A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions 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 main types of volcanic eruptions. Magmatic eruptions involve the decompression of gas within magma that propels it forward. Phreatic eruptions are driven by the superheating of steam due to the close proximity of magma. This type exhibits no magmatic release, instead causing the granulation of existing rock. Phreatomagmatic eruptions are driven by the direct interaction of magma and water, as opposed to phreatic eruptions, where no fresh magma reaches the surface. 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.
Mechanisms
Volcanic eruptions arise through three main mechanisms:
Gas release under decompression, causing magmatic eruptions Ejection of entrained particles during steam eruptions, causing phreatic eruptions Thermal contraction from chilling on contact with water, causing phreatomagmatic eruptions In terms of activity, there are explosive eruptions and effusive eruptions. The former are characterized by gas-driven explosions that propel magma and tephra. The latter pour out lava without significant explosion.
Impact Volcanic eruptions vary widely in strength. On the one extreme there are effusive Hawaiian eruptions, which are characterized by lava fountains and fluid lava flows, which are typically not very dangerous. On the other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events. Volcanoes are not bound to one eruptive style, and frequently display many different types, both passive and explosive, even in the span of a single eruptive cycle. Volcanoes do not always erupt vertically from a single crater near their peak, either. Some volcanoes exhibit lateral and fissure eruptions. Notably, many Hawaiian eruptions start from rift zones. Scientists believed that pulses of magma mixed together in the magma chamber before climbing upward—a process estimated to take several thousands of years. Columbia University volcanologists found that the eruption of Costa Rica's Irazú Volcano in 1963 was likely triggered by magma that took a nonstop route from the mantle over just a few months.
Volcanic explosivity index
The volcanic explosivity index (commonly shortened to VEI) is a scale, from 0 to 8, for measuring the strength of eruptions but does not capture all of the properties that may be perceived to be important. It is used by the Smithsonian Institution's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows. It operates in a way similar to the Richter scale for earthquakes, in that each interval in value represents a tenfold increasing in magnitude (it is logarithmic). The vast majority of volcanic eruptions are of VEIs between 0 and 2.
Magmatic Magmatic eruptions produce juvenile clasts during explosive decompression from gas release. They range in intensity from the relatively small lava fountains on Hawaii to catastrophic Ultra-Plinian eruption columns more than 30 km (19 mi) high, bigger than the eruption of Mount Vesuvius in 79 AD that buried Pompeii.
Hawaiian
Hawaiian eruptions are a type of volcanic eruption named after the Hawaiian volcanoes, such as Mauna Loa, where this eruptive type is a hallmark. Hawaiian eruptions are calm volcanic events, characterized by the effusive eruption of very fluid basalt-type lavas with low gaseous content. The volume of ejected material from Hawaiian eruptions is less than half of that found in other eruptive types. Steady production of small amounts of lava builds up the large, broad form of a shield volcano. Eruptions are not centralized at the main summit as with other volcanic types, and often occur at vents around the summit and from fissure vents radiating out of the center. Hawaiian eruptions often begin as a line of vent eruptions along a fissure vent, a so-called "curtain of fire." These die down as the lava begins to concentrate at a few of the vents. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery scoria fragments; when the air is especially thick with clasts, they cannot cool off fast enough due to the surrounding heat, and hit the ground still hot, the accumulation of which forms spatter cones. If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long lived; Puʻu ʻŌʻō, a volcanic cone on Kilauea, erupted continuously for over 35 years. Another Hawaiian volcanic feature is the formation of active lava lakes, self-maintaining pools of raw lava with a thin crust of semi-cooled rock.
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