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Chapter 4
Volcanoes
Introduction
Over a time span longer than human record, volcanoes have played a key role in forming and
modifying the planet upon which we live. More than 80 percent of the earth’s surface—above
and below sea level—is of volcanic origin. Gaseous emissions from volcanic vents over
hundreds of millions of years formed the earth’s earliest oceans and atmosphere, which
supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic
eruptions have produced mountains, plateaus, and plains, which subsequent erosion and
weathering have sculpted into majestic landscapes or reduced to fertile soils.
Ironically, these volcanic soils and inviting terrains have attracted, and continue to attract,
people to live on the flanks of volcanoes. Thus, as population density increases in regions of
active or potentially active volcanoes, an awareness of the hazards must increase so people will
learn not to “crowd” the volcanoes. People living in the shadow of volcanoes must live in
harmony with them, expecting and planning for periodic violent unleashings of their pent-up
energy. 1
A volcano is a vent or chimney to the earth’s surface from a reservoir of molten matter, known
as magma, in the depths of the crust of the earth. The material ejected through the vent
frequently accumulates around the opening, building up a cone, called the volcanic edifice. The
tallest mountains on earth are volcanic edifices. The term volcano includes both the vent and
the accumulation (cone) around it.
Volcanic eruptions vary between two extremes. In one, the lava rises more or less quietly to the
surface and overflows the lip of the crater. The gases bubble through the lava and escape
undramatically, or, in some instances, rush out with sufficient force to form lava fountains
hundreds of meters in height. Nevertheless, the lava is not disrupted but flows away as a river
of lava, with little resulting damage except to objects in the path of its flow. On the other
extreme, tremendous explosions occur in the chimney, and as the lava rises into zones of less
pressure it “froths” and is ejected in the form of ash and pumice. Thus, in these volcanoes the
molten rock never reaches the surface as a liquid (lava) but is disrupted and ejected as ash.
The explosions are sometimes so severe that they disrupt the cone, frequently blowing away
large sections of it and spreading the debris over the countryside. The essential difference in
the two types is in the gas content and the manner in which the gas is released when the molten
rock reaches the surface. Another factor is the chemical composition of the magma.
The great majority of the volcanoes of the world are intermediate between the two extremes
described, yielding both lavas and fragmental products. 2
Historical Volcanic Eruptions
La Soufriere, St. Vincent Island, 1902, 1979
The volcano La Soufriere occupies the northern end of the island of St. Vincent, 140 kilometers
(87 miles) to the south of Martinique in the Caribbean. The summit of La Soufriere, which rises
1,277 meters (4200 feet) above sea level, consists of two craters, the 1812 crater and the socalled
“old” crater, immediately to the southwest, which was the site of the 1902 eruption.