textures_igneous_rocks - Textures of Igneous Rocks EENS 212...

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This document last updated on 12-Feb-2004 EENS 212 Petrology Prof. Stephen A. Nelson Tulane University Textures of Igneous Rocks Introduction to Igneous Rocks An igneous rock is any crystalline or glassy rock that forms from cooling of a magma. A magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase. Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock , or beneath the surface of the Earth, - in which case it produces a plutonic or intrusive igneous rock . Characteristics of Magma Types of Magma Types of magma are determined by chemical composition of the magma. Three general types are recognized, but we will look at other types later in the course: 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. 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) with 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 dissolved gas contents than basaltic magmas. Textures of Igneous Rocks 2/12/2004 Page 1 of 6
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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: 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
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textures_igneous_rocks - Textures of Igneous Rocks EENS 212...

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