Subglacial eruption: 1 water vapor cloud, 2 lake, 3 ice, 4 layers of lava and ash, 5 strata, 6 pillow lava, 7 magma conduit, 8 magma chamber, 9 dike.

A subglacial eruption is a volcanic eruption that has occurred under ice, or under a glacier. Subglacial eruptions can cause dangerous floods, lahars and create hyaloclastite and pillow lava. Subglacial eruptions sometimes form a subglacial volcano called a tuya. Tuyas in Iceland are called table mountains because of their flat tops. Tuya Butte, in northern British Columbia is an example of a tuya. A tuya may be recognized by its stratigraphy, which typically consists of a basal layer of pillow basalts overlain by hyaloclastite breccia, tuff, and capped off by a lava flow. The pillow lavas formed first as a result of subaqueous eruptions in glacial meltwater. Once the vent reaches shallower water, eruptions become phreatomagmatic, depositing the hyaloclastite breccia. Once the volcano emerges through the ice, it erupts lava, forming the flat capping layer of a tuya.

The thermodynamics of subglacial eruptions are very poorly understood. Rare published studies indicate that plenty of heat is contained in the erupted lava, with 1 unit-volume of magma sufficient to melt about 10 units of ice. However, the rapidity by which ice is melted is unexplained, and in real eruptions the rate is at least an order of magnitude faster than existing predictions.

Antarctica eruption

On January, 2008, the British Antarctic Survey (Bas) scientists led by Hugh Corr and David Vaughan, reported (in the journal Nature Geoscience) that 2,200 years ago, a volcano erupted under the Antarctica ice sheet (based on airborne survey with radar images). The biggest eruption in the last 10,000 years, the volcanic ash was found deposited on the ice surface under the Hudson Mountains, close to Pine Island Glacier. The ash covered an area the size of New Hampshire and was probably deposited from a 12 km high ash plume. Researchers have detected a mountainous peak some 100 meters beneath the surface believed to be the top of the tuya associated with this eruption.

List of volcanoes with Holocene subglacial eruptions

Iceland

• Bárðarbunga
• Eyjafjallajökull
• Grímsvötn
• Hveravellir
• Katla
• Kverkfjöll
• Prestahnúkur
• Snæfellsjökull
• Torfajökull
• Öræfajökull

South America

• Lautaro
• Sollipulli

Diagram of a Submarine eruption: 1. Water vapor cloud 2. Water 3. Stratum 4. Lava flow 5. Magma conduit 6. Magma chamber 7. Dike 8. Pillow lava.

Submarine eruptions are a type of volcanic eruption that occurs underwater. An estimated 75% of the total volcanic eruptive volume is generated by submarine eruptions near mid ocean ridges alone, however because of the problems associated with detecting deep sea volcanics, they remained virtually unknown until advances in the 1990s made it possible to observe them.

Submarine eruptions are generated by seamounts (underwater volcanoes), and are driven by one of two processes. Volcanoes near plate boundaries and mid-ocean ridges are built by the decompression melting of mantle rock that floats up to the crustal surface. Eruptions near subducting zones, meanwhile, are driven by subducting plates that adds volatiles to the rising plate, raising its melting point. Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic, whereas subduction flows are mostly calc-alkaline, and more explosive and viscous.

Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge, to up to 16 cm (6 in) along the East Pacific Rise. Higher spreading rates are a probably 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 acoustic waves, known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below a magnitude of 4, but acoustic waves travel well in water and long periods of time. A system in the North Pacific, maintained by the United States Navy and originally intended for the detection of submarines, has detected an event on average every 2 to 3 years.

The most common underwater flow is pillow lava, a circular lava flow named after its unusual shape. Less common are glassy, marginal sheet flows, indicative of larger-scale flows. Volcaniclastic sedimentary rocks 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 caps 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 exemplery seamounts are Loihi Seamount, Bowie Seamount, Cross Seamount, and Denson Seamount.

Surtseyan eruption: 1 water vapor cloud, 2 cupressoid ash, 3 crater, 4 water, 5 layers of lava and ash, 6 stratum, 7 magma conduit, 8 magma chamber, 9 dike.

A Surtseyan eruption is a type of volcanic eruption that takes place in shallow seas or lakes. It is named after the island of Surtsey off the southern coast of Iceland.

These eruptions are commonly phreatomagmatic eruptions, representing violent explosions caused by rising basaltic or andesitic magma coming into contact with abundant, shallow groundwater or surface water. Tuff rings, pyroclastic cones of primarily ash, are built by explosive disruption of rapidly cooled magma. Other examples of these volcanoes: Capelinhos, Faial Island, Azores; and Taal Volcano, Batangas, Philippines.

Characteristics of Surtseyan Eruption

Although similar in nature to phreatomagmatic eruptions, there are several specific characteristics:

• Physical nature of magma: viscous; basaltic.
• Character of explosive activity: violent ejection of solid, warm fragments of new magma; continuous or rhythmic explosions; base surges.
• Nature of effusive activity: short, locally pillowed, lava flows; lavas may be rare.
• Nature of dominant ejecta: lithic, blocks and ash; often accretionary lapilli; spatter, fusiform bombs and lapilli absent.
• Structures built around vent: tuff rings

Diagram of a Plinian eruption: 1. Ash plume 2. Magma conduit 3. Volcanic ash rain 4. Layers of lava and ash 5. Stratum 6. Magma chamber.

