Phreatic Eruption
"An explosive eruption driven by steam, occurring when magma heats groundwater or surface water, causing it to flash-boil instantenously."
A Phreatic Eruption (often called a steam blast) is one of the most dangerous and unpredictable types of volcanic activity. Unlike magmatic eruptions, which are driven by the pressure of new magma rising to the surface, phreatic eruptions are driven by water.
The Mechanism
- Heat Source: Hot rocks or magma reside just below the surface.
- Water Ingress: Groundwater, a crater lake, or melting ice seeps down into the hot rocks.
- Flash Boiling: The water is superheated beyond its boiling point but trapped under pressure.
- Failure: The overlying rock cracks, releasing the pressure. The water instantly flashes to steam, expanding 1,600 times in volume.
- Blast: The ground simply explodes, blasting pulverized old rock (ash), steam, and acidic mud into the air.
Why They Are Deadly
Phreatic eruptions give almost no warning. Because magma doesn’t need to move (which creates detectable earthquakes), monitoring equipment often stays silent until it’s too late. This was the cause of the tragedy at Whakaari / White Island in 2019 and Mount Ontake in 2014. In both cases, tourists were on the volcano when the groundwater system, without warning, detonated like a boiler explosion.
Key Characteristics
- No Fresh Magma: The ejected material is usually old, pulverized rock, not fresh lava.
- Speed: They happen in seconds.
- Steam Plumes: Often white or grey billowy clouds.