[Illustration: A sketch of the havoc wrought in St. Pierre Harbor on
Martinique during the eruption of Mont Pelée in 1902.]
Volcanic or lava domes are formed by relatively small, bulbous masses of
lava too viscous to flow any great distance; consequently, on extrusion,
the lava piles over and around its vent. A dome grows largely by
expansion from within. As it grows its outer surface cools and hardens,
then shatters, spilling loose fragments down its sides. Some domes form
craggy knobs or spines over the volcanic vent, whereas others form
short, steep-sided lava flows known as “coulees.” Volcanic domes
commonly occur within the craters or on the flanks of large composite
volcanoes. The nearly circular Novarupta Dome that formed during the
1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and
200 feet high. The internal structure of this dome—defined by layering
of lava fanning upward and outward from the center—indicates that it
grew largely by expansion from within. Mont Pelée in Martinique, Lesser
Antilles, and Lassen Peak and Mono domes in California are examples of
lava domes. An extremely destructive eruption accompanied the growth of
a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4
miles downslope to the south, was demolished and nearly 30,000
inhabitants were killed by an incandescent, high-velocity ash flow and
associated hot gases and volcanic dust. Only two men survived; one
because he was in a poorly ventilated, dungeon-like jail cell and the
other who somehow made his way safely through the burning city.
[Illustration: The Novarupta Dome formed during the 1912 eruption of
Katmai Volcano, Alaska.]
[Illustration: Schematic representation of the internal structure of a
typical volcanic dome.]