Volcanic eruptions

No two volcanic eruptions are identical. There are, however, sufficient similarities between some eruptions that they serve as models to describe the activity of the remainder. Quantification of these descriptive models has permitted a scientific basis for more accurate eruption forecasting, for interpreting past unwitnessed eruptions, and for assessing the nature of volcanic activity on extraterrestrial bodies.

Classification of Eruptions: Past Eruption Styles

There are many ways to classify volcanic eruptions, whether by the shape of the vent (linear versus point source), the location (submarine or subaerial), the composition of the erupted material, the form of the erupted material (lava, gas, or pyroclastics), or the style of the eruption. All these categories are interrelated to a certain degree. Volcanologists classify eruptions according to style in order to have tools for readily describing the particular type of activity of any given volcano.

By the 1920s, French mineralogist Alfred Lacroix had defined four distinct eruption styles, which he termed Hawaiian, Strombolian, Vulcan, and Peléan; these terms remain in common usage. The names refere the locations of volcanoes that typified the style of eruption described. Style terms that have been adopted since that time follow this system. (An exception is the term “Plinian,” which references Pliny the Elder, who died in 79 CE during the eruption of Vesuvius that destroyed the town of Pompeii, or to Pliny the Younger, who described the eruption.) For example, the eruption in the 1960s of Surtsey, off the coast of Iceland, led to the definition of the Surtseyan eruption style. Narrower subtypes may also be identified, such as when the May 18, 1980, eruption of Mount St. Helens led to the introduction of the term “directed blast eruption” to the volcanological literature (although a similar style had been previously exhibited by the volcano Bezymianny in Kamchatka).

These eruption styles can be generally ranked in order of eruptive strength. Hawaiian eruptions are the weakest, followed by Strombolian, Vulcanian, Surtseyan, Peléan, Plinian, and Ultra-Plinian. More powerful than any of these are supervolcanoes, the popular term for a number of types of extremely powerful volcanic formations. Eruptions may also be classified in three broad types based on the forces driving the eruption. Most familiar are magmatic eruptions, which include the Hawaiian, Strombolian, Vulcanian, Peléan, and Plinian types and are driven by the decompression of gas. Phreatomagmatic eruptions, including Surtseyan, submarine, and subglacial types, involve the compression of gases inside magma. Phreatic eruptions are caused by expanding steam and do not release magma.

Quantification of Eruption Styles

No attempts to quantify eruption style were made until the pioneering volcanology work of George Walker in the early 1970s, with subsequent refinements by Walker and by graduate students who worked with him. In reality, no volcano has a single eruption style. For example, at each of its three vents, the volcano Stromboli might be erupting in a different style at the same moment. Thus, it is more accurate to group together those volcanoes where a single eruption style predominates, those that exhibit a repeated pattern or sequence of eruption styles, and those that have several eruptive styles but no pattern of their occurrence.

Walker's attempt to quantify eruption style is based on five parameters of volcanism: magnitude, intensity, dispersive power, violence, and destructive potential. Magnitude has to do with either the quantity of material or the amount of energy released by an eruption. Because the purpose of quantifying style is to be able to apply measurable parameters to unwitnessed eruptions, the total volume emitted during a single event is a more useful parameter than is energy release. To facilitate comparison between volcanoes, the actual volume is converted to a dense rock equivalent (DRE), which makes allowance for the voids in the rock. Measured volumes range from less than 0.001 cubic kilometer to 1,000 cubic kilometers. Intensity is a measure of the discharge rate—that is, the volume emitted during a specified period of time. Measured intensities range from less than 0.1 to more than 10,000 cubic meters per second DRE. Intensity usually varies throughout the course of an eruption, so a more applicable measurement might be peak intensity. Although there is a strong correlation between intensity and magnitude, a wide range of magnitudes can exist for a given intensity. Dispersive power has to do with the area covered by the deposits of a single event. This parameter can be strongly affected by winds. Areas covered can range from less than 1 square kilometer to about 106 square kilometers. Violence is a measure of the momentum applied to particles in an eruption. Destructive potential refers to the areal extent of the damage to property and vegetation. Neither violence nor destructive potential is particularly applicable to past unwitnessed eruptions, but both are significant in volcanic hazard assessment.

