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Igneous Rocks With Some Graphics from Press et al., Understanding Earth, 4th Ed. (Copyright © 2004 by W. H. Freeman & Company) By: Abboud Suliman Ahmed Igneous Rocks Igneous rocks form from molten rock (magma) crystallizing below earth's surface or from volcanic activity. They commonly form at plate boundaries and are commonly exposed in mountainous areas. Igneous rocks form from molten rock (magma) crystallizing below earth's surface or from volcanic activity. They Igneous rocks form fromform crystallization of magma commonly at plate boundaries and at depth (within the earth's crust) orexposed at the surface (from volcanic are commonly in mountainous eruptions) areas. There are two (2) basic types or forms of igneous rocks: 1. Plutonic rocks = intrusive igneous rocks = igneous rocks that form from cooling magma at depth 2. Extrusive igneous rocks = igneous rocks that form from volcanic activity (at or near surface) Key Terminology Plutonic Intrusive Extrusive Volcanic Texture Phaneritic Aphanitic Porphyritic Glassy Vesicular Pyroclastic Magma Lava Bowen’s Reaction Series Assimilation Partial melting Fractional crystallization Discordant Concordant Dike Stock Batholith Sill Laccolith Lopolith Table. 5.2 Felsic Granite Rhyolite Intermediate Granodiorite Dacite Mafic Diorite Gabbro Andesite Basalt Viscosity Melting Temperature Bowen's Reaction SeriesPlagioclase (Ca-feldspar) Olivine Pyroxene Amphibole Biotite Plagioclase (Na-feldspar) Orthoclase (K-feldspar) Muscovite Quartz Common Minerals General characteristics of magma • Igneous rocks form as molten rock cools and solidifies • General characteristics of magma: • Parent material of igneous rocks • Forms from partial melting of rocks • Magma at surface is called lava General characteristics of magma • General characteristic of magma • Rocks formed from lava = extrusive, or volcanic rocks • Rocks formed from magma at depth = intrusive, or plutonic rocks The Rock CycleMinerals form rocks All rocks can be transformed into other rock types Rocks are divided into 3 categories Igneous- crystalline- forms as liquid cools Metamorphic- crystalline-forms as rocks are heated and squeezed Sedimentary- non-crystalline- smaller pieces or chemicals from other rocks Igneous formed from Magma and Lava Magma • molten rock below Earth's surface. L ava • magma on the Earth's surface. Pyroclastic material • (pyro = fire, clastic = debris) • Airborne lava — cools as it falls Igneous Rock Chemistry • • ■ Major elements: Approximately 99% of Igneous Rocks are comprised of only eight elements. Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium The amount of silica (SiO2) determines the mineral content and general color of igneous rocks. ■ Rare Elements: ■Trace elements are those which occur in very low concentrations in common rocks (usually < 0.1 % by weight). ■ Their concentrations are therefore commonly expressed in parts per million (ppm; 1 ppm = 10-4 weight%). ■ Unlike major elements, trace elements tend to concentrate in fewer minerals, and are therefore more useful in formulating models for magmatic differentiation, and in some cases, in predicting the source of a particular magma. ■ Trace elements most commonly used for the interpretation of the petrogenesis of igneous rocks include: Ni, Cr, Sc, V, Rb, Ba, Sr, Zr, Y, Nb and the rare earth elements (La to Lu). ■عندما تتبلور المعادن االساسية من الصهير تسمح بعض المعادن المكونة للعناصر النادرة بدخول بنيتها البلورية ■حيث وضع العلم Goldschmidtعام 1937قاعدة توضح سلوك العناصر النادرة تسمى قاعدة ( )Goldschmidtو هى كاالتى: -1عناصر مستترة ( :)Camouflageويسمى االحالل بالتخفى ايون اساسى ( )Aو ايون نادر ()BA++ = B++ AR = BR مثال : mg++و co++ R=0.80 A R=0.83 A حالتين: )Capture( االسر-2 A R = BR ● A++ ≠ B++++ K+ Ba++ R=1.33 A R=1.34 A AR > BR A+++ = B+++ ● حالتين: )Admission( القبول-3 AR = BR ● A+++ = B+ mg++ Li+ :مثال R=0.68 A R=0.66A AR < BR A+++ = B+++ ● Sources of Heat for Melting Heat from below : Heat moves upward a. __________________ (by conduction and convection) from the very hot (>5000 ̊ C) core through the mantle and