GEOL: CHAPTER 4

Igneous Rocks and Intrusive Igneous Activity

• 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

Learning Outcomes

• 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

Igneous Rocks

• 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

Magma and Lava

• Silicate rocks usually the source• Silica is primary constituent• Other constituents:

– Aluminum– Calcium– Sodium– Iron– Magnesium– Potassium

Composition 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

Three Types of Magma

• 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

Magma/Lava Temperatures

• 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

Viscosity

• 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

Origination of Magma

• 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

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

Bowen’s Reaction Series, cont.

Fig. 4-3, p. 69

Stepped Art

Reaction

Pyroxene(augite)

Reaction

Biotitemica

Reaction

Amphibole(hornblende)

Potassiumfeldspar

Muscovitemica

Quartz

Types ofmagma

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Sodium-richplagioclase

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Calcium-richplagioclaseOlivine

Intermediate(53–65% silica)

Felsic(>65% silica)

• 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 Spreading Ridges

• 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

Magma at Subduction Zones

• Interior portions of plates

• Mantle plumes: rising magma from the core-mantle boundary

• Creates volcanoes– Hawaiian Islands

Hot-Spot Magma

• 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: Crystal Settling

• Magma reacts with country rock

• Country rock melts and changes composition of magma

• Inclusions of incompletely melted country rock

Changing Magma Composition: Assimilation

• A volcano can erupt lavas of different composition

• Some of these magmas mix, which changes composition

Changing Magma Composition: Magma Mixing

• Mineral appearance

• Size most important– Cooling rate of magma or lava

• Shape

• Arrangement

Igneous Rock Textures

• Rapid cooling

• Mineral nuclei form faster than mineral growth

• Fine-grained

• Lava flows: extrusive

Aphanitic Texture

• Slow cooling

• Magma underground

• Mineral growth faster than nuclei formation

• Coarse-grained

• Plutons: intrusive

Phaneritic Texture

• Minerals of markedly different sizes

• Phenocrysts = large minerals

• Groundmass = small minerals

• Complex cooling history

• Porphyry

Porphyritic Texture

• Lava

• Very rapid cooling

• No ordered 3-D framework of minerals

• Natural glass

Glassy Texture

• Magma can contain water vapor and other gases

• Gasses trapped in cooling lava

• Vesicular: many small holes from gases

Vesicles

• Also called fragmental texture

• Explosive volcanic activity

• Consolidated ash from eruptions

Pyroclastic Texture

Fig. 4-7, p. 73

Stepped Art

Phenocrysts

• Texture– Aphanitic to Phaneritic

• Composition– Ultramafic <45% silica– Mafic 45% to 52% silica– Intermediate 53%-65% silica– Felsic >65% silica

Classifying Igneous Rocks

• <45% silica

• Mostly ferromagnesian silicates

• Darker minerals: dark rocks

• Peridotite: mostly olivine

• Pyroxenite: mostly pyroxene

• Komatiites: very old lava flows

Ultramafic Rocks

• Mafic magma: 45% to 52% silica

• Basalt: aphanitic, lava flows

• Gabbro: phaneritic, lower part oceanic crust

• Large proportion ferromagnesian silicates

• Dark color

Basalt-Gabbro

• Intermediate magma: 53%-65% silica

• Andesite: aphanitic, convergent plate boundary volcanoes

• Diorite: phaneritic, in crust

• Plagioclase feldspar with amphibole or biotite

Andesite-Diorite

• Felsic magma: >65% silica

• Rhyolite: aphanitic, uncommon, explosive eruptions

• Granite: phaneritic, most common intrusive rock

• Potassium feldspar, sodium-rich plagioclase, quartz

Rhyolite-Granite

• Texture, not composition

• Typically granitic composition

• Minerals at least 1 cm across

Pegmatite

• Volcanoes erupt fragmental material

• Ash: <2 mm

• Tuff

• Rhyolite tuff

• Welded tuff

Other Extrusive Igneous Rocks

• Volcanic glass• Obsidian

– Color varies– Conchoidal fracture

• Pumice– Vesicular, floats

• Scoria– Vesicular

Other Extrusive Igneous Rocks, cont.

• 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

Plutons

• Dikes

• Sills

• Laccoliths

• Volcanic pipes and necks

• Batholiths

• Stocks

Pluton Types

• Dike:– Discordant– Up to 100 m thick– Intrude into fractures

• Sill:– Concordant– Often intrude sedimentary rocks

• Laccolith:– Inflated sill, domed upward

Dikes, Sills, Latholiths

• Volcano pipe: central conduit of volcano

• Volcanic neck: – Hardened magma of volcanic pipe– Exposed through erosion

Volcanic Pipes and Necks

• Batholith: 100 km2 or larger

• Stock: smaller

• Mostly discordant

• Usually granitic

• Near convergent plate boundaries

• Mineral resources

Batholiths and Stocks

• Forceful injection:– Rises slowly– Forces aside country rock– Some country rock fills in underneath

• Stoping: – Rising magma detaches and engulfs

country rock

How Batholiths Intrude Crust