A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when magma heats ground or surface water. The extreme temperature of the magma (anywhere from 600 to 1,170 °C (1,112 to 2,138 °F)) causes near-instantaneous evaporation to steam, resulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helens, hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano. In 1949, Thomas Jaggar described this type of activity as a steam-blast eruption.

Phreatic eruptions typically include steam and rock fragments; the inclusion of lava is unusual. The temperature of the fragments can range from cold to incandescent. If molten material is included, the term phreato-magmatic may be used. These eruptions occasionally create broad, low-relief craters called maars. Phreatic explosions can be accompanied by carbon dioxide or hydrogen sulfide gas emissions. The former can asphyxiate at sufficient concentration; the latter is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 142 people, most of whom were overcome by poisonous gases.