<P> Several types of volcanic eruptions--during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are 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 . </P> <P> 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 . Phreatomagmatic 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 . </P> <P> Within these wide - defining 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 Volcanic Explosivity Index (VEI), an order of magnitude scale ranging from 0 to 8 that often correlates to eruptive types . </P>

What propels magma out of a volcano during an eruption