crust. Minerals start to crystallize from a cooling magma between 1200 ̊ C - 600 ̊ C. Geothermal Gradient b. ___________________________________ : 3o C /100 m (30o C/km) At great depth temperature alone would melt rock BUT high pressure may cause it to remain solid. iii. Not the same everywhere (i.e., It’s higher in volcanic regions). i. ii. c) Radioactive Decay • Heat byproduct during decay. • High concentration may cause temperature to increase with depth at a rate greater than the geothermal gradient. d) Friction • Rock grinding past rock • Active Mountain building regions. Friction of moving and shifting rock masses in regions of mountain building may combine with heat from other sources to melt rock. Melting due to the Addition of Volatiles Viscosity of Magma/ Lava Viscosity- important for volcanic activity • the resistance of a liquid to flow — high viscosity = thick and stiff — low viscosity = thin and "runny". Convergent Margins- flux melting Divergent Boundary Hot Spots (e.g. Hawaii) Silica Tetrahedron Silicon Oxygen Silicates are classified on the basis of Si-O polymerism • ■The Silicate structures are known from X-rays. ■The complexity of igneous rocks are attributed to complexity of silicate structures. ■ Decompositions of mineral depends on silicate structures [SiO4]4- Independent tetrahedra Nesosilicates Si : O 1:4 Example: Olivine group (Fe,mg)2SiO4 Garnet A3B2Si3O12 Usually B is Aluminum, A divalent (Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12 characteristic colors: Pyrope Mg3Al2Si3O12 – deep red to black Almandine Fe3Al2Si3O12 – deep brownish red Spessartine Mn3Al2Si3O12 – brownish red to black Grossular Ca3Al2Si3O12 – yellow-green to brown Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black Uvarovite Ca3Cr2Si3O12 – emerald green Common Nesosilicates: The Aluminosilicates Kyanite, Sillimanite, Andalusite Al2SiO5 = Al2 (SiO4)O Topaz Al2SiO4(F,OH)2, Si2O7]6- Double tetrahedra Sorosilicates Si : O 2:7 • Double silicon tetrahedra linked • by one bridging oxygen Sorosilicates commonly also contain independent silica tetrahedra (SiO4)-4 • Typically monoclinic symmetry Epidote Group Zoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH) Epidote – Ca2(Fe,Al)Al2O(SiO4)(Si2O7)(OH) Cyclosilicates n[SiO3]2- n = 3, 4, 6 Si : O 1:3 N=3, [Si3O9]6N=4, [Si4O12]8N=6, [Si6O18]12- Beryl, Be3Al2 (Si6O18) Inosilicates single chains [SiO3]2- Si : O 1:3 Example: Pyroxine group (Fe,mg) SiO3 Diopside , camg si2o6 Augite , ca (mg, Fe) si2o6 Inosilicates Double- chains [Si4O11]6Si : O 4 : 11 Example: amphibole minerals Actinolite, ca2(mg,Fe)5Si8O22(OH)2 Phyllosilicates Sheets of tetrahedra [Si2O5]2Si : O 2:5 Mica group: Muscovite, KAl2(AlSi3)O10(OH)2 Tectosilicates [SiO2] Si : O 1:2 3-D frameworks of tetrahedral: fully polymerized quartz and the silica minerals feldspars feldspathoids zeolites Examples: Quartz, SiO2. Orthoclase, KAlSi3O8. Plagioclase, CaAl2Si2O8 Plate Tectonic - Igneous Genesis 1. Mid-ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-arc Basins 6. Ocean Island Basalts 7. Miscellaneous IntraContinental Activity kimberlites, carbonatites, anorthosites... Binary phase diagram for a solid solution of Olivine Fayallite (Fa) % Fo (Mg2SiO4) Forsterite (Fo) quartz hornblende feldspar Olivine (Mg,Fe)2SiO4 Single tetrahedra No cleavage Pyroxene (Mg,Fe)SiO3 Amphibole Biotite Mica Ca2(Mg,Fe)5Si8022(OH)2 K(Mg,Fe)3AlSi3O10(OH)2 Single chains 2 cleavages (90o) Double chains 2 cleavages (60o and 120o ) Sheets 1 cleavage Olivine – Found in mafic and ultramafic igneous rocks, can be found as beach sand from volcanic islands, used in manufacturing of basic refractories, or as magnesium ore, gem quality peridot Micas Found in igneous (pegmatities and granites) and metamorphic rocks (schists and amphibolites), used for elect and heat insulation, paper products, fireproof paint and dry lubricants Hematite – Found in extrusive igneous rocks and secondary mineral of sed deposits (oxidizing environment), most important iron ore, red ochre pigments and polishing powders Hornblende – a member of the amphibole group Potassium feldspar Plagioclase feldspar