Plinian eruptions, also known as 'Vesuvian eruptions', are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in AD 79 (as described in a letter written by Pliny the Younger, and which killed his uncle Pliny the Elder).

Plinian eruptions are marked by columns of gas and volcanic ash extending high into the stratosphere, a high layer of the atmosphere. The key characteristics are ejection of large amount of pumice and very powerful continuous gas blast eruptions.

Short eruptions can end in less than a day, but longer events can take several days to months. The longer eruptions begin with production of clouds of volcanic ash, sometimes with pyroclastic flows. The amount of magma erupted can be so large that the top of the volcano may collapse, resulting in a caldera. Fine ash can deposit over large areas. Plinian eruptions are often accompanied by loud noises, such as those generated by Krakatoa.

The lava is usually rhyolitic and rich in silicates. Basaltic lavas are unusual for Plinian eruptions; the most recent example is the 1886 eruption of Mount Tarawera.

Pliny's description

Pliny described his uncle's involvement from the first observation of the eruption:

On the 24th of August, about one in the afternoon, my mother desired him to observe a cloud which appeared of a very unusual size and shape. He had just taken a turn in the sun and, after bathing himself in cold water, and making a light luncheon, gone back to his books: he immediately arose and went out upon a rising ground from whence he might get a better sight of this very uncommon appearance. A cloud, from which mountain was uncertain, at this distance (but it was found afterwards to come from Mount Vesuvius), was ascending, the appearance of which I cannot give you a more exact description of than by likening it to that of a pine tree, for it shot up to a great height in the form of a very tall trunk, which spread itself out at the top into a sort of branches; occasioned, I imagine, either by a sudden gust of air that impelled it, the force of which decreased as it advanced upwards, or the cloud itself being pressed back again by its own weight, expanded in the manner I have mentioned; it appeared sometimes bright and sometimes dark and spotted, according as it was either more or less impregnated with earth and cinders. This phenomenon seemed to a man of such learning and research as my uncle extraordinary and worth further looking into.

– Sixth Book of Letters, Letter 16

Pliny the Elder set out to rescue the victims from their perilous position on the shore of the Bay of Naples, and launched his galleys, crossing the bay to Stabiae (near the modern town of Castellammare di Stabia). Pliny the Younger provided an account of his death, and suggested that he collapsed and died through inhaling poisonous gases emitted from the volcano. His body was found interred under the ashes of the Vesuvius with no apparent injuries on 26 August, after the plume had dispersed, confirming asphyxiation or poisoning.

Ultra Plinian

According to the Smithsonian Institution's Volcanic Explosivity Index, a VEI of 6 to 8 is classified as "Ultra Plinian." They are defined by ash plumes over 25 km (16 mi) high and a volume of erupted material 10 km³ (2 cu mi) to 1,000 km³ (200 cu mi) in size. Eruptions in the "Ultra Plinian" category include Lake Toba (74 ka), Tambora (1815), and Krakatoa (1883).

Examples of Plinian Eruption

• The 2010 eruptions of Eyjafjallajökull in Iceland
• The June 2009 eruption of Sarychev Peak in Russia
• The 1991 Mount Pinatubo eruption in Luzon in the Philippines;
• The 1980 eruption of Mount St. Helens in USA
• The 1883 eruption of Krakatoa in Indonesia
• The 1815 eruption of Mount Tambora in Indonesia
• The 1667 and 1739 eruptions of Mount Tarumae in Japan
• The AD 79 eruption of Mount Vesuvius in Italy, which was the prototypical Plinian eruption.
• The 1645 BC eruption of Santorini in Greece
• The 4860 BC eruption forming Crater Lake in USA
• The Long Valley Caldera eruption in USA over 760,000 years ago.

Peléan eruption: 1 Ash plume, 2 Volcanic ash rain, 3 Lava dome, 4 Volcanic bomb, 5 Pyroclastic flow, 6 Layers of lava and ash, 7 Strata, 8 Magma conduit, 9 Magma chamber, 10 Dike

Peléan eruptions are a type of volcanic eruption. They can occur when viscous magma, typically of rhyolitic or andesitic type, is involved, and share some similarities with Vulcanian eruptions. The most important characteristics of a Peléan eruption is the presence of a glowing avalanche of hot volcanic ash, a pyroclastic flow. Formation of lava domes is another characteristical feature. Short flows of ash or creation of pumice cones may be observed as well.

The initial phases of eruption are characterized by pyroclastic flows. The tephra deposits have lower volume and range than the corresponding Plinian and Vulcanian eruptions. The viscous magma then forms a steep-sided dome or volcanic spine in the volcano's vent. The dome may later collapse, resulting in flows of ash and hot blocks. The eruption cycle is usually completed in few years, but in some cases may continue for decades, like in the case of Santiaguito.

The 1902 explosion of Mount Pelée is the first described case of a Peléan eruption, and gave it its name.

