PHOTOS of Volcanoes below texts.
This lesson describes different types of volcanic eruptions. Several different classification schemes are presented because different aspects of an eruption are of interest. Eruptions can occur at different locations on a volcano and can show an amazing range of characteristics. The lesson starts with a description of the location of eruptions. This section is followed by a description of water-related eruptions. The lesson concludes with a classification of eruptions based on their character. The primary sources for the following classifications are MacDonald (1972), McClelland and others (1989), and Williams and McBirney (1979).
In contrast, a central vent is the "opening at the Earth's surface of a volcanic conduit of cylindrical 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 symmetrical volcano" (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 1967-1968 lava lake in Halemaumau. The 1969-1974 Mauna Ulu eruption began as a fissure eruption 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 similar pattern, 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 location where volcanic material reaches the surface. Central eruptions can also be called summit eruptions, if they are located at the volcano's summit. Summit eruptions are the most common type of volcanic eruption. The explosive eruptions of Mount St. Helens and Mount Pinatubo are examples of summit eruptions. The most recent summit eruption of Kilauea occurred in 1982. Volcanic material can also be erupted from the side of a volcano to produce a flank eruption. Flank eruptions are common in Hawaii where magma travels in 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 the longest and most voluminous flank eruption in historical time.
Few submarine eruptions have been documented because of the difficulty in monitoring submarine volcanoes. Several lines of evidence indicate that submarine eruptions occur. Seismicity characteristic of volcanic eruptions and intrusions has been recorded at some seamounts (submarine volcanoes). Fresh volcanic material and floating rafts of pumice also indicate submarine eruptions. Boiling seawater is also evidence of shallow eruptions. A submarine eruption was reported near Necker Island in 1955. Passengers aboard 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 found an 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 "dry land" as a pumice raft that subsequently became water-logged and sank. This photograph shows 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 eruption occurred on the ocean ridge off the coast of Oregon.
Shallow water eruptions are characterized by steam explosions that produce islands made of tephra. 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 not occurred 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 to reach 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 and others (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 pillow basalts (Porter, 1987). There is no evidence of subglacial eruptions on Mauna Loa Volcano. 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 was cold 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 Survey Professional Paper 1350.
The summits of some volcanoes contain crater lakes. Close proximity of magma to the lake can lead to explosive activity. Crater lakes 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 an eruption 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. Phreatic explosion at Ruapehu in 1992. Photo by Christian Treber.
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 the explosive 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. Geological Survey, January 5, 1968.
Since early in this century, eruptions have been classified by their resemblance to specific volcanoes, 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 for Pliny 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 some volcanoes 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. Click here for a summary of the characteristics of each type of eruption.
Hawaiian eruptions are characterized by quiet, effusive eruptions that result from the low viscosity, low gas content, and high eruption temperatures of Hawaiian magmas.
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 lava fountains are commonly a few tens to hundreds of feet (meters) high. Less common are fountains 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 not truly 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 or cinder cone. The 1959 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 spaced lava 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 be pahoehoe 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 high lava fountain during phase 12 of the Mauna Ulu eruption. The ongoing eruption of Kilauea Volcano is typical of Hawaiian eruptions, with the exception of its long duration and great volume. Photograph courtesy of the U.S. Geological Survey, December 30, 1969.
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 eruptions produced 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 result from basaltic flood eruptions are called plateau or flood basalts (Williams and McBirney, 1979). This photo shows a stack of lava flows in the Columbia River Flood Basalt along the Snake River 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 for Stromboli volcano off the west coast of Italy, where a typical eruption consist of the rhythmic ejection of incandescent cinder, lapilli, and bombs to heights 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 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 is basaltic or andesitic and has a higher viscosity than Hawaiian magmas. Because of the higher viscosity, gas has greater difficulty escaping. Gas bubbles burst at the top of the magma column, producing small explosions 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 a common 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 constructed by Strombolian eruptions.
Vulcanian eruptions are named after the cone of Vulcano in the Lipari Islands west of Italy. Vulcanian eruptions can involve almost any type of magma but felsic magma, magma with relatively 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 from the 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).
