Igneous Rocks - Intrusive

8/13/2019 Igneous Rocks - Intrusive

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The Rock Cycle

A rockis a naturally formed,consolidated material usually

composed of grains of one or

more minerals

The rock cycleshows how onetype of rocky material gets

transformed into another

Representation of how rocks are

formed, broken down, and processed

in response to changing conditions Processes may involve interactions

of geosphere with hydrosphere,

atmosphere and/or biosphere

Arrows indicate possible process

paths within the cycle


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Imagine the first rock and the cycles that it

has been through.


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The Rock Cycle and PlateTectonics

Magmais created by melting of rock

above a subduction zone

Less dense magma rises and cools

to form igneous rock Igneous rock exposed at surface

gets weathered into sediment

Sediments transported to low areas,

buried and hardened into sedimentary rock Sedimentary rock heated and squeezed at depth to form

metamorphic rock

Metamorphic rock may heat up and melt at depth to form magma

Convergent plate boundary


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Igneous Rocks

Magma is molten rock

Igneous rocks form when magma

cools and solidifiesIntrusiveigneous rocks form when

magma solidifies underground

Granite is a common example

Extrusiveigneous rocks form whenmagma solidifies at the Earths

surface (lava)

Basalt is a common example

Granite

Basalt


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Igneous Rock Textures

Texture refers to the size, shape and

arrangement of grains or other constituents

within a rock

Texture of igneous rocks is primarilycontrolled by cooling rate

Extrusive igneous rockscool quickly at or

near Earths surface and are typicallyfine-

grained(most crystals 1 mm) Coarse-grained igneous rock

Fine-grained igneous rock


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Fast cooling

Slow cooling large xtals

small xtals

The basics of igneous rock textures

It is a function of viscosity ofthe melt, which is controlled by

composition and temperature.


7/46Coarse grained igneous rock


8/46Fine grained igneous rock


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Special Igneous Textures

Apegmatiteis an extremely coarse-grainedigneous rock (most crystals >5 cm) formed

when magma cools veryslowlyat depth

Aglassy texture contains no crystals at all,and is formed by extremely rapid cooling

Aporphyritictexture includes two distinct

crystal sizes, with the larger having formedfirst during slow cooling underground and

the small forming during more rapid cooling

at the Earths surface

Pegmatitic igneous rock

Porphyritic igneous rock


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Pegmatite:

Very coarse grained igneous rock


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Porphyritic igneous rock:

Big xtals in a fine grain matrix


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Igneous Rock Identification

Igneous rock names are based on texture(grain size) and

mineralogic composition

Textural classification

Plutonicrocks (gabbro-diorite-granite) are coarse-grained and cooled

slowly at depth

Volcanicrocks (basalt-andesite-rhyolite) are typically fine-grained and

cooled rapidly at the Earths surface

Compositional classification

Maficrocks (gabbro-basalt) contain abundant dark-coloredferromagnesian minerals

Intermediaterocks (diorite-andesite) contain roughly equal amounts of

dark- and light-colored minerals

Felsic rocks (granite-rhyolite) contain abundant light-colored minerals


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Igneous Rock Identification Igneous rock names are based on texture(grain size) and

mineralogic composition


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Source: http://csmres.jmu.edu/geollab/Fichter/IgnRx/simpclass.PDF


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Quartz

PlagioclaseFeldspar

Alkali(Ca,Na)Feldspar

Plutonic Rocks


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Quartz

PlagioclaseFeldspar

Alkali(Ca,Na)Feldspar

Volcanic Rocks


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http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgnRx.html


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Igneous Rock Chemistry

Rock chemistry, particularlysilica(SiO2) content, determinesmineral content and general color of igneous rocks

Maficrocks have ~50% silica, by weight, and contain dark-colored

minerals that are abundant in iron, magnesium and calcium

Intrusive/extrusive mafic rocks - gabbro/basalt

Felsic(silicic)rocks have >65% silica, by weight, and contain light-

colored minerals that are abundant in silica, aluminum, sodium and

potassium

Intrusive/extrusive felsic rocks - granite/rhyolite

Intermediaterocks have silica contents between those of mafic and

felsic rocks

Intrusive/extrusive intermediate rocks - diorite/andesite

Ultramaficrocks have


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Intrusive Rock Bodies

Intrusive rocks exist in bodies or structures that penetrate

or cut through pre-existing country rock

Intrusive bodiesare given names based on their size,

shape and relationship to country rock

Shallow intrusions:Dikes and sills

Form


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Intrusive Rock Bodies

Intrusive rocks exist in bodies or structures that penetrate

or cut through pre-existing country rock

Intrusive bodiesare given names based on their size,

shape and relationship to country rock

Deep intrusions: Plutons

Form at considerable depth beneath

Earths surface when rising blobs of

magma (diapirs) get trapped within

the crust

Crystallize slowly in warm

country rock

Generally composed of

coarse-grained rocks


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Intrusive Rock Bodies Volcanic neck

