VOLCANOES IN ERUPTION
Perhaps no force of nature arouses more awe and wonder than that of a volcanic eruption. Volcanoes can be ruthless destroyers. Primitive people offered sacrifices to stem the tide of such eruptions and many of their legends were centered around volcanic activity. Volcanoes are also benefactors. Volcanic processes have liberated gases of the atmosphere and water in our lakes and oceans from the rocks deep beneath Earth's surface. The fertility of the soil is greatly enhanced by volcanic eruptive products. Land masses such as islands and large sections of continents may owe their existence entirely to volcanic activity.
The word "volcano" is used to refer to the opening from which molten rock and gas issue from Earth's interior onto the surface, and also to the cone, hill, or mountain built up around the opening by the eruptive products. The molten rock material generated within Earth that feeds volcanoes is called magma and the storage reservoir near the surface is called the magma chamber. Eruptive products include lava (fluid rock material) and pyroclastics or tephra (fragmentary solid or liquid rock material). Tephra includes volcanic ash, lapilli (fragments between 2 and 64 mm), blocks, and bombs.
Low viscosity lava can spread great distances from the vent. Higher viscosity produces thicker lava flows that cover less area. Lava may form lava lakes of fluid rock in summit craters or in pit craters on the flanks of shield volcanoes. When the lava issues vertically from a central vent or a fissure in a rhythmic, jet-like eruption, it produces a lava fountain.
Pyroclastic (fire-broken) rocks and rock fragments are products of explosive eruptions. These may be ejected more or less vertically, then fall back to Earth in the form of ash fall deposits. Pyroclastic flows result when the eruptive fragments follow the contours of the volcano and surrounding terrain. They are of three main types: glowing ash clouds, ash flows, and mudflows. A glowing ash cloud (nuee ardente) consists of an avalanche of incandescent volcanic fragments suspended on a cushion of air or expanding volcanic gas. This cloud forms from the collapse of a vertical ash eruption, from a directed blast, or is the result of the disintegration of a lava dome. Temperatures in the glowing cloud can reach 1,000 C and velocities of 150 km per hour. Ash flows resemble glowing ash clouds; however, their temperatures are much lower. Mudflows (lahars) consist of solid volcanic rock fragments held in water suspension. Some may be hot, but most occur as cold flows. They may reach speeds of 92 km per hour and extend to distances of several tens of kilometers. Large snow-covered volcanoes that erupt explosively are the principal sources of mud flows.
Explosions can give rise to air shock waves and base surges. Air shock waves are generated as a result of the explosive introduction of volcanic ejecta into the atmosphere. A base surge may carry air, water, and solid debris outward from the volcano at the base of the vertical explosion column.
Volcanic structures can take many forms. A few of the smaller structures built directly around vents include cinder, spatter, and lava cones. Thick lavas may pile up over their vents to form lava domes. Larger structures produced by low viscosity lava flows include lava plains and gently sloping cones known as a shield volcanoes. A strato volcano (also known as a composite volcano) is built of successive layers of ash and lava. A volcano may consist of two or more cones side by side and is referred to as compound or complex. Sometimes a violent eruption will partially empty the underground reservoir of magma. The roof of the magma chamber may then partially or totally collapse. The resulting caldera may be filled by water. The volcanic structure tells us much about the nature of the eruptions.
|Barcena, Mexico Barcena, Mexico; 19.27 N 110.80 W; 375 m elevation.
Barcena is on San Benedicto Island, which lies off the coast of Mexico,
south of Baja California and west of Mexico City. This photograph (taken
in 1952) shows Barcena in eruption. This is the only eruption in recent
times. It produced a glowing avalanche and a lava flow. [Photo credit:
|Cerro Negro, Nicaragua Cerro Negro, Nicaragua; 12.50 N 86.70
W; 675 m elevation. This cinder cone in western Nicaragua has a name that
means "black hill." It has erupted more than 20 times since its birth in
1850. Explosive eruptions from the central crater are often accompanied
by lava flows from the base of the cone. It is the youngest of four cinder
cones scattered along a 20 km line east-southeast of Telica. This view
was taken on February 10, 1971, on the eighth day of an eleven-day eruption
cycle. The eruption has produced an enlargement of the summit crater and
a broadening of the cone due to ashfalls. [Photo credit: R.E. Wilcox, U.S.
|Etna, Sicily Etna, Sicily; 37.73 N 15.00 E; 3,290 m elevation.
This transitional shield-to-strato-volcano in northeastern Sicily has released
lava flows more than 150 times since activity was first recorded in 1500
B.C. Small-to-moderate explosive eruptions commonly occur at the summit.
One in 1979 took nine lives. Etna has the longest documented record of
volcanism in the world. [Photo credit: University of Colorado.]
|Irazu, Costa Rica Irazu, Costa Rica; 9.98 N 83.85 W; 3,432 m
elevation. Located 25 km from San Jose, Irazu is the highest volcano in
Costa Rica and also has the country's earliest historic eruption (1772).
During the lengthy eruption of 1963 to 1965, ash falls were common over
San Jose. Coffee plantations in the area were damaged. This view, taken
in early 1964, shows a dense cloud erupting from the crater. [Photo credit:
|Izalco, El Salvador Izalco, El Salvador; 13.82 N 89.63 W: 1,965
m elevation. This slide, taken in December 1949, shows a small steam eruption
and a view of the older lava flow on the side of the cone in the foreground.
Izalco is a young strato volcano on the south flank of Santa Ana Volcano
in western El Salvador. Its continuous small explosive eruptions (beginning
in 1770) caused it to be known as the "Lighthouse of the Pacific." The
activity consisted of ejection of cinder showers and bombs with occasional
lava flows from the lateral vents. The activity stopped shortly after a
hotel was built nearby to accommodate tourists. [Photo credit: Howell Williams.]
|Kilauea, HI Kilauea, Hawaii, U.S.; 19.42 N 155.29 W; 1,222 m
elevation. This is a lava fountain from the 1959 eruption of Kilauea Iki.
Such lava fountains completely blocked several of the roads in the Hawaii
Volcanoes National Park. The basaltic shield volcano on the Island of Hawaii
is among the most extensively studied volcanoes in the world. The volcano,
with its summit caldera, is located on the east flank of giant Mauna Loa.
Activity began with a violent explosive eruption in 1790. In the 19th and
early 20th Centuries its lava lake eruptions attracted widespread attention.
Nearly half of the world's known lava lake eruptions occur here. The summit
and rift zones have produced extensive lava flows. [Photo credit: University
|Kilauea, Hawaii Rift Eruption Kilauea, Hawaii, U.S.; 19.42 N
155.29 W; 1,222 m elevation. The east rift area has been one of the most
active areas on Kilauea. Shown here are fire fountains in a night view
of the east rift area (in January 1983). Forty-eight eruptive episodes
occurred in this area in 1983, and have continued to the present (1997).
This eruption is the largest of Kilauea's historic rift eruptions. A number
of prehistoric eruptions are indicated as well, by tephra chronology and
carbon-14 dating methods. [Photo credit: University of Colorado.]
|Krishima, Japan Kirishima, Japan; 31.93 N 130.87 E; 1,700 m
elevation. The shield strato volcano consists of more than 20 eruptive
centers over a 20 x 30 km area that also includes Japan's first national
park. Sixty-nine eruptions have been documented since 742 A.D. Kirishima
is one of Japan's 31 active volcanoes, and has erupted 19 times since 1700.
The slide shows two ponds in the crater bottom and gas activity higher
on the crater walls. [Photo credit: University of Colorado.]
|Lamington, Papua New Guinea Lamington, Papua New Guinea; 8.94
S 148.17 E; 1,780 m elevation. This dormant strato volcano in Papua New
Guinea suddenly exploded in 1951. Nuees ardentes (glowing avalanches) shot
down the mountain at 100 km per hour, devastated over 200 km2, and killed
about 3,000 people. Temperatures of the cloud were so high that the deposits
were still hot two years later. Following the explosive eruption, a 500-meter-high
lava dome grew in the explosion crater until 1956. This slide, taken in
1951, shows a nuee ardente. [Photo credit: University of Colorado.]
|La Soufriere, Guadeloupe La Soufriere, Guadeloupe; 16.05 N 61.67
W 1,467 m elevation. La Soufriere is a strato volcano with a summit dome,
on the southern part of Guadeloupe Island. It has erupted explosively about
ten times since 1400. An eruption in 1976 prompted the evacuation of 70,000
people for several months, but produced only minor explosions. This oblique
aerial view, taken in August 1976, shows a plume of vapor and ash boiling
from the summit. The villages and towns that can be seen in the background
were totally evacuated during the crisis. [Photo credit: F.C. Whitmore,
U.S. Geological Survey.]
|Las Pilas, Nicaragua Las Pilas, Nicaragua; 12.48 N 86.68 W;
938 m elevation. This volcano is located only 5 km southeast of Cerro Negro
(refer to Slide 2). Its composite dome is larger than Cerro Negro but the
volcano's only known eruptions were in 1952 and 1954. This aerial view
taken on October 23, 1952, shows steam escaping from the rift at Las Pilas
during a fissure eruption. Photograph credit: Howell Williams.
|Lassen Peak, CA Lassen Peak, California; 40.49 N 121.51 W; 3,186
m elevation. This view of Lassen Peak from Anderson, California (more than
33 km from the volcano) was taken on March 22, 1915. A series of explosive
eruptions from 1914 through 1915 culminated in hot avalanches in May 1915.
It is one of the largest lava domes on record and is nearly two km across
at the base and 606 m high. The sides of the dome are mantled with crumble
breccia (rock composed of large angular fragments) and tephra. A lava flow
melted snow and ice and the resulting mudflow swept 50 km down the valleys
of Hat and Lost Creeks. [Photo credit: R.I. Meyers.]
|Masaya, Nicaragua Masaya, Nicaragua; 11.95 N 86.15 W; 635 m
elevation. Masaya is a 6 x 11 km caldera containing several small central
strato-volcanoes 20 km southeast of Managua in southwestern Nicaragua.
It has erupted nearly 20 times since the first documented eruption in 1524
with varied activity including explosions, lava flows, and lava lakes.
It exhibited an active lava lake from 1965 to 1979 and became Nicaragua's
first national park in 1979. Its prehistoric record indicates explosive
violence unusual in basaltic volcanoes. Shown here is a view of the Santiago
crater within Masaya taken in April 1951. Photograph credit: U.S. Geological
Survey (R.E. Wilcox).
|Mayon, Luzon, Philippines Mayon, Luzon, Philippines; 13.26 N
123.68 E; 2,462 m elevation. Mayon is a classic strato-volcano cone in
the central Philippines. It has erupted explosively nearly 50 times since
1616, produced nuees ardentes from 18 eruptions, and numerous lava flows.
The region is densely populated and at least twelve eruptions have resulted
in deaths. Historically Mayon has been the most active volcano in the Philippines.
The slide, taken May 2, 1968, shows vertical ejection of incandescent material
to about 600 m above the vent. The falling material feeds nuees ardentes.
[Photo credit: J.G. Moore, U.S. Geological Survey.]
|Ngauruhoe, New Zealand Ngauruhoe, New Zealand; 39.16 S 175.6
E; 2,291 m elevation. This strato-volcano with a near perfect cone is located
in central North Island on the southwest flank of the Tangariro massif.
