kinds of volcanic eruptions

8/9/2019 Kinds of Volcanic Eruptions

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KINDS OF VOLCANIC ERUPTIONS

Even people with only a little interest in science are aware that there are several kinds ofvolcanic eruptions. Most television viewers have seen the dramatic footage of the lateral blast of

the 1980 Mount St. Helens eruption. The recent destruction of the Wahaula Visitor CenterinHawaii Volcanoes National Park, the Royal Gardens subdivision, and the village of Kalapana

brought Hawaiian eruptions to the nation's attention. Footage from the 1991-1993 Etna eruptionin Italy showed the successful diversion of lava away from a village. The 1991 explosive

eruption ofMount Pinatubo in the Philippines forced the evacuation of American servicemen,their families, and thousands of Filipinos. Newspaper articles have described the discovery of a

large submarine volcanic field on the Pacific Ocean floor. These examples illustrate the range indifferent types of volcanic eruptions reported by the media. Photograph of Hawaiian eruption at

Puu Oo, a central vent, on the flank of Kilauea copyrighted by Paul Buklarewicz.


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Lesson at a glance, Key Concepts, and Lesson Outcomes are available by clicking here.

Background:

Thomas Jaggar established a volcano observatory in Hawaii because the eruptions could beviewed relatively safely. Jaggar was familiar with the violent characteristics of other types ofvolcanoes, having visited Soufriere and Mont Pelee in Martinique after the 1902 eruptions. He

hoped that knowledge gained from Hawaiian volcanoes would be used to monitor moredangerous volcanoes and reduce the hazard to people living near them.

This lesson describes different types of volcanic eruptions.

Several different classification schemes are presented becausedifferent aspects of an eruption are of interest. Eruptions can

occur at different locations on a volcano and can show anamazing range of characteristics. The lesson starts with a

description of the location of eruptions. This section is followedby a description of water-related eruptions. The lesson

concludes with a classification of eruptions based on theircharacter. The primary sources for the following classifications

are MacDonald (1972), McClelland and others (1989), andWilliams and McBirney (1979).

ClassificationsBasedon Formand Locationofthe Vent

Eruptions can be classified by the opening through which volcanic

material flows or is ejected. Lava or pyroclastic material can beerupted from a fissure or a central vent. A fissure is a "surface of

fracture or a crack in rock along which there is distinct separation"(Bates and Jackson, 1980, p. 232). A fissure eruption is an

"eruption that takes place from an elongate fissure" (Bates andJackson, 1980, p. 232). Fissure eruptions are common along mid-

ocean ridges and produce pillow basalts. Fissure eruptions are alsocommon in Hawaii and often produce curtains of fire as lava

fountains along a fissure. This slide shows a fissure eruption near Halemaumau on April, 30,1982. Photograph by Norm Banks, U.S. Geological Survey.

In contrast, a central vent is the "opening at the Earth's surface of a volcanic conduit ofcylindrical or pipelike form" (Bates and Jackson, 1980, p. 102). A central eruption is the

"ejection of debris and lava flows from a central point, forming a more or less symmetricalvolcano" (Bates and Jackson, 1980, p. 102). The 1991 eruption of Mount Pinatubo and the 1980

explosive eruption of Mount St. Helens, after the lateral blast, are examples of central eruptions.A central eruption continued at Mount St. Helens as a volcanic dome grew within the crater of

the volcano. Central eruptions also occur in Hawaii, such as the explosive eruption of 1790 or the


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1967-1968 lava lake in Halemaumau. The 1969-1974 Mauna Ulu eruption began as a fissureeruption and evolved to a central vent. The 1983 eruption of Kilauea Volcano began as a fissure

eruption and, over the course of several months, became localized at a central vent . A similarpattern, although of shorter duration, occurred during the 1959 eruption at Kilauea Iki Crater.

This photo shows an eruption from a central vent at Paricutin in Mexico. Photograph by K.

Segerstrom of the U.S. Geological Survey,

Eruptions can also be classified by the locationwhere volcanic material reaches the surface.

Central eruptions can also be called summiteruptions, if they are located at the volcano's

summit. Summit eruptions are the mostcommon type of volcanic eruption. The

explosive eruptions of Mount St. Helens andMount Pinatubo are examples of summit

eruptions. The most recent summit eruption ofKilauea occurred in 1982. Volcanic material

can also be erupted from the side of a volcanoto produce a flank eruption. Flank eruptions

are common in Hawaii where magma travelsin rift zones to the flank of the volcano. Since 1955, most eruptions of Kilauea Volcano have

been on the east flank (rift zone) of the volcano. The ongoing eruption of Kilauea Volcano is thelongest and most voluminous flank eruption in historical time.

