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Volcanoes
The word "volcano" comes from the little island of Vulcano
in the
Mediterranean Sea off Sicily. Centuries ago, the people living in this area
believed that Vulcano was the chimney of the forge of Vulcan -- the blacksmith
of the Roman gods. They thought that the hot lava fragments and clouds of dust
erupting form Vulcano came from Vulcan's forge as he beat out thunderbolts for
Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia
the people attributed eruptive activity to the beautiful but wrathful Pele,
Goddess of Volcanoes, whenever she was angry or spiteful. Today we know that
volcanic eruptions are not super-natural but can be studied and interpreted by
scientists.
The Nature of Volcanoes
Volcanoes are mountains,
but they are very different from other
mountains; they are not formed by folding and crumpling or by uplift and
erosion. Instead, volcanoes are built by the accumulation of their own eruptive
products -- lava, bombs (crusted over lava blobs), ashflows, and tephra
(airborne ash and dust). A volcano is most commonly a conical hill or
mountain built around a vent that connects with reservoirs of molten rock below
the surface of the Earth. The term volcano also refers to the opening or
vent through which the molten rock and associated gases are expelled.
Driven by buoyancy and gas pressure the molten rock,
which is lighter than the
surrounding solid rock, forces its way upward and my ultimately break through
zones of weaknesses in the Earth's crust. If so, an eruption begins, and the
molten rock may pour from the vent as nonexplosive lava flows, or it may shoot
violently into the air as dense clouds of lava fragments. Larger fragments fall
back around the vent, and accumulations of fallback fragments may move downslope
as ash flows under the force of gravity. Some of the finer ejected materials
may be carried by the wind only to fall to the ground many miles away. The
finest ash particles may be injected miles into the atmosphere and carried many
times around the world by stratospheric winds before settling out.
-- Excerpts from:
Tilling, 1985, Volcanoes: USGS General Interest Publication
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