Global Volcanism Program | Image GVP-09197
Escaping fumarolic gases pour from the sulfur-lined wall of a lava dome at the bottom of the active crater of Poás. The water filling the turquoise-colored crater lake is among the most acidic natural waters on Earth (pH = 0). The hot (roughly 50 degrees Centigrade) lake has been the source of frequent phreatic eruptions that produce geyser-like plumes of steam, ash, and mud. Pyroclastic sulfur has also been ejected.
Photo by Guillermo Alvarado (Instituto Costarricense de Electricidad).