A volcano is a vent in the earth from which molten rock and gas erupt. The molten rock that erupts from the volcano forms a hill or mountain around the vent. The lava may flow out as a viscous liquid or it may explode from the vent as solid or liquid particles.
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Kinds of Volcanic Materials
Three basic materials that may erupt from a volcano are; 1. lava, 2. rock fragments, and 3. gas.
Lava is the name for magma that has been released onto the Earth’s surface. When lava comes to the Earth’s surface, it is red hot and may have temperatures of more than 2012 degrees Fahrenheit. Fluid lava flows swiftly down a volcano’s slopes. Sticky lava flows more slowly. As the lava cools, it may harden into many different formations. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe. Pahoehoe 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 are spatter cones and lava tubes. Spatter cones are steep hills that can get up to 100 feet high. They build up from the spatter of geyser-like eruptions of thick lava. Lava tubes are tunnels formed from fluid lava. As the lava flows, its exterior covering cools and hardens. But the lava below continues to flow. After the flowing lava drains away, it leaves a tunnel. Rock Fragments Rock fragment are usually called tephra and are formed from sticky magma. This magma is so sticky that its gas cannot easily escape when the magma approaches the surface or central vent. Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra consists of volcanic dust, volcanic ash, and volcanic bombs, (from smallest to largest size particle). Volcanic dust consists of particles less than one one-hundredth inch in diameter. Volcanic dust can be carried for great distances. In 1883, the eruption of Krakatau in Indonesia shot dust 17 miles into the air. The dust was carried around the Earth several times and produced brilliant red sunsets in many parts of the world. Some scientists assume large quantities of volcanic dust can affect the climate by reducing the amount of sunlight that reaches the Earth. Volcanic ash is made up of fragments less than one fifth inch in diameter. Nearly all volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may speeds up to 60 miles per hour and can be remarkably shattering. Volcanic bombs are large fragments. Most of them range from the size of a baseball to the size of a basketball. The largest bombs can measure up to more than four feet across and weigh up to 100 short tons. Small volcanic bombs are generally called cinders.
Gas pours out of volcanoes in large quantities during almost all eruptions. The gas is made up particularly of steam, but may also include carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano’s magma, but some steam may also be produced when rising magma heats water in the ground. Volcanic gas carries a large sum of volcanic dust. This alliance of gas and dust looks like black smoke.
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Types of Volcanoes
The magmas that are the most liquefied erupt quietly and flow from the vent to form sloping shield volcanoes, a name that is conceived because they look like the shields of ancient German warriors. The lava that flows from shield volcanoes is usually only one to ten meters thick, but the lava may extend for great distances away from the vent. The volcanoes of Hawaii and Iceland are typical shield volcanoes.
Magma with high gas contents and high viscosities are usually more explosive than the lava that flows from shield volcanoes. This gas-rich lava in many occurrences is blown very high into the air during an eruption. The magma falls as volcanic bombs, which accumulate around the vent and form steep-sided but relatively small cinder cones. Volcanic bombs range in size from fine-grained ash to house-size blocks. Cinder cones most commonly consist of volcanic fragments anywhere from ash to small- pebble size which is less than three centimeters in diameter.
Most of the tallest volcanoes are composite volcanoes, which are also called stratovolcanoes. These form a cycle of quiet eruptions of fluid lava followed by explosive eruptions of viscous lava. The fluid lava creates an erosion resistant shell over the explosive debris, which forms, strong, steep-sided volcanic cones.
In the past, giant eruptions of extremely fluent basaltic lava from extensive systems of fissures in the Earth have occurred. These series of eruptions formed large plateaus of basaltic lava. In India, the Deccan basalts cover 260,000 square kilometers, and in Oregon and Washington the Columbia Plateau basalts cover approximately 130,000 square kilometers. No eruptions of this extent have ever been observed during historical times. Even more voluminous accumulations of basaltic lava, nevertheless, are currently being formed at the mid-ocean ridges.
How a volcano is formed: The Beginning
A volcano begins as lava inside the Earth. This lava is created from extreme temperatures in the Earth’s interior. Most magma forms 50 to 100 miles beneath the Earth’s surface. Some magma develops at depths of 15 to 30 miles below the Earth’s surface. The magma, which is now filled with gas from combining with the other rock inside the Earth, progressively rises toward the Earth’s surface because it is less dense than the solid rock around it. As the magma rises, it melts gaps in the surrounding rock and forms a large room as close as two miles to the surface. The magma room that is formed is the reservoir from which volcanic materials erupt.
The gas-filled lava in the reservoir is now under great pressure from the weight of the solid rock around it. The pressure causes the gas to blast or melt a channel in a fractured or weakened part of the rock. The magma now moves through the channel to the surface. When the magma gets near the surface, the gas in the magma is released. The gas and magma blast out an opening called the central vent. Most of the lava and other volcanic materials then erupt through this vent. The materials gradually pile up around the vent, and form a volcanic mountain, or a volcano. After the eruption stops, a bowllike crater usually forms at the top of the volcano. The vent lies at the bottom of the crater. Once a volcano has formed, not all the lava from later eruptions reaches the surface through the central vent. As the magma rises, some of it may break through the channel wall and branch out into smaller channels in the rock. The magma in these channels may escape through a vent made in the side of the volcano, or it may rest below the surface. Volcanoes are very wondrous and amazing. They are one of the most destructive and one of the most beautiful things on this Earth. They contain gas, lava, and tunnels that go many miles into the Earth. They can form new islands or gigantic mountains. The materials that volcanoes erupt can help scientists understand about the inner Earth.
Bullard, Fred M. Volcanoes of the Earth. Austin: University of Texas Press, 1962. Decker, Robert and Barbara. Volcanoes. San Francisco: W.H. Freeman and company, 1981. Decker, Robert and Barbara. Volcanoes. New York: W.H. Freeman and company, 1981. Macdonald, Gordon A. Volcanoes. New Jersey: Prentice-Hall, inc., 1972. “Volcano”, The World Book Encyclopedia, 1993, Volume 20, pages 438-440.