Chapter 4: Igneous Rocks: Product of Earths Internal Fire

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Chapter 4 Igneous Rocks Product of Earths
Internal Fire
2
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 been extruded.

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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 grains
  and 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 or
  more 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 into
  minerals.
• A mineraloid forms instead (mineral-like solid
  that lacks either a crystal structure or a
  definite composition or both).
• Extrusive igneous rocks that are largely or
  wholly glassy are called obsidian.
• They display a distinctive conchoidal fracture
  (smooth, curved surface).

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Figure 4.5
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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 is
  determined, its name will depend on its mineral
  assemblage. All common igneous rocks consist
  largely 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
  total feldspar 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 formed by 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 both the
  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 potassium
  feldspar.
• Dacites contain a predominance of plagioclase.
• Dacites can only be distinguished from rhyolites
  through microscopic examination.

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Granite
Rhyolite
Figure 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-grained
  gabbro, fine-grained basalt is the most common
  kind of extrusive igneous rock.
• The dominant rock of the oceanic crust.

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Andesite
Diorite
Figure 4.7 B
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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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Basalt
Gabbro
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 Range
  batholith 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 are abundant
  on the continental crust.
• The process that forms rhyolitic magma does not
  occur in oceanic crust.
• The process that form rhyolitic magma must be
  restricted 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 on
  both 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 the
  plate 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
  erupt basaltic magma.
• Some large basaltic volcanoes are not located
  along midocean ridges. The Hawaiian volcanic
  chain is believed to have formed over the past 70
  million years as the Pacific Plate moved slowly
  northwestward across a 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 the
  incomplete 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 contain
  water in their formula.
• Water content of basaltic magma rarely exceeds
  0.2 percent.
• 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 garnet
  peridotites.

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Figure B4.1
44
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 continental
  crust.

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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 plunges back
  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 are
  confined to the continental crust or to regions
  of andesitic volcanism.
• Volcanoes that extrude rhyolitic magma give off a
  great deal of water vapor.
• Intrusive igneous rocks formed from rhyolitic
  magma (granite) contain significant quantities of
  OH-bearing (hydrous) minerals, such as mica and
  amphibole.

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Origin of Rhyolitic Magma (2)

• The generation of rhyolitic magma probably
  involves 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 it
  is very viscous, with a high SiO2 content (70
  percent).
• Most rhyolitic magma solidifies underground and
  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 of
  ways
• Compression can squeeze melt out of a
  crystal-melt mixture.
• Dense, early crystallized minerals may sink to
  the bottom of a magma chamber, thereby forming a
  solid mineral layer covered by melt.
• However a separation occurs, the compositional
  changes it causes are called magmatic
  differentiation 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 because
  of 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 between
  crystals and melts a continuous reaction series.

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Bowens Reaction Series (3)

• Bowen identified several sequences of reactions
  besides 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
  residual liquid 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 Soviet
  Union.
• 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 immiscible
  liquids.
• One, a sulfide liquid rich in iron, copper, and
  nickel, sinks to the floor of the magma chamber
  because it is denser.
• The resulting igneous rock is rich in copper or
  nickel ore.
• Many of the worlds great nickel deposits, in
  Canada, 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
  a hot mass of rock is under pressure and the
  pressure suddenly decreases, decompression
  melting can occur.
• The oceanic crust varies very little in
  composition around the world.
• It is simply referred to as MORB, an acronym for
  midocean ridge basalt.
• The ridge and seafloor are everywhere covered by
  water 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
  two colliding 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 from
  which it forms by melting, rises buoyantly
  upward, bringing with it the nutrients on which
  life depends.
• A continent unaffected by any process of surface
  renewal, such as uplift or volcanic eruptions,
  but subjected to erosion for a hundred million
  years, would finish with low relief and almost
  barren soils.

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Figure 4.23