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Igneous Rocks GLG 101 - Physical Geology Bob Leighty These notes and web links are your primary “lecture” content in this class. Additionally, various articles are assigned each week to supplement this “lecture” information. I believe you’ll have enough information to reference without having to purchase a costly textbook. These lecture notes are very similar to the ones I use in my traditional classes. You’ll find they are loaded with imagery and streamlined text that highlight the most essential terms and concepts. The notes provide a framework for learning and, by themselves, are not meant to be a comprehensive source of information. To take advantage of the global knowledge base known as the Internet, I have included numerous hyperlinks to external web sites (like the Wikipedia, USGS, NASA, etc.). Follow the links and scan them for relevant info. The information from linked web sites is meant to supplement and reinforce the lecture notes – you won’t be responsible for knowing everything contained in them. As a distance learning student, you need to explore and understand the content more independently than in a traditional class. As always, I will help guide you through this learning adventure. Remember, email Dr. Bob if you have any questions about today’s lecture ([email protected]). Leave no questions behind! Explore and have fun! Igneous Rocks The Life of Magma Most magma initially rises because of its greater buoyancy > cool and crystallize at depth as intrusions > migrate to the surface and erupt as extrusions Igneous Rocks Composition & Texture Composition (mineralogy) - mostly related to what was melted Texture (crystal size) - mostly related to the cooling rate (largely controlled by where the magma cools) rhyolite fine-grained texture Cool slowly at depth Crustal melting felsic magmas granite coarse-grained texture Igneous Rocks Naming Igneous Rocks click on the chart Igneous Rocks Igneous Compositions Mafic Mafic magmas are rich in Fe, Mg, & Ca, but poor in Si, Na, & K O O Ca Si O Ca O Mg O Fe magma O Fe O Si O Mg Igneous Rocks Igneous Compositions Mafic Upon cooling, mafic magmas crystallize mafic minerals… …which compose most mafic igneous rocks olivine basalt pyroxene Ca-feldspar gabbro Igneous Rocks Igneous Compositions Mafic & Ultramafic Abundant in the oceanic crust & mantle basalt gabbro peridotite Igneous Rocks Igneous Compositions Mafic & Ultramafic Mafic rocks form by partial melting of ultramafic mantle basalt gabbro peridotite Igneous Rocks Igneous Compositions Felsic Felsic magmas are poor in Fe, Mg, & Ca, but rich in Si, Na, & K O K Si O O K O Na O Si magma O Si O Si O Na Igneous Rocks Igneous Compositions Felsic Upon cooling, felsic magmas crystallize felsic minerals… …which compose most felsic igneous rocks mica NaFeldspar rhyolite K-feldspar quartz granite Igneous Rocks Igneous Compositions Felsic Abundant in the continental crust rhyolite pumice granite Igneous Rocks Igneous Compositions Felsic Form by melting continental crust rhyolite pumice granite Igneous Rocks Igneous Compositions Intermediate “Combo deal” includes some mafic & felsic elements O Ca Si O O K O Na O Fe magma O Si O Si O Mg Igneous Rocks Igneous Compositions Intermediate Upon cooling, intermediate magmas crystallize int. minerals… …which compose intermediate igneous rocks amphibole andesite Ca/Na Feldspar diorite Igneous Rocks Igneous Compositions Intermediate Abundant in the continental crust andesite diorite Igneous Rocks Igneous Compositions Intermediate Form by “wet” melting of the mantle and crust andesite diorite Igneous Rocks Igneous Textures Big crystals (coarse-grained texture) > slow cooling at depth (intrusive) microscope view granite Igneous Rocks Igneous Textures Small crystals (fine-grained texture) > fast cooling on the surface (extrusive) rhyolite basalt Igneous Rocks Igneous Textures No crystals (glassy texture) > ultra-fast to cool (quenching) on the surface (extrusive) obsidian Igneous Rocks Igneous Textures Big & small crystals (porphyritic texture) > two stages of cooling: slower & faster cooling (int & ext) microscope view porphyritic andesite Igneous Rocks Igneous Textures Holes or vesicles (vesicular texture) > gas bubbles escaping the magma (extrusive) scoria Igneous Rocks Igneous Textures Pieces of rock (pyroclastic texture) > explosive eruption (extrusive) Volcanic breccia, lapilli, & tuff increasing explosiveness Igneous Processes WWW Links in this Lecture > Magma - http://en.wikipedia.org/wiki/Magma > Buoyancy - http://en.wikipedia.org/wiki/Buoyancy > Intrusions - http://en.wikipedia.org/wiki/Intrusion > Extrusions - http://en.wikipedia.org/wiki/Extrusive > Mafic - http://en.wikipedia.org/wiki/Mafic > Basalt - http://en.wikipedia.org/wiki/Basalt > Gabbro - http://en.wikipedia.org/wiki/Gabbro > Oceanic crust - http://en.wikipedia.org/wiki/Oceanic_crust > Mantle - http://en.wikipedia.org/wiki/Mantle_%28geology%29 > Peridotite - http://en.wikipedia.org/wiki/Peridotite > Felsic - http://en.wikipedia.org/wiki/Felsic > Rhyolite - http://en.wikipedia.org/wiki/Rhyolite > Granite - http://en.wikipedia.org/wiki/Granite > Continental Crust - http://en.wikipedia.org/wiki/Continental_crust > Pumice - http://en.wikipedia.org/wiki/Pumice > Andesite - http://en.wikipedia.org/wiki/Andesite > Diorite - http://en.wikipedia.org/wiki/Diorite > Obsidian - http://en.wikipedia.org/wiki/Obsidian > Scoria - http://en.wikipedia.org/wiki/Scoria > Tuff - http://en.wikipedia.org/wiki/Tuff