Lecture 03 igeneous rock

Contents
 Igneous Rocks
 classification on the basis of solidification
 Igneous Rock Textures
 Igneous Rocks Mineral Composition
 Bowens Reaction Series
 Ferromagnesian Silicates
 Non-Ferromagnesian Silicates
 Igneous Rocks: (examples )

What are Rocks?
 Most rocks are an aggregate of one or more minerals and
a few rocks are composed of non-mineral matter.
 There are three major rock types:
 1. Igneous
 2. Metamorphic
 3. Sedimentary
Table of Contents

Igneous Rocks
 Igneous rocks are formed by the cooling of molten rock.
 There are two major states of molten rock: Magma and Lava.
 Magma is a form of molten rock that exists below the Earth’s
surface.
 Lava is the term given to magma once it reaches the Earth’s
surface, usually in the form of a volcanic eruption.
 There are two major classifications of igneous rocks:
o Intrusive igneous rocks
o Extrusive igneous rocks
Table of Contents

 Intrusive igneous rocks are formed by magma that cools
below the Earth’s surface.
 Intrusive igneous rocks generally cool very slowly deep
below the earth’s surface or as the magma is rising to
the earth’s surface.
 Plutonic rocks .pluton (greek god of underworld)
Intrusive igneous rocks

 Commonly observed forms of Plutonic (intrusive) rocks
observed in the field are:
 Dykes
 Sills
 Laccoliths
 Bysmaliths
 Batholiths
 Phacoliths
 Lopolith
 Volcanic necks
 Chonoliths
Types of intrusive /plutonic rocks

• Based on the attitudes of the associated country
rocks the forms are called either as Concordant
(parallel ) or Discordant (prependiculer).

Dykes
• They are discordant
• Cut across the bedding of the rocks in which
they intrude
• Vertical to steeply inclined and sheetlike body
(extensive in lateral dimension)
• Thickness vary widely from an inch upto
hundred of feet
• Injected through fractures, joints, and weak
planes

 Quartz-Dolerite dykes of Midland
valley of Scotland are about 50-
60 km long and upto 30m thick.
Few places some dykes are very
short upto few meters and as
thin as few cm.

 Sills are relatively thin tabular sheet like body that penetrates
parallel to the bedding planes
 Laterally it may extends for 100s of km and upto 10 km in
width.
 Lateral extend mainly depends on the hydrostatic force,
temperature, degree of fluidity or viscosity, weight of
overlying sediment column.
 Since basic magma are more fluid then acidic magma-
mostly sills are made up of gabbros, dolerites and basalts.
Sills

Spreads parallel to the bedding
planes of the rocks, hence
concordant in nature.
mudstone
sandstone
limestone

When viscous magma is
injected rapidly along
the bedding, as it cannot
spreads it pushes up the
overlying layers and
keep on piling up.
It causes folding of the
overlying rock layers.
It is a concordant body, with flat bottom and
convex upward. It is dome shaped.
LACCOLITHS

BYSMALITH:
 It is cylindrically shaped body.
 It is developed when highly viscous magma is injected,
because the lateral spreading along the bedding is less it
acquires to move upwards and form cylindrical shape.
 Causes breaking of overlying rock layers.
Department of

 These are the largest kind of plutons, irregular in shape and
occupies large area.
 Their side sloping away from each other which makes them
larger and large downwards extending to greater depth.
 Their occurrence is commonly associated with the
mountain-building process
 These are either granites or granodiorites in composition
BATHOLITHS

Stocks: Are smaller irregular
bodies with 10 km in maximum
dimension, and are associated
with batholiths.

These are concordant bodies that occurs along the
crests and troughs of the folded sedimentary strata.
PHACOLITHS

These are basin or saucer-shaped concordant bodies with top
nearly flat and convex bottom
LOPOLITHS

:
It is cylindrical conduit that fed magma upward to a
volcanic vent or it is a conduit of the ancient volcano.
Vary in diameter from a few 100s of m to a kilometer or
more. These are filled up with crystalline rocks.
Shape-circular, elliptical or irregular.
VOLCANIC NECK or VOLCANIC PLUGS

CHONOLITHS
This term is applied to all other intrusive igneous bodies
with irregular shape, i.e. the body with no specific shape.

Texture
 Texture is a term used to describe the size, shape, and
arrangement of interlocking crystallized mineral grains
in an igneous rock.
 Two major factors affect the size of crystal grains in an
igneous rock:
 1) Rate at which molten rock cools; slow or fast
 2) Amount of dissolved gases or fluids in the magma.

