Types of volcanic eruptions
Several types of volcanic eruptions—during which material is expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.
"Eruption" redirects here. For other uses, see Eruption (disambiguation).
There are three main types of volcanic eruption:
Within these broad eruptive types are several subtypes. The weakest are Hawaiian and submarine, then Strombolian, followed by Vulcanian and Surtseyan. The stronger eruptive types are Pelean eruptions, followed by Plinian eruptions; the strongest eruptions are called Ultra-Plinian. Subglacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is the Volcanic Explosivity Index an order-of-magnitude scale, ranging from 0 to 8, that often correlates to eruptive types
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[63] and thrusting out a mixture of steam, water, ash, volcanic bombs, and volcanic blocks.[64] 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.[65] 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.[63]
Often a precursor of future volcanic activity,[66] phreatic eruptions are generally weak, although there have been exceptions.[65] Some phreatic events may be triggered by earthquake activity, another volcanic precursor, and they may also travel along dike lines.[63] 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.[66]
Volcanoes known to exhibit phreatic activity include: