 Types of volcanic eruptions. Several types of volcanic eruptions during which lava, tetrahash, alopea-like, volcanic bombs and volcanic blocks and assorted gases are expelled from a volcanic vent or fissures 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. There are three different types of eruptions. The most well-observed are magmatic eruptions, which involve the decompression of gas within magma that propels it forward. The three other magmatic eruptions are another type of volcanic eruption, driven by the compression of gas within magma, the direct opposite of the process-powering magmatic activity. The third eruptive type is the phreatic eruption, which is driven by the superheating of steam via contact with magma. These eruptive types often exhibit no magmatic release. Instead causing the granulation of existing rock. Within these wide-defining eruptive types are several subtypes. The weakest are Hawaiian and Submarine, then Strombland, followed by Volcanian and Certician. The stronger eruptive types are Pylite eruptions, followed by Plinian eruptions. The strongest eruptions are called Ultraplinian. Glacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is volcanic explosivity index, the iron order of magnitude scale, ranging from 0 to 8 that often correlates to eruptive types.