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Igneous Rocks 1/26/2011 Igneous rock‐forming environments •Earth’s crust is 4/5 igneous rock. •Every igneous rock begins life as magma. Igneous rock is formed when •As magma migrates toward the surface, some of it chills and molten, or partially molten, hardens underground into various types of igneous rocks. rock solidifies. •Magma that makes it to the surface erupts in either flowing or explosive volcanoes, generating lava or pyroclastic debris. Igneous Rocks (two categories) Pillow Basalt Intrusive Extrusive Magma crystallized slowly within the crust. Lava and Pyroclastic Debris No exposure to the cool atmosphere. Extruded at surface or at very Plutonic –intrusive igneous rock at shallow levels. great depth within crust or mantle. Granite is Intrusive Basalt is Extrusive As magma crystallizes Texture Igneous Rocks a network of interlocking minerals develops. • Igneous Rocks are named on the basis of their Phaneritic texture - with large minerals texture and composition. The composition and (Granite) texture of the resulting Texture of a rock is the size and rock is determined by arrangement of the minerals it these minerals. contains. Composition of a rock is the assemblage of minerals it contains. Large crystals had a long time to crystallize. Therefore, this is an intrusive rock 1 1/26/2011 Texture Texture Texture Vesicular texture – Porphyritic Aphanitic texture - many pits from gas texture - with 2 mineral grains too escape (Basalt) small to see with the distinct grain sizes, unaided eye (Basalt) large and small (Andesite Porphyry) What is the cooling history? Small crystals had a short time to crystallize. Extrusive rock. Therefore, this is an extrusive rock Texture Texture is estimated using visual grain size Composition (depends on crystallization history) Igneous color (gray scale) is used to estimate chemical composition Glassy texture- Felsic Intermediate Mafic without obvious minerals (Obsidian) No crystals. This is an extrusive rock. Low Fe/Mg content…………………………………………High Fe/Mg content High Si/O content……………………………………………Low Si/O content Composition Composition Olivine Plagioclase How do igneous rocks form in a cooling magma chamber? Feldspar (Ca/Na) Felsic – oxygen, silicon, sodium enriched Remember partial melting? iron, magnesium, calcium depleted Igneous Rock‐Forming Source – partial melting, continental crust Silica (felsic) compounds melt first – therefore in a cooling magma chamber Minerals they must crystallize last Orthoclase Pyroxene Granite ‐ Feldspar Intermediate Iron‐rich (mafic) compounds melt last – therefore in a cooling magma chamber Diorite (pink is considered felsic) they must crystallize first Mafic Felsic last Mafic –Fe/Mg/Ca first Mafic –Fe/Mg/Ca Minerals Minerals Amphibole Mafic ‐ iron, magnesium, calcium enriched Muscovite oxygen, silicon, sodium depleted Crystallizing Melting Mica Source –mantle, oceanic crust Basalt first Felsic –Si/O/Na last Felsic –Si/O/Na Biotite Mica Ultramafic Therefore cooling magma will become enriched in Quartz Peridotite Si/O as crystallization proceeds 2 1/26/2011 How do igneous rocks form? The Igneous Types of Igneous rock is a ubiquitous component of Earth’s crust Minerals Bowen’s Reaction Series Rocks formed because it evolves as a product of tectonic processes. Ultramafic Basalt Hot Olivine Ca ‐ plagioclase Gabbro Mafic Pyroxene Plagioclase Granite Diorite Gabbro Intermediate feldspar Andesite Amphibole Intrusive The Igneous Diorite Rocks Biotite Na ‐ plagioclase Felsic Rhyolite Andesite Basalt Peridotite Texture Orthoclase feldspar Rhyolite Granite Extrusive Muscovite Cool Quartz Felsic Intermediate Mafic Ultramafic Composition 3.
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