Texture
 Igneous minerals vary greatly in grain size. Grain-size
classes are similar to the sedimentary scale, but there are
fewer divisions with a greater range of size.
 Phenocrysts are grains in an igneous rock that are larger
than the other grains that make up the rest of the rock.
Grain Size
Categories
Grain Size Divisions
fine grained = < 1 mm
medium grained = 1 mm < 5 mm
coarse grained = 5 mm < 3 cm
very coarse-grained = > 3 cm
Phenocrysts Texture
microphenocrysts = 0.03 mm – 0.3 mm
phenocrysts = 0.3 mm – 5 mm
megaphenocrysts = > 5 mm
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Igneous Rock Textures
Phaneritic (Intrusive)
Aphanitic (Extrusive)
Porphyritic (Intrusive and Extrusive)
Vesicular texture (Extrusive)
Glassy (Extrusive)
Pegmatitic (Intrusive)
Pyroclastic Materials (Extrusive)
Aa Lava (Extrusive)
Pahoehoe Lava (Extrusive)

Phaneritic Texture
 Phaneritic (Intrusive)
 Phaneritic rocks are coarse-grained rocks which form below the Earth’s
surface.
 The individual crystals are relatively even-sized and large enough for
scientists to identify the different mineral grains that compose the rock.
Quartz Crystals:
(White)
Feldspar Crystals:
(Pink)
Biotite Crystals:
(Black)
Granite rock with a phaneritic texture
Copyright © Dr. Richard Busch
Table of Contents

Coarse-grained
Phaneric
- > 5mm
Medium-grained
Phaneric
- 1 mm - 5mm
Fine-grained
Phaneric
<1 mm
Is characterized by LARGE SIZE MINERALS which can be
easily seen by Naked eye (size at least 2mm or greater)
PHANERIC TEXTURE

Aphanitic Texture
 Aphanitic rocks are very
fine-grained and contain
crystals that are too small
to distinguish without the
aid of a magnifying lens.
 Aphanitic rocks are often
described by how light or
dark the rock appears.
Lighter colored aphanitic
rocks contain mostly non-
ferromagnesian silicate
minerals. Darker colored
aphanitic rocks contain
mostly ferromagnesian
silicate minerals.

Porphyritic Texture
 Porphyritic rocks contain both
coarse- and fine-grained textures
indicating different environmental
conditions which formed the rock.
 The coarse grains in a porphyritic
rock develop as the magma is
cooling below the surface of the
earth.
 The fine-grained component of a
porphyriic rocks forms when the
magma or lava cools faster.
 The large coarse-grained crystals
are referred to as phenocrysts.
 The small fine-grained crystals are
referred to as groundmass.
Rhyolite rock with
porphyritic texture containing
phenocrysts of olivine and pyroxene
and a gabbro groundmass.

Vesicular Texture
Aphanitic rocks may also contain vesicles of remnant gas that
give the rock a vesicular texture. Vesicles form when the rock
cools very quickly and preserves the openings formed by the
expansion of trapped gas bubbles.

Glassy Texture
 Glassy textured rocks are formed by very
rapid cooling of magma.
 Glassy rocks often form from magmas
with high silica content that arranges into
long chainlike structures before
crystallization occurs. These silica chains
increase the viscosity of the magma and it
once it eventually cools it forms a glassy
textured rock.
 Glassy rocks can be considered
amorphous because they have no
crystalline structure.
 Glassy rocks are classified by the amount
of glass contained by the rock:
 Glass-bearing: 0-20% glass
 Glass-rich: 20-50% glass
 Glassy: 50 – 100% glass
 Obsidian is a common glassy rock.
Obsidian rock with a glassy
texture and conchoidal fractures

Pegmatitic Texture
 Pegmatitic (Intrusive)
 Pegmatitic rocks contain large interlocking
crystalline grains > 1-2 centimeter in diameter.
 Pegmatites are commonly composed of quartz,
feldspar, and mica minerals.
 Pegmatities form from a combination of
hydrothermal and igneous processes; and is
dependant on the presence of fluids and
volatiles such as water, chlorine, bromine,
sulfur, and fluorine.
 Pegmatites form late in the crystallization
process when there are a lot of fluids present in
the molten rock. The fluids enable individual
ions to move around more freely, ultimately
bonding to form very large and sometimes
exotic crystals.
 Pegmatitic dikes form around the margins of
intrusive plutons, or occasionally as veins of
rock which extend into the pluton.
Examples of pegmatitic veins
extending through rock

Pyroclastic Materials
 Pyroclastic materials form when
individual rock fragments are ejected
during a violent volcanic eruption and
consolidate into larger rock composites
when they deposit on the surface.
 Pyroclastic rocks contain at least 75%
pyroclastic fragments with the
remainder consisting of other inorganic
sediments or organic materials.
 Pyroclastic rocks contain a mixture of
different types of particles that are not
cohesively joined by interlocking
crystals, but instead are consolidated
masses of multiple rock fragments.
 Tephra is the term used to describe
pyroclastic sediments.
Tuff rock with pyroclastic material.

