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Transcript
Igneous Rocks Fig. 7.16 Complete the following table by identifying which of the characteristics in the left-hand column are present in volcanic and/or plutonic igneous rocks by stating yes or no for the appropriate number. One characteristic has been completed as an example. Characteristic May form from basaltic magma Form at Earth’s surface Have texture Made of small grains Granite is an example Form as a result of melting Present at Earth’s surface only after erosion Contains minerals Classified based on color Dark-colored examples have low silica content Contain visible grains Volcanic igneous Plutonic igneous rocks rocks 1. 3. 2. 4. 5. 7. 9. 11. 13. 15. 17 19. 6. 8. 10. 12. 14. 16 18. 20. Overview of Igneous Rocks Form when minerals crystallize from magma IntrusivePlutonic ExtrusiveVolcanic Magmas derived from below the Earth’s surface in the mantle Magma is hot and buoyant Koryakskaya Sopka Volcano, Eastern Russia It’s all about heat and density Heat source?? Composition Geothermal Gradient Hot stuff Igneous Rock Classification Composition Texture Felsic Intermediate Mafic Ultramafic Phaneritic Granite Diorite Gabbro Peridotite Porphyritic Phaneritic Aphanitic Porphyritic Aphanitic Porphyritic Porphyritic Porphyritic Granite Diorite Gabbro Rhyolite Andesite Basalt Porphyritic Porphyritic Porphyritic Rhyolite Andesite Basalt Intrusive Vs. Extrusive Plutonic Formed within the Earth Magma Reach Surface by uplift and erosion of the Earth’s Crust Volcanic Formed at the Surface Lava Texture Related to the cooling history of the rock Really Fast = no grains glassy Fast = Fine-grained aphanitic Slow = Coarse-grained phaneritic Complex = Mixture porphyritic Why?? Other textures: Vesicular: trapped gases in lava Pyroclastic: ash and rock fragments formed explosively Texture Aphanitic Fine –grained Cooled quickly Crystallized at the Earth’s surface Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Peanut Butter or Sugar Cookie Aphanitic--peanut butter cookie Texture Aphanitic Porphyritic Aphanitic Two stages of cooling – 1st cooled slowly within the Earth (larger cyrstals Phenocrysts) – 2nd cooled rapidly on the Earth’s surface (fine-grained matrix) Phaneritic Porphyritic Phaneritic Chocolate Chip Cookie Porphyritic Aphanitic—Chocolate Chip Cookie Texture Aphanitic Porphyritic Aphanitic Phaneritic Coarse –grained Cooled slowly Crystallized within the Earth Porphyritic Phaneritic Ooopps!! I must have eaten the Oatmeal Cookie Phaneritic—Oatmeal Cookie Texture Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Two stages of cooling Oatmeal Raisin Cookie – 1st cooled slowly within the Earth (larger crystals Phenocrysts) – 2nd cooled faster but still slow enough that crystals fully develop – within the Earth (coarse-grained matrix) Porphyritic Phaneritic—Oatmeal Raisin Cookie Texture Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular –voids left by trapped gas Glassy Pyroclastic Texture Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular Glassy Very rapid cooling Ions do not have time to from crystalline structures Pyroclastic Texture Aphanitic Porphyritic Aphanitic Phaneritic Porphyritic Phaneritic Vesicular Glassy Pyroclastic – welded shards of rock & ash ejected from a vent during an eruption Composition of Igneous Rocks Silica (Si02) is primary Composition of Earth's Crust by Mass ingredient of all magmas Viscosity: Resistance to flow O-46.6% Silica content temperature All Others-9% Si-27.7 Ca-3.6% Fe-5.0% Al-8.1% Composition—Silica Content Felsic: Feldspar & Silica >65% silica High Viscosity Intermediate: 53-65% silica Intermediate Viscosity Mafic: Magnesium and Iron (Fe) 45-52% silica Low Viscosity Ultramafic: <45% silica Very Low Viscosity Composition Felsic-rhyolitic: <900EC; Na, K, Al-rich Light colored Composition Mafic-basaltic: >1100EC; Ca, Fe, Mg-rich Dark Colored Composition Intermediate-andesitic: 900-1100EC; Na, Al, Ca, Fe, Mg, K Salt & Pepper appearance Andesite Porphyry Diorite Bowen’s Reaction Series Hot Melting minerals crystallize from magmas at different temperatures Crystallization Different Cold Magmatic Differentiation Formation of more than one magma from a single parent magma Magmatic Differentiation Crystal Settling: crystallized minerals have a density greater than the magma and settle to the bottom due to gravity Because Fe and Mg are first removed, melt becomes rich in SiO2, Na, and K Marbles analogy Magmatic Differentiation Assimilation: magma reacts with the “country rock” which is adjacent to the magma chamber Magma composition is altered according to the composition of the assimilated country rock Inclusions are rocks Incompletely melted chunks of country rock Magmatic Differentiation Magma Mixing: Magmas of different compositions are mixed together Resulting magma is of a composition intermediate between the parents Magma Mixing Magma Mixing Magma Mixing Fig. 7.21 Composition Quiz Which type of lava would flow most easily? Mafic/Ultramafic Which type of volcano would erupt most violently? Felsic
