Igneous Rocks/EPCC/LM4

Igneous Rocks Igneous Rocks

Chapter 4Chapter 4

Just a quick review:Just a quick review:

What is a rock?What is a rock?

A rock (is most often)is hard and A rock (is most often)is hard and made up of minerals.made up of minerals.

Why most often?Why most often?

What is coal?What is coal?

What is obsidian?What is obsidian?

Rock Gypsum?Rock Gypsum? The are criteria has exceptions!The are criteria has exceptions!

Types of RocksTypes of Rocks

IgneousIgneousSedimentarySedimentaryMetamorphicMetamorphic

The Rock CycleThe Rock Cycle



Chapter 4Chapter 4

Igneous Rocks: Product of Igneous Rocks: Product of Earth’s Internal FireEarth’s Internal Fire

Igneous Rocks

What Is an Igneous Rock? What Is an Igneous Rock?

Igneous rocks vary greatly.Igneous rocks vary greatly.– Some contain large mineral grains.Some contain large mineral grains.

– Others contain grains so small they can barely be Others contain grains so small they can barely be seen under a high power microscope.seen under a high power microscope.

– Igneous rocks also vary greatly in color.Igneous rocks also vary greatly in color.

All igneous rocks are formed through the All igneous rocks are formed through the cooling and solidification of magma/lava.cooling and solidification of magma/lava.

Mode of FormationMode of Formation

Intrusive versus ExtrusiveIntrusive versus Extrusive

Intrusive Versus Extrusive Igneous Intrusive Versus Extrusive Igneous RocksRocks

Intrusive igneous rocks form when magma Intrusive igneous rocks form when magma cools within existing rocks in Earth’s cools within existing rocks in Earth’s crust.crust.

Intrusive Versus Extrusive Igneous Intrusive Versus Extrusive Igneous RocksRocks

Extrusive igneous rocks form when magma Extrusive igneous rocks form when magma cools on Earth’s surface, where they have cools on Earth’s surface, where they have been “extruded.”been “extruded.”

TextureTexture

Intrusive versus ExtrusiveIntrusive versus Extrusive

Textures in Igneous Rocks Textures in Igneous Rocks

Texture – size, shape and orientation of theTexture – size, shape and orientation of the

crystals within a rockcrystals within a rockSizes of mineral grains:Sizes of mineral grains:

– Intrusive rocks are coarse-grained. (Phaneritic)Intrusive rocks are coarse-grained. (Phaneritic) Magma that solidifies in the crust cools slowly and has Magma that solidifies in the crust cools slowly and has

sufficient time to form large mineral grains.sufficient time to form large mineral grains.

Coarse GrainedCoarse Grained

Textures in Igneous RocksTextures in Igneous Rocks

– Extrusive rocks are fine-grained.Extrusive rocks are fine-grained.Lava that solidifies on the surface usually Lava that solidifies on the surface usually

cools rapidly, allowing insufficient time cools rapidly, allowing insufficient time for large crystals to grow.for large crystals to grow.

– Fine-grained igneous rock is called an Fine-grained igneous rock is called an aphaniticaphanitic

Fine GrainedFine Grained


– A A porphyryporphyry is an igneous rock in which 50% or more is an igneous rock in which 50% or more of the rock is coarse mineral grains scattered through a of the rock is coarse mineral grains scattered through a mixture of fine mineral grains.mixture of fine mineral grains.

– Porphyritic texturePorphyritic texture

– The isolated large grains are The isolated large grains are phenocrystsphenocrysts..

PorphyriticPorphyritic

Phenocrysts


Glassy rocks.Glassy rocks.– Atoms lack time to organize themselves into Atoms lack time to organize themselves into

minerals.minerals.– Extrusive igneous rocks that are largely or wholly Extrusive igneous rocks that are largely or wholly

glassy are called obsidianglassy are called obsidian..They display a distinctive conchoidal fracture (smooth, They display a distinctive conchoidal fracture (smooth,

curved surface).curved surface).