American volcanologists Christopher Newhall and Steve Self developed a volcanic explosivity index (VEI), which attempts to integrate quantitative data with more subjective style descriptions. As its name implies, the index is most applicable to pyroclastic eruptions and is unsuitable for large-scale lava eruptions. Furthermore, the VEI is based partly on eruption column height, which is a function of intensity, and intensity cannot be correlated directly with magnitude. Although calling an eruption a VEI 5 provides little in the way of a mental image of the volcanic activity, it does provide an extremely useful means of computer-based comparisons of the thousands of dated eruptions.

Geoffrey Wadge proposed that lava eruptions could be subdivided into groups defined by magnitude and intensity: high effusion rate with low volume, low effusion rate with high volume, and, perhaps, high effusion rate with high volume and low effusion rate with low volume.

Attempts to quantify eruption style by direct measurement are restricted to subaerial (land) volcanoes on the earth. It has been estimated, however, that about 75 percent of the material erupted on the earth is generated in the deep sea, where pressure and temperatures are vastly different from those on land. Only with the development of manned submersibles did the variety of volcanic products on the ocean floor become apparent. “Smokers”—small, chimneylike structures issuing hot, mineral-laden gases—have been filmed. On other solar system bodies that exhibit volcanic activity, such as Io, there may be essentially zero pressure and much lower gravity. Even if eruption magnitude and intensity in such an environment were the same as those of a subaerial eruption, the style of eruption would be drastically different.

Study of Eruptions

With temperatures on the order of 1,000 degrees Celsius and velocities of some erupting materials of 100 meters per second, direct observation of active eruptions can be a hazardous undertaking. Fortunately, direct measurements to determine the magnitude and intensity of an eruption can be performed at a distance, using photographic or remote sensing techniques. The height of the volcanic column can be measured directly, using ground-based or aerial photography, to provide an estimate of the intensity of the eruption. The volume of solid particles in the eruption column can be measured by determining the amount of light that can be passed through it. Velocities of entrained particles can be measured through viewing high-speed films of the eruption column.

Techniques pioneered by Japanese geologists and extended by George Walker involve measurement of the ejected material after an eruption has ceased. These techniques are little different from those applied to sedimentary rocks; they include the measurement of thickness, the maximum grain size, the grain-size distribution, the proportion of different components, the crystal content of pumice, and the density and porosity. Measurements of thickness of a deposit are used to construct an isopach map—that is, a map showing where the deposit thickness falls between a series of upper and lower limits. Thickness measurements are taken at numerous locations, the number being determined by the size of the deposit and the level of accuracy required. The resultant map provides an indication of the deposit volume, the location of the vent, and the way the deposit was dispersed.

Maximum grain-size measurements are useful for determining the energetics of the eruption column and the effects of wind velocities. If numerous grains are measured over the area of the deposit, an isopleth map can be constructed, indicating areas in which maximum particle sizes are equal. An isopleth map can sometimes be more useful than an isopach map, for erosion tends to remove fine material from a deposit, lessening its thickness, while larger particles remain. The grain-size distribution is obtained by simply sieving numerous samples. A certain amount of care has to be exercised, because pumice and glass are extremely fragile. The result of this analysis is a determination of the degree of sorting of the deposit. This information can be employed to distinguish between certain types of eruptions.