CharacteristicsofWater-Related Eruptions

McClelland and others (1989) reported on 388 documented volcanic eruptions from 1975 to1985. They found four characteristic ways that water interacts with eruptions: submarineeruptions, formation of new islands, subglacial eruptions, and crater lakes.

Few submarine eruptions have been documented

because of the difficulty in monitoring submarinevolcanoes. Several lines of evidence indicate that

submarine eruptions occur. Seismicity characteristicof volcanic eruptions and intrusions has been recorded

at some seamounts (submarine volcanoes). Fresh

volcanic material and floating rafts of pumice alsoindicate submarine eruptions. Boiling seawater is alsoevidence of shallow eruptions. A submarine eruption

was reported near Necker Island in 1955. Passengersaboard a plane bound for Honolulu from Tokyo saw

what appeared to be a column of smoke rising from the water. On a closer approach, they foundan area of steaming, turbulent water about one mile (1.5 km) in diameter. Nearby was an area of

several thousand square yards that looked like dry land. MacDonald and others interpreted this


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"dry land" as a pumice raft that subsequently became water-logged and sank. This photographshows water discolored by pyroclasts and a steam plume rising above a submarine eruption at

Kavachi. Photograph is courtesy of the U.S. Geological Survey. In 1996, a submarine eruptionoccurred on the ocean ridge off the coast of Oregon.

Shallow water eruptions are characterized by steam explosions that produce islands made oftephra. In most cases, the islands are eroded by ocean waves. In some cases lava is erupted

subaerially and forms a protective cap on the island. Formation of a new Hawaiian island has notoccurred in the recent geologic past. However, this process is one stage in the growth of all

Hawaiian volcanoes. Loihi, a submarine volcano south of the island of Hawaii, is expected toreach this stage in a couple of hundred thousand years. Photograph of explosive interaction of

lava and water by Donna Donovan-O'Meara of Nature Stock.

Only five subglacial eruptions were reported in the ten-year interval studied by McClelland andothers (1989). They note that most of these eruptions occur in remote regions. Surprisingly, there

have been subglacial volcanic deposits in Hawaii. About 10,000 years ago, during the Ice Age,the summit of Mauna Kea Volcano was covered by a glacier. Subglacial eruptions produced


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pillow basalts (Porter, 1987). There is no evidence of subglacial eruptions on Mauna LoaVolcano. However, more recent eruptions may have buried the older subglacial volcanic

deposits. Glaciers did not form on Kilauea Volcano because of its low elevation, however it wascold enough to snow. This photograph shows pillow lava that formed beneath the ice on Mauna

Kea about 170,000 years ago. Photograph from S.C. Porter, Pleistocene subglacial eruptions on

Mauna Kea, U.S.Geological SurveyProfessional Paper 1350.

The summits of somevolcanoes contain crater

lakes. Close proximity ofmagma to the lake can lead

to explosive activity. Craterlakes can also generate

hazardous mudflows.McClelland and others (1989) reported 24 eruptions that occurred through crater lakes between

1975 and 1985. The presence of only one small lake on the Mauna Kea Volcano makes aneruption associated with a crater lake in Hawaii very unlikely. The recent eruption at Ruapehu in

New Zealand displayed many of the common characteristics of this type of eruption. Phreaticexplosion at Ruapehu in 1992. Photo by Christian Treber.

ClassificationsBasedonthe Characterofthe Eruption

Eruptions can be classified by how explosive they are. An explosive eruption is an "eruption ...

that is characterized by the energetic ejection of pyroclastic material" (Bates and Jackson, 1980,p. 217). This photograph shows the explosive eruption of Augustine volcano in Alaska on March

27, 1986. Photograph courtesy of the U.S. Geological Survey.


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An effusive eruption is characterized by the relatively quiet outpouring of lava (MacDonald,

1972, p. 210). A mixed eruption is an eruption "that includes both the emission of lava and theexplosive ejection of pyroclasts" (Bates and Jackson, 1980, p. 403). This photo shoes the gentle

outpouring of lava from a vent in Halemaumau Crater. Photograph by R. Fiske, U.S. GeologicalSurvey, January 5, 1968.

