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Igneous Rocks and Intrusive Igneous Activity
GEOL: CHAPTER 4 Igneous Rocks and Intrusive Igneous Activity
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View of the Sierra Nevada taken west of Lone Pine, California
View of the Sierra Nevada taken west of Lone Pine, California. The rocks in this view are part of the Sierra Nevada batholith— a huge mass of granite and related rocks made up of intrusive bodies. The high peak toward the right is Mount Whitney, which at 4,421 m is the highest peak in the continental United States.
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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
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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
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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
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Composition of Magma Silicate rocks usually the source
Silica is primary constituent Other constituents: Aluminum Calcium Sodium Iron Magnesium Potassium
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Three Types of Magma Felsic magma Mafic magma Intermediate 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
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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
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Figure 4.1 How Hot Is Lava? A geologist uses a remotely operated, handheld device for measuring temperature of a lava flow visible through an opening in the rocks in Hawaiian Volcanoes National Park, Hawaii.
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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
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Figure 4.2 Viscosity of Magma and Lava Temperature is an important control on viscosity, but so is composition. Mafic lava tends to be fluid, whereas felsic lava is much more viscous.
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Figure 4.2 Viscosity of Magma and Lava Temperature is an important control on viscosity, but so is composition. Mafic lava tends to be fluid, whereas felsic lava is much more viscous.
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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
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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
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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
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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
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Figure 4.3 Bowen’s Reaction Series Bowen’s reaction series consists of a discontinuous branch, along which a succession of ferromagnesian silicates crystallize as the magma’s temperature decreases, and a continuous branch, along which plagioclase feldspars with increasing amounts of sodium crystallize. Notice also that the composition of the initial mafic magma changes as crystallization takes place along the two branches.
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Continuous reaction series
Decreasing temperature Olivine Calcium-rich plagioclase Types of magma Continuous reaction series Sodium-rich plagioclase Continuous branch Plagioclase feldspars Reaction Pyroxene (augite) Mafic (45–52% silica) Discontinuous branch Reaction Amphibole (hornblende) Intermediate (53–65% silica) Reaction Biotite mica Figure 4.3 Bowen’s Reaction Series Bowen’s reaction series consists of a discontinuous branch, along which a succession of ferromagnesian silicates crystallize as the magma’s temperature decreases, and a continuous branch, along which plagioclase feldspars with increasing amounts of sodium crystallize. Notice also that the composition of the initial mafic magma changes as crystallization takes place along the two branches. Potassium feldspar Felsic (>65% silica) Muscovite mica Quartz Stepped Art Fig. 4-3, p. 69
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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
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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
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Figure 4.4 The Origin of Magma Magma forms beneath spreading ridges, because as plates separate, pressure is reduced on the hot rocks and partial melting of the upper mantle begins. Invariably, the magma formed is mafic. Magma also forms at subduction zones, where water from the subducted plate causes partial melting of the upper mantle. This magma is also mafic, but as it rises, melting of the lower crust makes it more felsic.
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Hot-Spot Magma Interior portions of plates
Mantle plumes: rising magma from the core-mantle boundary Creates volcanoes Hawaiian Islands
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Figure 4.5 Mantle Plume and Hot Spot A mantle plume beneath oceanic crust with a hot spot. Rising magma forms a series of volcanoes that become younger in the direction of plate movement.
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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
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Figure 4.6 Crystal Settling and Assimilation
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Changing Magma Composition: Assimilation
Magma reacts with country rock Country rock melts and changes composition of magma Inclusions of incompletely melted country rock
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Figure 4.6 Crystal Settling and Assimilation
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Changing Magma Composition: Magma Mixing
A volcano can erupt lavas of different composition Some of these magmas mix, which changes composition
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Igneous Rock Textures Mineral appearance Size most important Shape
Cooling rate of magma or lava Shape Arrangement
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Aphanitic Texture Rapid cooling
Mineral nuclei form faster than mineral growth Fine-grained Lava flows: extrusive
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Phaneritic Texture Slow cooling Magma underground
Mineral growth faster than nuclei formation Coarse-grained Plutons: intrusive
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Porphyritic Texture Minerals of markedly different sizes
Phenocrysts = large minerals Groundmass = small minerals Complex cooling history Porphyry
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Glassy Texture Lava Very rapid cooling
No ordered 3-D framework of minerals Natural glass
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Vesicles Magma can contain water vapor and other gases
Gasses trapped in cooling lava Vesicular: many small holes from gases
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Pyroclastic Texture Also called fragmental texture
Explosive volcanic activity Consolidated ash from eruptions
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Figure 4. 7 Textures of Igneous Rocks a
Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm.