A deposit contains three components: pumice, lithics, and crystals. The varying proportions of each of these components throughout the deposit can be used to infer the conditions in the magma chamber prior to eruption, and the relative proportions at different sites can be employed in determining the eruption dynamics. The physical separation of these components can be an extremely time-consuming task, involving sorting by hand under a microscope. The crystal content of pumice is a further indicator of the preeruption conditions in the magma chamber. To determine the crystal content, pumice lumps can be crushed and the more durable crystals removed. Measurements of density and porosity are important for calculating the dense rock equivalent. In the case of pumice, which often has unconnected pore spaces, these measurements can be rather complicated. Simply, samples are variously measured dry in air, after soaking for several days in water, and under water.

DateLocation5000 BCE MAZAMA, OREGON: The volcano that became erupts, sending pyroclastic flows as far as 37 miles (60 kilometers) from the vent; 12 to 17 cubic miles (50 to 70 cubic kilometers) of material is erupted as a caldera forms from the collapse of the mountaintop. c. 1470 BEC AEGEAN SEA: A volcanic eruption and caldera collapse leave a town buried and preserved intact, possibly causing the disappearance of the Minoan civilization on the island of Crete, the alleged location of the “lost continent of Atlantis.”Aug. 24, 79 CE ITALY: , burying Pompeii and Herculaneum. More than 13,000 dead, 4 cities completely buried, 270 square miles (700 square kilometers) devastated. c. 186 CE NEW ZEALAND: On North Island, a huge eruption with a volcanic explosivity index greater than 6 creates a crater that eventually becomes Lake Taupo.1169 SICILY: erupts with a volcanic explosivity index of 2 and leaves more than 15,000 dead.1362 ICELAND:Öroefajökull erupts with a volcanic explosivity index of 5; 200 die.1586 JAVA, INDONESIA: Kelut erupts with a volcanic explosivity index of 5, leaving 10,000 dead.1591 PHILIPPINES: Taal erupts with a volcanic explosivity index of 3, leaving thousands dead. Dec. 16, 1631 ITALY: Mount Vesuvius erupts with a volcanic explosivity index of 4; 4,000 die.Mar. 11, 1669 SICILY: Mount Etna erupts, leaving more than 20,000 dead, 14 villages destroyed, and 27,000 homeless.1741 ECUADOR: Cotopaxi erupts with a volcanic explosivity index of 2, leaving 1,000 dead.Sept. 29, 1759 MEXICO: Jorullo erupts with a volcanic explosivity index of 3, leaving hundreds dead.Oct. 23–28, 1766 LUZON, PHILIPPINES: Mayon erupts with a volcanic explosivity index of 3, leaving 2,000 dead.Aug. 11–12, 1772 JAVA, INDONESIA: Papandayan erupts with a volcanic explosivity index of 3, leaving 3,000 dead.Dec. 1779–Jan. 1780 JAPAN: Sakurajima erupts with volcanic explosivity index of 4, leaving 300 dead.1783 JAPAN: Asama erupts with a volcanic explosivity index of 4, leaving 1,377 dead.June 1783–Feb. 1784 SOUTHERN ICELAND: The Laki fissure eruption in Iceland produces the largest lava flow in historic time. Benjamin Franklin speculates on its connection to a cold winter in Paris the following year.1790 HAWAII: erupts with a volcanic explosivity index of 4, leaving 100 dead.Feb. 10, 1792 JAPAN: with a volcanic explosivity index of 2, leaving 14,500 dead.1794 RIOBAMBA, ECUADOR: Tunquraohua erupts with a volcanic explosivity index of 2, leaving 40,000 dead.Apr. 27, 1812 ST. VINCENT, WEST INDIES: La Soufrière erupts with a volcanic explosivity index of 4, leaving more than 1,000 dead.Feb. 1, 1814 LUZON, PHILIPPINES: Mayon erupts with a volcanic explosivity index of 4, leaving more than 2,200 dead.Apr. 5, 1815 SUMBAWA, INDONESIA: The dramatic explosion of , 248.6 miles (400 kilometers) east of Java, the largest volcanic event (VEI = 7) in modern history, produces atmospheric and climatic effects for the next two years; 92,000 die. Frosts occur every month in New England during 1816, the so-called Year Without a Summer.Oct. 8 and 12, 1822 JAVA, INDONESIA: Galung Gung erupts with a volcanic explosivity index of 5, leaving 4,000 dead.Jan. 22, 1835 NICARAGUA: Cosigüina erupts with a volcanic explosivity index of 5; hundreds die.1845 