igneous - Igneous Rocks Page 1 of 14 EENS 1110 Physical...

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This page last updated on 23-Jan-2012 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Magmas and Igneous Rocks Magma and Igneous Rocks Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types z Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth z Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth. Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and properties of a liquid. Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock . If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock . Types of Magma Chemical composition of magma is controlled by the abundance of elements in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO 2 is the most abundant oxide. 1. Mafic or Basaltic -- SiO 2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Intermediate or Andesitic -- SiO 2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Felsic or Rhyolitic -- SiO 2 65-75%, low in Fe, Mg, Ca, high in K, Na. Gases - At depth in the Earth nearly all magmas contain gas. Gas gives magmas their explosive character, because the gas expands as pressure is reduced. z Mostly H 2 O with some CO 2 z Minor amounts of Sulfur, Cl , and F z Felsic magmas usually have higher gas contents than mafic magmas. Temperature of Magmas z Mafic/Basaltic - 1000-1200 o C z Intermediate/Andesitic - 800-1000 o C z Felsic/Rhyolitic - 650-800 o C. Page 1 of 14 Igneous Rocks 1/23/2012 http://www.tulane.edu/~sanelson/eens1110/igneous.htm
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Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content. z Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas z Lower Temperature magmas have higher viscosity than higher temperature magmas. Summary Table Magma Type Solidified Volcanic Rock Solidified Plutonic Rock Chemical Composition Temperature Viscosity Gas Content Mafic or Basaltic Basalt Gabbro 45-55 SiO 2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 o C Low Low Intermediate or Andesitic Andesite Diorite 55-65 SiO 2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 o C Intermediate Intermediate Felsic or Rhyolitic Rhyolite Granite 65-75 SiO 2 %, low in Fe, Mg, Ca, high in K, Na 650 - 800 o C High High Origin of Magma As we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids.
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This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.

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igneous - Igneous Rocks Page 1 of 14 EENS 1110 Physical...

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