Types of Eruptions
Icelandic -
Volcanic eruptions may be classified in order of increasing violence. Icelandic eruptions
involve
the quiet outpouring of lava from long fissures. Most than 2 1/2 cu mi of lava flowed from
Iceland's Lake
Fissure in 1783, the largest emission of lava in historic times. Eruptions of the
Icelandic type during the
Tertiary Period built up vast basalt plateaus in many parts of the World. One of the
largest is the Columbia
Plateau, more than 20,000 sq mi in the northwestern United States.
Hawaiian - Lava
issues from a single vent in eruptions of the Hawaiian type. Each flow adds a layer of
rock to a broad, gently sloping mountain called a shielt volcano. In calderas at their
summits shield
volcanoes may contain lava lakes in which lava continuosly circulates from below. The
volcano Niragongo
in Africa contains a lava lake, as did Kilauea in Hawaii. The escape of gas at the vent
during Hawaiian
erputions often produces lava fountains several hundred feet high.
Strombolian -
Continuos eruptions that emit showeres of lava at rhythmic intervals are called
strombolian
after the volcano Stromboli, north of Sicily. Much of the erupted material is fragmental,
so that the cone is
steep sided. A cross section would show alternate layers of pyroclastic material and lava
flows, forming a
mountain known as a stratovolcano.
Vulcanian - In
Vulcanian eruptions, which also produce stratovolcanoes, the magma is more acid and less
fluid than in the preceding types. The opening phase is explosive, blowing away the thick
crust over the vent
and often destroying part of the mountain. Lava flows may emerge later. Both Vulcano, from
which such
eruptions are named, and Italy's Mount Vesuvius illustrate this type of activity. In 79
A.D. an eruption of
Mount Vesuvius buried the city of Pompeli under a layer of pumice, and many of its 20,000
inhabitants
were killed. A nearby city, Herculaneum, was buried by a lahar, or mudflow, caused by
rains that
accompanied the eruption. A single Vulcanian eruption where none has occurred before
produces a cinder
cone, or pyroclastic cone, surrounded by lava flows. Paricutin is an example.
Pelean - The
most explosive eruption is the Pelean type, named for the 1902 eruption of Mount Pelee on
the island of Martinique in the West Indies. On the morning of May 8 a blast of steam and
red-hot ash shot
laterally from the volcano and destroyed the city of Saint Pierre and all but two of more
than 26,000
inhabitants. This was the greatest loss of life ever known to result directly from an
eruption, although
80,000 lives were lost in the famine that was caused by the eruption in 1815 of Tambora in
the East Indies.
The blast that destroyed Saint Pierre was a nuee ardente, a stream of ash-laden gas that
forced a passage
through the viscous magma that clogged the crater. Some months later a volcanic spine, or
tower of stiff
lava, arose from the crater. It reached a height of 1,020 ft above the crater floor before
collapsing.
Phreatic -
Violent phreatic eruptions, similar to the Pelean type, are caused by the vaporization of
underground water by magma. In the phreatic eruption of Bandai-san, in Japan, 461 persons
lost their lives
in 1888.