Igneous rock ge 106

Igneous Rocks
By
Moustafa Hashad
Three Lectures
Lecture - 1
Attendance ??

Table of Contents
Lecture 1 Lecture 2
What are Rocks?
 Major Rock Types
 The Rock Cycle
 Nature, and Origin of
Igneous Rocks
 Igneous Rocks
 What happens to
molten rock as it
cools?
 Crystal size and
Cooling Rates
 Texture, Igneous Rock
Textures
 Phaneritic ,
Aphanitic ,
Porphyritic Glassy
, Pegmatitc
 Pyroclastic Aa Lava,
Pahoehoe
Lecture 3
Structures of
Igneous Rocks.
Mineral
Composition
Magma and
Bowen Reaction
Series
Mineral
Composition and
Magma
Ferromagnesian
Silicates
Non-
Ferromagnesian
Silicates
Classification of Igneous
Rocks.
*Color/Texture Igneous
Classification
**Modal Igneous
Classification
*** Normative Igneous
Classification
Igneous Rock Categories
Granitic, Basaltic,
Andesitic Igneous Rocks
Several types of Igneous
Rocks:
Granite, Rhyolite, Pumice,
Obsidian, Gabbro, Basalt,
Andesite, Diorite, Tuff.

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
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Major Rock Types
 Igneous rocks are formed by the cooling of molten
magma or lava near, at, or below the Earth’s surface.
 Sedimentary rocks are formed by the lithification
of inorganic and organic sediments deposited near or at
the Earth’s surface.
 Metamorphic rocks are formed when preexisting
rocks are transformed into new rocks by heat and
pressure, usually below the Earth’s surface.
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Table of Contents

Major Rock Types
Although the three rock
Categories are connected
in the Endless rock cycle,
igneous rocks
Are traditionally taught
first. Remember details of rock cycle

Nature, and Origin
of
Igneous Rocks

Igneous Rocks
How to Tell Igneous Rocks
 The main thing about Igneous rocks is that they were
once hot enough to melt. The following features are all
related to that.
 Because their mineral grains grew together tightly as
the melt cooled, they are relatively strong rocks.
 They're made of primary minerals that are mostly black,
white or gray. Any other colors they may have are pale.

Igneous Rocks
Origin of Igneous Rocks
The word "igneous" comes from the Latin for fire, and
1- Igneous rocks, in common, have formed by the cooling
and crystallization of a melt. 2- This material may have been:
a- lava erupted at the Earth's surface, and or
b- Magma (un-erupted lava) at depths of up to a few kilometers
(shallow magma), or c- Magma in deeper bodies (plutons).
Those three different settings create three main types of igneous
rocks:
1- Rock formed of lava is called Extrusive Rocks
2- Rock from shallow magma is called Hypabyssal
intrusive Rocks and
3- Rock from deep magma is called Plutonic intrusive
Rocks
The deeper the magma, the slower it cools and the larger the

Origin of Igneous Rocks
هذه الشريحة و التالية
تكرار للشريحة السابقة
 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:
Intrusive and Extrusive.
 Intrusive igneous rocks are formed by magma that
cools below the Earth’s surface: a - at shallow depth
(Old name Hypabyssal Rocks). And, b - at deep depth
.(Old name Plutonic Rocks)
 Extrusive igneous rocks are formed by lava that cools
at the Earth’s surface. (Old name Volcanic Rocks)
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Igneous Rocks
 Intrusive igneous rocks generally cool very slowly
below the earth’s surface or as the magma is rising
to the earth’s surface.
Hypabyssal
Hypabyssal igneous rocks are formed at a depth in between the
plutonic and volcanic rocks. These are formed due to cooling
and resultant solidification of rising magma just beneath the

earth surface. Hypabyssal rocks are less common than plutonic
or volcanic rocks and often form dikes, sills, laccoliths, lopoliths,
or phacoliths
 Extrusive igneous rocks generally cool quickly when they
reach the earth’s surface usually through volcanoes or
fissure.
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Igneous Rocks
Igneous rocks form in three main places:
1- where the tectonic plates
pull apart (at mid-ocean ridges),
2- where plates
come together
(at subduction zones)
and
3- where continents
are pushed together,
making the Earth's crust thicker
and allowing it to heat to melting.

What happens
to
molten rock
as
it cools?