Peleean eruptions are named for Mont Pelee in the West Indies, where this type of activity was first witnessed and described in 1902-1903. Peleean eruptions are associated with rhyolitic or andesitic 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 generally much less widespread than most Vulcanian and Plinian eruptions (Williams and McBirney, 1979).
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 hot block-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 Peleean eruption that has continued for decades. Photo shows a volcanic spine at the summit of the Mt. Pelee. Photograph by Heilprin.
Plinian eruptions are named for the famous Roman naturalist Pliny the Elder. He died during an eruption of Vesuvius in A.D. 79. Pliny the Elder's nephew described the eruption, which is characteristic 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, more voluminous 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 that the summit of the volcano collapses to produce a caldera. Classic examples of collapse to produce a caldera are Krakatau in 1883, Crater Lake about 7,000 years ago, and S antorini in 1500 B.C. During Plinian eruptions fine ash can be dispersed over very large areas. Total volume of tephra erupted during the formation of Crater Lake was 18 cubic miles (75 cubic km). The 1886 eruption of Tarawera 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. Geological Survey.
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 these eruptions is a result of hot ash flows. Macdonald cites the 1912 eruption of Mt. Katmai in Alaska as an example of a rhyolitic flood eruption. This eruption produced a caldera and greater than 1.8 cubic miles (7 cubic km) of ash. The area is now part of Katmai 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 new magma 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 of activity "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 the 1965 eruption of Taal in the Philippines are additional examples of ultravulcanian eruptions.
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1. Magma chamber
Mount St. Helens
From Picture Postcard Beauty . . .
Long before settlers arrived from the east, Mt. St. Helens was a sacred place to the local Indian tribes. They had been witnesses to its long history of eruptive behavior and ancient legends caused them to give the mountain a wide berth. Some of the names given to the mountain were Lawelatla ("One From Whom Smoke Comes"), Louwala-Clough ("Smoking Mountain"), Tah-one-lat-clah ("Fire Mountain") and the most commonly used name today Loo-wit ("Keeper of the Fire"). The local tribes would not fish in Spirit Lake, believing the fish, with heads like bears, held the souls of the evilest people who had ever lived. They also believed the lake shores were populated by a band of rogue demons. Only young warriors out to prove their bravery dared climb to the timberline and spend the night. Later, legends claimed the evil spirits of the mountain were punishing the local tribes for allowing the white men to settle at her feet.
In 1792, Captain George Vancouver of the British Royal Navy spotted the mountain from the deck of his ship Discovery as he sailed past the mouth of the Columbia River. He gave the peak its present name after a fellow countryman and friend, Alleyne Fitzherbert, who held the title Baron St. Helens and was at the time the British Ambassador to Spain.
To Scorned Woman . . .
May 18, 1980 dawned clear and bright. It was an amazingly beautiful day for May in the Pacific Northwest. Being a Sunday, there were only a few loggers, campers and scientists in the area. Many of these people had been lulled into a false sense of security because of the mountain's recent silence. Not even the scientists could predict exactly what was to come.
Both of the pictures below were taken from Johnston Ridge, just 5 miles northwest of the summit. It was here that U.S. Geological Survey vulcanologist, David Johnston, was taking measurements the morning of the 18th. At 8:32 a.m. (PST), Johnston radioed "Vancouver! Vancouver! This is it!", only moments before he was struck by the advancing wall of rock, ice and trees that swept laterally from the mountain at more than 500 miles per hour. His body has never been found. In May of 1997 an observation and education center was built on Johnston Ridge in his honor. It is the visitor's center closest to the mountain. (photos below by Harry Glicken, USGS)
At 8:32 a.m. a 5.1 magnitude quake struck one mile below the mountain. While there had been literally hundreds of earthquakes at the mountain since March 20th, the unstable north face could not sustain another. Within moments the largest landslide in recorded history removed more than 1,300 feet from the summit and swept away almost the entire north side of the mountain. The elevation of the mountain dropped from 9,677 feet to its present day 8,363 feet. What was once the 9th highest peak in Washington state was suddenly reduced to the 30th highest peak. The intense high pressure/high temperature steam that escaped, instantly turned more than 70% of the snow and glacial ice on the mountain to water. This massive movement of rock, ash, water and downed trees swept into Spirit Lake and down the north fork of the Toutle River Valley at speeds in excess of 175 miles per hour.