COLOMBIA: Nevado del Ruiz erupts with a volcanic explosivity index of 3, leaving 700 dead.June 24, 1853 TONGA ISLANDS: Niuafo'ou explodes, leaving 70 dead, and a village mostly destroyed.Apr. 24–26, 1872 ITALY: Mount Vesuvius erupts again, leaving 22 dead.June 26, 1877 ECUADOR: The eruption of Cotopaxi leaves 1,000 humans dead, thousands of animals dead, and buildings and bridges destroyed.Aug. 26, 1883 INDONESIA: A cataclysmic is heard 2,983 miles (4,800 kilometers) away; 36,417 die and two-thirds of the island is destroyed. Pyroclastic flows race over pumice rafts floating on the surface of the sea; many die from a tsunami.July 15, 1888 HONSHU, JAPAN: Bandai erupts, leaving 461 dead, 70 burned and scarred, and several villages buried.June 23, 1897 LUZON, PHILIPPINES: Mayon erupts, leaving 400 dead, and villages and animals destroyed.May 7, 1902 ST. VINCENT ISLAND, LESSER ANTILLES: La Soufrière erupts, leaving 1,500–1,700 dead, and producing great losses in livestock and crops.May 8, 1902 MARTINIQUE, CARIBBEAN: Mount Pelée, on the northern end of the island, sends violent pyroclastic flows into the city of St. Pierre, killing all but 4 of the 30,000 inhabitants.Oct. 24, 1902 GUATEMALA: Santa María erupts, leaving 6,000 dead, animals and crops destroyed, and buildings collapsed.1905–1906 ITALY: Vesuvius erupts, leaving dozens dead, and buildings destroyed.Jan. 4, 1906. NICARAGUA: Masaya erupts; thousands dieJan. 30, 1911 PHILIPPINES: Taal erupts; 1,335 dead, 200 injured.June 6, 1912 ALASKA: Katmai erupts with a volcanic explosivity index of 6. Ash covers the Valley of Ten Thousand Smokes.June 6, 1917 EL SALVADOR: Boquerón erupts, leaving 450 dead, and 100,000 homeless.May 20, 1919 JAVA, INDONESIA: Kelut's eruption kills 5,500 and destroys many villages.Apr. 17, 1926 HAWAII: Mauna Loa erupts, killing dozens and destroying the town of Hoopuloa.Aug. 4–5, 1928 PALUWEH, INDONESIA: Rokatenda erupts, killing 226 and destroying villages and boats.Dec. 13–28, 1931 JAVA, INDONESIA: Merapi erupts, killing more than 1,300.Feb. 20, 1943 MEXICO: Paricutín volcano comes into existence in a cultivated field; eruption continues for nine years.Jan. 17–21, 1951 GUINEA: Lamington erupts, killing 3,000.Dec. 2–8, 1951 PHILIPPINES: Hibok-Hibok erupts, killing 500.Mar. 30, 1956 KAMCHATKA PENINSULA: The volcano Bezymianny erupts with a violent lateral blast, stripping trees of their bark 18.6 miles (30 kilometers) away.Mar. 20, 1963 BALI, INDONESIA: Mount Agung erupts, killing more than 1,200 and leaving 200,000 homeless.Nov. 8, 1963–June 5, 1967 ICELAND: Surtsey Island is born from an eruption with a volcanic explosivity index of 3.Sept. 28, 1965 PHILIPPINES: Taal erupts, killing 200.July 1968 COSTA RICA: Arenal erupts with a volcanic explosivity index of 3, killing 80.Jan.–May 1973 ICELAND: Hundreds of homes are destroyed after an eruption on Heimaey Island.Jan. 10, 1977 ZAIRE: Nyiragongo erupts with a volcanic explosivity index of 1, killing more than 1,000.May 18, 1980 WASHINGTON STATE: , killing 57 humans, and an estimated 7,000 big-game animals, and destroying nearly 200 homes, more than 185 miles of road, and 4 billion board feet of timber. Ashfall is detected for 22,000 square miles.Mar. 28–Apr. 4, 1982 MEXICO: El Chichón erupts, killing about 2,000 and injuring hundreds. Hundreds are left homeless, thousands evacuated, 9 villages are destroyed, and more than 116 square miles of farm land is ruined.Nov. 13, 1985 COLOMBIA: Mudflows from the eruption of the Nevado del Ruiz volcano kill at least 23,000 people.Aug. 21, 1986 CAMEROON: After building up from volcanic emanations, carbon dioxide escapes from Lake Nyos, killing more than 1,700 people.June 12–15, 1991 LUZON, PHILIPPINES: , killing approximately 350 (mostly from collapsed roofs); extensive damage to homes, bridges, irrigation canal dikes, and cropland occurs; 20 million tons of sulfur dioxide spews into the stratosphere to an elevation of 15.5 miles.Nov. 22, 1994 JAVA, INDONESIA: Merapi eruption kills at least 31.Sept.–Nov. 1996 ICELAND: Eruption of lava beneath a glacier in the Grimsvötn Caldera melts huge quantities of ice, producing major flooding.June 25, 1997 MONTSERRAT, THE CARIBBEAN: kills 19, and 8,000 are evacuated.

