igneous rock study

7/29/2019 Igneous Rock Study

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Igneous Rocks: Product of EarthsInternal Fire


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Introduction: What Is an Igneous Rock?

Igneous rocks vary greatly.

Some contain large mineral grains.

Others contain grains so small they can barely be

seen under a high power microscope.

Igneous rocks also vary greatly in color.

All igneous rocks are formed through the cooling

and solidification of magma.


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Intrusive Versus Extrusive Igneous

Rocks

Intrusive igneous rocks form when magma cools

within existing rocks in Earths crust.

Extrusive igneous rocks form when magma cools

on Earths surface, where they have beenextruded.


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Texture In Igneous Rocks (1)

The two most obvious textural features of an

igneous rock are the size of its mineral grainsand how the mineral grains are packed together.

Sizes of mineral grains:

Intrusive rocks are coarse-grained.

Magma that solidifies in the crust cools slowly and has

sufficient time to form large mineral grains.


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Extrusive rocks are fine-grained.

Magma that solidifies on the surface usually cools rapidly,

allowing insufficient time for large crystals to grow.

Coarse-grained igneous rock is called a phanerite (from

the Greek word meaning visible).

Igneous rock that contains unusually large mineral grains

(2cm or larger) is called a pegmatite.

Fine-grained igneous rock is called an aphanite (from the

Greek word meaning invisible).


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The isolated large grains are phenocrysts.

A porphyry is an igneous rock in which 50% ormore of the rock is coarse mineral grains scattered

through a mixture of fine mineral grains.


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Glassy rocks.

Atoms lack time to organize themselves intominerals.

A mineraloid forms instead (mineral-like solidthat lacks either a crystal structure or a definitecomposition or both).

Extrusive igneous rocks that are largely or whollyglassy are called obsidian.

They display a distinctive conchoidal fracture (smooth,curved surface).


8/70Figure 4.5


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Another common variety of glassy igneous rock is

pumice, a mass of glassy bubbles of volcanic

origin.

Volcanic ash is also mostly glassy because the

fragments of magma cooled too quickly to

crystallize.


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Mineral Assemblage In Igneous Rocks

Once the texture of an igneous rock isdetermined, its name will depend on its mineralassemblage. All common igneous rocks consistlargely of:

Quartz.

Feldspar (both potassium feldspar and plagioclase).

Mica (both muscovite and biotite).

Amphibole.

Pyroxene.

Olivine.


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Color

The overall lightness or darkness of a rock is a

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

Quartz.

Feldspar.

Muscovite.

Dark-colored rocks are:

Biotite.

Amphibole.

Pyroxene.


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Intrusive (Coarse-grained) Igneous

Rocks (1)

Granite is quartz-bearing rock in which potassium

feldspar is at least 65 percent by volume of the totalfeldspar present.

Granodiorite is quartz-bearing rock in which

plagioclase is 65 percent or more of the total

feldspar present.


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Figure 4.6


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Rocks (2)

Granitic rocks include both granite and granodiorite.

Granitic rocks are only found in the continental crust. Granitic magma forms when continental crust is heated

to its melting temperature.

The most common place where such high temperatures are

reached is in the deeper portions of mountain belts formedby the collision of two masses of continental crust.


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Rocks (3)

Diorite:

The chief mineral in diorite is plagioclase.

Either or both amphibole and pyroxene are invariably

present.

Forms in the same way as granite and

granodiorite. It is found only in continental crust.


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Rocks (4)

Dark-colored diorite grades into gabbro.

In gabbro, dark-colored minerals pyroxene and olivine

exceed 50 percent of the volume of the rock.

A coarse-grained igneous rock in which olivine is

the most abundant mineral is called a peridotite.

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


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Extrusive (Fine-Grained) Igneous

Rocks (1)

Rhyolites and dacites are quartz-bearing.

Rhyolites contain a predominance of potassiumfeldspar.

Dacites contain a predominance of plagioclase.

Dacites can only be distinguished from rhyolites

through microscopic examination.


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Granite RhyoliteFigure 4.7 A


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Extrusive (Fine-Grained) Igneous

Rocks (2)

Andesite:

An igneous rock similar in appearance to a dacite,but lacking quartz.

Named for the Andes.

Basalt:

Compositionally equivalent to coarse-grainedgabbro, fine-grained basalt is the most common kindof extrusive igneous rock.

The dominant rock of the oceanic crust.


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DioriteAndesite


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Pyroclasts, Tephra, And Tuffs (1)

A fragment of rock ejected during a volcanic

eruption is called a pyroclast. Rocks formed from pyroclasts are pyroclastic

rocks.

Geologists commonly refer to a deposit of

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

pyroclasts.


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Tephra particles are categorized by size:

Bombs: greater than 64 mm in diameter Lapilli: between 2 and 64 mm

Ash: smaller than 2 mm.

Tephra is igneous when it goes up but

sedimentary when it comes down.