Some other examples include the following:

• the 1948-1951 eruption of Hibok-Hibok;
• the 1951 eruption of Mount Lamington, which remains the most detailed observation of this kind;
• the 1956 eruption of Bezymianny;
• the 1968 eruption of Mayon Volcano;
• and the 1980 eruption of Mount St. Helens.

Vulcanian Eruption: 1 Ash plume, 2 Lapilli, 3 Lava fountain, 4 Volcanic ash rain, 5 Volcanic bomb, 6 Lava flow, 7 Layers of lava and ash, 8 Stratum, 9 Sill, 10 Magma conduit, 11 Magma chamber, 12 Dike

The term Vulcanian was first used by Giuseppe Mercalli, witnessing the 1888-1890 eruptions on the island of Vulcano. His description of the eruption style is now used all over the world for eruptions characterised by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. Mercalli described vulcanian eruptions as "...Explosions like cannon fire at irregular intervals..." Their explosive nature is due to increased silica content of the magma. Almost all types of magma can be involved, but magma with about 55% or more silica (basalt–andesite) is most common. Increasing silica levels increase the viscosity of the magma which means increased explosiveness. They usually commence with phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground. This is usually followed by the explosive clearing of the vent and the eruption column is dirty grey to black as old weathered rocks are blasted out of the vent. As the vent clears, further ash clouds become grey-white and creamy in colour, with convolutions of the ash similar to those of plinian eruptions.

Characteristics

Vulcanian eruptions display several common characteristics. The mass of rock ejected during the eruption is usually between 10² - 10⁶ tonnes and contains a high proportion of non-juvinial material (> 50%). During active periods of volcanic activity, intervals between explosions vary from less than 1 minute (e.g. Anak Krakatoa) to about a day. Pyroclastic flows are also common features of this type of eruption. The gas streaming phase of these eruptions are characterised discrete canon-like explosions, which are a particular features of vulcanian eruptions. These expulsions of gas can reach supersonic velocities resulting in shock waves.

The tephra is dispersed over a wider areas than that from Strombolian eruptions. The pyroclastic rock and the base surge deposits form an ash volcanic cone, while the ash covers a large surrounding area. The eruption ends with a flow of viscous lava. Vulcanian eruptions may throw large metre-size blocks several hundred metres, occasionally up to several kilometres.

Vulcanian eruptions are dangerous to persons within several hundred metres of the vent. One feature of this type of eruption is the "Volcanic bomb." These can be blocks often 2 to 3 m in dimensions. At Galeras a vulcanian eruption ejected bombs which impacted with several volcanologists who were in the crater and many died or suffered terrible injuries.

1930 Eruption of Stromboli

The 11 September 1930 eruption of Stromboli was a vulcanian eruption. It started at 08:10 hours (local), when ash was vented for about 10 minutes. Then at 09:52 two incredibly powerful explosions occurred which shook the whole island. Blocks were hurled about 2 km. These fell out of the sky smashing through buildings etc. A tsunami 2 to 2.5m high was generated. By 10:40 the explosive phase of the eruption was over. Expulsion of lava followed, this flowed down the Sciara del Fuoco, lasting into the night. At the same time incandescent scoria flowed down the Vallonazzo Valley and entered the sea near Piscità.

By the end, 6 people had died. Four fishermen died at sea when the avalanches of hot scoria caused the sea to become very disturbed. One person was killed in Stromboli village by falling blocks, and the 6th was killed by the tsunami. It is believed that water entered the conduit due to a partial collapse of the conduit. The water flashed into steam and took the easiest "escape route," via the open conduit. As it expanded in the molten magma it generated the two very large explosions.

Diagram of a Strombolian eruption: 1. Ash plume 2. Lapilli 3. Volcanic ash rain 4. Lava fountain 5. Volcanic bomb 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Dike 10. Magma conduit 11. Magma chamber 12. Sill

Strombolian eruptions are relatively low-level volcanic eruptions, named after the Italian volcano Stromboli, where such eruptions consist of ejection of incandescent cinder, lapilli and lava bombs to altitudes of tens to hundreds of meters. They are small to medium in volume, with sporadic violence.

They are defined as "...Mildly explosive at discrete but fairly regular intervals of seconds to minutes..."

The tephra typically glows red when leaving the vent, but its surface cools and assumes a dark to black color and may significantly solidify before impact. The tephra accumulates in the vicinity of the vent, forming a cinder cone. Cinder is the most common product, the amount of volcanic ash is typically rather minor.

The lava flows are more viscous, and therefore shorter and thicker, than the corresponding Hawaiian eruptions; it may or may not be accompanied by production of pyroclastic rock.

Instead the gas coalesces into bubbles, called gas slugs, that grow large enough to rise through the magma column, bursting near the top due to the decrease in pressure and throwing magma into the air. Each episode thus releases volcanic gases, sometimes as frequently as a few minutes apart. Gas slugs can form as deep as 3 kilometers, making them difficult to predict.

Strombolian eruptive activity can be very long-lasting because the conduit system is not strongly affected by the eruptive activity, so that the eruptive system can repeatedly reset itself. For example, the Parícutin volcano erupted continuously between 1943-1952, Mount Erebus, Antarctica has produced Strombolian eruptions for at least many decades, and Stromboli itself has been producing Strombolian eruptions for several thousand years.

