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volcan&magma - Volcanoes Magma and Volcanic...

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This page last updated on 28-Jan-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Volcanoes, Magma, and Volcanic Eruptions Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth's surface, we must first discuss the characteristics of magma and how magmas form in the Earth. Characteristics of Magma Types of Magma Types of magma are determined by chemical composition of the magma. Three general types are recognized: 1. Basaltic magma -- SiO 2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Andesitic magma -- SiO 2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Rhyolitic magma -- SiO 2 65-75%, low in Fe, Mg, Ca, high in K, Na Gases in Magmas At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are: z Mostly H 2 O (water vapor) & some CO 2 (carbon dioxide) z Minor amounts of Sulfur, Chlorine, and Fluorine gases The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher gas contents than basaltic magmas. Temperature of Magmas Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows: Volcanoes, Magma, and Volcanic Eruptions 1/28/2004 Page 1 of 9
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z Basaltic magma - 1000 to 1200 o C z Andesitic magma - 800 to 1000 o C z Rhyolitic magma - 650 to 800 o C. Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature. z Higher SiO 2 (silica) content magmas have higher viscosity than lower SiO 2 content magmas (viscosity increases with increasing SiO 2 concentration in the magma). z Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.
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