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Gabbro Basalt

Figure 4.7 C


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Pyroclastic rocks are transitional between igneous

and sedimentary rocks.

When bomb-sized tephra are transformed into a

rock they are called agglomerates.

They are called tuffs when particles are either lapilli

or ash.


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Figure 4.8 B


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Tephra can be converted into pyroclastic rock in

two ways:

Through the addition of a cementing agent, such

as quartz or calcite, introduced by groundwater.

Through the welding of hot, glassy, ash particles.

Welded tuff.


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Plutons

All bodies of intrusive igneous rock, regardless of

shape or size, are called plutons, after Pluto, the

Greek god of the underworld.

Plutons are given special names depending on their

shapes and sizes.


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Figure 4.10


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Figure 4.11


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Minor Plutons: Dikes, Sills, and

Laccoliths

A dike is a tabular, sheet-like (thin but laterally

extensive) body of igneous rock that cuts across the

layering or fabric of the rock into which it intrudes.

A Sill is tabular and sheet-like, like a dike, but runs

parallel to the layering or fabric of the rocks into

which it intrudes.


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Minor Plutons: Dikes, Sills, and

Laccoliths (2)

A laccolith is parallel to the layering of the rocks

into which it intrudes, but forces the layers of rock

above it to bend, forming a dome.

A volcanic pipe is the roughly cylindrical conduit

that once fed magma upward to a volcanic vent.


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Major Plutons

A batholith is the largest kind of pluton. It is an

intrusive igneous body of irregular shape that cuts

across the layering or other fabric of the rock into

which it intrudes.

The largest batholith in North America,

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

The magma from which a batholith forms intrudes

upward from its source deep in the continental crust.


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Figure 4.14


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Xenoliths and Stocks

Rising magma can dislodge fragments of the

overlying rock, and the dislodged blocks, being

cooler and more dense than the magma, sink. This

process, called stoping, can produce xenoliths.

Any rock fragment still enclosed in a magmatic body

when it solidifies is a xenolith. Stocks are irregularly shaped intrusives no larger

than 10 km in maximum dimension.


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Figure 4.16


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Distribution of Volcanoes (1)

Rhyolitic magma:

Volcanoes that erupt rhyolitic magma areabundant on the continental crust.

The process that forms rhyolitic magma does not occur

in oceanic crust.

The process that form rhyolitic magma must berestricted to continental-type crust (including

those places in the ocean where new crust of

continental character is forming.


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Andesitic magma:

Volcanoes that erupt andesitic magma occur onboth oceanic and continental crust.

A line around the Pacific separates andesitic

volcanoes from those that erupt only basaltic lava.

This Andesite Line is generally parallel to theplate subduction margins.


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Figure 4.17


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Basaltic magma:

Volcanoes that erupt basaltic magma also occur on

both oceanic and continental crust.The source of basaltic magma, therefore, must be the

mantle.

Everywhere along the midocean ridges, volcanoes eruptbasaltic magma.

Some large basaltic volcanoes are not located alongmidocean ridges. The Hawaiian volcanic chain isbelieved to have formed over the past 70 million yearsas the Pacific Plate moved slowly northwestward acrossa midplate hot spot.


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Origin of Basaltic Magma (1)

When discussing the origin of basaltic magma,

geologists ask:

Was the rock that melted to form the magma wet

or dry?

the presence of water lowers the temperature at which

melting begins.

What kind of rock melted?

The kind of rock that melts controls the composition of

the magma that forms.

Did the rock melt completely or only partially?


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The process of forming magma through theincomplete melting of rock is known as chemical

differentiation by partial melting.

Basaltic magma is probably either a dry or a water-poor magma.

Olivine, pyroxene,and plagioclase do not containwater in their formula.

Water content of basaltic magma rarely exceeds 0.2percent.

The process must occur in the mantle.


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Laboratory experiments on the dry partial-

melting properties of garnet peridotite show

that, at asthenospheric pressures and

temperatures (100 km deep), a 5 to 10 percent

partial melts has a basaltic composition.

The upper portion of the mantle contains garnetperidotites.


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Figure B4.1


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Figure B4.2


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Origin of Andesitic Magma (1)

Andesitic magma is close to the average

composition of continental crust.

Igneous rocks formed from andesitic magma

commonly occur in the continental crust.

It is likely that andesitic magma forms by the

complete melting of a portion of the continentalcrust.


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In the laboratory, wet partial melting of mantle

rock under suitably high pressure yields a

magma of andesitic composition.

Andesitic magma can be extruded from

volcanoes that are far from the continental crust.

When a moving plate of lithosphere plungesback into the asthenosphere, it carries with it a

capping of basaltic oceanic crust saturated with

seawater.


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Wet partial melting that starts at a pressure that

is equivalent to a depth of about 80 km produces

a melt having the composition of andesitic

magma.

The andesitic line corresponds closely with plate

subduction margins.