Hawaiian eruption: 1: Ash plume, 2: Lava fountain, 3: Crater, 4: Lava lake, 5: Fumaroles, 6: Lava flow, 7 Layers of lava and ash, 8: Stratum, 9: Sill, 10: Magma conduit, 11: Magma chamber, 12: Dike

A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high temperature at the vent. Very little amount of volcanic ash is produced. This type of eruption occurs most often at hotspot volcanoes such as Kīlauea, though it can occur near subduction zones (e.g. Medicine Lake Volcano in California, United States) and rift zones. Another example of Hawaiian eruptions occurred on Surtsey from 1964 to 1967, when molten lava flowed from the crater to the sea.

Hawaiian eruptions may occur along fissure vents, such as during the eruption of Mauna Loa Volcano in 1950, or at a central vent, such as during the 1959 eruption in Kīlauea Iki Crater, which created a lava fountain 580 meters (1,900 ft) high and formed a 38 meter cone named Puʻu Puaʻi. In fissure-type eruptions, lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of lava can spurt to a height of 300 meters or more (heights of 1600 meters were reported for the 1986 eruption of Mount Mihara on Izu Ōshima, Japan).

Hawaiian eruptions usually start by formation of a crack in the ground from which a curtain of incandescent magma or several closely spaced magma fountains appear. The lava can overflow the fissure and form ʻaʻā or pāhoehoe style of flows. When such an eruption from a central cone is protracted, it can form lightly sloped shield volcanoes, for example Mauna Loa or Skjaldbreiður in Iceland.

Petrology of Hawaiian Basalts

The key factors in generating a Hawaiian eruption are basaltic magma and a low percentage of dissolved water (less than one percent). The lower the water content, the more peaceful is the resulting flow. Almost all lava that comes from Hawaiian volcanoes is basalt in composition. Hawaiian basalts that make up almost all of the islands are tholeiite. These rocks are similar but not identical to those that are produced at ocean ridges. Basalt relatively richer in sodium and potassium (more alkaline) has erupted at the undersea volcano of Lōʻihi at the extreme southeastern end of the volcanic chain, and these rocks may be typical of early stages in the "evolution" of all Hawaiian islands. In the late stages of eruption of individual volcanoes, more alkaline basalt also was erupted, and in the very late stages after a period of erosion, rocks of unusual composition such as nephelinite were produced in very small amounts. These variations in magma composition have been investigated in great detail, in part to try to understand how mantle plumes may work.

Safety

Hawaiian eruptions are usually the most attractive to tourists and are the safest because there is little danger from ash. However, Hawaiian eruptions are not always safe.

In 1790, 80 warriors marching on Kīlauea were killed in an eruption. On May 18, 1924, a plantation accountant named Truman Taylor who was sightseeing on Kīlauea's caldera, was hit with debris from an explosion. Although rushed to hospital, Taylor succumbed to his injuries later that day.

Diagram of a Phreatic eruption: 1. Water vapor cloud 2. Volcanic bomb 3. Magma conduit 4. Layers of lava and ash 5. Stratum 6. Water table 7. Explosion 8. Magma chamber

Phreatic eruptions or steam-blast eruptions are a type of eruption driven by the expansion of steam. When cold ground or surface water coming into contact with hot rock or magma it superheats and explodes, fracturing the surrounding rock and thrusting out a mixture of steam, water , ash, volcanic bombs, and volcanic blocks. 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. Because they are driven by the cracking of rock stata 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.

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 base surges, lahars, avalanches, and volcanic block "rain." They may also release deadly toxic gas able to suffocate anyone in range of the eruption.

Volcaoes known the exhibit Phreatic activity include:

• Mount St. Helens, which exhibited Phreatic activity just prior to its catastrophic 1980 eruption (which was itself Plinian).
• Taal Volcano, Philippines, 1965.
• La Soufrière of Guadeloupe (Lesser Antilles), 1975-1976 activity.

A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when rising magma makes contact with ground or surface water. The extreme temperature of the magma (anywhere from 600 to 1,170 °C (1,112 to 2,138 °F)) causes near-instantaneous evaporation to steam resulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helens, hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano. In 1949, Thomas Jaggar described this type of activity as a steam-blast eruption.

Phreatic eruptions typically include steam and rock fragments; the inclusion of lava is unusual. The temperature of the fragments can range from cold to incandescent. If molten material is included, the term phreato-magmatic may be used. These eruptions occasionally create broad, low-relief craters called maars. Phreatic explosions can be accompanied by carbon dioxide or hydrogen sulfide gas emissions. The former can asphyxiate at sufficient concentration; the latter is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 149 people, most of whom were overcome by poisonous gases.

It is believed that the 1883 eruption of Krakatoa, which obliterated most of the volcanic island and created the loudest sound in recorded history, was a phreatic event. Kilauea, in Hawaii, has a long record of phreatic explosions; a 1924 phreatic eruption hurled rocks estimated at eight tons up to a distance of one kilometer. Additional examples are the 1963–65 eruption of Surtsey, the 1965 eruption of Taal Volcano, and the 1982 Mount Tarumae eruption.