A volcanic eruption occurred at Mount Nyiragongo in the Democratic Republic of Congo and Zaire on January 17, 2002. Lava flows traveled up to 12 miles and resulted in 200 deaths. An eruption of Nevado del Huila, the highest volcano in Colombia, triggered avalanches of earth and debris and resulted in 16 deaths on November 20, 2008. The Chaitén volcano in Chile also erupted in 2008, with a VEI of four; it was the largest rhyolite eruption recorded since 1912 and destroyed much of the neighboring town. On October 24, 2010, volcanic eruptions at Mount Merapi in Indonesia resulted in extensive lava flows that resulted in 322 deaths. At least 137,000 people were affected. On June 4, 2011, the Puyehue-Cordón Caulle volcano erupted, causing major evacuations and disrupting air travel around the Southern Hemisphere with its ash cloud.

By January 2014, Mount Sinabung on the Indonesian island of Sumatra had erupted more than two hundred times over a short period, increasingly intensifying and causing more than twenty thousand local villagers to flee the immediate area. Another series of eruptions the following month proved deadly, with at least fourteen people reported dead following an engulfment of ash. That same year, a surprise phreatic eruption of Mount Ontake in Japan claimed the lives of at least sixty people, the majority of them hikers. This was the deadliest volcanic eruption to have occurred in Japan in decades. In 2015, Chile's Calbuco volcano erupted for the first time in more than forty years, displacing around four thousand people from the surrounding area. No one was killed, even as the volcano continued to erupt several more times over subsequent days. Still active, Mount Sinabung erupted again in May 2016, causing the deaths of at least seven more people.

The late 2010s and early 2020s saw a number of deadly and destructive volcanic eruptions around the world. In June 2018, for example, Guatemala experienced its deadliest volcanic eruption in over a century when Volcán de Fuego, located near the city of Antigua, erupted and killed at least 190 people. The December 2021 eruption of Mount Semeru in Indonesia killed fifty-seven people. However, neither of these eruptions were as powerful as the eruption of Tonga's Hunga Tonga–Hunga Haʻapai volcano, which began erupting in December 2021 and continued erupting into January of the following year, with its most powerful underwater explosion taking place on January 15, 2022. At least six people were killed by tsunamis triggered by the eruption, which was considered the most atmospherically-disruptive volcanic eruption since Krakatoa in 1883 as well as one of the most powerful explosions ever measured by modern instruments.

