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Chapter 3 Igneous Rocks PowerPoint Presentation Stan Hatfield . SW Illinois College Ken Pinzke . SW Illinois College Charles Henderson . University of Calgary Tark Hamilton . Camosun College Copyright (c) 2005 Pearson Education Canada, Inc. 3-1 Magma: The Parent Material of Igneous Rock Igneous rocks form as molten rock cools and solidifies General Characteristic of magma • Parent material of igneous rocks • Forms from partial melting inside the Earth • Magma that reaches the surface is called lava Copyright (c) 2005 Pearson Education Canada Inc. 3-2 Magma: The Parent Material of Igneous Rock General Characteristic of Magma • Generally formed by partial melting in Upper Mantle (~1200° C) or Lower Crust (~850° C) • Rocks formed from lava at the surface are classified as extrusive or volcanic rocks • Rocks formed from magma that crystallizes at depth are termed intrusive or plutonic rocks • Magmas are buoyant, gas laden & transport Heat • Flow rates vary over many orders of magnitude from cm/yr to supersonic Copyright (c) 2005 Pearson Education Canada Inc. 3-3 Magma: The Parent Material of Igneous Rock The Nature of Magma • Consists of three components: – A liquid portion, called melt, that is composed of mobile ions derived from the partial melting of minerals – Solids, if any, are silicate minerals that have already crystallized from the melt: – Phenocrysts are large, Microlites are small – Volatiles, which are gases dissolved in the melt, including water vapour (H2O), carbon dioxide (CO2), sulphur dioxide (SO2) & minor HF, HCl, He Copyright (c) 2005 Pearson Education Canada Inc. 3-4 Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock • Cooling of magma results in the systematic arrangement of ions into orderly patterns, cations + anions = minerals • The silicate minerals resulting from crystallization form in a predictable order • Textures & inclusion relations tells order of crystallization (early small crystals get surrounded by later larger phenocrysts) Copyright (c) 2005 Pearson Education Canada Inc. 3-5 Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock • Texture in igneous rocks is determined by the size and arrangement of mineral crystals • Igneous rocks are typically classified by – Textures – Mineral compositions & proportions – Rocks with similar textures can have different compositions (glasses all appear similar) – Rocks with similar compositions can have different textures (rhyolite and granite look different because of different cooling histories) Copyright (c) 2005 Pearson Education Canada Inc. 3-6 Textures + Minerals = Igneous Rock Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Factors affecting crystal size: • Cooling Rates – Fast cooling forms glass or may tiny crystals (microlites) – Slow cooling rates promote the growth of fewer larger phenocrysts • Volatiles are Solvents – Volatiles lower viscosity & increase diffusion leading to larger crystals • Nucleation – More crystal nuclei promote the growth of more, but smaller crystals which impinge on each other Copyright (c) 2005 Pearson Education Canada Inc. 3-7 Igneous Textures Factors affecting crystal size • Cooling Rate for magma – Slow cooling (°C/yr) allows crystals to chemically react with magma – Fast rate (~few °C/hr) forms many small crystals – Very fast rate (~hundreds of °C/sec) forms glass (disordered ions) • Amount of Silica (SiO2) present – Mafic (Low silica) magmas like basalts (<50% SiO2 ) flow easily – Felsic (High silica) magmas are stiff and explosive • Nucleation of crystals – Contamination with crustal rocks promotes nucleation • Amount of Dissolved Gases – Affects viscosity, diffusion and explosivity – <5% dissolved volatiles allows flows of km/day – More than ~5% volatiles exsolve and form explosive foams Copyright (c) 2005 Pearson Education Canada Inc. 3-8 Igneous Textures Types of igneous textures • Aphanitic (fine-grained) texture Volcanic! – Rapid rate of cooling of lava or magma (in air, water) – Microscopic crystals – May contain vesicles (holes from gas bubbles) and thus rocks that contain them have a vesicular texture • Porphyritic = large phenocrysts & smaller groundmass – Generally lava flows or sub-volcanic intrusions (dykes/sills) – Phenocrysts grew slowly then eruption quenched the lava • Phaneritic (coarse-grained) texture Plutonic! – Crystals can be identified without a microscope (>2mm) – Generally caused by slow cooling (heat loss at depth) • Pegmatitic (very coarse-grained) texture Plutonic! – – – – Crystals (>2cm) Generally caused by very slow cooling Also caused by abundant volatiles (increases