Phreatomagmatic eruptions are eruptions that arise from interactions between water and magma. They are driven from thermal contraction (as opposed to magmatic eruptions, which are driven by thermal expansion) of magma when it comes in contact with water. This temperature difference between the two causes violent water-lava interactions that make up the eruption. The products of phreatomagmatic eruptions are believed to be more regular in shape and finer grained than the products of magmatic eruptions because of the differences in eruptive mechanisms.

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 ultimetly lead to rapid cooling and explosive contraction-driven eruptions.

Surtseyan Eruptions

A Surtseyan eruption (or hydrovolcanic) is a type of volcanic eruption caused by shallow-water interactions between water and lava, named so after its most famous example, the eruption and formation of the island of Surtsey off the coast of Iceland in 1963. Surtseyan eruptions are the "wet" equivalent of ground-based Strombolian eruptions, but because of where they are taking place they are much more explosive. This is because as water is heated by lava, it flashes in steam and expands violently, fragmenting the magma it is in contact with into fine-grained ash. Surtseyan eruptions are the hallmark of shallow-water volcanic oceanic islands, however they are not specifically confined to them. Surtseyan eruptions can happen on land as well, and are caused by rising magma that comes into contact with an aquifer (water-bearing rock formation) at shallow levels under the volcano. The products of Surtseyan eruptions are generally oxidized palagonite basalts (though andesitic eruptions do occur, albeit rarely), and like Strombolian eruptions Surtseyan eruptions are generally continuous or otherwise rhythmic.

A distinct 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 vaperous eruptive column, one that is denser overall then a regular volcanic column. The densest part of the cloud is nearest to the vent, resulting a wedge shape. Associated with these laterally moving rings are dune-shaped depositions of rock left behind by the lateral movement. These are occasionally disrupted by bomb sags, rock that was flung out by the explosive eruption and followed a ballistic path to the ground. Accumulations of wet, spherical ash known as accretionary lapilli is another common surge indicator.

Over time Surtseyan eruptions tend to form maars, broad low-relief volcanic craters dug into the ground, and tuff rings, circular structures built of rapidly quenched lava. These structures are associated with a single vent eruption, however if eruptions arise along fracture zones a rift zone may be dug out; these eruptions tend to be more violent then the ones forming a tuff ring or maars, an example being the 1886 eruption of Mount Tarawera. Littoral cones 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:

• Surtsey, Iceland. The volcano built itself up from depth and emerged above the Atlantic Ocean off the coast of Iceland in 1963. Initial hydrovolcanics were highly explosive, but as the volcano grew out rising lava started to interact less with the water and more with the air, until finally Surtseyan activity waned and became more Strombolian in character.
• Ukinrek Maars in Alaska, 1977, and Capelinhos in the Azores, 1957, both examples of above-water Surtseyan activity.
• Mount Tarawera in New Zealand erupted along a rift zone in 1886, killing 150 people.
• Ferdinandea, a seamount in the Mediterranean Sea, breached sea level in July 1831 and was the source of a dispute over sovereignty between Italy, France, and Great Britain. The volcano did not build tuff cones strongly enough to withstand erosion, and disappeared back below the waves soon after it appeared.
• The underwater volcano Hunga Tonga in Tonga breached sea level in 2009. Both of its vents exhibited Surtseyan activity for much of the time. It was also the site of an earlier eruption in May 1988.

Submarine Eruptions

Submarine eruptions are a type of volcanic eruption that occurs underwater. An estimated 75% of the total volcanic eruptive volume is generated by submarine eruptions near mid ocean ridges alone, however because of the problems associated with detecting deep sea volcanics, they remained virtually unknown until advances in the 1990s made it possible to observe them.

Submarine eruptions are generated by seamounts (underwater volcanoes), and are driven by one of two processes. Volcanoes near plate boundaries and mid-ocean ridges are built by the decompression melting of mantle rock that floats up to the crustal surface. Eruptions near subducting zones, meanwhile, are driven by subducting plates that adds volatiles to the rising plate, raising its melting point. Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic, whereas subduction flows are mostly calc-alkaline, and more explosive and viscous.

Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge, to up to 16 cm (6 in) along the East Pacific Rise. Higher spreading rates are a probably 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 acoustic waves, known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below a magnitude of 4, but acoustic waves travel well in water and long periods of time. A system in the North Pacific, maintained by the United States Navy and originally intended for the detection of submarines, has detected an event on average every 2 to 3 years.

The most common underwater flow is pillow lava, a circular lava flow named after its unusual shape. Less common are glassy, marginal sheet flows, indicative of larger-scale flows. Volcaniclastic sedimentary rocks 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 caps 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 exemplery seamounts are Loihi Seamount, Bowie Seamount, Cross Seamount, and Denson Seamount.