Figure 4.5

Textures in Igneous RocksTextures in Igneous Rocks

– Another common variety of igneous rock is pumice, Another common variety of igneous rock is pumice, a mass of glassy bubbles of volcanic origin.a mass of glassy bubbles of volcanic origin.

– Texture is vesicular – the bubbles have “popped” Texture is vesicular – the bubbles have “popped” and left a vesicleand left a vesicle

– Volcanic ash is also mostly glassy because the Volcanic ash is also mostly glassy because the fragments of magma cooled too quickly to fragments of magma cooled too quickly to crystallizecrystallize..

Vesicular TextureVesicular Texture

Pumice

Basalt

TexturesTextures

Intrusive Igneous RocksIntrusive Igneous Rocks

1) Coarse grained or phaneritic1) Coarse grained or phaneritic Extrusive Igneous RocksExtrusive Igneous Rocks

1) Fine grained or aphanitic1) Fine grained or aphanitic

2) Porphyritic2) Porphyritic

3) Glassy3) Glassy

4) Vesicular4) Vesicular

Solidification of MagmaSolidification of Magma

Bowen’s Reaction SeriesBowen’s Reaction Series

Solidification of MagmaSolidification of Magma

A magma of a given composition can A magma of a given composition can crystallize into many different kinds of crystallize into many different kinds of igneous rock.igneous rock.

Solidifying magma forms several different Solidifying magma forms several different minerals which start to crystallize from the minerals which start to crystallize from the cooling magma at different temperaturescooling magma at different temperatures..

Solidification of MagmaSolidification of Magma Crystal-melt separation can occur in a number Crystal-melt separation can occur in a number

of ways:of ways:– Compression can squeeze melt out of a crystal-melt Compression can squeeze melt out of a crystal-melt

mixture. mixture. – Dense, early crystallized minerals may sink to the Dense, early crystallized minerals may sink to the

bottom of a magma chamber, thereby forming a solid bottom of a magma chamber, thereby forming a solid mineral layer covered by melt.mineral layer covered by melt.

However a separation occurs, the compositional However a separation occurs, the compositional changes it causes are called changes it causes are called magmatic magmatic differentiation by fractional crystallizationdifferentiation by fractional crystallization..


Canadian-born scientist N. L. Bowen Canadian-born scientist N. L. Bowen (1887-1956) first recognized the (1887-1956) first recognized the importance of importance of magmatic differentiation by magmatic differentiation by fractional crystallization.fractional crystallization.

Bowen argued that a single magma could Bowen argued that a single magma could crystallize into both basalt and rhyolite crystallize into both basalt and rhyolite because of fractional crystallizationbecause of fractional crystallization..

Bowen’s Reaction Series Bowen’s Reaction Series

Bowen knew that plagioclases that Bowen knew that plagioclases that crystallize from basaltic magma are usually crystallize from basaltic magma are usually calcium-rich (anorthitic).calcium-rich (anorthitic).

Plagioclases formed from rhyolitic magma Plagioclases formed from rhyolitic magma are commonly sodium-rich (albitic).are commonly sodium-rich (albitic).

Bowen called such a continuous reaction Bowen called such a continuous reaction between crystals and melts a continuous between crystals and melts a continuous reaction series.reaction series.

Bowen’s Reaction Series Bowen’s Reaction Series Bowen identified several sequences of Bowen identified several sequences of

reactions besides the continuous reaction reactions besides the continuous reaction series of the feldspars.series of the feldspars.

When basalt cools down, one of the earliest When basalt cools down, one of the earliest minerals to form is olivine.minerals to form is olivine.– Olivine contains about 40 percent SiOOlivine contains about 40 percent SiO22 by weight. by weight.– Basaltic magma contains 50 percent SiOBasaltic magma contains 50 percent SiO22..

Crystallization of olivine will leave the Crystallization of olivine will leave the residual liquid a little richer in silica.residual liquid a little richer in silica.


The solid olivine reacts with silica in the melt The solid olivine reacts with silica in the melt to form a more silica-rich mineral, pyroxene.to form a more silica-rich mineral, pyroxene.