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Phenocrysts Figure 4.7 Textures of Igneous Rocks a. Rapid cooling as in lava flows results in many small minerals and an aphanitic (fine-grained) texture.b. Slower cooling in plutons yields a phaneritic texture.c. These porphyritic textures indicate a complex cooling history.d. Obsidian has a glassy texture because magma cooled too quickly for mineral crystals to form.e. Gases expand in lava to yield a vesicular texture.f. Microscopic view of a rock with a fragmental texture. The colorless, angular particles of volcanic glass measure up to 2 mm. Stepped Art Fig. 4-7, p. 73
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Classifying Igneous Rocks
Texture Aphanitic to Phaneritic Composition Ultramafic <45% silica Mafic 45% to 52% silica Intermediate 53%-65% silica Felsic >65% silica
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Figure 4.8 Classification of Igneous Rocks This diagram shows the percentages of minerals, as well as the textures of common igneous rocks. For example, an aphanitic (fine-grained) rock of mostly calcium-rich plagioclase and pyroxene is basalt.
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Ultramafic Rocks <45% silica Mostly ferromagnesian silicates
Darker minerals: dark rocks Peridotite: mostly olivine Pyroxenite: mostly pyroxene Komatiites: very old lava flows
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Figure 4.9 Peridotite This specimen of the ultramafic rock peridotite is made up mostly of olivine. Notice in Figure 4.8 that peridotite is the only phaneritic rock that does not have an aphanitic counterpart. Peridotite is rare at Earth’s surface, but is very likely the rock that makes up the mantle.
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Basalt-Gabbro Mafic magma: 45% to 52% silica
Basalt: aphanitic, lava flows Gabbro: phaneritic, lower part oceanic crust Large proportion ferromagnesian silicates Dark color
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Figure 4.10 Mafic Igneous Rocks
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Figure 4.10 Mafic Igneous Rocks
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Andesite-Diorite Intermediate magma: 53%-65% silica
Andesite: aphanitic, convergent plate boundary volcanoes Diorite: phaneritic, in crust Plagioclase feldspar with amphibole or biotite
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Figure 4.11 Intermediate Igneous Rocks
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Figure 4.11 Intermediate Igneous Rocks
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Rhyolite-Granite Felsic magma: >65% silica
Rhyolite: aphanitic, uncommon, explosive eruptions Granite: phaneritic, most common intrusive rock Potassium feldspar, sodium-rich plagioclase, quartz
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Figure 4.12 Felsic Igneous RocksThese rocks are typically light colored, because they contain mostly nonferromagnesian silicate minerals. The dark spots in the granite specimen are biotite mica. The white and pinkish minerals are feldspars, whereas the glassy-appearing minerals are quartz.
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Figure 4.12 Felsic Igneous RocksThese rocks are typically light colored, because they contain mostly nonferromagnesian silicate minerals. The dark spots in the granite specimen are biotite mica. The white and pinkish minerals are feldspars, whereas the glassy-appearing minerals are quartz.
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Pegmatite Texture, not composition Typically granitic composition
Minerals at least 1 cm across
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Figure 4.13 Pegmatite
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Other Extrusive Igneous Rocks
Volcanoes erupt fragmental material Ash: <2 mm Tuff Rhyolite tuff Welded tuff
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Other Extrusive Igneous Rocks, cont.
Volcanic glass Obsidian Color varies Conchoidal fracture Pumice Vesicular, floats Scoria Vesicular
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Figure 4.14 Igneous Rocks Classified Primarily by Their Texture
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Figure 4.14 Igneous Rocks Classified Primarily by Their Texture
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Figure 4.14 Igneous Rocks Classified Primarily by Their Texture
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Figure 4.14 Igneous Rocks Classified Primarily by Their Texture
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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
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Pluton Types Dikes Sills Laccoliths Volcanic pipes and necks
Batholiths Stocks
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Figure 4.15 Plutons
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Dikes, Sills, Latholiths
Discordant Up to 100 m thick Intrude into fractures Sill: Concordant Often intrude sedimentary rocks Laccolith: Inflated sill, domed upward
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Figure 4.15 Plutons
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Volcanic Pipes and Necks
Volcano pipe: central conduit of volcano Volcanic neck: Hardened magma of volcanic pipe Exposed through erosion
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Figure 4.15 Plutons
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Batholiths and Stocks Batholith: 100 km2 or larger Stock: smaller
Mostly discordant Usually granitic Near convergent plate boundaries Mineral resources
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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
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Figure 4.16 Emplacement of a Batholith by Forceful Injection and Stoping
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Figure 4.16 Emplacement of a Batholith by Forceful Injection and Stoping
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