Subglacial Eruptions

Subglacial eruptions are a type of volcanic eruption characterized by interactions between lava and ice, often under a glacier. The nature of glaciovolcanism dictates that it occurs at areas of high latitude and high altitude. It has been suggested that subglacial volcanoes that are not actively erupting often dump heat into the ice covering them, producing meltwater. This meltwater mix means that subglacial eruptions often generate dangerous jökulhlaups (floods) and lahars.^[40]

The study of glaciovolcanism is still a relatively new field. Early accounts described the unusual flat-topped steep-sided volcanoes (called tuyas) 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. 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 glassy 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 imbedded in ice, as ice retracts worldwide there are concerns that tuyas and other structures may destabalize, resulting in mass landslides. Evidence of volcanic-glacial interactions are evident in Iceland and parts of British Columbia, and it's even possible that they play a role in deglaciation.

Glaciovolcanic products have been identified in Iceland, British Columbia, Hawaii and Alaska, the Cascade Range, South America and even on the planet Mars. Volcanoes known to have subglacial activity include:

• Mauna Kea in tropical Hawaii. There is evidence of past subglacial eruptive activity on the volcano in the form of a subglacial deposit on its summit. The eruptions originated about 10,000 years ago, during the last ice age, when the summit of Mauna Kea was covered in ice.
• In 2008, the British Antarctic Survey reported a volcanic eruption under the Antarctica ice sheet 2,200 years ago. It is believed to be that this was the biggest eruption in Antarctica in the last 10,000 years. Volcanic ash deposits from the volcano were identified through an airborne radar survey, buried under later snowfalls in the Hudson Mountains, close to Pine Island Glacier.
• Iceland, well known for both glaciers and volcanoes, is often a site of subglacial eruptions. An example an eruption under the Vatnajökull ice cap in 1996, which occurred under an estimated 2,500 ft (762 m) of ice.
• As part of the search for life on Mars, scientists have suggested that there may be subglacial volcanoes on the red planet. Several potential sites of such volcanism have been reviewed, and compared extensively with similar features in Iceland:

—Jack Farmer, Arizona State University

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 AD 79 eruption that buried Pompeii.

Hawaiian Eruptions

Hawaiian eruptions are a type of volcanic eruption, named after the Hawaiian volcanoes with which this eruptive type is hallmark. Hawaiian eruptions are the calmest types of volcanic events, characterized by the effusive eruption 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 beings 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; however, 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 splatter cones. If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long lived; Pu'u O'o, a cinder cone of Kilauea, has been erupting continuously since 1983. 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; there are currently only 5 such lakes in the world, and the one at Kīlauea's Kupaianaha vent is one of them.

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 then pahoehoe, but tend to move slower. Flows can measure 2 to 20 m (7 to 66 ft) 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 glassy 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 rods, known as Pele's hair. Sometimes basant 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 largest lava fountain ever recorded formed on the island of Izu Ōshima (on Mount Mihara) in 1986, a 1,600 m (5,249 ft) gusher that was more than twice as high as the mountain itself (which stands at 764 m (2,507 ft)).

Volcanoes known to have Hawaiian activity include:

• Pu'u O'o, a parasitic cinder cone located on Kilauea on the island of Hawaiʻi which has been erupting continuously since 1983. The eruptions began with a 6 km (4 mi)-long fissure-based "curtain of fire" on January 3. These gave way to centralized eruptions on the site of Kilauea's east rift, eventually building up the still active cone.
• For a list of all of the volcanoes of Hawaii, see List of volcanoes in the Hawaiian - Emperor seamount chain.
• Mount Etna, Italy.
• Mount Mihara in 1986 (see above paragraph)

Strombolian Eruptions

Strombolian eruptions are a type of volcanic eruption, named after the volcano Stromboli, which has been erupting continuously for centuries. Strombolian eruptions are driven by the bursting of gas bubbles within the magma. These gas bubbles within the magma accumulate and coalesce into large bubbles, called gas slugs. These grow large enough to rise through the lava column. Upon reaching the surface, the difference in air pressure causes the bubble to burst with a loud pop, throwing magma in the air in a way similar to a soap bubble. Because of the high gas pressures associated with the lavas, continued activity is generally in the form of episodic explosive eruptions accompanied by the distinctive loud blasts. During eruptions, these blasts occur as often as every few minutes.

The term "Strombolian" has been used indiscriminately to describe a wide variety of volcanic eruptions, varying from small volcanic blasts to large eruptive columns. 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 basaltic 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 volcanic bombs and lapilli fragments that travel in parabolic paths before landing around their source vent. The steady accumulation of small fragments builds cinder cones composed completely of basaltic pyroclasts. 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 eruptive columns, 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:

• Parícutin, Mexico, which erupted from a fissure in a cornfield in 1943. Two years into its life, pyroclastic activity began to wane, and the outpouring of lava from its base became its primary mode of activity. Eruptions ceased in 1952, and the final height was 424 m (1,391 ft). This was the first time that scientists are able to observe the complete life cycle of a volcano.
• Mount Etna, Italy, which has displayed Strombolian activity in recent eruptions, for example in 1981, 1999, 2002-2003, and 2009.
• Mount Erebus in Antarctica, the southernmost active volcano in the world, having been observed erupting since 1972. Eruptive activity at Erebrus consists of frequent Strombolian activity.
• Stromboli itself. The namesake of the mild explosive activity that it possesses has been active throughout historical time; essentially continuous Strombolian eruptions, occasionally accompanied by lava flows, have been recorded at Stromboli for more than a millennium.