Shallow intrusion formed when magmasolidifies in throat of volcano

Dike

Tabular intrusive structure that cuts across

any layering in country rock

Sill

Tabular intrusive structure that parallels

layering in country rock

Pluton

Large, blob-shaped intrusive body formed

of coarse-grained igneous rock, commonly

granitic

Small plutons (exposed over 100 km2) are called batholiths

Light-colored dikes

Basaltic sill

Sierra Nevada batholith


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Devils Tower; a volcanic neck, a feeder pipe


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Shiprock, New Mexico; a volcanic neck


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Rhumski, Cameroon; a volcanic neck


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Sill; parallels layers in the country rock


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Dike; cuts across layers in the country rock


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Half Dome; part of the Sierra Nevada batholith


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Magma formationWhy rocks melt

Heat from below

Heat upward (by

conduction and

convection) from the

very hot (>5000C) corethrough the mantle and

crust

Rate at which

temperature increaseswith increasing depth

beneath the surface is the

geothermal gradient


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Heat from the mantle causes rocks at depth to melt.


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Rocks can melt as they rise and decompress

Melting point of

minerals generally

increases withincreasing pressure.

Decompression

melting can occur

when hot mantle rockmoves upward and

pressure is reduced

enough to drop

melting point to the

temperature of therising rock body


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Hot water under

pressure

Water becomes

increasingly reactive

at higher

temperatures

At sufficient

pressures and

temperatures, highly

reactive water vapor

can reduce themelting point of

rocks by over 200C

Insert new Fig. 3.18 here

With H2O

Without H2O

Adding water lowers the temperature at which rocks melt.


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Mineral mixtures

Mixtures of

minerals, such as

quartz and

potassium feldspar,

can result in themelting of both at

temperatures

hundreds of degrees

lower than either

mineral would melton its own

Mixtures melt at lower temperatures


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Summary:

Crustal rocks are heated from below

Rocks can melt during decompression

Rocks can melt because water orvolatiles are added to them

Mixing magmas can reduce the meltingtemperature


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Magma Crystallization andMelting Sequence

Minerals crystallize in a predictable order (and meltin the reverse order), over a large temperature range,

as described byBowens Reaction Series

Discontinuous branch

Ferromagnesian minerals (olivine, pyroxene, amphibole,biotite) crystallize in sequence with decreasing temperature

As one mineral becomes chemically

unstable in the remaining magma,

another begins to form

Continuous branch

Plagioclase feldspar forms with a

chemical composition that evolves

(from Ca-rich to Na-rich) with

decreasing temperatureBowens Reaction Series


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Six common Igneous Rocks

Volcanic

Rocks

Silica

Content

MineralsPresent (in order of

abundance)

Plutonic

Rocks

Basalt low pyroxene,olivine, feldspar,

& amphibole

Gabbro

Andesite intermediate feldspar,amphibole,

pyroxene, biotitemica

Diorite

Rhyolite high feldspar, quartz,muscovite mica,

& amphibole

Granite

Solidifying Temperature 500 C1000 C

LighterCol

or

Increasing Grain Size


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Lessons from Bowens Reaction Series

Large variety of igneous rocks is produced by large

variety ofmagma compositions

Mafic magmaswill crystallize into basaltorgabbroif

early-formed minerals are not removed from the magma

Intermediate magmaswill similarly crystallize into

dioriteor andesiteif minerals are not removed

Separation of early-formed ferromagnesian minerals

from a magma body increases the silica content of the

remaining magma

Minerals melt in the reverse order of that in which they

crystallize from a magma >>>> partial melting!!!


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Magma Evolution

A change in the composition of a magmabody is known as magma evolution

Magma evolution can occur by

differentiation,partial melting,

assimilation, or magma mixing Differentiationinvolves the changing of

magma composition by the removal of

denser early-formed ferromagnesian

minerals by crystal settling Partial meltingproduces magmas less

mafic than their source rocks, because

lower melting point minerals are more

felsic in composition


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Magma chamber fills

Early formed mafic mineralscrystallize and settle (or areotherwise separated from theresidual melt)

The remaining melt isenriched in silica

Differentiation

M E l ti I


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Magma Evolution I

Assimilationoccurs

when a hot magma

melts and

incorporates morefelsic surrounding

country rock

Xenolith

Insert new

Fig. 3.22 here

M E l ti II


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Magma mixing

involves the

mixing of more

and less maficmagmas to produce

one of intermediate

composition

Magma Evolution II


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Rock types and tectonic setting

Igneous Activity and


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Igneous Activity andPlate Tectonics

Igneous activity occurs primarily at or

near tectonic plate boundaries

Mafic igneous rocks are commonly

formed at divergent boundaries

Increased heat flow and decreased

overburden pressure produce mafic magmas

from partial melting of the asthenosphere

Intermediate igneous rocks arecommonly formed at convergent

boundaries

Partial melting of basaltic oceanic crust

produces intermediate magmas

Igneous Activity and


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Igneous Activity andPlate Tectonics

Felsic igneous rocks are

commonly formed adjacent

to convergent boundaries

Hot rising magma causes

partial melting of the granitic

continental crust

Intraplate volcanism

Rising mantle plumes can

produce localized hotspots and

volcanoes when they produce

magmas that rise through

oceanic or continental crust

Hawaii is an example

Igneous Rocks - Intrusive

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