1. introduction
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
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 . A more recent example is the dramatic 1980
eruption of Mount St. Helens in the Cascade Range in Washington State.
5. January 2009 image of the rhyolitic lava dome of Chaitén Volcano, southern Chile
during its 2008-2009 eruption
6. How a volcano is formed
Powerful forces within the earth cause volcanoes. Scientists do not fully understand these
forces. But they have developed theories on how the forces create volcanoes. This section
describes how most scientists explain the beginning and eruption of a volcano.
The beginning of a volcano. A volcano begins as magma, melted rock inside the earth. Magma
results from the extreme heat of the earth's interior. At certain depths, the heat is so great it
partly melts the rock inside the earth. When the rock melts, it produces much gas, which
becomes mixed with the magma. Most magma forms 80 to 160 kilometres beneath the surface.
Some develops at depths of 25 to 50 kilometres.
The gas-filled magma gradually rises toward the earth's surface because it is lighter than the
solid rock around it. As the magma rises, it melts gaps in the surrounding rock. As more magma
rises, it forms a large chamber as close as 3 kilometres to the surface. This magma chamber is
the reservoir from which volcanic materials erupt.
The eruption of a volcano. The gas-filled magma in the reservoir is under great pressure from
the weight of the solid rock around it. This pressure causes the magma to blast or melt a
conduit (channel) in a fractured or weakened part of the rock. The magma moves up through
the conduit to the surface. When the magma nears the surface, the gas in the magma is
released. The gas and magma blast out an opening called the central vent. Most magma and
other volcanic materials then erupt through this vent. The materials gradually pile up around
the vent, forming a volcanic mountain, or volcano. After the eruption stops, a bowllike crater
generally forms at the top of the volcano. The vent lies at the bottom of the crater.
8. SOME FAMOUS VOLCANOES
Name Location Height above sea level interesting facts
El Chichon Mexico 1,060 Eruption in 1982 killed 1897
people & released a cloud of
Sulphur dioxide & gas high into
atmosphere.
Mount St. U.S.A. 2,549 In 1980 violent eruption occurred
Helence Lots of molted rocks & ash killed
about 546 people.
Mauna Loa Hawaii U.S.A. 4,169 Worlds largest volcano rises abo-
ut 9000m. From ocean. And 100
km. wide at the base.
Nevado del Columbia 5,400 Erupted in 1985 triggered mud
Ruiz slides & flood destroyed city of
Armero and killed about 25,000
people.
9. The Stromboli volcano off the coast of Sicily has
erupted continuously for thousands of years, giving rise
to the term strombolian eruption
10. • Kinds of volcanic materials
• Three basic kinds of materials may erupt from a volcano. They are (1)
lava, (2) rock fragments, and (3) gas. The material that erupts depends
chiefly on how sticky or fluid a volcano's magma is.
• Lava is the name for magma that has escaped onto the earth's surface.
When lava comes to the surface, it is red hot and may have a temperature
of more than 1100 °C. Highly fluid lava flows rapidly down a volcano's
slopes. Sticky lava flows more slowly. As the lava cools, it hardens into
many different formations. Highly fluid lava hardens into smooth, folded
sheets of rock called pahoehoe. Stickier lava cools into rough, jagged
sheets of rock called aa. Pahoehoe and aa cover large areas of
Hawaii, where the terms originated. The stickiest lava forms flows of
boulders and rubble called block flows. It may also form mounds of lava
called domes.
• Other lava formations include spatter cones and lava tubes. Spatter cones
are steep hills up 30 metres high. They build up from the spatter of
fountainlike eruptions of thick lava. Lava tubes are tunnels formed from
fluid lava. As the lava flows, its outer surface cools and hardens. But the
lava underneath continues to flow. After the flowing lava drains away, it
leaves a tunnel.