Aa Lava
 Aa is a basaltic lava flow that has a rough
surface, characterized by sharp, jagged blocks
and protruding spines of volcanic rock.
 Aa flows move slowly (5-50 meters per hour)
and are often several meters thick.
 As aa lava flows, the outer surface and
advancing edge cools first. The molten material
pushes through the cooled rocks and breaks the
fragments even more. As a result the lava flow
appears more like a mass of advancing rubble
as apposed to a viscous flow.
 Aa lava flows are common on the Hawaiian
Islands. The aa flows move so slowly that
tourists can walk up to them and take pictures.

Pahoehoe Lava
 Pahoehoe (pronounced pah-hoy-hoy) is
a basaltic lava flow that has a smooth
and twisty, rope-like surface.
 The characteristic ropy texture forms as
the surface lava cools while the molten
material beneath it is still moving. The
tension formed by the cooling lava
causes it to wrinkle as the subsurface
lava continues to flow. As a result the
surface cools in a series of overlapping,
ropy lobes.
 Pahoehoe lava flows move slow enough
(5-50 meters per hour) for observers to
watch the cooling lava as it advances
forward.
Photo Courtesy USGS
Pahoehoe lava flows in Hawaii.

What happens to molten rock as it cools?
 When the temperature of molten rock begins to drop there is a loss of energy that
causes ions to slow down. As the ions slow down, they group together and arrange
themselves into orderly crystalline structures. This process is referred to as
crystallization.
 During crystallization, the silicon and oxygen atoms are the first to link together
forming silicon-oxygen tetrahedrons, which are the building block of all silicate
minerals.
 As crystallization continues, these individual silicon-oxygen tetrahedrons join
with one another, and other ions, to form the basic structure of most minerals and
igneous rocks.
 Environmental conditions including temperature and the presence of water or
gases during crystallization affect the composition, the size, and the arrangement
of the mineral grains.
 The size and arrangement of mineral crystals, also referred to as grains, define the
texture of the rock.
 Geologists use mineral and textural classifications to infer information about the
environmental setting in which different igneous rocks are formed.

Crystal Size and Cooling Rates
 Slower cooling rates produce larger individual crystals in the rock
 Intrusive igneous rocks generally cool very slowly and tend to have large
crystals that produce a course-grained rock.
 Phaneritic rocks are coarse-grained rocks which contain individual crystals
that are relatively even in size and large enough for scientists to identify the
different mineral grains that compose the rock.
 Faster cooling rates produce smaller individual crystals in the rock
 Extrusive igneous rocks tend to cool quickly and are characterized by smaller
grains that produce a fine-grained rock.
 Aphanitic is the term used to describe very fine grained rocks.
 Porphyritic textured rocks contain both a coarse and fine-grained texture.
 The coarse grains in a porphyritic rock begin to develop as the magma is
cooling below the surface of the earth. Following eruption or exposure to
lower temperatures, the remaining magma or lava cools very quickly and
forms minerals with fine-grained textures. As a result, porphyritic textures
contain both coarse- and fine–grained minerals.

Igneous Rocks Mineral Composition
 The chemical composition of the magma during cooling determines the
mineral composition of the crystallized rocks.
 98% of all magma is composed primarily of silicate (SiO2) ions joined with
aluminum (Al), calcium (Ca), sodium (Na), potassium (K), magnesium
(Mg), and iron (Fe) ions.
 Magma may also contain trace amounts of other elements such as titanium
(Ti), manganese (Mn), gold (Au), silver (Ag), and uranium (U).
 During crystallization the minerals combine to form two major groups of
silicate minerals, these include the dark-colored ferromagnesian silicates
which crystallize at high temperatures and the light-colored
nonferromagnesian silicates which crystallize at lower temperatures.

