Eruption of Chaiten in Chile in October 2008
A volcanic eruption is regarded as violent release of material. Many volcanoes release steam and gas around the clock, year after year. These are regarded as releases not eruptions. In some volcanic areas steam and gas are emitted from vents called fumaroles. Some produce a the rotten egg smell of hydrogen sulphide and coat boulders with a layer of sulphur.

During a volcanic eruption, lava, tephra (ash, lapilli, volcanic bombs and blocks), and various gases are expelled from a volcanic vent or fissure.During an episode of activity, a volcano commonly displays a distinctive pattern of behavior. Some mild eruptions merely discharge steam and other gases, whereas other eruptions quietly extrude quantities of lava. The most spectacular eruptions consist of violent explosions that blast great clouds of gas-laden debris into the atmosphere.

A volcano erupts when magma in the magma chamber rises due to changes in pressure between the magma and the surrounding rock and changes in the chemical and physical composition of the magma. Before many eruptions the pressures inside the volcano are so great the entire mountain swells. Magma that is relatively viscid often contains a large amount of silica, a main ingredient of volcanic ash. When the volcano erupts, a great amount of ash is also ejected.

There are basically two general kinds of eruptions: 1) explosive ones in which large amounts of ash, gas, and steam are ejected in a relatively short period id time; and 2) less explosive “effusive” ones in which volcanic rock emerges like a fountain or fireworks explosion or simple pours from a crater or vent. During small eruptions material is ejected in rhythmic bursts that occur several seconds or a few minutes apart. When the material is released over a period of time a lava dome often forms around where the material emerges. Large eruptions often produces wild streaks of pink, blue and yellow lightning caused by a build-up of static electricity in the cloud of erupting ash, produced by friction among the swirling particles with the cloud.

Sometimes the worst explosions occur after the initial eruption when a solidified lava domes plugs the volcano’s vent like the lid of a pressure cooker, causing pressure to build up in the vent and produce a huge explosion when it builds up to a critical level.

Websites and Sources on Volcanoes: USGS Volcanoes volcanoes.usgs.gov ; Volcano World volcano.oregonstate.edu ; Volcanoes.com volcanoes.com ; Volcano Tourism volcanolive.com ;Wikipedia Volcano article Wikipedia , Smithsonian Global Volcanism Program volcano.si.edu operated by the Smithsonian has descriptions of volcanoes around the globe and a catalog of over 8,000 eruptions in the last 10,000 years.; Volcano Pictures Volcano Photo Gallery decadevolcano.net/photos ; Archive of Volcano Photos doubledeckerpress.com Book: “Volcanoes in Human History” by Jelle Zeilinga de Boer and Donald Theodore Sanders (Princeton University Press, 2002)

What Makes a Volcano Erupt

one explanation for
causes of volcanic eruptions
Scientists are only now beginning to understand the mechanism of what makes a volcano erupt and have gained insight into why the same volcano sometimes erupts with massive, violent explosions and other times ooze out lava in relatively peaceful “effusive” eruptions. [Source: James Glanz, New York Times, November 18, 2003]

Scientists believe the mechanism for an en explosive eruption are as follows: 1) Magma becomes saturated with dissolved gases. 2) Bubbles form and enlarge as the magma rises and pressure on it drops. Sometimes the bubbles occupy as much 90 percent of the volume of the magma 3) Highly viscous magma, rising swiftly and cools, forming a plug. An explosive eruption occurs when the pressure of the bubbling magma exceed the strength of the plug and blows out the plug like a cork on a bottle of champagne.

Scientists believe the explosive energy of volcano is associated with the effect of bubbles and water on the viscosity of the magma, with the viscosity increasing as the presence of bubble and water rises. Sometimes the difference between a violent explosion and peaceful one can be slight differences in amount of bubbles. If the lava remains fluid it can ooze peacefully out of the volcano. If it becomes too viscous it behaves like a solid and is more likely to form a potentially explosive plug Sometimes changes in the amount of water in magma-lava by a factor of 10 can affect the viscosity of the magma-lava by a factor of a million.

