VOLCANIC ERUPTIONS: CHARACTERISTICS, PHASES, CAUSES, AFTERWARDS

ERUPTING VOLCANOES

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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

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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

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.

Volcanic “Scream” Before an Eruption

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another explanation
expanding magma Harmonic tremors are continuous seismic vibrations that can precede volcanic eruptions. At Alaska’s Redoubt Volcano, these vibrations increase in frequency until they abruptly stop shortly before another explosion. Scientists believe the phenomenon could provide a limited early warning of eruptions. [Source: Ker Than, National Geographic, July 16, 2013]

Geophysicist Alicia Hotovec-Ellis of the University of Washington and her colleagues proposed that Redoubt’s unusually high-pitched tremors occur as thick magma is forced under great pressure through a narrow underground passage. The magma sticks to the surrounding rock until pressure forces it upward in sudden movements, each producing a small earthquake. As pressure increases, the earthquakes become smaller and more frequent, blending into a continuous, rising sound. According to the researchers, the sound comes from the moving rocks rather than vibrating magma bubbles.

The tremors suddenly stop when the rocks begin sliding smoothly against each other, possibly indicating that the earthquakes can no longer keep pace with the movement. At Redoubt, the volcano had already been erupting for several days when the tremors became especially intense, so the phenomenon might provide only minutes or hours of warning before another explosion.

The researchers published their findings in the Journal of Volcanology and Geothermal Research and Nature Geoscience. They hoped their model would improve understanding of volcanic activity and help explain similar tremors at other volcanoes, including Soufrière Hills on Montserrat. Further research would investigate why pressure becomes concentrated in particular locations within a volcano.

Singing of Kilauea Before It Erupted in 2018

In 2007, lava began accumulating inside Hawaii’s Kilauea volcano. Over the following decade, rocks falling from the crater rim produced waves that traveled through the lava pool. These vibrations were recorded by seismic instruments and later studied for clues about conditions inside the volcano before its major eruption in 2018, which lasted from May through August. Josh Crozier and Leif Karlstrom of the University of Oregon’s Department of Earth Sciences analyzed the waves and how they propagated through the lava. Their research, published in Science Advances, examined whether the vibrations could reveal information about the volcano’s internal conditions, similar to how the tones produced by tapping a glass vary with the liquid’s properties. [Source: Cassidy Ward, SYFY, June 12, 2022]

After the 2018 eruption, the researchers analyzed a decade of data from seismic sensors, GPS stations and visual observations. They developed a computer model and tested different combinations of temperature and dissolved gas content to reproduce the recorded vibrations. Their results showed that the characteristics of the waves were related to both the lava’s temperature and its dissolved gas content.

Because direct measurements of these properties are often difficult to obtain before an eruption, analyzing seismic vibrations could help scientists infer conditions inside a volcano. The findings may improve understanding of volcanic processes and contribute to future eruption monitoring, although further research is needed to determine how reliably these measurements can help predict eruptions.

Volcanic Lightning

Volcanic lightning occurs when electrical charges build up within an ash plume during an eruption. When Chile’s Calbuco volcano erupted in April 2015 for the first time since 1972, its ash plume rose more than 14 miles (23 kilometers) into the atmosphere and produced thousands of lightning flashes. Powerful eruptions can generate spectacular electrical storms, but not all eruptions produce lightning. The intensity of the eruption and the availability of water and ice influence whether lightning develops. [Source: Irene Berman-Vaporis and Jason Treat, National Geographic, February 2021]

As magma rises toward the surface, expanding water vapor breaks molten rock into tiny particles. Near the volcanic vent, these particles can produce small electrical discharges detectable through continuous radio-frequency signals. As the ash plume rises, rock fragments collide and scrape against one another, exchanging electrons and becoming electrically charged. This process, known as tribocharging, helps separate positive and negative charges within the plume.

