Igneous Rocks. Characteristics of magma Igneous rocks form as molten rock cools and solidifies Characteristics of magma (molten rock) Parent material.

Igneous Rocks

Igneous Rocks

Characteristics of magma

Igneous rocks form as molten rock cools and solidifies

Characteristics of magma (molten rock)• Parent material of igneous rocks• Forms from partial melting of rocks inside

the Earth• Magma that reaches the surface is lava

through a volcano or crack (fissure)

Characteristics of magma

Molten rock (magma) originates in the mantle andcrust as a result of partial melting of solid rock

Characteristics of magma

General Characteristic of magma• Rocks formed from lava at the surface are

classified as extrusive, or volcanic rocks• Rocks formed from magma that

crystallizes at depth are termed intrusive, or plutonic rocks

Extrusive Igneous Rock - Lava

Extrusive Igneous Rock - Lava (Hawaii)

Intrusive Igneous Rock

Most magma never reaches the surface. Rather it crystallizes at depth to produce intrusive or plutonic igneous rocks

Intrusive Igneous Rock (Granite)

Intrusive Igneous Rock

Although intrusive igneous rocks form at depth, upliftingand erosion often expose them at the surface

Characteristics of magma

The nature of magma• Consists of three components:

– A liquid portion, called melt, that is composed of mobile ions

– Solids, if any, are silicate minerals that have already crystallized from the melt

– Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

Characteristics of magma

Crystallization of magma• Cooling of magma results in the

systematic arrangement of ions into orderly patterns

• The silicate minerals resulting from crystallization form in a predictable order

Crystallization of minerals in magma bodies

Bowen’s reaction series and the composition of igneous rocks

• N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points

• A listing of the most common silicate minerals in igneous rocks

• Different minerals crystallize from magmas at different temperatures

Bowen’s reaction series

Bowen’s reaction series also lists the minerals in the order in which they melt with increasing temperatures

Minerals higher on the reaction series crystallize before minerals lower on the reaction series

Bowen’s reaction series has two branches. They are:Discontinuous reaction series, from olivine to biotite

Continuous reaction series, from Ca plagioclase to Na plagioclase

Bowen’s reaction series

The discontinuous reaction series involves the dark-colored ferromagnesian minerals:

OlivinePyroxeneAmphiboleBiotite

Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure

Bowen’s reaction series

The continuous reaction series involves the plagioclase feldsparsPlagioclase feldspars consists of two end members:

Albite or Na plagioclase, the sodium end memberAnorthite or Ca plagioclase, the calcium end member

The series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure

Bowen’s reaction series

Bowen’s reaction series help us to understand why certain minerals tend to occur together in igneous rocks

For example, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar

These are all minerals which crystallize at high temperatures

Bowen’s reaction series

Bowen’s reaction series help us to understand why certain minerals do Not occur together in igneous rocks

For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature, and quartz is a low temperature mineral

Bowen’s Reaction Series

GraniteHand Sample

GraniteThin Section

Order ofCrystallization

Zoned feldspar (plagioclase) showing change in composition with time in magma chamber

(calcic in core to sodic at rim)

Processes responsible for changing a magma’s composition ( تغير عن المسؤولة العمليات

الماجما ( تركيب

Crystal Settling ( البلورات (ترسيبAssimilation ( في خارجية مواد ادخال

النارية (الحشوةMagma Mixing ( الماجما (اختالط

Magmatic Differentiation( الصخور من انواع عدة (تكوين

The formation of several different rock types from one initial magma, through separation of earlier-formed crystals, causing the magma to evolve to become more silica-rich

Magmatic DifferentiationCrystal Settling: As a magma cools, the early-formed crystals may settle to the bottom of the magma chamber. This would produce a rock type at the bottom of the magma chamber that is dominated by early-formed minerals such as olivine, pyroxene, and calcic plagioclase

Crystallized minerals have a density greater than the magma and settle to the bottom due to gravity

Because Fe and Mg are first removed, melt becomes rich in SiO2, Na, and K

Magmatic DifferentiationAssimilation: Magma reacts with the “country rock” ( مضيف which is adjacent to the magma (صخرchamber ( الصهارة (غرفة

Magma composition is altered according to the composition of the assimilated country rockInclusions are rocksincompletely melted chunks of country rock

Magmatic DifferentiationMagma Mixing: Magmas of different compositions are mixed together

Resulting magma is of a composition intermediate between the parents

Magma Mixing

Magma Mixing

Characteristics of magma ( خصائص(الماجما

Crystallization of magma ( الماجما (تبلور• Texture in igneous rocks is determined by the

size and arrangement of mineral grains• Igneous rocks are typically classified by both

( على يعتمد النارية الصخور :(تصنيف– Texture ( النسيج) – Mineral composition ( المعدني (التركيب

Igneous texturesالنارية ) الصخور (انسجة

Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals ( متداخلة (معادنFactors affecting crystal size ( التي العوامل

البلورات حجم على (تؤثر• Rate of cooling ( التبريد (معدل

– Slow rate promotes the growth of fewer but larger crystals (thousands to millions of years)

