Post on 03-Apr-2018
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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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Texture In Igneous Rocks (2)
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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Texture In Igneous Rocks (3)
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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Texture In Igneous Rocks (4)
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).
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Texture In Igneous Rocks (5)
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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Intrusive (Coarse-grained) Igneous
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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Intrusive (Coarse-grained) Igneous
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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Intrusive (Coarse-grained) Igneous
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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Pyroclasts, Tephra, And Tuffs (2)
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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Pyroclasts, Tephra, And Tuffs (3)
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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Pyroclasts, Tephra, And Tuffs (4)
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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Distribution of Volcanoes (2)
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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Distribution of Volcanoes (3)
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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Origin of Basaltic Magma (2)
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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Origin of Basaltic Magma (3)
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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Origin of Andesitic Magma (2)
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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Origin of Andesitic Magma (3)
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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Bowens Reaction Series (2)
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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Bowens Reaction Series (3)
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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Bowens Reaction Series (4)
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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Valuable Magmatic Mineral Deposits
(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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Valuable Magmatic Mineral Deposits
(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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Valuable Magmatic Mineral Deposits
(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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