diffusion rates) Rare metals (Au, B, Be, Sn) & unusual minerals can occur Copyright (c) 2005 Pearson Education Canada Inc. 3-9 Igneous Textures: Volcanic Fe Staining: Weathering Hand Specimen Andesite with twinned Plagioclase laths, birefringent Augite, Magnetite & Glass Copyright (c) 2005 Pearson Education Canada Inc. Thin Section 3-10 Igneous Textures: Plutonic Granite with: Pink K-Feldspar, White Plagioclase, Grey Quartz & Black Biotite Phaneritic Biotite-birefringent, Qtz-grey/white Feldspars-Twinned Black/white Copyright (c) 2005 Pearson Education Canada Inc. 3-11 Igneous Textures: Volcanic Types of igneous textures • Porphyritic texture: 2 different crystal sizes – Minerals form at different temperatures as well as crystallizing at differing rates – Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass – Sudden loss of volatiles can arrest crystallization • Glassy texture (Vitreous & Conchoidal fracture) – Very rapid cooling to volcanic rock in air or water – This is common in very viscous, Hi-Silica magmas – Resulting rock is called obsidian Copyright (c) 2005 Pearson Education Canada Inc. 3-12 Volcanic Textures Flattened & fused Glassy shards Indicate flow & Horizontal directions Black Magnetite as dust sized particles darkens the glass Pale green patches are altered to chlorite. Copyright (c) 2005 Pearson Education Canada Inc. 3-13 Igneous Textures Types of igneous textures • Pyroclastic texture (volcanic) – Various fragments ejected during a violent volcanic eruption (rocks, crystals, glass shards, foams) – Textures often appear more similar to sedimentary rocks, but usually angular & partly glassy • Pegmatitic texture (plutonic) – Exceptionally coarse-grained – Form in late stages of crystallization of magmas • Xenolithic / Xenocrystic texture (plutonic) – Accidental rock fragments from mantle or crust – Included crystals from other rocks or magmas Copyright (c) 2005 Pearson Education Canada Inc. 3-14 Igneous Textures: Plutonic Large K-Feldspars, White Plagioclase, Grey Quartz, Brown-Green Hornblende, Black Magnetite Phaneritic Porphyritic Copyright (c) 2005 Pearson Education Canada Inc. 3-15 Igneous Compositions Igneous rocks are composed primarily of silicate minerals • Dark (or ferromagnesian) silicates, ∑= Colour Index • Depends on Mg + Fe content of magma • Crystallize in order of falling Temperature • May react with magma to form a lower T°C phase – Spinel or Magnetite (oxides not silicates) Fe3O4 – Olivine (Mg,Fe)SiO4 , lone tetrahedra – Pyroxene Ca(Mg,Fe)Si2O6 , single chains – Amphibole Ca2(Mg,Fe)5Si8O22(OH)2 , double chains – Biotite mica K(Mg,Fe)3(Al,Si)3O10(OH,F)2 , sheets Copyright (c) 2005 Pearson Education Canada Inc. 3-16 Igneous Compositions Igneous rocks are composed primarily of silicate minerals • Light (or nonferromagnesian) silicates (with falling Temperature) – Plagioclase Feldspar: framework silicate – Anorthite CaAl2 Si2O8 to Albite NaAlSi3O8 – K-Feldspar KAlSi3O8 , framework silicate – Quartz SiO2 , framework silicate – Muscovite mica K(Mg,Fe)3(Al,Si)3O10(OH,F)2 , sheets, (this mineral is only found in plutonic rocks) Copyright (c) 2005 Pearson Education Canada Inc. 3-17 Igneous Compositions Felsic versus Mafic Compositions • Rhyolitic composition – Common in explosive strato-volcanoes (arcs) • Granitic composition – Composed of light-coloured silicates – Designated as being felsic (feldspar and silica) in composition – Contains high amounts of silica (SiO2), >68% – Low temperature melts but high viscosity – Major rock type in continental crust – Common in batholiths of continental margin arcs Copyright (c) 2005 Pearson Education Canada Inc. 3-18 Igneous Compositions Mafic versus Felsic Compositions • Basaltic (or Gabbroic) composition – Composed of dark silicates and calcium-rich feldspar – Designated as being mafic (magnesium and ferrum, for iron) in composition – High Temperature magmas but low viscosity – More dense than granitic rocks – Comprise the ocean crust as well as many volcanic islands Copyright (c) 2005 Pearson Education Canada Inc. 3-19 Igneous Compositions Other compositional groups • Intermediate (or andesitic) composition – Contain at least 25 percent dark silicate minerals – Associated with explosive volcanic activity – Present in arc volcanoes and plutons • Ultramafic composition – Composition that is high in MgO and FeO > 55% – Dense, high Temperature, Low viscosity melts – Composed entirely of >90% ferromagnesian silicates – Common in mantle (plutonic) but rare in crust – More common in lower crust and in Precambrian rocks Copyright (c) 2005 Pearson Education Canada Inc. 3-20 Igneous Compositions Silica Content as an