Igneous Rocks

Sawtooth Mtns. near Stanley, ID-example of a intrusive igneous rocks….not to mentiona very cool place to do fieldwork……

Lava flow in Hawaii[magma @ the Earth’ssurface]

Pyroclastic FlowMontserrat Volcano

What is a rock? [Yes, you really do care.]

The Nature of Igneous Rocks• Form from Magma [Greek=“paste”]

– Hot, partially molten mixture of solid liquid and gas

– Gases: H2O, CO2, etc.

– less dense than

solid rock

– solidify upon

cooling

“The beer I had for breakfast wasn’t badso I had one more for desert.”-Sunday Morning Coming Downby Kris Kristofferson

• Magma vs. Lava– Magma: molten rock beneath the surface

– Lava: molten rock that has reached the surface

– Magma: form intrusive igneous rocks

– Lava: form extrusive igneous rocks

• Composition varies widely– Oxygen plus major elements

– Generally a silica (SiO2) melt

– Silica and water content control viscosity

– Silica content used in classification

Mafic Magmas

• Silica content ~ 50%

• High Fe, Mg and Ca

• High temperature molten magma– 1000o to 1200oC

• Major minerals:– Olivine - Ca Plagioclase

– Pyroxene

Silicic Magma

• Silica content: 65-77%

• High Al, Na and K

• Lower temperature magmas– Less than 850oC

• Major minerals:– Feldspars - Micas

– Quartz

Magma Viscosity-resistance to flow

• Controlled by silica and water content, and temperature

• As magma cools-silica tetrahedron form links

• Linkages control

viscosity:•High Silica=high viscosity•Low Silica=low viscosity

Magma Viscosity• High Silica=high viscosity/Low Silica=low viscosity

• Cooler Temperatures=higher viscosity/Higher Temperatures=lower viscosity

• Water and volatiles break linkages=lower viscosity

Occurrence of Igneous Rocks

• Found globally

• Formed in discrete geologic settings– Convergent plate margins

– Divergent plate margins

– Mantle plumes

Igneous Textures• Texture - the size, shape and relationship of minerals in the

rock

• Cooling history of the magma or lava

• Crystal size increases as rate of cooling slows

• Texture - the size, shape, and arrangement of interlocking minerals

• Factors affecting crystal size

• Rate of coolingSlow rate = fewer but larger crystalsFast rate = many small crystalsVery fast rate forms glass

Types of igneous textures Aphanitic (fine-grained) texture

Rapid rate of cooling Microscopic crystals May contain vesicles (holes from gas bubbles)

Phaneritic (coarse-grained) texture Slow cooling Large, visible crystals Kinds of igneous textures

Porphyritic texture Glassy texture Pyroclastic texture Pegmatitic texture

Exceptionally coarse grained Form in late stages of crystallization of granitic magmas

Glassy Texture• Very rapid cooling - quenched

– Volcanic glass

– Conchoidal fracture

– Rock is called obsidian

• No apparent crystalsLava flow quenchingin ocean water.

Glassy texture in obsidian

Crystalline Textures

• Crystal growth requires time for ions to migrate - form minerals

• Slow rate of cooling=time for crystal growth

• Crystals grow until melt is quenched or completely solidified

Aphanitic Texture• Fine grained texture

• Few crystals visible in hand specimen

• Relatively rapid rate of cooling

Aphanitic texture in rhyolite

Phaneritic Texture

• Coarse grained texture

• Relatively slow rate of cooling

• Equigranular, interlocking crystals

• Slow cooling = crystallization at depth

Phaneritic texture in granite

Pegmatites from the Sawtooth Mtns.-unusually largecrystals

Porphyritic Texture

Minerals form at different temperatures Large crystals (phenocrysts) are embedded in a matrix

of smaller crystals (groundmass)

• Well formed crystals (phenocrysts)

• Fine grained matrix (groundmass)

• Complex cooling history– Initial stage of slow cooling

– Later stage of rapid cooling

Pyroclastic Texture

• Fragmental appearance produced by violent volcanic eruptions

• Explosive volcanic eruptions

• Appear porphyritic with visible crystals– Crystals show breakage or distortion

• Matrix dominated by glassy fragments– Hot fragments may “weld” together

Granitic versus basaltic compositions Granitic [felsic] composition

Light-colored silicates Termed felsic (feldspar and silica) in composition High silica (SiO2) content Major constituent of continental crust

