Volcanoes

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Term Paper Title	Volcanoes
# of Words	1823
# of Pages (250 words per page double spaced)	7.29

Volcanoes

By Tricia Severson
2nd hour Science
4/30/98

A volcano is a vent, or opening, in the surface of the Earth through which magma and
associated gases and ash erupt. The word also refers to the form or structure, usually
conical, produced by accumulations of erupted material. Volcanoes occur mainly near
plate tectonic boundaries and are especially common around the Pacific basin, called the
Pacific Ring of Fire (see Plate Tectonics).

   Humanity has long been awed by this powerful force of nature. The Romans attributed
volcanic events to Vulcan, the god of fire and metalworking. In AD 79 the eruption of
Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. Polynesians
believe volcanoes to be ruled by the fire goddess Pele. One of the most spectacular
volcanic eruptions in recorded history occurred in 1883 with the explosion of Krakatoa,
an island in the Sunda Strait near Java (see Krakatoa). A more recent example is the
dramatic 1980 eruption of Mount St. Helens in the Cascade Range in Washington State.

Volcano Formation and Eruptions

Volcanic eruptions may be violent, even catastrophic, or relatively mild. The most
explosive eruptions are essentially blasts of steam that create spectacular displays.
Quieter fissure eruptions occur when molten rock pushes through long cracks in the
Earth's crust and floods the surrounding landscape. Such repeated outpourings of lava can
fill surrounding valleys and bury low hills, creating thick lava sequences that eventually
become plateaus (see Plateau).

   The origin of molten rock, referred to by geologists as magma, is not clearly
understood. About 80 percent of all magma is composed of basalt rock. Geophysical
research suggests that volcanic magma forms near the base of the Earth's crust and moves
upward to a shallow magma chamber before erupting at the surface. Magmas rise
because they are less dense than the rocks at lower depths, and their heat probably
weakens surrounding rocks. The upward movement of magma may also be due to
expanding gases within the molten rock or to chemical reactions that dissolve rocks
above the magma. Volcanic material moves toward the surface through channelways, or
volcanic conduits, and is extruded through vents at the Earth's surface. (See also Lava
and Magma.)

   Eruptions take different forms depending on the composition of the magma when it
reaches the surface. Sudden eruptions are often associated with low-viscosity (more
...

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