Vulcanian Eruptions

Vulcanian eruptions are a type of volcanic eruption, named after the volcano Vulcano, which also gives its name to the word Volcano. It was named so following Giuseppe Mercalli's observations of its 1888-1890 eruptions. In Vulcanian eruptions, highly viscous magma within the volcano make it difficult for vesiculate gases to escape. Similar to Strombolian eruptions, this leads to the buildup of high gas pressure, eventually popping the cap holding the magma down and resulting in an explosive eruption. However, unlike Strombolian eruptions, ejected lava fragments are not aerodynamical; this is due to the higher viscosity of Vulcanian magma and the greater incorporation of crystalline material broken off from the former cap. They are also more explosive than their Strombolian counterparts, with eruptive columns often reaching between 5 and 10 km (3 and 6 mi) high. Lastly, Vulcanian deposits are andesitic to dacitic rather than basaltic.

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 volcanic bombs and blocks. 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 pyroclastic material down the volcano's slope.

Deposits near the source vent consist of large volcanic blocks and bombs, 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 glassy or fine-grained shell, but the inside continues to cool and vesiculate. The center of the fragment expands, cracking the exterior. However the bulk of Vulcanian deposits are fine grained 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:

• Sakurajima, Japan has been the site of Vulcanian activity near-continuously since 1955.
• Tavurvur, Papua New Guinea, one of several volcanoes in the Rabaul Caldera.
• Irazú Volcano in Costa Rica exhibited Vulcanian activity in its 1965 eruption.

Peléan Eruptions

Peléan eruptions (or nuée ardente) are a type of volcanic eruption, named after the volcano Mount Pelée in Martinique, the site of a massive Peléan eruption in 1902 that is one of the worst natural disasters in history. In Peléan eruptions, a large amount of gas, dust, ash, and lava fragmets are blown out the volcano's central crater, driven by the collapse of rhyolite, dacite, and andesite lava dome collapses that often create large eruptive columns. An early sign of a coming eruption is the growth of a so-called Peléan or lava spine, a bulge in the volcano's summit preempting its total collapse. The material collapses upon itself, forming a fast-moving pyroclastic flow (known as a block-and-ash flow) that moves down the side of the mountain at tremendous speeds, often over 150 km (93 mi) per hour. These massive landslides make Peléan eruptions one of the most dangerous in the world, capable of tearing through populated areas and causing massive loss of life. The 1902 eruption of Mount Pelée caused tremendous destruction, killing more than 30,000 people and competely destroying the town of St. Pierre, the worst volcanic event in the 20th century.

Peléan eruptions are characterized most prominently by the incandescent 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:

• Mount Pelée, Martinique. The 1902 eruption of Mount Pelée completely devastated the island, destroying the town of St. Pierre and leaving only 3 survivors.^[25] The eruption was directly preceded by lava dome growth.
• Mayon Volcano, the Philippines most active volcano. It has been the site of many different types of eruptions, Peléan included. Approximarly 40 ravines radiate from the summit and provide pathways for frequent pyroclastic flows and mudslides to the lowlands below. Mayon's most violent eruption occurred in 1814 and was responsible for over 1200 deaths.
• The 1951 Peléan eruption of Mount Lamington. Prior to this eruption the peak had not even been recognized as a volcano. Over 3,000 people were killed, and it has become a benchmark for studying large Peléan eruptions.

Plinian Eruptions

Plinian eruptions (or Vesuvian) are a type of volcanic eruption, named for the historical AD 79 eruption of Mount Vesuvius that buried the Roman towns of Pompeii and Herculaneum, and specifically for its chronicler Pliny the Younger. The process powering Plinian eruptions starts in the magma chamber, where dissolved volatile gases are stored in the magma. The gases vesiculate and accumulate as they rise through the magma conduit. These bubbles agglutinate and once they reach a certain size (about 75% of the total volume of the magma conduit) they explode. The narrow confines of the conduit force the gases and associated magma up, forming an eruptive column. Eruption velocity is controlled by the gas contents of the column, and low-strength surface rocks commonly crack under the pressure of the eruption, forming a flared outgoing structure that pushes the gases even faster.

These massive eruptive columns are the distinctive feature of a Plinian eruption, and reach up 2 to 45 km (1 to 28 mi) 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 prevailing winds drive the plume in a direction away from the volcano.

These highly explosive eruptions are associated with volatile-rich dacitic to rhyolitic lavas, and occur most typically at stratovolcanoes. Eruptions can last anywhere from hours to days, with longer eruptions being associated with more felsic volcanoes. Although they are associated with felsic magma, Plinian eruptions can just as well occur at basaltic volcanoes, given that the magma chamber differentiates and has a structure rich in silicon dioxide.

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 lava fountains 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 0.5 to 50 km³ (0 to 12 cu mi) 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.

However the most dangerous eruptive feature are the pyroclastic flows generated by material collapse, which move down the side of the mountain at extreme speeds of up to 700 km (435 mi) 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 lahars, fast moving mudslides with the consistency of wet concrete that move at the speed of a river rapid.

