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The Nature of Igneous Rocks • Form from Magma – Hot, partially molten mixture of solid liquid and gas – Mineral crystals form in the magma making a crystal “slush” – Gases - H2O, CO2, etc. - are dissolved in the magma – Magma is less dense than solid rock The Nature of Igneous Rocks • Magma vs. Lava – Magma is molten rock beneath the surface – Lava is molten rock that has reached the surface – Magma solidifies to form intrusive igneous rocks – Lava solidifies to form extrusive igneous rocks The Nature of Igneous Rocks • Composition varies widely – Silica and water content control viscosity – 2 end members are: • Mafic magmas • Silicic magmas Mafic Magmas • Silica content of ~ 50% • High concentrations of Fe, Mg and Ca • High temperature of molten magma – 1000o to 1200oC • Major minerals – Olivine, pyroxene, Ca plagioclase • Fluid flow (low viscosity) Silicic Magma • Silica content of 65-77% • High concentrations of Al, Na and K • Lower temperature magmas – Less than 850oC • Major minerals – Feldspars, quartz, micas • Viscous, thicker than mafic magmas Figure 4.2. Distribution of igneous rocks in North America Igneous Textures • Texture - the size, shape and relationship of minerals in the rock • Relates the cooling history of the magma or lava – fast vs. slow • Large crystals – slow cooling; • Small/microscopic crystals – fast cooling Glassy Texture • Very rapid cooling - quenched – Volcanic glass – Conchoidal fracture • No apparent crystals – embryonic crystals may be present • Dark color from low concentrations of Fe - generally silicic composition Figure 4.3A. Glassy texture in obsidian Crystalline Textures • Crystal growth requires time for ions to migrate - form minerals • Slow rate of cooling provides time for crystal growth • Crystals grow until melt is quenched or is completely solidified Aphanitic Texture • Fine grained texture • Few crystals visible in hand specimen • Relatively rapid rate of cooling • Vesicles may be formed by gases trapped in cooling magma Figure 4.3B. Aphanitic texture in rhyolite Phaneritic Texture • Coarse grained texture • Relatively slow rate of cooling • Equigranular, interlocking crystals • Slow cooling = crystallization at depth Figure 4.3C. Phaneritic texture in granite Porphyritic Texture • Well formed crystals (phenocrysts) • Fine grained matrix (groundmass) • Complex cooling history – Initial stage of slow cooling • Large, well formed crystals form – Later stage of rapid cooling • Remaining magma crystallizes more rapidly Porphyritic andesite Porphyritic olivine basalt Pyroclastic Texture • Produced by explosive volcanic eruptions • May appear porphyritic with visible crystals – Crystals show breakage or distortion • Matrix may be dominated by glassy fragments – Fragments also show distortion – Hot fragments may “weld” together Figure 4.3D. Pyroclastic texture Classification of Igneous Rocks – Texture • Aphanitic • Phanaritic – Composition • Silicic • Intermediate • Mafic • Ultramafic Combination of Texture and Composition produces rock name Figure 4.4. Classification of common igneous rocks Extrusive Rock Bodies • Form of extrusive bodies influenced by magma properties – Composition • Silica content – Viscosity • Volatile content • Temperature Basaltic Eruptions • Low Silica + High T = Low Viscosity • Produce – Lava Flows - Pahoehoe or Aa – Flood basalts – Fissure eruptions – Spatter cones; cinder cones (v. small) – Shield Volcanoes (v. large) – Pillow lavas Aa flow Pahoehoe flow Figures 4.6 A & B Beginnings of a spatter cone (Fig 4.6F) Large cinder cone (Fig 4.8) Fig 4.7. Flood basalts with several thick and thin layers. Each layer represents a separate eruption. Formation of pillow lavas (Fig 4.12) Intermediate & Silicic Eruptions • Higher Silica + Lower T = Higher Viscosity • Produce – Lava (Rhyolite) Domes - small – Composite volcanos - medium – Ash Flow Calderas - large Formation of Volcanic Domes (Fig. 4.13 A & B) Fig 4.14. Mt. St. Helen's prior to 1980 eruption, a classic composite volcano Process of formation of ash flow caldera - e.g., Crater Lake, OR (Fig 4.15) Fig. 4.9. Size comparison of various volcanic features Intrusive Rock Bodies • Less dense magmas rise through the crust • Rising magmas slowly cool – Viscosity increases – Density increases • Intrusions form as magma solidifies beneath the surface Intrusive Rock Bodies • Intrusions are classified by their size, shape and relative age • Large intrusions • Batholiths • Stocks • Small intrusions • Dikes • Sills • Laccoliths Figure 4.18. Types of magmatic intrusions Figure 4.2. Distribution of igneous rocks in North America Plate Tectonic Setting of Igneous Rocks • Divergent Plate Boundaries – mid-ocean ridges and continental rifts – Partial melting of mantle produces basaltic magma • Convergent Plate Boundaries – Subduction and partial melting of basalt, sediments and the surrounding mantle forms overlying volcanoes – Andesitic and rhyolitic magma generated Plate Tectonic Setting of Igneous Rocks • Mantle Plumes aka Hot Spots – Partial melting of rising plumes of solid mantle material – If located in oceanic crust then basaltic magmas ex. Hawaiian Islands – If located in continental crust then either rhyolite calderas (Yellowstone Nat’l Park) or flood basalts (Snake River/Columbia Plateau) Igneous Rocks and Plate Tectonics Convergent margins (cont. & oceanic) Igneous Rocks and Plate Tectonics Divergent (oceanic crust) End of Chapter 4