Chapter 6 - Chapter 6 Volcanoes and Other Mountains 1...

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Chapter 6 Volcanoes and Other Mountains 1 Chapter 6: Volcanoes and Other Mountains Civilization exists by geological consent, subject to change, without notice. Will Durant One ought never to turn one’s back on a threatened danger and try to run away from it. If you do that, you will double the danger. But if you meet it promptly and without flinching, you will reduce the danger by half. Winston Churchill, U.K. Prime Minister during World War II
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Chapter 6 Volcanoes and Other Mountains 2 Chapter 6.2 Magma Viscosity pg. 148 KEY POINTS: Molten rock below Earth’s surface is magma; on Earth’s surface, it is called lava. The violence of volcanic eruptions is largely controlled by the viscosity of the magma. Composition (silica content) and temperature control magma viscosity. Volcanic gases escape more easily from low – viscosity magmas. The resistance to flow is called viscosity. Low – viscosity fluids flow easily, while high – viscosity fluids resist flow (flow slower). We can reduce viscosity by heating a material and increase viscosity by cooling it. Viscosity and Heat: Viscosity is an important property influencing volcanic activity, particularly as it relates to the flow of magma, molten rock below. Earth’s surface, and lava, molten rock of earth’s surface. Just as with honey, the viscosity of magma increases as its temperature decreases. Magma works its way upward through pipes and fractures in Earth’s crust because it is warmer and less dense than the surrounding material. Low – viscosity magmas can flow more rapidly and may form lava flows that cover thousands of square feet. On Earth’s surface, lava solidifies (cools and hardens) to form volcanic igneous rocks. If magma solidifies within the crust, plutonic igneous rock forms. There are three common types of lava, each with a different viscosity: basalt, andesite, and rhyolite. The difference in viscosity between low – viscosity basalt lava and high viscosity rhyolite lava in similar to the difference between water and thick peanut butter. Viscosity and Chemical Composition: Magma contains considerable volumes of dissolved gases, most commonly water vapor, carbon dioxide, and sulfur dioxide. These gases are dissolved in the magma under pressure deep in the crust. As the magma rises, the pressure decreases, and the gas comes out of the solution as bubbles that help push the magma to the surface. While it is tempting to consider that lava is just magma at Earth’s surface, there is a key difference between these hot, fluid materials. Magma releases gases approaching the surface as pressure decreases. The loss of gases changes the composition of the material, resulting in some differences in the rocks formed from solidified magma compared to hardened lava. The chemical content (composition) of magma is more important than temperature in determining viscosity.
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This note was uploaded on 06/14/2011 for the course PHSC 1001 taught by Professor Gabrielclay during the Fall '09 term at Walden University.

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Chapter 6 - Chapter 6 Volcanoes and Other Mountains 1...

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