0603 on 18 June and ejected water, steam, and debris no more than 50 m above the lake's surface. Phreatic explosions were also registered on 19 June.

According to the Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA), a small phreatic explosion from the lake was recorded at 0819 on 25 July 2016. The explosion ejected material 50 m above the lake surface.

News accounts (Q Costa Rica, La Prensa Libre) reported that at 1409 local time on 16 August 2016 an explosion sent a column of gas to a height of 100 m above the crater; the activity lasted 2 minutes. An OVSICORI-UNA video of this explosion was posted in the news articles.

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/); La Prensa Libre (URL: https://www.laprensalibre.cr/); Prensa Latina (URL: http://www.plenglish.com/); Q Costa Rica News (URL: http://qcostarica.com/).