Events Before an Eruption

another explanation
expanding magma
Chris Newhall, a volcanologist with USGS and the University of Washington, told National Geographic that six events occur in order of appearance before a volcano eruptions: 1) large amounts of carbon dioxide are released from a volcano’s fumaroles, indicating that magma is rising below the surface; 2) a slight swelling occurs over a large area of the volcano’s surface, showing the magma is rising closer to the surface; 3) hundreds of small earthquakes shake the mountain, indicating the magma and volcanic fluids are forcing their way through cracks in the Earth towards the surface.

As the volcano gets nearer to erupting Newall said: 4) the small earthquakes suddenly decrease, an event sometimes accompanied by a rise in sulfur dioxide emissions, indicating that possibly the magma has stalled just below the surface and an explosive event is imminent; 5) a large bulge within a few hundred meters of the volcano’s vent forms, indicating an influx of magma or an increase magma pressure near the surface; and 6) the release of large amounts as steam as the magma comes into contact with trapped pocket of groundwater, which pulverize the last bits of rock between the magma and the surface.

On Mt. St. Helens earthquakes began rumblings weeks before the eruption, followed by plumes of smoke. On the eve of the eruption the northern flank of the mountain began bulging outward up to two meters a day.

Types of Volcanic Eruptions

Several types of volcanic eruptions have been categorized by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. These include "Strombolian," "Vulcanian," "Vesuvian," "Pelean," and "Hawaiian" eruptions Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity — others may display an entire sequence of types. [Source: Wikipedia; [Source: Asahi Shimbun, September 28, 2014]

Pelee eruption in 1902

There are two main eruptive forms. 1) magmatic and 2) phreatic. The most common are magmatic eruptions in which magma is released out of the mountain surface. They involve the decompression of gas within magma that propels it forward. Magmatic eruptions typically involve a swelling of a volcano and minor tremors when magma rises up through a volcanic vent from its chamber. When magma is ejected from the surface, it releases a torrent of lava and pyroclastic flows. The 1991 eruption of Mount Unzen in Japan, which killed dozens of people, is an example of this.

Within these wide-defining eruptive types are several subtypes (from weakest to strongest): 1) Hawaiian. 2) submarine, 3) Strombolian, 4) Vulcanian, 5) Surtseyan; The stronger eruptive types are 6) Pelean eruptions, 7) Plinian eruptions and 8) "Ultra Plinian," the strongest eruptions. 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), a magnitudic scale ranging from 0 to 8 that often correlates to eruptive types.

The eruptive activity of Parícutin Volcano in 1947 demonstrated a "Vulcanian"-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone. In a Plinian, or "Vesuvian" eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash-laden gas are violently discharged to form cauliflower-shaped cloud high above the volcano.

Phreatic Eruptions

Phreatic (or steam-blast) eruptions result from the interaction of magma and groundwater beneath the surface. Driven by the superheating of steam that comes in contact with magma, they do not involve a major magma movement and spew only substances already deposited inside the mountain. The eruptions are caused driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. Phreatic explosions occur when the heat of rising magma causes underground water to boil and steam pressure rises.

Phreatomagmatic eruptions are driven by the compression of gas within magma, the direct opposite of the process powering magmatic activity. Explosive eruptions characteristic of phreatomagmatic explosions are caused when magma and underground water directly interact.

The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak, but can be quite violent in some cases, such as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76 activity at La Soufrière, Guadeloupe (Lesser Antilles).

Hawaiian Eruption

1954 Kilauea eruption
A Hawaiian eruption in a relatively gentle, low level eruption and feature lava flows from vents and few if nay dramatic explosions. They are so named because they are 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. Little volcanic ash is produced. [Source: Wikipedia, USGS]

"Hawaiian" eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950; or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent 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 fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.

