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GEOL: CHAPTER 4 Igneous Rocks and Intrusive Igneous Activity Learning Outcomes • LO1: Describe the properties and behavior of magma and lava • LO2: Explain how magma originates and changes • LO3: Identify and classify igneous rocks by their characteristics • LO4: Recognize intrusive igneous bodies, or plutons • LO5: Explain how batholiths intrude into Earth’s crust Igneous Rocks • Molten rock (magma or lava) that cools and crystallizes to form minerals • Intrusive: underground, magma, plutons • Extrusive: above ground, lava, volcanic eruptions • Large parts of continents • All of oceanic crust Magma and Lava • Magma: molten rock below surface • Less dense than surroundings and wants to rise • Most solidifies underground: plutons • Lava flows: when magma reaches the surface • Volcanic rocks = extrusive igneous rocks – Lava flows – Pyroclastic materials Composition of Magma • Silicate rocks usually the source • Silica is primary constituent • Other constituents: – Aluminum – Calcium – Sodium – Iron – Magnesium – Potassium Three Types of Magma • Felsic magma – >65% silica – Considerable sodium, potassium, aluminum – Little calcium, iron, magnesium • Mafic magma – <52% silica – Silica poor – Proportionally more calcium, iron, magnesium • Intermediate magma – Composition between felsic and mafic magma Magma/Lava Temperatures • Lava usually 700ºC to 1,200ºC • Magma hotter, but can’t measure reliably • Mafic lava nonexplosive, easier to measure • Felsic more explosive, harder to measure • New igneous rocks take years or millennia to cool Viscosity • Resistance to flow • Higher temperatures reduce viscosity – Hotter magma/lava moves more readily • Increased silica content increases viscosity – Mafic lavas flow far – Felsic lavas don’t flow far • Higher amounts of dissolved gases reduce viscosity Origination of Magma • Can be 100-300 km deep • Usually shallower: upper mantle and lower crust • Accumulates in magma chambers • Some magma cools: plutons • Some rises through surface: volcanic Bowen’s Reaction Series • Minerals crystallize from cooling magma in a predictable sequence • Discontinuous branch • Continuous branch • Crystallization occurs on both branches simultaneously • Continued crystallization changes the composition of the melt Bowen’s Reaction Series, cont. Discontinuous branch • Ferromagnesian silicates only • One mineral changes to another over specific temperature ranges • Olivine to pyroxene to amphibole to biotite • Reactions often incomplete, so can have all ferromagnesian silicates in one rock Bowen’s Reaction Series, cont. Continuous branch • Plagioclase feldspar silicates only • Calcium-rich plagioclase crystallizes first • Then increasing amounts of sodium are incorporated until all sodium and calcium are gone • Rapid cooling gives calcium-rich core surrounded by zones of increasingly rich sodium Calcium-rich plagioclase Olivine Types of magma Reaction Mafic (45–52% silica) Pyroxene (augite) temperature Decreasing Reaction Amphibole (hornblende) Intermediate (53–65% silica) Reaction Biotite mica Sodium-rich plagioclase Potassium feldspar Felsic (>65% silica) Muscovite mica Quartz Stepped Art Fig. 4-3, p. 69 Magma at Spreading Ridges • Geothermal gradient: 25ºC/km • Lower pressure at ridges allows melting • Ultramafic rocks undergo partial melting – Release more silica-rich minerals (Bowen’s reaction series) – Create mafic magma Magma at Subduction Zones • Volcanoes and plutons near leading edge of overriding plate • Partial melting at depth – Releases water from hydrous minerals – Water rises and enhances melting – Mafic rocks melt, creating intermediate and felsic magma Hot-Spot Magma • Interior portions of plates • Mantle plumes: rising magma from the core-mantle boundary • Creates