Igneous Rocks

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What are Igneous Rocks?

from the Latin word for “fire” - ignis

Thus, rocks that are “fire-formed”

Molten rock (magma) cools to form a solid rock

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What is magma?

Magma is molten rock, and contains:

Melt

Liquid, composed of mobile ions

Solids

Silicate minerals that have already crystallized from

the melt

Volatiles

gases dissolved in the melt,

including water vapor (H2O),

carbon dioxide (CO2), and sulfur

dioxide (SO2)

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Identifying Igneous Rocks

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How to classify igneous rocks

1. Composition

2. Texture

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Igneous Rocks

Rocks formed on the surface

Volcanic (extrusive) rocks

Rocks formed inside Earth

Plutonic (intrusive) rocks

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Igneous Rocks (two types)

Extrusive

Lava and Pyroclastic Debris

Extruded at surface or at very shallow levels.

Granite is Intrusive

Basalt is Extrusive

Intrusive

Magma crystallized slowly within the crust.No exposure to the cool atmosphere.

Plutonic– intrusive igneous rock at great depth within crust or mantle.

Magma Compositions

Dark iron and/or magnesium

Olivine

Pyroxene

Amphibole

Biotite mica

Light potassium, sodium, & calcium

Quartz

Muscovite mica

Feldspars

Magma is composed mainly of silicate minerals

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Bowen’s Reaction Series

Magma crystallizes over a temperature range of several hundred degrees

Minerals crystallize in a predictable order

Last minerals to crystallize are very different in composition from the earlier formed minerals

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Bowen’s Reaction Series

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Bowen’s Reaction Series Animation

Bowen’s Reaction Series

Mg

Fe

Mg

Fe

Mg

Fe

Mg

Fe

Mg

Fe MgFe

Mg

Fe

Ca

K

Na

Ca

K

Na

Ca

K

Na

Ca

K

Na

Ca

K

Na

Na

SiSi

Si

Si

Si

Si

Si

Si

Si

AlAl

Al

AlAl

Al

Al

Al

Olivine

Fe Mg

Pyroxene

Amphibole

Plagioclase

SiBiotite

K

AlCa

Si

Muscovite

Orthoclase

Quartz12 / 51

Partial MeltingSilica-rich compounds melt before other compounds.

Magma DifferentiationFour types:•Magma Mixing•Crystal Settling•Magma Assimilation•Magma Migration

Changing a Magma’s Composition

Crystal settling

Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the

magma chamber

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Changing a Magma’s Composition

Assimilation

Changing a magma’s composition by the

incorporation of foreign matter

(surrounding rock bodies) into a magma

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Changing a Magma’s Composition

Magma mixing

Involves two bodies of magma intruding one another

Two chemically distinct magmas may produce a composition quite different from either original

magma

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Texture is estimated using visual grain size(depends on crystallization history)

Composition is estimated using visual colorCan be misleading

Mafic minerals crystallize early and felsic minerals crystallize late in magma

Minerals at the top of Bowen’s Reaction Series:

• Dark in color• Mafic to ultramafic • Iron and magnesium

Minerals at the bottom of Bowen’s Reaction Series:

• Light in color• Felsic• Sodium plagioclase, potassium

plagioclase, and quartz are light.

Igneous rocks are named on the basis of their texture and composition

Igneous Textures

The size, shape, and arrangement of the minerals. Glassy – Made of solid glass or glass shards. Interlocking crystals – Minerals that fit like jigsaw

pieces. Fragmental – Pieces of pre-existing rocks.

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Igneous Textures

Aphanitic

Phaneritic

Porphyritic

Glassy

Pyroclastic

Pegmatitic

• Rapid rate of cooling of lava or magma

• Microscopic crystals• Typically occurs in extrusive /

volcanic rocks

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Basalt: Aphanitic Texture

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Igneous Textures

AphaniticPhaneritic

Porphyritic

Glassy

Pyroclastic

Pegmatitic

• Slow cooling• Crystals can be identified

without a microscope• Typically occurs in intrusive /

plutonic rocks

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Granite: Phaneritic Texture

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Thin section showing APHANITIC TEXTURE

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Thin section showing PHANERITIC TEXTURE

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Igneous Textures

Aphanitic

PhaneriticPorphyritic

Glassy

Pyroclastic

Pegmatitic

• Minerals form at different temperatures & rates

• Large crystals (phenocrysts) are embedded in a matrix of smaller crystals

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Granodiorite: Porphyritic Texture