As charged particles move at different speeds, positive and negative charges become separated, creating an electric field that can produce lightning. If the plume rises high enough into the atmosphere, water freezes into ice crystals. Collisions among ice particles, hail and supercooled liquid droplets can increase electrical charging and produce especially intense lightning activity. Water-rich plumes may generate more ice crystals, but scientists have not yet established exactly how much ash or ice is needed to produce lightning.

Research on eruptions in Alaska and Indonesia has helped scientists understand how ice contributes to volcanic lightning, but many aspects of the process remain uncertain. Researchers continue to monitor major eruptions to determine why some ash plumes generate extensive lightning displays while others do not.

Earthquake-Volcano Links

Anak Krakatau Major earthquakes may trigger volcanic activity, although scientists have difficulty establishing a statistical relationship because both events are relatively rare. In 1960, a magnitude-9.5 earthquake struck Chile, and Puyehue Volcano erupted two days later. The volcano erupted again in June 2011, following a magnitude-8.8 earthquake in February 2010. Research by former Japan Meteorological Agency volcanologist Masaaki Churei found that 12 of 13 eruptions recorded in northeastern Japan between 1841 and 1996 occurred within eight years before or after four magnitude-8-class earthquakes off the Sanriku coast. [Source: Taizan Emura, Yomiuri Shimbun, July 28, 2011]

Earthquakes and volcanoes are linked through tectonic plate movements. Off northeastern Japan, the Pacific Plate sinks beneath the overriding plate along the Japan Trench. Water released from the descending plate helps surrounding rocks melt and form magma. However, magma can take thousands or tens of thousands of years to reach the surface, so this process alone does not explain volcanic activity occurring shortly after a major earthquake.

Following the March 11, 2011, earthquake in Japan, underground volcanic activity increased at 20 volcanoes, including Mount Yakedake, Mount Hakone and Mount Aso. The Japan Meteorological Agency suggested that seismic waves shook underground magma chambers, causing gases within the magma to form bubbles and trigger swarms of small earthquakes. Another hypothesis is that earthquake-related movements in the Earth's crust alter pressure around magma chambers, forcing magma upward. Researcher Eisuke Fujita investigated how the March 2011 earthquake changed the gravitational pressure exerted by surrounding rocks near Mount Fuji's magma chamber. Mount Fuji had erupted 49 days after the 1707 Hoei Earthquake. Fujita's computer simulation suggested that movements along nearby faults could deform a magma chamber located about 18 kilometers (11 miles) underground, squeezing magma out and potentially contributing to an eruption.

Return of Life After a Volcanic Eruption

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.

greenery on Anak Krakatoa

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.

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.

Return of Life to Krakatoa and Mount St. Helens

Despite the devastation caused by the 1980 eruption of Mount St. Helens, some organisms there survived. According to Charlie Crisafulli of the U.S. Forest Service, nocturnal animals such as mice and voles remained underground during the morning explosion. Some birds had not yet migrated to their nesting grounds, while snow and ice protected certain plants and aquatic species. These survivors, along with organic material from dead trees and insects that helped build soil, provided a foundation for ecological recovery. [Source: Lyn Garrity, Smithsonian magazine, January 2012]

The avalanche created hummocks and depressions that eventually formed two lakes and about 150 ponds. Within a few years, frogs and toads colonized the new water bodies. Northern pocket gophers also survived and helped restore the devastated landscape by bringing buried plant material and soil microbes to the surface as they burrowed. Their mounds of dark, nutrient-rich soil stood out against the surrounding gray ash and trapped windblown seeds. Elk later helped create additional refuges for amphibians by stepping into gopher tunnels.

On the pumice plain, prairie lupine became a pioneering plant species. This flowering legume added nitrogen to the nutrient-poor soil, allowing other plants to establish themselves. Millions of lupines now grow across the plain alongside penstemons, grasses, willows and young conifers, demonstrating how surviving organisms and pioneer species helped the ecosystem recover.

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.

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 October 2026