Eruption Prediction

Determination of the type of a volcanic eruption is important for two major reasons: to permit theoretical reconstructions of unwitnessed eruptions and so that scientists can predict the consequences of future eruptions at an active volcano. After the magnitudes and intensities have been established for the unwitnessed eruptions at a specific volcano, such as Mount St. Helens, by combining these data with age data for each deposit, it becomes possible to determine the eruptive history. The volcanologist is then armed with information regarding the largest magnitude eruption that has taken place in the past and the frequency at which volcanic events occur. It is then possible to forecast that a specific volcano can produce an eruption of a given magnitude in the future and to ascertain the probability of such an event taking place within a given period of time.

If a volcano goes through a regular sequence of eruptive events prior to a paroxysmal eruption, recognition of this precursor activity may assist authorities in their decision to evacuate the local populace. The effects of failure to undertake such an analysis and to heed warning signs is well illustrated by the 1951 eruption of Mount Lamington, which had been regarded as an extinct volcano, in Papua New Guinea. A cloud of smoke was seen above the vent on January 15 of that year. Five days later came the paroxysmal eruption, which caused six thousand deaths. In those areas with a high population density or rich agricultural land, insurance brokers are particularly interested in knowing details of possible eruptions to minimize their economic losses.

From the scientific viewpoint, types of volcanic activity provide windows into the interior of the planet and allow assessments of what is happening beneath plate margins, continental interiors, and the ocean floors—and beneath the surfaces of alien planets and satellites. Climatic changes and mass extinctions have been attributed to large-scale volcanic eruptions. Long-term changes in the magnitude and styles of eruption reflect important changes taking place in planetary interiors.

Principal Terms

crystal: a solid with a regular atomic arrangement

lithic: having to do with rock

pumice: pyroclastic rock full of vesicles (spherical voids originally occupied by gas)

pyroclastic rock: fragmented rock produced during a volcanic eruption (the term includes essentially all volcanic products except lava flows)

Bibliography

de Boer, Jella Zeilinga, and Donald Theodore Sanders. Volcanoes in Human History: The Far-Reaching Effects of Major Eruptions. 4th ed. Princeton UP, 2012.

Global Volcanism Program. Smithsonian Inst., Natl. Museum of Natural History, 2024, volcano.si.edu. Accessed 12 Jul. 2024.

Grattan, John, and Robin Torrence, editors. Living Under the Shadow: Cultural Impacts of Volcanic Eruptions. Routledge, Taylor & Francis Group, 2016.

Lipman, Peter W., and D. R. Mullineaux, eds. The 1980 Eruptions of Mount St. Helens, Washington. US Geological Survey Professional Paper 1250. Department of the Interior, 1981. doi.org/10.3133/pp1250. Accessed 15 Apr. 2023.

Lockwood, J. P., and Richard W. Hazlett. Volcanoes: Global Perspectives. 2nd ed. Wiley-Blackwell, 2022.

Oppenheimer, Clive. Eruptions That Shook the World. Cambridge UP, 2015.

Sigurdsson, Haraldur, ed. Encyclopedia of Volcanoes. 2nd ed. Academic Press, 2015.

"Volcanic Eruptions." World Health Organization, www.who.int/health-topics/volcanic-eruptions. Accessed 12 Jul. 2024.

Wei-Haas, Maya. "Tonga's Strange Volcanic Eruption Was Even More Massive Than We Knew." National Geographic, 20 Nov. 2022, www.nationalgeographic.com/science/article/tonga-volcano-largest-eruption-pacific-ocean-tallest-plume. Accessed 12 Jul. 2024.

Winch, Jessica. "What Went Wrong on Mount Ontake?" The Telegraph, 28 Sept. 2014, www.telegraph.co.uk/news/worldnews/asia/japan/11127112/What-went-wrong-on-Mount-Ontake.html. Accessed 15 Apr. 2023.