Basaltic [mafic] composition Dark silicates and Ca-rich feldspar Termed mafic (magnesium and ferrum, for iron) in

composition Higher density than granitic rocks Comprise the ocean floor and many volcanic islands

Other compositional groups Intermediate (or andesitic) composition

Contain 25% or more dark silicate minerals Associated with explosive volcanic activity

Ultramafic composition Rare composition that is high in magnesium and iron Composed entirely of ferromagnesian silicates

Silica content as an indicator of composition Crustal rocks exhibit a considerable range

45% to 70%

Silica content influences magma behavior Granitic magmas = high silica content and

viscous Basaltic magmas = much lower silica content

and more fluid-like behavior

Igneous rocks form as molten rock cools and solidifies General characteristics of magma

Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava Rocks formed from lava = extrusive, or volcanic rocks Rocks formed from magma at depth = intrusive, or plutonic

rocks The nature of magma

Consists of three components: Liquid portion = melt Solids, if any, are silicate minerals Volatiles = dissolved gases in the melt, including water vapor (H2O),

carbon dioxide (CO2), and sulfur dioxide (SO2)

Classification of Igneous Rocks

– Texture• Aphanitic

• Phanaritic

– Composition• Silicic

• Intermediate

• Mafic

• Ultramafic

Combination of Texture and Compositionproduces rock name

Origin of MagmasGenerating magma from solid rock

• Role of heat Temperature increases in the upper crust (geothermal gradient) average between 20oC to

30oC per kilometer Rocks in the lower crust and upper mantle are near their melting points Additional heat may induce melting

• Role of pressure Increases in confining pressure increases a rock’s melting temperature When confining pressures drop, decompression melting occurs

• Role of volatiles Volatiles (primarily water) cause melting at lower temperatures Important factor where oceanic lithosphere descends into the mantle

• Solid rock is at equilibrium with its surrounding• Changes in the surroundings cause solid rock magma [melting]• Lowering P

Mantle convection moves deep mantle rocks upwards• Raising T

Hot mafic magma intrudes into the crust• Changing composition

Adding small amounts of water

How do rocks melt?

Heat: Geothermal Gradient

Temperature of mostSilicic magmas [600-800oC]-about the base of crust

Temperature of MaficMagmas [1100-1200oC]-upper mantle conditions

A single volcano may extrude lavas exhibiting very different compositions

Bowen’s reaction series Minerals crystallize in a systematic fashion

based on their melting points During crystallization, the composition of the

liquid portion of the magma continually changes

Processes responsible for changing a magma’s composition

Magmatic differentiation Separation of a melt from earlier formed crystals

Assimilation Changing a magma’s composition by incorporating

surrounding rock bodies into a magma

Magma mixing Two chemically distinct magmas may produce a

composition quite different from either original magma

Magma Differentiation

• Magmas, and the resulting igneous rocks, show a wide range of compositions

• Source Rock – variations cause major and minor

variations in the magma

• Magma Mixing

• Assimilation

• Partial Melting– Individual minerals in source rock melt at

different T

– Magma is enriched in many elements, especially SiO2

How do we know this?Norman Bowen [1887-1956]-Experimental Petrologist-developed Bowen’s Reaction Series

Bowen’s reaction series

• Fractional Crystallization– Individual minerals precipitate from

magma at different T

– Magma is enriched in many elements, especially SiO2

– Magma may migrate [buoyancy]

Bowen’s reaction series

Plate Tectonic Setting of Igneous Rocks• Divergent Plate Boundaries

– Partial melting of mantle produces basaltic magma

• Convergent Plate Boundaries

– Subduction produces partial melting of basalt, sediments, parts of mantle

– Andesitic and rhyolitic magma

– Ascending magma assimilates lower crustal material

• Mantle Plumes

– Partial melting of plumes of mantle material

– Basaltic magma is produced

– Rising magma produce

• Intraplate island chains

• Flood basalt [Columbia River Basalts]