Since early in this century, eruptions have been classified by their re-semblance to specificvolcanoes, where certain types of activity are common. Thus, Hawaiian, Strombolian, Vulcanian,

and Peleean eruptions are named for the volcanoes of Hawaii, Stromboli (Italy), Vulcano (Italy),and Mt. Pelee (Martinique, West Indies). Additional classifications are based on the nature and

scale of activity, for example, basaltic flood and gas eruptions. Plinian eruptions are named forPliny the Elder, a Roman naturalist who died in an eruption of Vesuvius in the year A.D. 79.

MacDonald (1972) noted that there are gradations between each type of eruption and that somevolcanoes can display more than one type of activity.

In the paragraphs that follow, each type of eruption is described. A review of the terms used to

describe tephra may be useful. Clickhere for a summary of the characteristics of each type oferuption.

Hawaiian eruptions are characterized by quiet, effusive eruptions that result from the low

viscosity, low gas content, and high eruption temperatures of Hawaiian magmas.


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During some eruptions, hydrostatic pressure (pressure from magma at higher levels in the

system) and the expansion of gas shoot lava high into the air. These \

ava fountains are commonly a few tens to hundreds of feet (meters) high. Less common arefountains that reach over 1,000 feet (300 m) in height. The highest 1959 lava fountain at Kilauea

Iki reached 1,900 feet (580 m) in height. Macdonald (1972) states that lava fountains are nottruly explosive and are best thought of as jets of incandescent lava shot into the air like water

from a fire hose. The lava accumulates near the vent to produce a spatter orcinder cone. The1959 Kilauea Iki eruption made Puu Puai, a 125-foot (38 m) high cinder and spatter cone.

Hawaiian eruptions commonly start as fissure eruptions with a curtain of fire or closely spacedlava fountains. During this stage of an eruption, spatter ramparts can form. If the lava level in the

fissure is high enough, lava can overflow. Because of their basaltic composition, the lava flows

are thin, fluid, and extensive. The flows can bepahoehoe or aa.

If the eruption is from a central vent, repeated overflowings can form a gently sloped mound of

lava, much like a small "shield volcano" (Macdonald, 1972, p. 215). Mauna Ulu, on the upper

east rift zone of Kilauea Volcano, is an example of this type of feature. This photo shows a highlava fountain during phase 12 of the Mauna Ulu eruption. The ongoing eruption ofKilauea

Volcano is typical of Hawaiian eruptions, with the exception of its long duration and greatvolume. Photograph courtesy of the U.S. Geological Survey, December 30, 1969.


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Basaltic flood eruptions are similar to Hawaiian eruptions in general character but differ by the

very large volume of lava produced. In the northwestern United States, basaltic flood eruptionsproduced flows with an average thickness of 80 ft (25 m) that can be over 60 miles (100 km)long. Individual flows can cover more than 15,600 square miles (40,000 square km)(Swanson

and others, 1975). The thick accumulation of laterally extensive basaltic lava flows that resultfrom basaltic flood eruptions are calledplateau or flood basalts (Williams and McBirney, 1979).

This photo shows a stack of lava flows in the Columbia River Flood Basalt along the SnakeRiver south of Asotin, Washington. Photograph by Robert Wickman.

Photograph of Strombolian eruption at Stromboli copyrighted by Steve O'Meara of Nature Stock.

Strombolian eruptions are named forStromboli volcano off the west coast of Italy, where a

typical eruption consist of the rhythmic ejection of incandescent cinder, lapilli, and bombs toheights of a few tens or hundreds of feet (meters). The effusion of lava flows may or may not

accompany the ejection of pyroclastic material. Lava flows from Strombolian eruptions are


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typically more viscous than Hawaiian lava flows and thus are somewhat shorter and thicker(Macdonald, 1972). Tephra is glowing red when it leaves the vent but becomes black and nearly

solid before hitting the ground. Cinder is most common with less abundant bombs and lapilli.Ash may be present in relatively minor amounts. The tephra accumulates near the central vent

and builds a cinder cone. Magma associated with Strombolian activity isbasaltic or andesitic and

has a higher viscosity than Hawaiian magmas. Because of the higher viscosity, gas has greaterdifficulty escaping. Gas bubbles burst at the top of the magma column, producing smallexplosions and throwing clots of molten lava into the air. Strombolian eruptions can last from a

few hours or days to a few months or a few years. The long duration of Strombolian activity is acommon characteristic. Paricutin volcano in Mexico erupted continuously from 1943 to 1952,

producing a cone made of cinder, bombs, lapilli, and ash. Izalco, in El Salvador, was constructedby Strombolian eruptions.

