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A volcanic eruption occurs when material is expelled from a volcanic vent or fissure vent. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous 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 , followed by ; 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.
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.
| +Volcanic eruptions by VEI index
! VEI
! Plume height
! Eruptive volume *
! Typical eruption type ! Frequency ** ! Example | |||||
| 0 | < | Hawaiian | Continuous | Kīlauea | |
| 1 | Hawaiian/Strombolian | Daily | Stromboli | ||
| 2 | † | Strombolian/Vulcanian | Fortnightly | Galeras (1992) | |
| 3 | Vulcanian | 3 months | Nevado del Ruiz (1985) | ||
| 4 | Vulcanian/Peléan | 18 months | Eyjafjallajökull (2010) | ||
| 5 | > | Plinian eruption | 10–15 years | Mount St. Helens (1980) | |
| 6 | > | Plinian/Ultra-Plinian | 50–100 years | Mount Pinatubo (1991) | |
| 7 | > | Ultra-Plinian | 500–1000 years | Mount Tambora (1815) | |
| 8 | > | Supervolcano | 50,000+ years (2025). 9781444332605, Wiley-Blackwell. ISBN 9781444332605 (2025). 9781473601703, Teach Yourself. ISBN 9781473601703 | Lake Toba (74 k.y.a.) | |
| * This is the minimum eruptive volume necessary for the eruption to be considered within the category. ** Values are a rough estimate. † There is a discontinuity between the 1st and 2nd VEI level; instead of increasing by a magnitude of 10, the value increases by a magnitude of 100 (from 10,000 to 1,000,000). | |||||
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 (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 , 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 , self-maintaining pools of raw lava with a thin crust of semi-cooled rock.
Flows from Hawaiian eruptions are basaltic, and can be divided into two types by their structural characteristics. Pahoehoe lava is a relatively smooth lava flow that can be billowy or ropey. They can move as one sheet, by the advancement of "toes", or as a snaking lava column. A'a lava flows are denser and more viscous than pahoehoe, and tend to move slower. Flows can measure thick. A'a flows are so thick that the outside layers cools into a rubble-like mass, insulating the still-hot interior and preventing it from cooling. A'a lava moves in a peculiar way—the front of the flow steepens due to pressure from behind until it breaks off, after which the general mass behind it moves forward. Pahoehoe lava can sometimes become A'a lava due to increasing viscosity or increasing rate of shear, but A'a lava never turns into pahoehoe flow.
Hawaiian eruptions are responsible for several unique volcanological objects. Small volcanic particles are carried and formed by the wind, chilling quickly into teardrop-shaped volcanic glass fragments known as Pele's tears (after Pele, the Hawaiian volcano deity). During especially high winds these chunks may even take the form of long drawn-out strands, known as Pele's hair. Sometimes basalt aerates into reticulite, the lowest density rock type on earth.
Although Hawaiian eruptions are named after the volcanoes of Hawaii, they are not necessarily restricted to them; the highest lava fountain recorded was during the 23 November 2013 eruption of Mount Etna in Italy, which reached a stable height of around for 18 minutes, briefly peaking at a height of .
Volcanoes known to have Hawaiian activity include:
The term "Strombolian" has been used indiscriminately to describe a wide variety of volcanic eruptions, varying from small volcanic blasts to large eruption column. In reality, true Strombolian eruptions are characterized by short-lived and explosive eruptions of lavas with intermediate viscosity, often ejected high into the air. Columns can measure hundreds of meters in height. The lavas formed by Strombolian eruptions are a form of relatively viscous lava, and its end product is mostly scoria. The relative passivity of Strombolian eruptions, and its non-damaging nature to its source vent allow Strombolian eruptions to continue unabated for thousands of years, and also makes it one of the least dangerous eruptive types.
Strombolian eruptions eject and lapilli fragments that travel in parabolic paths before landing around their source vent. The steady accumulation of small fragments builds composed completely of basaltic . This form of accumulation tends to result in well-ordered rings of tephra.
Strombolian eruptions are similar to Hawaiian eruptions, but there are differences. Strombolian eruptions are noisier, produce no sustained eruption column, do not produce some volcanic products associated with Hawaiian volcanism (specifically Pele's tears and Pele's hair), and produce fewer molten lava flows (although the eruptive material does tend to form small rivulets).
