GE 106
Lecture 2
Igneous RocksBy
Moustafa Hashad
Three Lectures
Lecture - 1
Attendance ??
Table of ContentsLecture 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.
Major Rock Types
and
Part 1
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 moltenmagma or lava near, at, or below the Earth’s surface.
Sedimentary rocks are formed by the lithificationof inorganic and organic sediments deposited near or atthe Earth’s surface.
Metamorphic rocks are formed when preexistingrocks are transformed into new rocks by heat andpressure, usually below the Earth’s surface.
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Major Rock TypesAlthough the three rock
Categories are connected
in the Endless rock cycle,
igneous rocksAre traditionally taught
first.Remember details of rock cycle
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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 RocksOrigin 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 mineral crystals that form in it.
Origin of 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: 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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هذه الشريحة و التالية ةتكرار للشريحة السابق
Igneous Rocks Intrusive igneous rocks generally cool very slowly
below the earth’s surface or as the magma is risingto the earth’s surface.
Extrusive igneous rocks generally cool quickly when theyreach the earth’s surface usually through volcanoes orfissure.
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HypabyssalHypabyssal 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
Igneous RocksIgneous 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 beginsto drop there is a loss of energy that causesions to slow down. As the ions slow down, theygroup together and arrange themselves intodefinit crystalline structures. This process isreferred to as crystallization.
So, What is the definition of crystallization ??
Definition:
Crystallization:
It is the process where the temperature of moltenrock begins to drop, so, there is a loss ofenergy that causes ions to slow down. As theions slow down, they group together andarrange 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-oxygentetrahedrons, which are the building block of all silicateminerals.
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 temperatureand 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 texturalclassifications to infer information about theenvironmental setting in which different igneousrocks are formed.
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Crystal size
and
Cooling Rates
Crystal Size and Cooling Rates Slower cooling rates produce
larger individual crystals inthe rock
Intrusive igneous rocksgenerally cool very slowlyand tend to have largecrystals that produce acourse-grained rock.
Phaneritic rocks arecoarse-grained rockswhich contain individualcrystals that are relativelyeven in size and largeenough for scientists toidentify the differentmineral grains thatcompose the rock.
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Crystal Size and Cooling Rates
Faster cooling rates producesmaller individual crystals inthe rock
Extrusive igneous rockstend to cool quickly andare characterized bysmaller grains thatproduce a fine-grainedrock.
Aphanitic is the termused to describe very finegrained 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 rockbegin to develop as the magma is coolingbelow the surface of the earth. Followingeruption or exposure to lowertemperatures, the remaining magma or lavacools very quickly and forms minerals withfine-grained textures.
As a result, porphyritic textures containboth coarse- and fine–grained minerals.
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Textureand
Igneous Rock Textures
TextureDefinition:
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.25
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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.
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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 mmTable 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 TexturePhaneritic (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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Igneous Rock Textures - 2
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.
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Basalt rock with an
aphanitic
and vesicular texture
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Igneous Rock Textures - 3
Porphyritic Texture Porphyritic (Intrusive and Extrusive)
Definition: Porphyritic rockscontain 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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Igneous Rock Textures - 4
Porphyritic
Texture
Definition:
Large crystals
called
phenocrysts are
surrounded by
an aphanitic
matrix or
groundmass.
Images for porphyritic textures
Images for porphyritic textures
Glassy Texture Glassy (Extrusive)
Definition: 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.36
Obsidian rock with a glassy
texture and conchoidal
fractures
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Igneous Rock Textures - 5
Pegmatitic Texture
Pegmatitic (Intrusive)
Definition: Pegmatitic rocks
contain large interlocking
crystalline grains > 1-2 centimeter in
diameter.
Pegmatites are commonly composed of quartz, feldspar, and mica minerals.
Pegmatites form from a combination of hydrothermaland igneous processes; and is dependant on thepresence of fluids and volatiles such as water,chlorine, bromine, sulfur, and fluorine. 37
Examples of pegmatitic
veins extending through rock
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Igneous Rock Textures - 6
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.
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Examples of pegmatitic veins
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.
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.39
Tuff rock with
pyroclastic material.
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Igneous Rock Textures - 7
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).40
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Igneous Rock Textures - 8
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.
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Aa lava in Hawaii’s
Volcanoes National Park.
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Igneous Rock Textures - 9
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.42
Pahoehoe lava
flows in Hawaii.
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Igneous Rock Textures - 10
IGNEOUS INTRUSIONS / PLUTONS
intrusions/ plutonsAs 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
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EndOf
Lecture 2
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