The pyroxene in turn can react to form The pyroxene in turn can react to form amphibole.amphibole.

Amphibole can react to form biotite.Amphibole can react to form biotite. Such a series of reactions is called a Such a series of reactions is called a

discontinuous reaction seriesdiscontinuous reaction series..

Bowens Reaction SeriesBowens Reaction Series

Bowens Reaction SeriesBowens Reaction Series

Mineral AssemblagesMineral Assemblages

What it tells usWhat it tells us

Mineral Assemblage In Igneous Mineral Assemblage In Igneous RocksRocks

Once the texture of an igneous rock is Once the texture of an igneous rock is determined, its name will depend on its determined, its name will depend on its mineral assemblage. All common igneous mineral assemblage. All common igneous rocks consist largely of:rocks consist largely of:– Quartz. Quartz. – Feldspar (both potassium feldspar and plagioclase).Feldspar (both potassium feldspar and plagioclase).– Mica (both muscovite and biotite).Mica (both muscovite and biotite).– Amphibole.Amphibole.– Pyroxene.Pyroxene.– Olivine.Olivine.

ColorColor The overall lightness or darkness of a rock The overall lightness or darkness of a rock

is a valuable indicator of its makeup.is a valuable indicator of its makeup.– Light-colored rocks are:Light-colored rocks are:

Quartz. Quartz. Feldspar. Feldspar. Muscovite.Muscovite.

– Dark-colored rocks are:Dark-colored rocks are:Biotite. Biotite. Amphibole. Amphibole. Pyroxene.Pyroxene.

Igneous RocksIgneous Rocks

One more lookOne more look

Intrusive (Coarse-grained) Igneous Intrusive (Coarse-grained) Igneous Rocks Rocks

GraniteGranite is quartz-bearing rock in which is quartz-bearing rock in which potassium feldspar is at least 65 percent by potassium feldspar is at least 65 percent by volume of the total feldspar present.volume of the total feldspar present.

Intrusive (Coarse-grained) Igneous Intrusive (Coarse-grained) Igneous Rocks Rocks

DioriteDiorite: : — The chief mineral in diorite is plagioclase.The chief mineral in diorite is plagioclase.— Either or both amphibole and pyroxene are invariably Either or both amphibole and pyroxene are invariably

present.present.

Intrusive Igneous Rocks Intrusive Igneous Rocks

Dark-colored diorite grades into Dark-colored diorite grades into gabbro.gabbro.— In gabbro, dark-colored minerals pyroxene and olivine In gabbro, dark-colored minerals pyroxene and olivine

exceed 50 percent of the volume of the rockexceed 50 percent of the volume of the rock.. A coarse-grained igneous rock in which A coarse-grained igneous rock in which

olivine is the most abundant mineral is olivine is the most abundant mineral is called a peridotite.called a peridotite.

Gabbros and peridodites can be found in Gabbros and peridodites can be found in both the oceanic and the continental crust.both the oceanic and the continental crust.

Extrusive Igneous RocksExtrusive Igneous Rocks

RhyolitesRhyolites are quartz-bearing. are quartz-bearing. Rhyolites contain a predominance of Rhyolites contain a predominance of

potassium feldspar.potassium feldspar.

Extrusive Igneous RocksExtrusive Igneous Rocks

Andesite:Andesite:– Compositionally equal to a dioriteCompositionally equal to a diorite– Named for the Andes. Named for the Andes.

Basalt:Basalt: – Compositionally equivalent to coarse-grained Compositionally equivalent to coarse-grained

gabbro, fine-grained basalt is the most common kind gabbro, fine-grained basalt is the most common kind of extrusive igneous rock. of extrusive igneous rock.

– The dominant rock of the oceanic crustThe dominant rock of the oceanic crust..