Potassium Feldspar Phenocryst

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Igneous Textures

Aphanitic

Phaneritic

PorphyriticGlassy

Pyroclastic

Pegmatitic

• Very rapid cooling of molten rock

• Resulting rock is called obsidian

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Obsidian: Glassy Texture

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Igneous Textures

Aphanitic

Phaneritic

Porphyritic

GlassyPyroclastic

Pegmatitic

• Various fragments ejected during a violent volcanic eruption

• Textures often appear similar to sedimentary rocks

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Igneous Textures

Aphanitic

Phaneritic

Porphyritic

Glassy

PyroclasticPegmatitic

• Exceptionally coarse grained• Form in late stages of

crystallization of granitic magmas

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Quartz Vein: Pegmatitic Texture

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Crystal Shapes

• Euhedral - grains completely bounded by crystal faces

• Subhedral - grains only partly bounded by crystal faces

• Anhedral - grains completely devoid of crystal boundaries

Source: 1, 2

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Igneous Textures

Arrangement of the grains

Equigranular inequigranularSlightly inequigranular

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Igneous Textures

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Magma Compositions

Igneous rocks are often classified by mineral (chemical) composition

Ultramafic

Mafic (or basaltic)

Intermediate (or andesitic)

Felsic (or granitic)

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Ultramafic Compositions

Magma Composition:Rare

high in magnesium and iron

composed entirely of ferromagnesian silicates

Fe

Mg

Fe

MgFe

Mg

Mg

FeOlivine

Pyroxene

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Magma Composition: Mafic: magnesium and ferrum (iron)

Contains substantial dark silicate minerals and calcium-rich plagioclase feldspar

More dense than granitic rocks

Mafic (or basaltic) Compositions

Fe

Mg

Fe

MgFe

Mg

Mg

Fe Olivine

PyroxeneCa

Plagioclase

Ca

Ca

Na

Na

Na

Al

Al

Si

Si

Si

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Mafic (or basaltic) Compositions

Extrusive / Volcanic Rock:

Basalt

Aphanitic texture Composed mainly of pyroxene

and calcium-rich plagioclase Most common extrusive

igneous rock

Intrusive / Plutonic Rock:

Gabbro

Phaneritic texture Composed of pyroxene and

calcium-rich plagioclase Makes up a significant portion

of the oceanic crust

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Intermediate (or andesitic) Compositions

Magma Composition

Contains at least 25% dark silicate minerals

Associated with explosive volcanic activity

Fe

Mg

Fe

Mg

Mg

Fe

Ca

Ca

Ca

Na

Na

Na

Al

Al

Si

Si

Si

Amphibole

Plagioclase

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Intermediate (or andesitic) Compositions

Extrusive / Volcanic Rock:

Andesite

Aphanitic texture Dark grey color

May have plagioclase phenocrysts

Intrusive / Plutonic Rock:

Diorite

Phaneritic texture Composed of intermediate

feldspar and hornblende “Black and white makes

DIORITE”

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Felsic (or granitic) Compositions

Magma Composition

Felsic: feldspar and silica

Composed almost entirely of light-colored silicates

Fe

Mg

Fe

Mg

Mg

Fe

Ca

Ca

Ca

Na

Na

Na Al

SiSi

Amphibole

Plagioclase

Biotite

Muscovite

Orthoclase

Quartz

K

K

K

Al

Si

Al

K AlSi

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Felsic (or granitic) Compositions

Extrusive / Volcanic Rock:

Rhyolite

Aphanitic texture Buff or pink color

May have glass phenocrysts

Intrusive / Plutonic Rock:

Granite

Phaneritic texture Over 25 % quartz, about 65 %

or more feldspar The term “granite” covers a

wide range of mineral compositions

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Varied Compositions

Obsidian

Volcanic Dark colored Glassy texture

Pumice

Volcanic Glassy texture

Frothy appearance with numerous voids

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Varied Compositions

Pyroclastic rocks

Composed of fragments ejected during a volcanic

eruption

Tuff – ash-sized fragments

Volcanic breccia – particles larger than ash

lvo.wr.usgs.gov/gallery/BishopTuff_1.html

www.gc.maricopa.edu 48 / 51

Igneous Rock Classification

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All rocks on Earth have evolved from the first igneous rocks

All Igneous Rocks Result from Magma Differentiation.

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