Indicator of Composition • Silica content in crustal rocks exhibits a considerable range – A lower than 45% in ultramafic rocks – Over 75% percent in some felsic rocks – Tends to increase during fractional crystallization Copyright (c) 2005 Pearson Education Canada Inc. 3-21 Igneous Rock Classification Rock Names Depend on Mineral %’s & Textures Plutonic/Volcanic Mineralogy of common igneous rocks Copyright (c) 2005 Pearson Education Canada Inc. and the magmas from which they form. 3-22 Igneous Compositions Silica content influences a magma’s behaviour • Granitic magma – High silica content > 68% – Extremely viscous, flows slowly, explosive – Liquid exists at temperatures as low as 700oC – Forms by differentiation from more Mafic magmas – Can also form by partial melting of Lower Crust in collisional orogens – Often high volatile contents: H2O, CO2 etc. Copyright (c) 2005 Pearson Education Canada Inc. 3-23 Igneous Compositions Silica content influences a magma’s behaviour • Basaltic magma – Much lower silica content <54% – Fluid-like behaviour – Low volatile content < a few %, usually not explosive – Crystallizes at higher temperatures – Most common Magma on Earth (or Moon!) – Partial melt of Peridotite (5% to 25%) Copyright (c) 2005 Pearson Education Canada Inc. 3-24 Igneous Compositions Naming Igneous Rocks – Felsic (Granitic) Rocks • Granite – Phaneritic – Over 25 percent quartz, about 65 percent or more feldspar – May exhibit a porphyritic texture – Very abundant as it is often associated with mountain building – The term granite covers a wide range of mineral compositions but mostly alkali feldspar & quartz Copyright (c) 2005 Pearson Education Canada Inc. 3-25 Plutonic Igneous Compositions Granite: K-spar > Plagioclase CI < 15 Collisional varieties have 2 micas Copyright (c) 2005 Pearson Education Canada Inc. 3-26 Igneous Compositions Naming Igneous Rocks – Felsic (Granitic) Rocks • Rhyolite – Extrusive equivalent of granite – Found in stratovolcanoes & calderas – May contain glass fragments and vesicles – Aphanitic texture – Less common and less voluminous than granite Copyright (c) 2005 Pearson Education Canada Inc. 3-27 Volcanic Igneous Compositions Rhyolite Aphanitic may be any colour Vesicles & glass are common May contain Quartz phenocrysts Copyright (c) 2005 Pearson Education Canada Inc. 3-28 Felsic Volcanic Compositions Naming Igneous Rocks – Felsic (Granitic) Rocks • Obsidian – Dark-coloured, often flow banded – Glassy texture, conchoidal fracture – Gas bubbles (vesicles) are common but flattened • Pumice – Low density & Light Coloured Vesicular Volcanic – Glassy texture with few if any crystals – Frothy appearance with numerous voids Copyright (c) 2005 Pearson Education Canada Inc. 3-29 Felsic Volcanic Compositions Obsidian with palagonite (Fe-clays) in fractures Copyright (c) 2005 Pearson Education Canada Inc. 3-30 Felsic Volcanic Compositions Pumice Copyright (c) 2005 Pearson Education Canada Inc. 3-31 Intermediate Volcanic Compositions Naming Igneous Rocks – Intermediate (Andesitic) Rocks • Andesite – Volcanic origin usually in arc stratovolcanoes – Aphanitic or aphanitic-porphyritic texture – Often crystal rich with 25% to 40% phenocrysts – Essential Plagioclase with Pyroxene or Hornblende – Often resembles rhyolite when pyroclastic (tuffaceous) Copyright (c) 2005 Pearson Education Canada Inc. 3-32 Intermediate Volcanic Compositions Felsic Volcanic Compositions Andesite From Black to White! Abundant Plagioclase phenocrysts with small hornblende microphenocrysts. Magnetite is opaque. Groundmass is smaller by 50X. High phenocryst content makes these viscous & explosive. Copyright (c) 2005 Pearson Education Canada Inc. 3-33 Intermediate Plutonic Compositions Naming Igneous Rocks – Intermediate (Andesitic) Rocks • Diorite – Plutonic equivalent of andesite – Coarse-grained – Intrusive (like in the roots of the Andes!) – Composed mainly of intermediate plagioclase feldspar and amphibole – K-feldspar is minor if present, < 1/3 of total feldspar – 16 < CI < 45 is intermediate Copyright (c) 2005 Pearson Education Canada Inc. 3-34 Intermediate Plutonic Compositions Diorite Phaneritic Grey Rocks ½ Plagioclase & ½ Pyroxene or Hornblende Copyright (c) 2005 Pearson Education Canada Inc. 3-35 Igneous Compositions Naming Igneous Rocks – Mafic (Basaltic) Rocks • Basalt – Volcanic origin – Dark green to black in colour – Aphanitic texture – Composed mainly of pyroxene and calciumrich plagioclase feldspar – Most common extrusive igneous rock Copyright (c) 2005 Pearson Education Canada Inc. 3-36 Igneous Compositions Basalt Copyright (c) 2005 Pearson Education Canada Inc. 3-37 Igneous