introductionA volcano is a vent, or opening, in the surface of the Earth through which magma andassociated gases and ash erupt. The word also refers to the form or structure, usuallyconical, produced by accumulations of erupted material. Volcanoes occur mainly nearplate tectonic boundaries and are especially common around the Pacific basin, calledthe Pacific Ring of FireHumanity has long been awed by this powerful force of nature. The Romans attributedvolcanic events to Vulcan, the god of fire and metalworking. In AD 79 the eruption ofMount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. Polynesiansbelieve volcanoes to be ruled by the fire goddess Pele. One of the most spectacularvolcanic 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 1980eruption of Mount St. Helens in the Cascade Range in Washington State.
January 2009 image of the rhyolitic lava dome of Chaitén Volcano, southern Chileduring its 2008-2009 eruption
How a volcano is formedPowerful forces within the earth cause volcanoes. Scientists do not fully understand theseforces. But they have developed theories on how the forces create volcanoes. This sectiondescribes 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. Magmaresults from the extreme heat of the earths interior. At certain depths, the heat is so great itpartly melts the rock inside the earth. When the rock melts, it produces much gas, whichbecomes 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 earths surface because it is lighter than thesolid rock around it. As the magma rises, it melts gaps in the surrounding rock. As more magmarises, it forms a large chamber as close as 3 kilometres to the surface. This magma chamber isthe reservoir from which volcanic materials erupt.The eruption of a volcano. The gas-filled magma in the reservoir is under great pressure fromthe weight of the solid rock around it. This pressure causes the magma to blast or melt aconduit (channel) in a fractured or weakened part of the rock. The magma moves up throughthe conduit to the surface. When the magma nears the surface, the gas in the magma isreleased. The gas and magma blast out an opening called the central vent. Most magma andother volcanic materials then erupt through this vent. The materials gradually pile up aroundthe vent, forming a volcanic mountain, or volcano. After the eruption stops, a bowllike cratergenerally forms at the top of the volcano. The vent lies at the bottom of the crater.
SOME FAMOUS VOLCANOES Name Location Height above sea level interesting factsEl 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 occurredHelence 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 mudRuiz slides & flood destroyed city of Armero and killed about 25,000 people.
The Stromboli volcano off the coast of Sicily haserupted continuously for thousands of years, giving rise to the term strombolian eruption
• 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 volcanos magma is.• Lava is the name for magma that has escaped onto the earths 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 volcanos 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.