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Poás volcano (Costa Rica): sudden explosive eruption yesterday morning came without warning

View of Poás volcano the morning before the eruption with visitors near the crater where it occurred later at night (image: OVSICORI-UNA)

View of Poás volcano the morning before the eruption with visitors near the crater where it occurred later at night (image: OVSICORI-UNA)

An eruption occurred yesterday (7 April) morning at 2.42 a.m. local time from the volcano’s crater northern area. The event lasted about 3 minutes and generated a steam plume that rose 500 m from the crater to approx. 3200 m altitude.
There was apparently no warning for the eruption, which came without detectable precursors, such as increases seismic activity, an increase in temperature at the fumaroles or other geophysical changes. During other eruptions of Poás, the temperature at the northern crater rim’s fumaroles often had been seen increasing from 100 to 900°C, volcanologist Javier Pacheco from Costa Rica’s monitoring agency OVSICORI-UNA explained in an interview with La Nación. This had not been the case yesterday. As the eruption seems to have not involved any new magma, but almost purely ejected steam, it was likely a phreatic (steam-driven) explosion.



It is actually rather typical that such phreatic explosions occur with little or no warning at an active volcano: Fluids, water and gasses, contained in the subsurface system of cracks and cavities normally form a circulating system that allows heat and excess gasses that can no longer be stored in solution to be transferred to the surface, where degassing takes place, often in the form of fumaroles or hot crater lakes.
If this system is disturbed, however, for example if pockets of fluids become blocked or overheated, they might suddenly explode into steam or overcome the strength of the surrounding conduit and generate violent explosions, that involve no magma itself, but eject steam and fragmented rock material as well as large blocks to considerable distances. Such sudden eruptions are the largest near-area hazard at active volcanoes that are currently not erupting.

Video of the eruption: