During an episode of activity, a volcano commonly displays a distinctive
pattern of behavior. Some mild eruptions merely discharge steam and
other gases, whereas other eruptions quietly extrude quantities of lava.
The most spectacular eruptions consist of violent explosions that blast
great clouds of gas-laden debris into the atmosphere.
The type of volcanic eruption is often labeled with the name of a
well-known volcano where characteristic behavior is similar—hence the
use of such terms as “Strombolian,” “Vulcanian,” “Vesuvian,” “Peléan,”
“Hawaiian,” and others. Some volcanoes may exhibit only one
characteristic type of eruption during an interval of activity—others
may display an entire sequence of types.
In a Strombolian-type eruption observed during the 1965 activity of
Irazú Volcano in Costa Rica, huge clots of molten lava burst from the
summit crater to form luminous arcs through the sky. Collecting on the
flanks of the cone, lava clots combined to stream down the slopes in
fiery rivulets.
[Illustration: Irazú Volcano, Costa Rica, 1965.]
In contrast, the eruptive activity of Parícutin Volcano in 1947
demonstrated a “Vulcanian”-type eruption, in which a dense cloud of
ash-laden gas explodes from the crater and rises high above the peak.
Steaming ash forms a whitish cloud near the upper level of the cone.
In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius
in Italy in A.D. 79, great quantities of ash-laden gas are violently
discharged to form a cauliflower-shaped cloud high above the volcano.
[Illustration: Parícutin Volcano, Mexico, 1947.]
[Illustration: Mount Vesuvius Volcano, Italy, 1944.]
In a “Peléan” or “Nuée Ardente” (glowing cloud) eruption, such as
occurred on the Mayon Volcano in the Philippines in 1968, a large
quantity of gas, dust, ash, and incandescent lava fragments are blown
out of a central crater, fall back, and form tongue-like, glowing
avalanches that move downslope at velocities as great as 100 miles per
hour. Such eruptive activity can cause great destruction and loss of
life if it occurs in populated areas, as demonstrated by the devastation
of St. Pierre during the 1902 eruption of Mont Pelée on Martinique,
Lesser Antilles.
“Hawaiian” eruptions may occur along fissures or fractures that serve as
linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii
in 1950; or they may occur at a central vent such as during the 1959
eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In
fissure-type eruptions, molten, incandescent lava spurts from a fissure
on the volcano’s rift zone and feeds lava streams that flow downslope.
In central-vent eruptions, a fountain of fiery lava spurts to a height
of several hundred feet or more. Such lava may collect in old pit
craters to form lava lakes, or form cones, or feed radiating flows.
[Illustration: Mauna Loa Volcano, Hawaii, 1950.]
“Phreatic” (or steam-blast) eruptions are driven by explosive expanding
steam resulting from cold ground or surface water coming into contact
with hot rock or magma. The distinguishing feature of phreatic
explosions is that they only blast out fragments of preexisting solid
rock from the volcanic conduit; no new magma is erupted. Phreatic
activity is generally weak, but can be quite violent in some cases, such
as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76
activity at La Soufrière, Guadeloupe (Lesser Antilles).
The most powerful eruptions are called “plinian” and involve the
explosive ejection of relatively viscous lava. Large plinian
eruptions—such as during 18 May 1980 at Mount St. Helens or, more
recently, during 15 June 1991 at Pinatubo in the Philippines—can send
ash and volcanic gas tens of miles into the air. The resulting ash
fallout can affect large areas hundreds of miles downwind. Fast-moving
deadly pyroclastic flows (“nuées ardentes”) are also commonly associated
with plinian eruptions.
[Illustration: Kilauea Volcano, Hawaii, 1959.]
[Illustration: Taal Volcano, Philippines, 1965.]