Hawaiian eruptions most often occur at hotspot volcanoes such those in Hawaii and can occur near subduction zones such as Medicine Lake Volcano in California, United States, and rift zones. The Surtsey eruption from 1964 to 1967, when molten lava flowed from the crater to the sea, is another example of a Hawaiian eruption

Hawaiian eruptions usually start by the 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 aa- or pa-hoehoe style of flows. When such an eruption from a central cone is protracted, it can form lightly sloped shield volcanoes such as Mauna Loa or Skjaldbreiður in Iceland. 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. The 1959 eruption in Ki-lauea Iki Crater created a lava fountain 580 meters (1,900 ft) high and formed a 38 meter cone named Puu Puai. Lava reportedly spurted 1,600 meters into the air during the 1986 eruption of Mount Mihara on Izu O-shima, Japan.

Strombolian Eruptions

Stromboli eruption
Strombolian eruptions are relatively low-level volcanic eruptions, named after the Italian volcano Stromboli, where such eruptions frequently occur and 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, and are defined as "Mildly explosive at discrete but fairly regular intervals of seconds to minutes." [Source: Wikipedia, USGS]

Tephra typically glows red when leaving the vent, but its quickly cools and often turns black and solidifies before it hits the ground. The tephra accumulates in the vicinity of the vent, forming a cinder cone. Most of the tephra is in the form of cinder. The amount of volcanic ash is usually relatively low. In the Strombolian-type eruption observed during the 1965 activity of Irazú Volcano in Costa Rica, huge clots of molten lava burst from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

Lava flows are more viscous, and therefore shorter and thicker, than those associated with Hawaiian eruptions. Sometimes pyroclastic rock is formed. Sometimes, 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. These can occur as frequently as a few minutes apart. Gas slugs can form as deep as three kilometers, making them difficult to predict.

Strombolian eruptions can last a long time the conduit system is not strongly affected by the eruptive activity. This allow the eruptive system to repeatedly reset itself. Mount Erebus in Antarctica has produced Strombolian eruptions for many decades. Stromboli itself has been producing Strombolian eruptions for over two thousand years. The Romans referred to it as the "Lighthouse of the Mediterranean."

Peléan Eruptions

Model of a Pelean eruption
In a "Peléan" or "Nuée Ardente (glowing cloud) eruption, such as occurred on the Mayon Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move downslope at velocities as great as 100 miles per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mont Pelée on Martinique, Lesser Antilles.

Peléan eruptions occur when viscous magma, typically of rhyolitic or andesitic type, is involved.The most important and dangerous feature of a Peléan eruption is the presence of a pyroclastic flow, glowing avalanche of hot volcanic ash. There are often associated with with the formation of lava domes. Short flows of ash or creation of pumice sometimes occur. [Source: Wikipedia

The initial phases of eruption are characterized by pyroclastic flows. 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 a few years, but in some cases may continue for decades.

The 1902 eruption of Mount Pelée in Martinique, which killed around 30,000 people, was the first described case of a Peléan eruption, and gave it its name. Other examples include 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.

Plinian and Ultra-Plinian Eruptions

The most powerful eruptions are called "plinian" and involve the explosive ejection of relatively viscous lava. Large plinian eruptions — such as during 18 May 1980 at Mount St. Helens or, more recently, during 15 June 1991 at Pinatubo in the Philippines — can send ash and volcanic gas tens of miles into the air. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving deadly pyroclastic flows ("nuées ardentes") are also commonly associated with plinian eruptions.