As the north face slid away it let loose the trapped gases like a cork removed from a well shaken bottle of champagne. The three pictures below are from a series of photos taken by Gary Rosenquist who was camping at Bear Meadows, 11 miles north east of the mountain. Rosenquist and the rest of his party survived the eruption even though the lateral bast was rushing straight in their direction at speeds nearing 650 miles per hour. Luckily, after rolling over ridge after ridge, the blast suddenly turned. In the last picture you can see the lateral blast overtaking the landslide. In only three minutes the blast flattened 230 square miles of old growth forest in a fan shape north of the mountain.
Mt. St. Helens is on an ocean-continent subduction boundary (the Juan de Fuca plate is subducting under the N. American plate). Mt. St. Helens is an active strato volcano. The easiest answer to what caused the eruption is that a body of magma moved into the shallow part of the volcano between the first earthquakes and minor eruptions in March, and by May 18th had become so pressurized that the north face of the mountain failed and this magma was able to erupt.The slow moving collision of the continental rock of North America and the Oceanic Rock of the Juan de Fuca tectonic plate, which is slowly moving beneath the Pacific Northwest is slowly refueling Mount St. Helens for another eruption. Scientists estimate that there is a large reservoir of magma four miles beneath the crater. No one knows enough about the volcano to predict when it will erupt again or what kind of volcanic activity the next eruption might bring.
A Return To Beauty . . .
photo by Lyn Topinka, USGS
Despite the devastation left behind on May 18th, not everything in the blast zone was destroyed. Areas that seemed like they would never recover have surprised scientists with the resilience of nature. Many animals and plants who were fortunate enough to be beneath the spring snowpack or underground, soon found their way through the thick ash to the surface. Several lakes that were still frozen over went virtually untouched even though all the life around them was decimated. One of the first plants to reappear, appropriately enough, was the fireweed (pictured above). The presence of plant life enticed deer and elk to return to the area. With their wanderings, they stirred up even more ash, freeing seeds and shoots of plants still buried. Even though there were over 1500 elk killed as a result of the eruption, the elk population had returned threefold by the early 1990’s thanks to mild winters, an abundance of food on the debris avalanche, and the lack of human interference.
Last Updated: July 13, 2000.
The new Landsat 7 satellite is now taking high resolution images of the Earth. This 1999 press release picture shows Mt. St. Helens. The green is forest and fields, the white is glaciers and snow, and the grey shows areas destroyed by flowing ash in the 1980's eruption that have still not recovered their vegetation covering. Spirit Lake is still partly covered by floating logs. Image by EROS Data Center, Sioux Falls, SD.
Mount St. Helens is a stratovolcano. When Mount St. Helens erupted on 18 May 1980, the top 1,300 ft. disappeared within minutes. The blast area covered an area of more than 150 sq. miles and sent thousands of tons of ash into the upper atmosphere. Image taken on 10/16/94 from the Space Shuttle.
A hot, fast moving and high-density (thick like a dust storm) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions is called a pyroclastic flow. Gases released from the volcano vent expand into the atmmosphere and carry the debris. Heavier materials flow along the surface while lighter ash and pumice pieces are thrown up and outward. At Mount St. Helens these flows were strong enough to flow uphill and over Spirit Lake. In the satellite image below the pyroclastic flows moved directly to the right of the mountain, over the Debris Avalanche Deposit and Pumice Plain and beyond Spirit Lake.
NASA/USGS Mount St. Helens image taken from space in September, 1994,
showing lava dome, new lakes, and pyroclastic flows.
Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers. (Description from the SI/USGS Global Volcanism Program)
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