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GY 111: Physical Geology UNIVERSITY OF SOUTH ALABAMA GY 111: Physical Geology Lecture 9: Extrusive Igneous Rocks Instructor: Dr. Douglas W. Haywick Last Time 1) The chemical composition of the crust 2) Crystallization of molten rock 3) Bowen's Reaction Series Web notes 8 Chemical Composition of the Crust Element Wt% % of atoms Oxygen 46.6 60.5 Silicon 27.7 20.5 Aluminum 8.1 6.2 Iron 5.0 1.9 Calcium 3.6 1.9 Sodium 2.8 2.5 Potassium 2.6 1.8 Magnesium 2.1 1.4 All other elements 1.5 3.3 Crystallization of Magma http://myweb.cwpost.liu.edu/vdivener/notes/igneous.htm Bowen’s Reaction Series Source http://www.ltcconline.net/julian Igneous Rock Composition Source: http://hyperphysics.phy-astr.gsu.edu Composition Formation Dominant Silica content Temperature Minerals Ultramafic Very high Olivine, pyroxene Very low (<45%) Mafic High Olivine, pyroxene, low Ca-plagioclase Intermediate Medium Na-Plagioclase, moderate amphibole, biotite Felsic Medium-low Orthoclase, quartz, high (>65%) muscovite, biotite Igneous Rock Texture Extrusive Rocks (Rapid Cooling; non visible* crystals) Intrusive Rocks (slow cooling; 100 % visible crystals) *with a hand lens Igneous Rock Texture Igneous Rock Texture Today’s Agenda 1) Pyro-what? (air fall volcanic rocks) 2) Felsic and Intermediate Extrusive Rocks 3) Mafic Extrusive Rocks Web notes 9 Pyroclastic Igneous Rocks Pyroclastic Igneous Rocks Pyroclastic: Pyro means “fire”. Clastic means particles; both are of Greek origin. Pyroclastic Igneous Rocks Pyroclastic: Pyro means “fire”. Clastic means particles; both are of Greek origin. Pyroclastic rocks are usually erupted from composite volcanoes (e.g., they are produced via explosive eruptions from viscous, “cool” lavas) Pyroclastic Igneous Rocks Pyroclastic: Pyro means “fire”. Clastic means particles; both are of Greek origin. Pyroclastic rocks are usually erupted from composite volcanoes (e.g., they are produced via explosive eruptions from viscous, “cool” lavas) The eruptions that produce pyroclastic rocks include phreatic, plinian and ultraplinian. Pyroclastic Igneous Rocks Pyroclastic: Pyro means “fire”. Clastic means particles; both are of Greek origin. Pyroclastic rocks are usually erupted from composite volcanoes (e.g., they are produced via explosive eruptions from viscous, “cool” lavas) The eruptions that produce pyroclastic rocks include phreatic, plinian and ultraplinian. These rocks cool extremely rapidly. Textures are glassy to aphanitic. Pyroclastic Igneous Rocks Pyroclastic Igneous Rocks Volcanic ash Pyroclastic Igneous Rocks Volcanic ash Pumice Pyroclastic Igneous Rocks Volcanic ash Obsidian Pyroclastic Igneous Rocks Nuee ardents and pyroclastic flows may deposit volcanic breccias or ignimbrites http://ougseurope.org/trips/santorini/santorini/Picture04.jpg Ignimbrite Pyroclastic Igneous Rocks During pyroclastic eruptions, “volcanic gas” is produced in copious quantities (more so than ash and rock). •water vapor (H20) •carbon dioxide (CO2) •hydrogen chloride (HCl) •sulfur dioxide (SO2) •nitrogen oxide (N2O) •Hydrogen cyanide (HCN) •argon (Ar) •helium (He) Other “pyroclastic” rocks Volcanic Bombs Tuff Rhyolite Chalk board Felsic/Intermediate Extrusive Igneous Rocks Felsic/Intermediate Extrusive Igneous Rocks Rhyolite Mt. St. Helens Lava dome Felsic and intermediate rocks are classified by the amount of SiO2 in their bulk chemistry (they have relatively high SiO2 content). We can distinguish them on the basis of mineral composition and color. Andesite Mafic Extrusive Igneous Rocks Mafic Extrusive Igneous Rocks Basalt http://z.about.com/d/geology/1/0/F/W/scoria.jpg Mafic extrusive igneous rocks have relatively low SiO2 contents in their bulk chemistry. Ultramafic extrusive igneous rocks are relatively rare today. Scoria Mafic Extrusive Igneous Rocks Mafic extrusive igneous rocks erupted under water produce a unique structure called “pillows”. Pillow Basalt Pillow basalt forms at divergent plate boundaries like the Mid- Atlantic Ridge Today’s Homework 1. Study; Lecture test 1 next Tuesday Next Time 1. Quiz 5; fill in the blanks 2. Intrusive igneous rocks GY 111: Physical Geology Lecture 9: Extrusive Igneous Rocks Instructor: Dr. Doug Haywick [email protected] This is a free open access lecture, but not for commercial purposes. For personal use only. .
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