Major Plinian eruptive events include:

• The historical AD 79 eruption of Mount Vesuvius buried the Roman towns of Pompeii and Herculaneum under a layer of ash and tephra. It is the model Plinian eruption. Mount Vesuvius has erupted multiple times since then, for example in 1822.
• The 1980 eruption of Mount St. Helens in Washington, which ripped apart the volcano's summit, was a Plinian eruption of Volcanic Explosivity Index (VEI) 5.
• The strongest types of erupions, with a VEI of 8, are so-called "Ulta-Plinian" eruptions, such as the most recent one at Lake Toba 74 thousand years ago, which put out 2800 times the material erupted by Mount St. Helens in 1980.
• Hekla in Iceland, an example of basaltic Pilian volcanism being its 1947-48 eruption. The past 800 years have been a pattern of violent initial eruptions of pumice followed by prolonged extrusion of basaltic lava from the lower part of the volcano.
• Pinatubo in the Philippines on 15 June 1991, which produced 5 km³ (1 cu mi) of dacitic magma, a 40 km (25 mi) high eruption column, and released 17 megatons of sulfur dioxide.

Volcanic eruptions arise through three main mechanisms:

• Gas release under decompression causing magmatic eruptions.
• Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions.
• Ejection of entrained particles during steam eruptions causing phreatic eruptions.

There are two types of eruptions in terms of activity, explosive eruptions and effusive eruptions. Explosive eruptions are characterized by gas-driven explosions that propels magma and tephra. Effusive eruptions, meanwhile, are characterized by the outpouring of lava without significant explosive eruption.

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 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, and some of the strongest Surtseyan eruptions develop along fracture zones.

Volcano explosivity index

The Volcanic Explosivity Index (commonly shortened VEI) is a scale, from 0 to 8, for measuring the strength of eruptions. 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.

Volcanic eruptions by VEI index

VEI	Plume height	Eruptive volume *	Eruption type	Frequency **	Example
0	<100 m (330 ft)	1,000 m3 (35,300 cu ft)	Hawaiian	Continuous	Kilauea
1	100–1,000 m (300–3,300 ft)	10,000 m3 (353,000 cu ft)	Hawaiian/Strombolian	Months	Stromboli
2	1–5 km (1–3 mi)	1,000,000 m3 (35,300,000 cu ft) †	Strombolian/Vulcanian	Months	Galeras (1992)
3	3–15 km (2–9 mi)	10,000,000 m3 (353,000,000 cu ft)	Vulcanian	Yearly	Nevado del Ruiz (1985)
4	10–25 km (6–16 mi)	100,000,000 m3 (0.024 cu mi)	Vulcanian/Peléan	Few years	Eyjafjallajökull (2010)
5	>25 km (16 mi)	1 km3 (0.24 cu mi)	Plinian	5–10 years	Mount St. Helens (1980)
6	>25 km (16 mi)	10 km3 (2 cu mi)	Plinian/Ultra Plinian	1,000 years	Krakatoa (1883)
7	>25 km (16 mi)	100 km3 (20 cu mi)	Ultra Plinian	10,000 years	Tambora (1815)
8	>25 km (16 mi)	1,000 km3 (200 cu mi)	Ultra Plinian	100,000 years	Lake Toba (74 ka)

Volcanoes can cause widespread destruction and consequent disaster through several ways. The effects include the volcanic eruption itself that may cause harm following the explosion of the volcano or the fall of rock. Second, lava may be produced during the eruption of a volcano. As it leaves the volcano the lava destroys any buildings and plants it encounters. Third, volcanic ash generally meaning the cooled ash - may form a cloud, and settle thickly in nearby locations. When mixed with water this forms a concrete-like material. In sufficient quantity ash may cause roofs to collapse under its weight but even small quantities will harm humans if inhaled. Since the ash has the consistency of ground glass it causes abrasion damage to moving parts such as engines. The main killer of humans in the immediate surrounding of an volcanic eruption is the pyroclastic flows, which consist of a cloud of hot volcanic ash which builds up in the air above the volcano and rushes down the slopes when the eruption no longer supports the lifting of the gases. It is believed that Pompeii was destroyed by a pyroclastic flow. A lahar is a volcanic mudflow or landslide. The 1953 Tangiwai disaster was caused by a lahar, as was the 1985 Armero tragedy in which the town of Armero was buried and an estimated 23,000 people were killed.

A specific type of volcano is the supervolcano. According to the Toba catastrophe theory 70 to 75 thousand years ago a super volcanic event at Lake Toba reduced the human population to 10,000 or even 1,000 breeding pairs creating a bottleneck in human evolution. It also killed three quarters of all plant life in the northern hemisphere. The main danger from a supervolcano is the immense cloud of ash which has a disastrous global effect on climate and temperature for many years.

During a volcanic eruption, lava, tephra (ash, lapilli, volcanic bombs and blocks), and various gases are 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 different metatypes 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 last eruptive metatype 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 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 erruptive strength is Volcanic Explosivity Index (VEI), a magnitudic scale ranging from 0 to 8 that often correlates to eruptive types.