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