volcanoes – Hawaiian Islands Changing Magma Composition: Crystal Settling • Physical separation of minerals by crystallization and settling • Olivine, first formed, denser than magma, so it sinks • Makes remaining magma less mafic, more felsic Changing Magma Composition: Assimilation • Magma reacts with country rock • Country rock melts and changes composition of magma • Inclusions of incompletely melted country rock Changing Magma Composition: Magma Mixing • A volcano can erupt lavas of different composition • Some of these magmas mix, which changes composition Igneous Rock Textures • Mineral appearance • Size most important – Cooling rate of magma or lava • Shape • Arrangement Aphanitic Texture • Rapid cooling • Mineral nuclei form faster than mineral growth • Fine-grained • Lava flows: extrusive Phaneritic Texture • Slow cooling • Magma underground • Mineral growth faster than nuclei formation • Coarse-grained • Plutons: intrusive Porphyritic Texture • • • • • Minerals of markedly different sizes Phenocrysts = large minerals Groundmass = small minerals Complex cooling history Porphyry Glassy Texture • Lava • Very rapid cooling • No ordered 3-D framework of minerals • Natural glass Vesicles • Magma can contain water vapor and other gases • Gasses trapped in cooling lava • Vesicular: many small holes from gases Pyroclastic Texture • Also called fragmental texture • Explosive volcanic activity • Consolidated ash from eruptions Phenocrysts Stepped Art Fig. 4-7, p. 73 Classifying Igneous Rocks • Texture – Aphanitic to Phaneritic • Composition – Ultramafic <45% silica – Mafic 45% to 52% silica – Intermediate 53%-65% silica – Felsic >65% silica Ultramafic Rocks • • • • • • <45% silica Mostly ferromagnesian silicates Darker minerals: dark rocks Peridotite: mostly olivine Pyroxenite: mostly pyroxene Komatiites: very old lava flows Basalt-Gabbro • Mafic magma: 45% to 52% silica • Basalt: aphanitic, lava flows • Gabbro: phaneritic, lower part oceanic crust • Large proportion ferromagnesian silicates • Dark color Andesite-Diorite • Intermediate magma: 53%-65% silica • Andesite: aphanitic, convergent plate boundary volcanoes • Diorite: phaneritic, in crust • Plagioclase feldspar with amphibole or biotite Rhyolite-Granite • Felsic magma: >65% silica • Rhyolite: aphanitic, uncommon, explosive eruptions • Granite: phaneritic, most common intrusive rock • Potassium feldspar, sodium-rich plagioclase, quartz Pegmatite • Texture, not composition • Typically granitic composition • Minerals at least 1 cm across Other Extrusive Igneous Rocks • • • • • Volcanoes erupt fragmental material Ash: <2 mm Tuff Rhyolite tuff Welded tuff Other Extrusive Igneous Rocks, cont. • Volcanic glass • Obsidian – Color varies – Conchoidal fracture • Pumice – Vesicular, floats • Scoria – Vesicular Plutons • Magma cools below the surface • Exposed at surface through uplift and erosion • Geometry – Tabular – Cylindrical – Irregular • Concordant: boundaries parallel to country rock • Discordant: boundaries cut across country rock Pluton Types • • • • • • Dikes Sills Laccoliths Volcanic pipes and necks Batholiths Stocks Dikes, Sills, Latholiths • Dike: – Discordant – Up to 100 m thick – Intrude into fractures • Sill: – Concordant – Often intrude sedimentary rocks • Laccolith: – Inflated sill, domed upward Volcanic Pipes and Necks • Volcano pipe: central conduit of volcano • Volcanic neck: – Hardened magma of volcanic pipe – Exposed through erosion Batholiths and Stocks • • • • • • Batholith: 100 km2 or larger Stock: smaller Mostly discordant Usually granitic Near convergent plate boundaries Mineral resources How Batholiths Intrude Crust • Forceful injection: – Rises slowly – Forces aside country rock – Some country rock fills in underneath • Stoping: – Rising magma detaches and engulfs country rock