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Igneous Rocks What is Igneous Rock? Earth’s crust is 4/5 igneous rock. Every igneous rock begins life as molten magma deep in the mantle. As magma migrates toward the surface, some of it chills and hardens Igneous underground into granite and other types of igneous rocks. Rocks Magma that makes it to the surface erupts in either flowing or explosive volcanoes, generating lava, geysers, and hot springs. http://www.soest.hawaii.edu/coasts/lecture/gg101/index.html Igneous rock crystallizes in Earth’s magma locations Extrusive igneous rock Intrusive igneous rock crystallizes within Earth’s crust. Intrusive igneous rock Extrusive igneous rock crystallizes upon Earth’s crust. 1 As magma crystallizes a network of interlocking Igneous Rocks minerals develops. The composition and • Igneous Rocks are named on the basis of texture of the resulting rock is determined by the ir tttexture and composition. these minerals. Composition of a rock is the assemblage of minerals it contains. Texture of a rock is the size and arrangement of the minerals it contains. Texture Texture Aphanitic texture - mineral grains too small to see with the unaided eye (Basalt) Phaneritic texture - with large minerals (Granite) Large crystals had a long time to crystallize. Small crystals had a short time to crystallize. Therefore, this is an intrusive rock Therefore, this is an extrusive rock 2 Texture Texture Glassy texture - without Vesicular texture – many pits obvious minerals (Obsidian) from gas escape (Basalt) No crystals. This is an extrusive rock. Extrusive rock. Composition Texture Igneous color (gray scale) is used to estimate chemical composition Porphry texture – with 2 Felsic Intermediate Mafic distinct grain sizes, large and small (Andesite Porphyry) What is the cooling history? Low Fe/Mg content…………………………………………High Fe/Mg content High Si/O content……………………………………………Low Si/O content 3 Remember, partial melting produces magma that is enriched in Composition Silicon and Oxygen and depleted in Iron and Magnesium Felsic – oxygen, silicon, sodium enriched iron, magnesium, calcium depleted Source – partial melting, continental crust Granite - pink is considered felsic Intermediate Diorite Mafic - iron, magnesium, calcium enriched Basalt oxygen, silicon, sodium depleted Source – mantle, oceanic crust Ultramafic Dunite Olivine Plagioclase Composition Feldspar (Ca/Na) How do igneous rocks form in a cooling magma chamber? Remember partial melting? Igneous Rock-Forming Minerals Silica (felsic) compounds melt first – therefore in a cooling magma chamber they must crystallize last Pyroxene Orthoclase Feldspar Iron-rich (mafic) compounds melt last – therefore in a cooling magma chamber they must crystallize first Mafic Felsic Minerals Minerals last Mafic – Fe/Mg/Ca first Mafic – Fe/Mg/Ca Amphibole Muscovite Melting Crystallizing Mica first Felsic – Si/O/Na last Felsic – Si/O/Na Biotite Therefore a cooling magma chamber will become enriched in Si/O as it crystallizes Mica Quartz 4 How do igneous rocks form? The Igneous Bowen’s Reaction Series Types of Minerals Rocks formed Ultramafic Basalt Hot Olivine Ca - plagioclase Gabbro Mafic Pyroxene Intrusive Granite Diorite Gabbro Plagioclase Intermediate feldspar Andesite plutonic Amphibole Diorite The Igneous Biotite Na - plagioclase Rhyolite Andesite Basalt Rocks Texture Felsic Extrusive volcanic Orthoclase feldspar Rhyolite Granite Muscovite Felsic Intermediate Mafic Cool Quartz Composition Where do the Igneous Rocks form? Intrusive Granite Diorite Gabbro Extrusive Rhyolite Andesite Basalt Felsic Intermediate Mafic 5.
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