Compositions Naming Igneous Rocks – Mafic (Basaltic) Rocks • Gabbro – Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene and calcium-rich plagioclase – Makes up a significant percentage of the oceanic crust Copyright (c) 2005 Pearson Education Canada Inc. 3-38 Igneous Compositions Naming Igneous Rocks – Pyroclastic Rocks • Composed of fragments ejected during a volcanic eruption • Varieties – Tuff – ash-sized fragments – Volcanic breccia – particles larger than ash Copyright (c) 2005 Pearson Education Canada Inc. 3-39 Origin of Magma Highly debated topic Generating magma from solid rock • Produced from partial melting of rocks in the crust and upper mantle • Role of Temperature – Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20oC to 30oC per kilometre Copyright (c) 2005 Pearson Education Canada Inc. 3-40 Origin of Magma Estimated temperatures in the crust and mantle. Copyright (c) 2005 Pearson Education Canada Inc. 3-41 Origin of Magma • Role of Temperature – Rocks in the lower crust and upper mantle are near their melting points – Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting Copyright (c) 2005 Pearson Education Canada Inc. 3-42 Origin of Magma • Role of Pressure – An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature – When confining pressures drop, decompression melting occurs Copyright (c) 2005 Pearson Education Canada Inc. 3-43 Origin of Magma Copyright (c) 2005 Pearson Education Canada Inc. Decompression melting 3-44 Origin of Magma • Role of volatiles – Volatiles (primarily water) cause rocks to melt at lower temperatures – This is particularly important where oceanic lithosphere descends into the mantle Copyright (c) 2005 Pearson Education Canada Inc. 3-45 How Magmas Evolve A single volcano may extrude lavas exhibiting very different compositions Bowen’s reaction series and the composition of igneous rocks • N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points Copyright (c) 2005 Pearson Education Canada Inc. 3-46 How Magmas Evolve Bowen’s Reaction Series shows the sequence in which minerals crystallize from a magma. Copyright (c) 2005 Pearson Education Canada Inc. 3-47 How Magmas Evolve Bowen’s reaction series • During crystallization, the composition of the liquid portion of the magma continually changes – Composition changes due to removal of elements by earlier-forming minerals – The silica component of the melt becomes enriched as crystallization proceeds – Minerals in the melt can chemically react and change Copyright (c) 2005 Pearson Education Canada Inc. 3-48 How Magmas Evolve Processes responsible for changing a magma’s composition • Magmatic differentiation – Separation of a melt from earlier formed crystals to form a different composition of magma • Assimilation – Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma Copyright (c) 2005 Pearson Education Canada Inc. 3-49 Processes responsible for changing a magma’s composition • Magma mixing – Involves two bodies of magma intruding one another – Two chemically distinct magmas may produce a composition quite different from either original magma Copyright (c) 2005 Pearson Education Canada Inc. 3-50 How Magmas Evolve Magma mixing, assimilation and magmatic differentiation. Copyright (c) 2005 Pearson Education Canada Inc. 3-51 How Magmas Evolve Partial Melting and Magma Formation • Incomplete melting of rocks is known as partial melting • Formation of a Mafic Magma (basaltic) – Most originate from partial melting of ultramafic rock in the mantle – Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones Copyright (c) 2005 Pearson Education Canada Inc. 3-52 How Magmas Evolve Partial Melting and Magma Formation • Formation of Basaltic Magmas – As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature – Large outpourings of basaltic magma are common at Earth’s surface Copyright (c) 2005 Pearson Education Canada Inc. 3-53 How Magmas Evolve Partial Melting and Magma Formation • Formation of Andesitic Magmas – Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition – Andesitic magma may also evolve by magmatic differentiation Copyright (c) 2005 Pearson Education Canada Inc. 3-54 How Magmas Evolve Partial Melting and Magma Formation • Formation of Felsic (granitic) Magmas – Most likely form as the end product of crystallization of andesitic magma – Granitic magmas are higher in silica and therefore more viscous than other magmas – Because of their viscosity, they lose their mobility before reaching the surface – Tend to produce large plutonic structures Copyright (c) 2005 Pearson Education Canada Inc. 3-55 End of Chapter 3 Copyright (c) 2005 Pearson Education Canada Inc. 3-56