Plinian eruptions are also known as Vesuvian eruptions. They are noted for their similarity to the eruption of Mount Vesuvius in A.D. 79, which killed Pliny the Elder, and was described in a letter by his nephew Pliny the Younger. On August 24, A.D. 79,Vesuvius Volcano suddenly exploded and destroyed the Roman cities of Pompeii and Herculaneum. Although Vesuvius had shown stir-rings of life when a succession of earthquakes in A.D. 63 caused some damage, it had been literally quiet for hundreds of years and was considered "extinct." Its surface and crater were green and covered with vegetation, so the eruption was totally unexpected. Yet in a few hours, hot volcanic ash and dust buried the two cities so thoroughly that their ruins were not uncovered for nearly 1,700 years, when the discovery of an outer wall in 1748 started a period of modern archeology. Vesuvius has continued its activity intermittently ever since A.D 79 with numerous minor eruptions and several major eruptions occurring in 1631, 1794, 1872, 1906 and in 1944 in the midst of the Italian campaign of World War II.

Mt. St. Helens eruption in 1980
Plinian eruptions typically feature columns of gas and volcanic ash that extending high into the air, often reaching the stratosphere, high layer of the atmosphere that begins 6,000 to 20,000 meters above the ground. There are also ejections of large amount of pumice and powerful continuous gas blast eruptions. “The lava is usually rhyolitic and rich in silicates.

Short eruptions can begin and end in less than a day. Longer ones can extend over several days, weeks or 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 be deposited over large areas. Plinian eruptions are often accompanied by loud noises, such as the one from Krakatoa heard thousands of kilometers away

Sometimes the term Ultra Plinian is used to describe really big Plinian eruptions, the largest eruptions known. According to the Smithsonian Institution's Volcanic Explosivity Index, a VEI of 6 to 8 is classified as "Ultra Plinian." These have ash plumes over 25 kilometers (16 miles) high and eject 10 square kilometers (two cubic miles) to 1,000 square kilometers (200 cubic miles) of material.

Ultra Plinian eruptions include: 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 1912 eruption of Novarupta in Alaska; 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[2]; The 180 AD Lake Taupo eruption New Zealand; The AD 79 eruption of Mount Vesuvius in Italy, which was the prototypical Plinian eruption.; The 400s BC eruption of the Bridge River Vent in British Columbia, Canada; 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.

Volcanoes After They Erupt

Anak Krakatau
A volcano after an eruption if one of the world's most desolate and lifeless places. The seeds for life the form of water and chemicals exist in the volcanic material left behind. Often what determines how life will take hold is composition of this volcanic material. Life has difficulty taking root on basalt flows, which are and hard and shed water and don't have many cracks where seeds can lodge themselves. Sometimes it takes centuries for life to establish itself.

Life has an easier time taking root in loose lava and ash, which absorbs water and has plenty of cracks and loose material where seeds can lodge themselves and plants can take root. Even so it still takes a while for life to establish itself. This is mainly because the ash and lava are so loose they are often washed away in rainstorms.

The first arrivals in areas that are devoid of life after an eruption are plants with light fluffy seeds that can be blown hundreds of miles and insects like moths and flies that get blown to the volcanic areas by accident. These early arrivals die soon because there are no nutrients for them or food to eat. They however produced nutrients that accumulate in cracks and depressions and provide nutrients for forms of life that appear later.

The rejuvenation process can often be surprisingly swift. The islands of Krakatoa were completely sterilized by the eruptions there in 1883. Areas with rich rain forests were replaced by barren wasteland. In some places the ash was heaped in piles 200 feet thick. In 1884 all that could be found on Rakata was one microscopic spider. But it didn’t take long for life to return. The first organisms to arrive were blue-green algae and ferns germinated from wind-borne spores. Next came grasses. Within three years 28 plant species had established themselves. In 1888, a large monitor lizard was spotted.

In the long term, volcanoes often a positive effect. Volcanic lava and ash produces rich soil that can produce multiple harvest year after year without fertilizer. Ash produce alkaline materials that increase the fertility of soil. High volcanic peaks can generate rain.

Image Sources: Wikimedia Commons, United States Geological Survey (USGS); Volcano Research Center University of Tokyo (the Japan pictures); San Diego State University, U.S. Geological Survey (non-Japan pictures)

Text Sources: United States Geological Survey (USGS), New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated April 2022

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