Volcanoes known to have Strombolian activity include:
Initial Vulcanian activity is characterized by a series of short-lived explosions, lasting a few minutes to a few hours and typified by the ejection of and volcanic block. These eruptions wear down the lava dome holding the magma down, and it disintegrates, leading to much more quiet and continuous eruptions. Thus an early sign of future Vulcanian activity is lava dome growth, and its collapse generates an outpouring of material down the volcano's slope.
Deposits near the source vent consist of large and volcanic bomb, with so-called "bread-crust bombs" being especially common. These deeply cracked volcanic chunks form when the exterior of ejected lava cools quickly into a volcanic glass or fine-grained shell, but the inside continues to cool and vesiculate. The center of the fragment expands, cracking the exterior. The bulk of Vulcanian deposits are fine grained volcanic ash. The ash is only moderately dispersed, and its abundance indicates a high degree of fragmentation, the result of high gas contents within the magma. In some cases these have been found to be the result of interaction with meteoric water, suggesting that Vulcanian eruptions are partially hydrovolcanic.
Volcanoes that have exhibited Vulcanian activity include:
Vulcanian eruptions are estimated to make up at least half of all known Holocene eruptions.
Peléan eruptions are characterized most prominently by the incandescence pyroclastic flows that they drive. The mechanics of a Peléan eruption are very similar to that of a Vulcanian eruption, except that in Peléan eruptions the volcano's structure is able to withstand more pressure, hence the eruption occurs as one large explosion rather than several smaller ones.
Volcanoes known to have Peléan activity include:
These massive eruptive columns are the distinctive feature of a Plinian eruption, and reach up into the atmosphere. The densest part of the plume, directly above the volcano, is driven internally by gas expansion. As it reaches higher into the air the plume expands and becomes less dense, convection and thermal expansion of volcanic ash drive it even further up into the stratosphere. At the top of the plume, powerful winds may drive the plume away from the volcano.
These highly explosive eruptions are usually associated with volatile-rich dacitic to rhyolitic lavas, and occur most typically at . Eruptions can last anywhere from hours to days, with longer eruptions being associated with more felsic volcanoes. Although they are usually associated with felsic magma, Plinian eruptions can occur at volcanoes, if the magma chamber differentiates with upper portions rich in silicon dioxide, or if magma ascends rapidly.
Plinian eruptions are similar to both Vulcanian and Strombolian eruptions, except that rather than creating discrete explosive events, Plinian eruptions form sustained eruptive columns. They are also similar to Hawaiian in that both eruptive types produce sustained eruption columns maintained by the growth of bubbles that move up at about the same speed as the magma surrounding them.
Regions affected by Plinian eruptions are subjected to heavy pumice airfall affecting an area in size. The material in the ash plume eventually finds its way back to the ground, covering the landscape in a thick layer of many cubic kilometers of ash.
The most dangerous eruptive feature are the generated by material collapse, which move down the side of the mountain at extreme speeds of up to per hour and with the ability to extend the reach of the eruption hundreds of kilometers. The ejection of hot material from the volcano's summit melts snowbanks and ice deposits on the volcano, which mixes with tephra to form , fast moving with the consistency of wet concrete that move at the speed of a rapids.
Major Plinian eruptive events include:
There is debate about the exact nature of phreatomagmatic eruptions, and some scientists believe that fuel-coolant reactions may be more critical to the explosive nature than thermal contraction. Fuel coolant reactions may fragment the volcanic material by propagating stress waves, widening cracks and increasing surface area that ultimately leads to rapid cooling and explosive contraction-driven eruptions.
A defining feature of a Surtseyan eruption is the formation of a pyroclastic surge (or base surge), a ground hugging radial cloud that develops along with the eruption column. Base surges are caused by the gravitational collapse of a water vapor eruptive column, one that is denser overall than a regular volcanic column. The densest part of the cloud is nearest to the vent, resulting in a wedge shape. Associated with these laterally moving rings are sand dune-shaped depositions of rock left behind by the lateral movement. These are occasionally disrupted by , rock that was flung out by the explosive eruption and followed a ballistics path to the ground. Accumulations of wet, spherical ash known as accretionary lapilli are another common surge indicator.