Bowen’s Reaction Series
 In the early 1900’s N.L. Bowen and other geologists conducted a
series of experiments to determine the order at which different
silicate minerals crystallize from magma. Their results produced
a generalized mineral crystallization model that is recognized as
Bowen’s Reaction Series, and it states that mineral
crystallization will occur in a predictable manner.
 Bowen’s Reaction Series is a model that describes the formation
of igneous rocks with an emphasis on the effect of temperature
changes, melting points, and cooling rates, on the types of
minerals crystallizing and their resultant rock compositions.

 Once crystallization begins, the composition of the
liquid magma changes. Minerals with higher
melting points will begin to solidify leaving behind a
liquid from which minerals with lower melting
temperatures will eventually solidify.
 An ideal discontinuous crystallizing series
progresses from the minerals olivine - pyroxenes -
amphiboles – biotite.
 An ideal continuous series progresses from calcium
to sodium-rich plagioclase feldspar.
 Both series merge and are followed by orthoclase
feldspar, muscovite, and quartz, with quartz
exhibiting the lowest crystallization temperature.

Bowen’s Reaction Series
Table of Contents
Crystallization
Temperature
1400 ºC
800 ºC
Mafic
Felsic
Intermediate
Olivine
Pyroxene
Amphibole
Biotite
Quartz
Orthoclase Feldspar
Muscovite mica
Calcium rich
Sodium rich
Discontinuous Series Continuous Series

Classification of Igneous Rocks by Mineral Composition
and Texture
Chemical Composition Felsic
(Granitic)
Intermediate
(Andesitic)
Mafic
(Basaltic)
Ultramafic
Dominant Minerals
Quartz,
Potassium
Feldspar,
Sodium-rich
plagioclase
feldspar
Amphibole,
Sodium-and
calcium-rich
plagioclase
feldspar
Pyroxene,
Calcium-rich
plagioclase
feldspar
Olivine,
Pyroxene
Accessory Minerals
Amphibole,
Muscovite,
Biotite
Pyroxene,
Biotite
Amphibole,
Olivine
Calcium-rich
plagioclase
feldspar
Rock Color
(% of dark minerals)
0-25 % 25 – 45 % 45 – 85 % 85 – 100 %
Phaneritic (coarse-grained) Granite Diorite Gabbro Peridotite
Aphanitic (fine- grained) Rhyolite Andesite Basalt Komatiite
Porphyritic Porphyritic used to describe abundant presence of
phenocrysts in Granite, Diorite, Gabbro, Peridotite, Rhyolite,
Andesite, and Basalt
Uncommon
Glassy Obsidian (compact) and Pumice (frothy-like)
Pyroclastic Tuff (fine grained) and Volcanic Breccia (coarse grained)
T
e
xt
u
r
e

Ferromagnesian Silicates
 Ferromagnesian silicates crystallize at higher temperatures than non-
ferromagnesian silicates.
 Ferromagnesian silicates contain greater amounts of iron (Fe) and
magnesium (Mg) and less silica (Si O2) than non-ferromagnesian
silicates .
 Ferromagnesian minerals are generally dark in color and can be
greenish, black, or dark grey.
 Common ferromagnesian silicate minerals include olivine, pyroxene,
amphibole, biotite, hornblende, augite, and peridote.
Gabbro rock with olivine (yellowish crystals) and
Pyroxene (darker crystals) phenocrysts

Non-Ferromagnesian Silicates
 Non-ferromagnesian silicates crystallize at lower temperatures that ferromagnesian
silicates.
 Non-ferromagnesian silicates contain greater amounts of potassium (K), sodium (Na),
and calcium (Ca) in combination with more silica (Si O2) than ferromagnesian silicates.
 Non-ferromagnesian minerals are generally light colored, and may be white, pink, or
light grey.
 Common non-ferromagnesian silicate minerals include quartz, muscovite, and
feldspars.
Granite composed of non-
ferromagnesian
silicates including feldspar
(pink crystals) and quartz
(white crystals).