Granite RhyoliteFigure 4.7 A

DioriteAndesite

Figure 4.7 B

Gabbro Basalt

Figure 4.7 C

Pyroclasts, Tephra, And TuffsPyroclasts, Tephra, And Tuffs

A fragment of rock ejected during a volcanic A fragment of rock ejected during a volcanic eruption is called a eruption is called a pyroclastpyroclast..– Rocks formed from pyroclasts are pyroclastic Rocks formed from pyroclasts are pyroclastic

rocksrocks.. Geologists commonly refer to a deposit of Geologists commonly refer to a deposit of

pyroclasts as pyroclasts as tephratephra, a Greek name for ash., a Greek name for ash.– Tephra is a collective term for all airborne pyroclasts.Tephra is a collective term for all airborne pyroclasts.


Tephra particles are categorized by size:Tephra particles are categorized by size:– Bombs: greater than 64 mm in diameterBombs: greater than 64 mm in diameter

– Lapilli: between 2 and 64 mmLapilli: between 2 and 64 mm

– Ash: smaller than 2 mm.Ash: smaller than 2 mm.

Tephra is igneous when it goes up but Tephra is igneous when it goes up but sedimentary when it comes down.sedimentary when it comes down.


Pyroclastic rocks are transitional between Pyroclastic rocks are transitional between igneous and sedimentary rocks.igneous and sedimentary rocks.

When bomb-sized tephra are transformed into When bomb-sized tephra are transformed into a rock they are called a rock they are called agglomeratesagglomerates..

They are called They are called tuffstuffs when particles are either when particles are either lapilli or ash.lapilli or ash.

Figure 4.8 B

Igneous BodiesIgneous Bodies

Plutons Plutons

All bodies of intrusive igneous rock, All bodies of intrusive igneous rock, regardless of shape or size, are called regardless of shape or size, are called plutons,plutons, after Pluto, the Greek god of the underworld.after Pluto, the Greek god of the underworld.

Plutons are given special names depending on Plutons are given special names depending on their shapes and sizes.their shapes and sizes.

Figure 4.11

Minor Plutons: Dikes, Sills, and Minor Plutons: Dikes, Sills, and LaccolithsLaccoliths

A A dike dike is a tabular, sheet-like (thin but is a tabular, sheet-like (thin but laterally extensive) body of igneous rock that laterally extensive) body of igneous rock that cuts across the layering or fabric of the rock cuts across the layering or fabric of the rock into which it intrudes. into which it intrudes.

AA Sill Sill is tabular and sheet-like, like a dike, is tabular and sheet-like, like a dike, but runs parallel to the layering or fabric of but runs parallel to the layering or fabric of the rocks into which it intrudes.the rocks into which it intrudes.

Dikes and SillsDikes and Sills

Minor Plutons: Dikes, Sills, and Minor Plutons: Dikes, Sills, and Laccoliths (2)Laccoliths (2)

A A laccolith laccolith is parallel to the layering of the is parallel to the layering of the rocks into which it intrudes, but forces the rocks into which it intrudes, but forces the layers of rock above it to bend, forming a layers of rock above it to bend, forming a dome. dome.

A A volcanic pipevolcanic pipe is the roughly cylindrical is the roughly cylindrical conduit that once fed magma upward to a conduit that once fed magma upward to a volcanic vent. volcanic vent.

LaccolithLaccolithMt. Cristo Rey in El PasoMt. Cristo Rey in El Paso

Volcanic NeckVolcanic Neck

Major Plutons Major Plutons A A batholithbatholith is the largest kind of pluton. It is is the largest kind of pluton. It is

an intrusive igneous body of irregular shape an intrusive igneous body of irregular shape that cuts across the layering or other fabric of that cuts across the layering or other fabric of the rock into which it intrudes.the rock into which it intrudes.– The largest batholith in North America, The largest batholith in North America,

approximately 1500 km long, is the Coast Range approximately 1500 km long, is the Coast Range batholith of British Columbia and southern Alaska.batholith of British Columbia and southern Alaska.

– The magma from which a batholith forms intrudes The magma from which a batholith forms intrudes upward from its source deep in the continental crust.upward from its source deep in the continental crust.

Figure 4.14

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Igneous Rocks/EPCC/LM4