Vulcanian eruptions are named after the cone ofVulcano in the Lipari Islands west of Italy.

Vulcanian eruptions can involve almost any type of magma but felsic magma, magma withrelatively high silica content, is most common (Williams and McBirney, 1979). This type of

eruption usually begins with steam explosions that remove old, solid lithic (rock) material fromthe central vent. The main phase of the eruption is characterized by the eruption of viscous, gas-

rich magma that forms vitric (glassy) ash. An eruption cloud, a cauliflower- or mushroom-shaped cloud of ash, develops above the vent. The eruption cloud can be gray or black. Lightning

in the eruption cloud is common during Vulcanian eruptions. Airfall, pyroclastic flow, and base-surge deposits can form a cone of ash, surrounded by wide sheets of ash. Tephra deposits from

Vulcanian eruptions are more widely dispersed than deposits from Hawaiian or Strombolian

eruptions. The eruption of thick, viscous lava flows indicates the end of the eruptive cycle(Williams and McBirney, 1979).


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Peleean eruptions are named forMont Pelee in the West Indies, where this type of activity was

first witnessed and described in 1902-1903. Peleean eruptions are associated with rhyolitic orandesitic magmas. The two characteristic features of Peleean eruptions are the formation of

domes and glowing avalanches (Macdonald, 1972). During the opening stages of the eruption,violent glowing avalanches of hot ash travel down the flanks of the volcano. These incandescent

avalanches can start fires and are powerful enough to topple walls. Tephra deposits are generallymuch less widespread than most Vulcanian and Plinian eruptions (Williams and McBirney,

1979).


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Following the initial explosive stage, viscous magma forms a steep-sided dome or volcanic spine

in the volcanic vent. Gravity or internal pressure can cause the dome to collapse, resulting in hotblock-and ash flows. Peleean eruptions generally complete their eruptive cycle in only a few

years (Williams and McBirney, 1979). Santiaguito, in Guatemala, is an example of a Peleeaneruption that has continued for decades. Photo shows a volcanic spine at the summit of the Mt.

Pelee. Photograph by Heilprin.


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Plinian eruptions are named for the famous Roman naturalist Pliny the Elder. He died during an

eruption ofVesuvius in A.D. 79. Pliny the Elder's nephew described the eruption, which ischaracteristic of Plinian eruptions. Two key characteristics are an exceptionally powerful,

continuous gas blast eruption and the ejection of large volumes of pumice (Walker and Crosdale,1971). Plinian eruptions can last less than a day, such as the short-lived explosions of gas-rich,

siliceous magma prior to the eruption of fluid basaltic lava flows in Iceland. Longer-lived, morevoluminous Plinian eruptions can last for weeks or months. The longer eruptions start with

showers of ash followed by glowing avalanches. In some cases, so much magma is erupted thatthe summit of the volcano collapses to produce a caldera. Classic examples of collapse to

produce a caldera are Krakatau in 1883, CraterLake about 7,000 years ago, and S antorini in1500 B.C. During Plinian eruptions fine ash can be dispersed over very large areas. Total volume

of tephra erupted during the formation of CraterLake was 18 cubic miles (75 cubic km). The1886 eruption ofTarawera is a rare case of a basaltic Plinian eruption. Photograph shows the

Plinian eruption of Mount St. Helens on May 18, 1980. Photograph courtesy of U.S. GeologicalSurvey.


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Rhyolitic flood eruptions are characterized by the production of large volumes of rhyolitic

material that spread great distances from their vents to produce broad, nearly level plains(Macdonald,1972). Rhyolitic flood eruptions are from fissure vents. The fluidity of theseeruptions is a result of hot ash flows. Macdonald cites the 1912 eruption ofMt. Katmai in Alaska

as an example of a rhyolitic flood eruption. This eruption produced a caldera and greater than 1.8cubic miles (7 cubic km) of ash. The area is now part ofKatmai National Park.

Ultravulcanian eruptions are characterized by the eruption of solid rock and steam. The

fragments can be from ash to blocks in size and cold to incandescent in temperature. No newmagma is involved. These eruptions are also called phreatic, based on the assumption that the

steam originates from the contact of groundwater with hot rock. Jaggar (1949) called this type ofactivity "steam-blast" eruptions. The 1924 explosive eruption at Halemaumau is an excellent

example of an ultravulcanian eruption. The 1963-1965 eruption of Surtsey in Iceland and the1965 eruption of Taal in the Philippines are additional examples of ultravulcanian eruptions.

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