Welcome to You Ask Andy

Jon Reeve, age 8, of Lansing, Mich., for his question:

HOW DOES A VOLCANO ERUPT?   

The word volcano comes from Vulcan, the name of the ancient Roman god of fire. The Romans believed that the god lived beneath a volcanic island off the Italian coast, and they called the island Volcano. Powerful forces inside the earth cause volcanoes. Scientists don't completely understand these forces although they know how hot gases and rock fragments erupt.

Molten rock inside the earth is called magma. At certain depths within the earth, heat is so great that it partly melts the rocks. When the rock melts, it produces much gas which becomes mixed with the magma.

Most magma forms 50 to 100 miles beneath the surface of the earth. Some develops at depths of 15 to 30 miles.

Gas filled magma often 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 rocks. As more magma rises, it forms a large chamber as close as two miles to the surface. This magma chamber is the reservoir from which volcanic materials erupt.

Gas filled magma in a reservoir is under much pressure from the weight of the solid rock around it. The pressure sometimes causes the magma to blast or melt a conduit or channel in a fracture or weakened part of the rock. The magma then moves up through the conduit to the surface.

As the magma nears the surface of the earth, the gas is released. Then gas and magma blasts out an opening that is called the central vent. Most of the magma and other volcanic materials are then sent flying out. 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 this crater.

Three kinds of materials may erupt from a volcano: lava, rock fragments and gas. 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 2012 degrees Fahrenheit. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehe. Stickier lava cools into rough, jagged sheets of rock called aa.   

That last word, aa, is Hawaiian, and it is pronounced with two syllables: AH ah. Both pahoehe and as cover large areas of Hawaii.

Among volcanic rock fragments are ball shaped pieces called bombs. The largest can be more than four feet across and weigh up to 100 tons. Some are the size of baseballs and are generally called cinders.

 

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