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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