It has been New Zealand's most active volcano in historic times, with 61
eruptive episodes since its first recorded eruption in 1839. This slide
shows a pyroclastic flow (nuee ardente) and eruption cloud of January 1974
eruption. Photograph credit: Univ. of Colorado.
|Pacaya, Guatemala Pacaya, Guatemala; 14.38 N 90.60 W; 2552 m
elevation. Pacaya is a volcanic complex of two small strato-volcano cones
and older lava domes in Southern Guatemala south southwest of Guatemala
City. It has erupted over twenty-two times since its birth in 1565 and
nearly annually since 1965. Eruptions are generally characterized by explosions,
but recent eruptions have also produced lava flows. This view shows an
ash eruption shortly after the February 4, 1976, magnitude 7.5 earthquake.
Photograph credit: U.S. Geological Survey.
|Paricutin, Mexico Paricutin, Mexico; 19.48 N 102.25 W; 3,170
m elevation. A cornfield in central Mexico was the birthplace of this cinder
cone. During its brief nine-year lifespan (1943-1952), Paracutin developed
a 410-meter-high cone with extensive lava fields around the base of the
cone. Most of the 2 km2 of eruptive products (ash, cinders, lava) were
produced in the first few years. Cinder cones such as this one are commonly
sites of only one eruption. Each subsequent eruption in the same area forms
its own cinder cone. This spectacular night view taken in 1944 shows lava
bursting far above the crater rim and also visible on the flanks of the
cone. [Photo credit: R.E. Wilcox, U.S. Geological Survey.] File:vo-eruption1-17
|St. Helens, WA St. Helens, Washington, U.S.; 46.20 N 122.18
W; 2,549 m elevation. This strato-volcano is in southern Washington, 165
km south of Seattle and 80 km north northeast of Portland, Oregon. Although
mid-19th century eruptions had been documented, and volcanologists recognized
the volcano's potential danger, the snow-covered mountain was primarily
known for its quiet beauty. However, in early 1980 the volcano reawakened.
In its gigantic explosive eruption on May 18, 1980, a directed blast leveled
400 km2 of forest, and formed a deep horseshoe crater facing north. A major
debris flow filled a valley for 24 km. Sixty-two people were dead or missing.
Total economic losses were estimated at $1.2 billion. This photo shows
the May 18, 1980 eruption viewed from the east. Photograph credit: Department
of Natural Resources.
|Stromboli, Italy Stromboli, Italy; 38.79 N 15.21 E; 926 m elevation.
This strato-volcano island is located west of Italy. Its nearly continuous
eruptions for over 2,000 years have given it the title, "Lighthouse of
the Mediterranean." Ships passing by are guided by the small explosions
of glowing lava hurled up from the crater every 15 to 30 minutes. Larger
eruptions, some with lava flows, take place at intervals of several years.
The name "strombolian" has been applied to all such small explosions that
hurl incandescent lava above the crater rim. This spectacular view of a
night eruption of Stromboli was taken in March 1951. Photograph credit:
|Surtsey, Iceland Surtsey, Iceland; 63.30 N 20.62 W; 170 m elevation.
This photo, taken on November 30, 1963, shows the sixteen- day-old cone
which became the Island of Surtsey, off the southern coast of Iceland.
The island's north shore touches the Arctic Circle. Born from the sea,
it has provided scientists a laboratory to observe how plants and animals
establish themselves in new territory. The eruption began 130 m below sea
level, where it proceeded quietly until the height of the volcano approached
the sea surface. Then the explosive activity could no longer be quenched
by the sea. A black column of volcanic ash announced the island's birth
on November 14, 1963. Jets of dense black ash shot skyward and the towering
eruption cloud rose to a height of 9 km. By April of 1965, ash had blocked
sea water from the crater area. Lava flows became prominent, forming a
hard cap of solid rocks over the lower slopes of Surtsey. This prevented
the waves from washing away the island. The three and one- half year eruption
was over in June 1967. [Photo credit: Howell Williams.] File:vo-eruption1-20