Phreatic eruptions occur when magma, lava, hot rock, or volcanic deposits suddenly heat or boil surface or groundwater to steam, causing an explosion of steam, liquid water, ash, and volcanic blocks.
The boiling and evaporation of water occurs at extremely high temperatures of 500°C (932°F) and 1170°C (2138°F). Such temperatures cause the water to evaporate or flash into steam almost instantaneously.
Phreatic eruptions are also known as steam-blast eruptions, phreatic explosions, or ultravulcanian eruptions.
These eruptions will form a jet of steam, water, ash, volcanic bombs, and other debris. Some may create lateral surges.
Lateral surges are weaker pyroclastic flows, i.e., fast-moving, ground-hugging hot gases laden with particles.
Usually, ultravulcanian eruptions don’t involve direct interaction of magma or lava with external water. Instead, the steam will violently pulverize the confining near-country rocks.
Some authors consider this explosion synonymous with hydrothermal eruptions. It often forms maars.
The term phreatic comes from the Greek word phrear or phreat, which means water well or spring. It denotes the involvement of external water in the eruption.
Phreatic vs. phreatomagmatic eruptions
Phreatic and phreatomagmatic are hydrovolcanic eruptions driven by steam explosions involving external water. However, they are not the same.
Phreatomagmatic involves directly interacting ground or shallow surface water with rising magma. Therefore, it will produce mainly juvenile fragments. Juvenile means they form from rising magma.
However, it can have accidental clasts torn from country rocks. Also, the proportion of accidental clasts goes high with an increase in the depth of the explosion.
On the other hand, phreatic eruptions result from magma, magmatic fluids, material, or hot rocks heating water to steam that causes the explosion.
Although the heat source is magma, no direct interaction occurs. Therefore, the ejected clasts will have mainly accidental clasts, not juvenile magmatic materials.
Another thing about phreatic eruptions is that steam explosions often occur at lower temperatures than phreatomagmatic ones. However, they, too, can be quite violent and produce a lot of ash.
Examples of phreatic eruptions
Phreatic eruptions are common in volcanoes, tectonic rifts, and calderas. In some cases, they precede volcanic eruptions like Plinian, Hawaiian, Strombolian, or effusive eruptions. However, they can also occur alone.
For instance, phreatic eruptions include Mount St. Helens, Washington, USA (1980), preceding the Plinian eruption. Also, the Soufrière Hills in 1995 in Saint Vincent and the Grenadines preceded the lava flow at the summit.
Notable examples of phreatic eruption are Krakatoa (1883), Indonesia, Kilauea region in Hawaii, USA, and Ritter Island (1888) in Papua New Guinea.
Krakatoa destroyed most of the island and produced the loudest sound ever, and Ritter Island represents the largest lateral spreading of a volcanic cone known to humankind.
On the other hand, Kilauea is famed for its long history of steam explosions and 1924 eruptions that hurled an 8-ton rock a kilometer away.
More examples are Surtsey (1963-1965), Whakaari/White Island (2019), and Ruapehu (2007) in New Zealand, Mount Tarumae in Japan (1982), and Karthala (1991) in Comoros.
Others are the Taal Volcano eruption (1965, 1977, and 2020), the Mayon Volcanic eruption (2013), and Mt Bulusan (2022) in the Philippines and Poás (2006–2017) and Turrialba (2010-2017) in Costa Rica.
Lastly, these eruptions occurred in Cumbre Vieja (2021) in La Palma.
Hazards
Phreatic eruptions are relatively small, with less than <0.01 km3 ejected materials. However, they are hazardous since they suddenly occur without warning.
Also, some may emit poisonous gases. For instance, too much hydrogen sulfide or carbon dioxide will cause asphyxiation.
In 1979, toxic gases from these eruptions killed 179 people in Java, Indonesia.