<P> Phreatic eruptions (or steam - blast eruptions) are a type of eruption driven by the expansion of steam . When cold ground or surface water come into contact with hot rock or magma it superheats and explodes, fracturing the surrounding rock and thrusting out a mixture of steam, water, ash, volcanic bombs, and volcanic blocks . The distinguishing feature of phreatic explosions is that they only blast out fragments of pre-existing solid rock from the volcanic conduit; no new magma is erupted . Because they are driven by the cracking of rock strata under pressure, phreatic activity does not always result in an eruption; if the rock face is strong enough to withstand the explosive force, outright eruptions may not occur, although cracks in the rock will probably develop and weaken it, furthering future eruptions . </P> <P> Often a precursor of future volcanic activity, phreatic eruptions are generally weak, although there have been exceptions . Some phreatic events may be triggered by earthquake activity, another volcanic precursor, and they may also travel along dike lines . Phreatic eruptions form base surges, lahars, avalanches, and volcanic block "rain ." They may also release deadly toxic gas able to suffocate anyone in range of the eruption . </P> <P> Volcanoes known to exhibit phreatic activity include: </P> <Ul> <Li> Mount St. Helens, which exhibited phreatic activity just prior to its catastrophic 1980 eruption (which was itself Plinian). </Li> <Li> Taal Volcano, Philippines, 1965 . </Li> <Li> La Soufrière of Guadeloupe (Lesser Antilles), 1975--1976 activity . </Li> <Li> Soufrière Hills volcano on Montserrat, West Indies, 1995--2012 . </Li> <Li> Poás Volcano, has frequent geyser like phreatic eruptions from its crater lake . </Li> <Li> Mount Bulusan, well known for its sudden phreatic eruptions . </Li> <Li> Mount Ontake, all historical eruptions of this volcano have been phreatic including the deadly 2014 eruption . </Li> </Ul>

What are the 6 types of volcanic eruptions