What happens to molten rock as it cools?
1 - 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
definit crystalline structures. This process is
referred to as crystallization.
 So, What is the definition of crystallization ??
 Definition:
 Crystallization:
It is the process where the temperature of molten
rock begins to drop, so, there is a loss of
energy that causes ions to slow down. As the
ions slow down, they group together and
arrange themselves into a crystalline structures.
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What happens to molten rock as it cools? Cont.
2 - 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.
3 - 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.

What happens to molten rock as it cools? Cont.
4 - 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,
(usually referred to crystals as grains), define the
texture of the rock.
So,
 Geologists use mineral and textural
classifications to infer information about the
environmental setting in which different igneous
rocks are formed.
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Crystal size
and
Cooling Rates

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.
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Crystal Size and Cooling Rates
 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.
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Crystal Size and Cooling Rates- C0nt.
 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.
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Texture
and
Igneous Rock Textures

Texture
Definition:
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 of cooling at which molten rock cools;
Slow …… or ….. fast
 2) Amount of dissolved gases or fluids in the
magma.
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Texture Cont.
 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.
26
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
Table of Contents

Igneous Rock Textures - 1
 Phaneritic (Intrusive)
 Aphanitic (Extrusive)
 Porphyritic (Intrusive and Extrusive)
 Glassy (Extrusive)
Pegmatitic (Intrusive)
 Pyroclastic Materials (Extrusive)
 Aa Lava (Extrusive)
 Pahoehoe Lava (Extrusive)
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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.
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Quartz Crystals:
(White)
Feldspar Crystals:
(Pink)
Biotite Crystals:
(Black)
Granite rock with a phaneritic texture
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Aphanitic Texture
 Aphanitic (Extrusive)
 Definition: 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.
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.
29
Basalt rock with an
aphanitic
and vesicular texture
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Porphyritic Texture
 Porphyritic (Intrusive and Extrusive)
Definition: 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.
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Rhyolite rock with
porphyritic texture containing
phenocrysts of olivine and
pyroxene and a gabbro
groundmass.
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Porphyritic
Texture
Definition:
Large crystals
called
phenocrysts are
surrounded by
an aphanitic
matrix or
groundmass.

Images for porphyritic textures

Glassy Texture
 Glassy (Extrusive)
 Definition: Glassy textured rocks are formed by very rapid cooling of
magma.
Obsidian rock with a glassy
texture and conchoidal
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.
fractures
 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. 36
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Pegmatitic Texture
 Pegmatitic (Intrusive)
veins extending through rock
 Definition: Pegmatitic rocks
contain large interlocking
crystalline grains > 1-2 centimeter in
diameter.
Examples of pegmatitic
Pegmatites are commonly composed of quartz,
feldspar, and mica minerals.
Pegmatites 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. 37
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Pegmatitic Texture
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 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
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extending through rock
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Igneous Rock Textures – 6a

Pyroclastic Materials
 Pyroclastic (Extrusive)
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.
Tuff rock with
pyroclastic material.
 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.
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Pyroclastic Fragments
 Pyroclastic materials do not conform well to igneous grain size
classifications. Instead, geologists use the following terms to
describe pyroclastic fragments:
 Bombs: fragments with a mean diameter > 64 mm, and a
rounded shape.
 Blocks: fragments with a mean diameter > 64 mm, and a
blocky, angular shape.
 Lapilli: fragments in any shape with a mean diameter of 2-
64 mm.
 Ash: grains of pyroclastic fragments with a mean diameter
< 2 mm. This includes coarse ash grains (0.032 -2 mm) and
fine ash grains (< 0.032 mm).
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Aa 0r aa Lava
 Aa Lava (Extrusive)
 Aa is a basaltic lava flow that has a rough surface,
characterized by sharp and jagged blocks.
 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
 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.
Aa lava in Hawaii’s
Volcanoes National Park.
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Pahoehoe Lava
 Pahoehoe Lava (Extrusive)
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.
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Pahoehoe lava
flows in Hawaii.
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IGNEOUS INTRUSIONS / PLUTONS
intrusions/ plutons
As magma cools underground, it will begin to solidify within
Earth's crust. Once completely crystallized, these masses of
igneous intrusive rock are referred to as intrusions or as
plutons (after Pluto, Greek god of the underworld). Both terms
are used interchangeably by geologists, so take your pick.
Dikes - are relatively small
igneous intrusions formed as
magma penetrates into rock
fractures and solidifies into thin,
sheet-like bodies. Where dikes
are very abundant they are
referred to as dike swarms.
Three-inch thick dike
formed within a larger mass
of granite

Igneous rock ge 106

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