Over time Surtseyan eruptions tend to form , broad low-relief dug into the ground, and , circular structures built of rapidly quenched lava. These structures are associated with single vent eruptions. If eruptions arise along , may be dug out. Such eruptions tend to be more violent than those which form tuff rings or maars, an example being the 1886 eruption of Mount Tarawera. are another hydrovolcanic feature, generated by the explosive deposition of basaltic tephra (although they are not truly volcanic vents). They form when lava accumulates within cracks in lava, superheats and explodes in a steam explosion, breaking the rock apart and depositing it on the volcano's flank. Consecutive explosions of this type eventually generate the cone.
Volcanoes known to have Surtseyan activity include:
Submarine eruptions may produce , which may break the surface and form volcanic islands.
Submarine volcanism is driven by various processes. Volcanoes near plate boundaries and are built by the decompression melting of mantle rock that rises on an upwelling portion of a convection cell to the crustal surface. Eruptions associated with subduction, meanwhile, are driven by subducting plate tectonics that add volatiles to the rising plate, lowering its melting point. Each process generates different rock; mid-ocean ridge volcanics are primarily , whereas subduction flows are mostly calc-alkaline, and more explosive and viscous.
Spreading rates along mid-ocean ridges vary widely, from per year at the Mid-Atlantic Ridge, to up to along the East Pacific Rise. Higher spreading rates are a probable cause for higher levels of volcanism. The technology for studying seamount eruptions did not exist until advancements in hydrophone technology made it possible to "listen" to , known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based cannot detect sea-based earthquakes below a Richter scale of 4, but acoustic waves travel well in water and over long periods of time. A system in the North Pacific, maintained by the United States Navy and originally intended for the detection of , has detected an event on average every 2 to 3 years.
The most common underwater flow is pillow lava, a rounded lava flow named for its unusual shape. Less common are volcanic glass, marginal sheet flows, indicative of larger-scale flows. Volcaniclastic are common in shallow-water environments. As plate movement starts to carry the volcanoes away from their eruptive source, eruption rates start to die down, and water erosion grinds the volcano down. The final stages of eruption cap the seamount in alkalic flows. There are about 100,000 deepwater volcanoes in the world, although most are beyond the active stage of their life. Some exemplary seamounts are Kamaʻehuakanaloa (formerly Loihi), Bowie Seamount, Davidson Seamount, and Axial Seamount.
The study of glaciovolcanism is still a relatively new field. Early accounts described the unusual flat-topped steep-sided volcanoes (called ) in Iceland that were suggested to have formed from eruptions below ice. The first English-language paper on the subject was published in 1947 by William Henry Mathews, describing the Tuya Butte field in northwest British Columbia, Canada. The eruptive process that builds these structures, originally inferred in the paper, begins with volcanic growth below the glacier. At first the eruptions resemble those that occur in the deep sea, forming piles of pillow lava at the base of the volcanic structure. Some of the lava shatters when it comes in contact with the cold ice, forming a volcanic glass breccia called hyaloclastite. After a while the ice finally melts into a lake, and the more explosive eruptions of Surtseyan activity begins, building up flanks made up of mostly hyaloclastite. Eventually the lake boils off from continued volcanism, and the lava flows become more effusive and thicken as the lava cools much more slowly, often forming columnar jointing. Well-preserved tuyas show all of these stages, for example Hjorleifshofdi in Iceland.
Products of volcano-ice interactions stand as various structures, whose shape is dependent on complex eruptive and environmental interactions. Glacial volcanism is a good indicator of past ice distribution, making it an important climatic marker. Since they are embedded in ice, as glacial ice retreats worldwide there are concerns that and other structures may destabilize, resulting in mass . Evidence of volcanic-glacial interactions are evident in Iceland and parts of British Columbia, and it is even possible that they play a role in ice age.
Glaciovolcanic products have been identified in Iceland, the Canadian province of British Columbia, the U.S. states of Hawaii and Alaska, the Cascade Range of western North America, South America and even on the planet Mars. Volcanoes known to have subglacial activity include:
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 , , , and volcanic block "rain." They may also release deadly toxic gas able to suffocate anyone in range of the eruption.
Volcanoes known to exhibit phreatic activity include:
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