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Figure 4.18


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Origin of Rhyolitic Magma (1)

Volcanoes that extrude rhyolitic magma areconfined to the continental crust or to regions of

andesitic volcanism.

Volcanoes that extrude rhyolitic magma give offa great deal of water vapor.

Intrusive igneous rocks formed from rhyoliticmagma (granite) contain significant quantities ofOH-bearing (hydrous) minerals, such as mica andamphibole.


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Origin of Rhyolitic Magma (2)

The generation of rhyolitic magma probablyinvolves some sort of wet partial melting of rock

having the composition of andesite. Once a rhyolitic magma has formed, it starts to

rise. However, the magma rises slowly because itis very viscous, with a high SiO2 content (70

percent).

Most rhyolitic magma solidifies undergroundand forms granitic batholiths.


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Solidification of Magma (1)

A magma of a given composition can crystallize

into many different kinds of igneous rock.

Solidifying magma forms several different

minerals which start to crystallize from the

cooling magma at different temperatures.


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Solidification of Magma (2)

Crystal-melt separation can occur in a number ofways:

Compression can squeeze melt out of a crystal-meltmixture.

Dense, early crystallized minerals may sink to thebottom of a magma chamber, thereby forming a solid

mineral layer covered by melt. However a separation occurs, the compositional

changes it causes are called magmaticdifferentiation by fractional crystallization.


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Bowens Reaction Series (1)

Canadian-born scientist N. L. Bowen (1887-

1956) first recognized the importance of

magmatic differentiation by fractional

crystallization.

Bowen argued that a single magma could

crystallize into both basalt and rhyolite becauseof fractional crystallization.


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Bowen knew that plagioclases that crystallize

from basaltic magma are usually calcium-rich

(anorthitic).

Plagioclases formed from rhyolitic magma are

commonly sodium-rich (albitic).

Bowen called such a continuous reaction betweencrystals and melts a continuous reaction series.


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Bowen identified several sequences of reactionsbesides the continuous reaction series of the

feldspars. When basalt cools down, one of the earliest

minerals to form is olivine.

Olivine contains about 40 percent SiO2 by weight.

Basaltic magma contains 50 percent SiO2.

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


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Figure 4.19 A


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Figure 4.19 B


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The solid olivine reacts with silica in the melt to

form a more silica-rich mineral, pyroxene.

The pyroxene in turn can react to form amphibole.

Amphibole can react to form biotite.

Such a series of reactions is called a discontinuous

reaction series.


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Figure 4.20


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Valuable Magmatic Mineral Deposits

(1)

The processes of partial melting and fractional

crystallization in magmas sometimes lead to

formation of large and potentially valuable

mineral deposits.

An important example of this kind of

concentration process is provided by pegmatites,especially those formed through crystallization

of rhyolitic magma.


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(2)

Pegmatites may contain significant enrichments

of rare elements such as beryllium, tantalum,

niobium, uranium, and lithium.

Most of the worlds chromium ores were formed

in this manner by accumulation of the mineral

chromite (FeCr2O4).


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(3)

The largest known chromite deposits are in

South Africa, Zimbabwe,and the former SovietUnion.

Vast deposits of ilmenite (FeTiO3), a source of

titanium, were formed by magmatic

differentiation.


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Figure 4.21 B


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(4)

Certain magmas separate into two immiscibleliquids.

One, a sulfide liquid rich in iron, copper, andnickel, sinks to the floor of the magma chamberbecause it is denser.

The resulting igneous rock is rich in copper ornickel ore.

Many of the worlds great nickel deposits, inCanada, Australia, Russia,and Zimbabwe,formed in this manner.


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Figure 4.21


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Revisiting Plate Tectonics And The

Earth System (1)

The melting of a rock increases with pressure. If ahot mass of rock is under pressure and the pressure

suddenly decreases, decompression melting canoccur.

The oceanic crust varies very little in compositionaround the world.

It is simply referred to as MORB, an acronym formidocean ridge basalt.

The ridge and seafloor are everywhere covered bywater except in a few places such as Iceland, where

the midocean ridge stands above the sea level.


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Revisiting Plate Tectonics And The

Earth System (2)

In places where a plate collision has caught up and

crushed a fragment of oceanic crust between twocolliding continental masses, the minerals that are

characteristic of basalt are transformed into an

assemblage dominated by a green, fibrous mineral

called serpentine.

Serpentine-dominated fragments of oceanic crust

found on continents are called ophiolites, from the

Greek word for serpent, ophis.


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Figure 4.22


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Igneous Rock And Life on Earth

Life requires nutrients such as potassium, sulfur,calcium, and phosphorus.

Magma, which is less dense than the rock fromwhich it forms by melting, rises buoyantlyupward, bringing with it the nutrients on whichlife depends.

A continent unaffected by any process of surfacerenewal, such as uplift or volcanic eruptions, butsubjected to erosion for a hundred million years,would finish with low relief and almost barren

soils.


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igneous rock study

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