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Alaska's Volcanoes
Volcanoes are mountains which are connected to the molten rock in the center of the earth. There are three types of volcanoes: cinder cone, shield, and composite cone. Each kind of volcano forms in a slightly different way but each one can erupt with amazing force. Magma makes the volcano tick… The center of the earth is composed of hot, liquid rock called magma. Magma is stored in large pockets or chambers in the earth's crust. Magma can be pushed up through cracks in the earth by pressure or by melting the rocks above it. When magma reaches the surface it is called lava. Lava cools as it is exposed to the atmosphere. Repeated eruptions cause the lava to build up creating a volcano. At the top of the volcano, a steep-sided hole called a crater may form. The opening that allows the magma to flow out on the earth's surface is called a vent. Just as people are different, so are volcanoes. The lava from some volcanoes is dark, thin, and runny. It flows gently from the volcano's crater. Lava that is light-colored doesn't contain much water. It tends to harden in the vent and block the flow of magma from beneath. Steam and new magma become trapped and build up tremendous pressure. When the pressure becomes too great an explosion occurs. Dust…Ash…Bombs… A volcanic explosion can send small fragments and dust high into the air. Very fine fragments of debris, up to .25 millimeters in diameter, are known as volcanic dust. Volcanic ash is somewhat larger, up to 5 millimeters in size. Debris larger that 5 millimeters are called volcanic bombs. They range in size with smaller bombs classified as cinders.Volcanic Personalities There are three types of volcanoes. Cinder Cones are small mountains with steep sides. When they erupt, cinders and rock particles are blown into the air. Paricutin in Mexico is an example of a cinder cone. Mauna Loa in Hawaii is an example of a shield volcano. Mauna Loa also is the largest volcano on earth. It makes up about 1/2 of the area of the island of Hawaii. A shield volcano covers a large area, is dome-shaped, and has gently sloping sides. The lava is dark, thin, and runny so it flows quietly from the vent. Composite Cones are large mountains with steep sides. They are cone-shaped. Eruptions from composite cones are violent, sending up volcanic bombs, cinders, and ash. A quiet volcanic flow usually follows the explosion and helps to build up the mountain. Mount St. Helen's in Washington and Mount Vesuvius in Italy are examples of composite cone volcanoes. Alaskan Volcanoes Alaska lies along an area called the Pacific Ring of Fire. Volcanoes circle the Pacific Ocean from the tip of South America, along the west coast of the U.S., through the Kamchatka Peninsula, down through Japan, Indonesia, and the islands in the South Pacific. Alaska contains more than 100 volcanoes and volcanic field. These have been active within the last 1.5 million years. More than 40 of these have been active in historic times. These volcanic "hotspots" make up 80 percent of all of the active volcanoes in the U.S. and 8 percent of all active above-water volcanoes on earth. Most all of Alaska's volcanoes are located along the 1,500 mile Aleutian Arc that extends westward to Kamchatka and forms the northern part of the Pacific Ring of Fire. Other volcanoes that have been active during the last few thousand years are located in southeastern Alaska and in the Wrangell Mountains. Smaller volcanoes, some which have been active in the last 10 thousand years, are located in the interior and in western Alaska as far north as the Seward Peninsula. Eruptions in Alaska have greatly affected the weather throughout the world. According to the U.S. Geological Survey, the largest eruption on Earth in the 20th Century occurred in Alaska at Novarupta Volcano, near Mount Katmai, in June 1912. The eruption created the Katmai Caldera and the Valley of Ten Thousand Smokes. Volcanic ash, more than from all other historical eruptions in Alaska combined, devastated areas hundreds of miles away. Major eruptions along the Aleutian Arc occur almost every year. Eruptions in unpopulated areas often go unnoticed by everyone except those who study volcanoes. These scientists are called volcanologists. They monitor volcanoes to try to warn people about eruptions and to learn more about how the earth works. |
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