Igneous Rock Categories:
Felsic to Mafic
 Igneous rocks are divided into three broad groups Granitic, Basaltic,
and Andesitic depending on their proportion of felsic (light-colored)
to mafic (dark-colored) minerals.
 Granitic rocks contain more light-colored feldspars and silica than
dark- colored minerals. Because of the high feldspar and silica
content of Granitic rocks, geologists refer to them as being felsic (fel
for feldspar and si for silica).
 The primary minerals in granitic rocks include quartz, feldspar, biotite,
and amphibole.
 Granitic rocks make up about 70% of the Earth’s crust.
 Basaltic rocks contain mostly darker silicate minerals and calcium-
rich plagioclase feldspar and little quartz. Because of the high
percentage of ferrromagnesian minerals in basaltic rocks, geologist
refer to them as mafic (ma for magnesium and f for ferrum).
 Basaltic rocks are dark colored and tend to be more dense than granitic
rocks.
 Andesitic rocks have a composition between granites and basalts.
 They generally contain about 25% dark silicate minerals (amphibole,
pyroxene, and biotite mica) with the remaining 75% consisting of
plagioclase feldspar.
Felsic
Mafic

Igneous Rocks
 Granite
 Pumice
 Obsidian
 Gabbro
 Basalt
 Diorite
 Tuff

Granite
 Granite is a felsic intrusive igneous rock and has either a phaneritic or porphyritic texture.
 Granite cools very slowly and often forms large masses of rock that are referred to as
plutons or batholiths.
 Granite usually contains about 20-50% quartz, 30-60% feldspar, and the remaining 5-10%
darker minerals such as biotite.
 The quartz grains are usually spherical in shape and are a white to grayish color.
 The feldspars grains are mostly potassium and sodium rich varieties with individual
rectangular shaped grains. The feldspars are often white, grey, or pinkish in color
depending on the chemical composition.
 The remaining darker minerals usually consist of muscovite, biotite and amphibole
and are generally black.
Coarse-grained granite Fine-grained granite
Feldspar
Quartz
Biotite
Table of Contents

Pumice
 Pumice is a felsic, extrusive igneous rock with a glassy, vesicular texture
formed from a combination of rapid cooling and a high gas content.
 Pumice forms in similar condition as obsidian, and the two can often be
found in close proximity.
 Pumice is so light from the presence of lots of gas bubbles pockets that
it often floats when placed in water.
Copyright © 2006 Andrew Alden, geology.about.com,
reproduced under educational fair use."
Pumice with a vesicular texture
Table of Contents

Obsidian
 Obsidian is a felsic, extrusive igneous rock with a glassy texture.
 Obsidian forms very quickly from the rapid cooling of silica-rich lava.
 Unlike other minerals and rocks, the ions that form obsidian are
unordered, or amorphous, meaning they have no structure, and as a
result it produces a conchoidal fracture when broken.
 Thin sections of obsidian appear translucent and it is the presence of
various metallic ions that give it an overall dark appearance.
Copyright © 2006 Andrew Alden, geology.about.com,
reproduced under educational fair use
Obsidian
Table of Contents

Gabbro
 Gabbro is a mafic, intrusive medium to coarse-grained igneous rock
with a phaneritic texture.
 Gabbro is composed primarily of pyroxene, with calcium-rich
plagioclase feldspar and small amounts of olivine and amphibole.
 Large gabbro intrusions are often sources of economically valuable
nickel, chromium, and platinum.
Medium-grained gabbro
Table of Contents
Department of

Basalt
 Basalt is a mafic, extrusive fine-grained dark green to black volcanic
rock with a porphyritic texture.
 Basalt is composed primarily of pyroxene, and calcium-rich plagioclase
with small amounts of olivine and amphibole.
Table of Contents

Diorite
 Diorite is an intermediate, intrusive igneous rock with a predominantly
coarse-grained phaneritic texture .
 Diorite is composed of quartz, sodium-rich plagioclase, and amphibole
or biotite.
 The composition of diorite looks similar to granite, except that diorite
contains a greater concentration of darker mafic minerals.
Table of Contents
http://www.mii.org/index.html

Tuff
 Tuff is an extrusive, pyroclastic rock composed of an aggregate of tiny ash
fragments ejected during volcanic eruption.
 A mixture of various other extrusive rock fragments may weld with tuff
making a cemented mass of ash and other rock/mineral fragments.
 Tuff may also be used as a descriptor along side other rocks depending on
the relative concentration of rock to ash ratio, for example a rhyolite tuff.
Table of Contents

Igneous Rocks in the Landscape
Mount Rushmore in the Black Hills of South Dakota
is a Precambrian igneous, granitic batholith.
Copyright © David Spear
Table of Contents
Devils Tower at Devils Tower National
Monument in Wyoming is an intrusive igneous
rock formation that is exposed from millions of
years of weathering and erosion of the
surrounding landscape.
Copyright © Louis Maher University of Wisconsin

Resources
Courtesy :department of natural resources south Carolina geological survey
Table of Contents

Lecture 03 igeneous rock

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