Igneous texturesالنارية ) الصخور (انسجة

Factors affecting crystal size• Rate of cooling ( التبريد (معدل

– Fast rate forms many small crystals (hours, days, weeks)

– Very fast rate forms glass (seconds, minutes)

• Amount of silica (SiO2) present ( نسبة(السليكا

• Amount of dissolved gases ( الغازات (كمية

Types of Igneous textures( النارية الصخور انسجة (انواع

Types of igneous textures• Aphanitic (fine-grained) texture ( دقيقة نارية صخور(النسيج

– Rapid rate of cooling of lava or magma

– Microscopic crystals

– May contain vesicles ( الغازات تسرب عن الناتجة holes) (الفتحاتfrom gas bubbles in the lava or magma)

– Very porous; may resemble a sponge; low density; may float on water

• Phaneritic (coarse-grained) texture ( خشنة نارية صخور(النسيج

– Slow cooling

– Crystals can be identified without a microscope

Aphanitic texture

Phaneritic texture

Igneous textures

Types of igneous textures• Porphyritic texture ( بورفيرية نارية صخور(النسيج

– Minerals form at different temperatures as well as differing cooling rates

– Large crystals, called phenocrysts ( كبيرة ,(حبيباتare embedded in a matrix of smaller crystals, called the groundmass ( الصخور من ناعمة حبيبات(النارية

• Glassy texture ( النسيج زجاجية نارية (صخور– Very rapid cooling of molten rock– Resulting rock is called obsidian

Porphyritic texture

Porphyritic texture

Granite

Glassy texture

Obsidian

Types of Igneous textures

Types of igneous textures• Pyroclastic texture ( نارية فتاتيه نارية صخور(النسيج

– Various fragments ejected during a violent volcanic eruption– Textures often appear to more similar to sedimentary rocks

• Pegmatitic texture ( متباينة نارية صخور(النسيج

– Exceptionally coarse grained (many over 2 cm)– Form in late stages of crystallization of granitic magmas– Granite pegmatite or pegmatitic granite

Pyroclastic Rock

Pegmatitic texture

Igneous Compositions ( التركيب(المدني

Igneous rocks are composed primarily of silicate minerals that include:

• Dark (or ferromagnesian) silicates

( اللون داكنة سليكاتية (معادن– Olivine

– Pyroxene

– Amphibole

– Biotite mica

Igneous Compositions ( المدني (التركيب

Igneous rocks are composed primarily of light silicate minerals that include:

( اللون ناصعة سليكاتية (معادن– Quartz

– Muscovite mica

– Feldspars

Igneous Compositions( المدني (التركيب

Igneous compositions ( المدني (التركيب

Granitic versus basaltic compositions• Granitic composition

– Composed of light-colored silicates

– Designated as being felsic (feldspar and silica) in composition

– Contains high amounts of silica (SiO2)

– Major constituents of continental crust

Igneous compositions

Naming igneous rocks – granitic (felsic) rocks

• Granite– Phaneritic– Over 65 percent quartz, about 25 percent or

more feldspar– Abundant and often associated with

mountain building– The term granite covers a wide range of

mineral compositions

Granite

Granite rocks contain 70% silica and are abundant in the continental crust

Granite

Igneous compositions

Naming igneous rocks – granitic (felsic) rocks

• Rhyolite– May contain glass fragments and vesicles

– Aphanitic texture

– Less common and less voluminous than granite

– Phenocrysts can include quartz and feldspar

Rhyolite

Igneous compositions

Granitic versus basaltic compositions• Basaltic composition

– Composed of dark silicates and calcium-rich feldspar

– Designated as being mafic (magnesium and ferrum, for iron) in composition

– More dense than granitic rocks

– Comprise the ocean floor as well as many volcanic islands

Igneous compositions

Naming igneous rocks – basaltic (mafic) rocks

• Basalt– Volcanic origin

– Aphanitic texture

– Composed mainly of pyroxene and calcium-rich plagioclase feldspar

– Most common extrusive igneous rock

Basalt

Basaltic rocks are rich in dark minerals and contain about 50% silica. Basalt makes up the ocean floor as well as lava flows and volcanic structures.

Basalt

Basalt (Scoria with Gas Bubbles)

Basalt (Scoria with Gas Bubbles)

Igneous compositions

Naming igneous rocks – basaltic (mafic) rocks

• Gabbro– Intrusive equivalent of basalt

– Phaneritic texture consisting of pyroxene and calcium-rich plagioclase

– Makes up a significant percentage of the oceanic crust

Gabbro - a maficIgneous rock

Igneous compositions

Other compositional groups• Intermediate (or andesitic) composition

– Contain at least 25 percent dark silicate minerals

– Associated with explosive volcanic activity

• Ultramafic composition– Rare composition that is high in magnesium

and iron– Composed entirely of ferromagnesian

silicates

Igneous compositions

Naming igneous rocks – intermediate rocks

• Andesite– Volcanic origin

– Aphanitic texture

– Often resembles rhyolite

Andesite

Igneous compositions

Naming igneous rocks – intermediate rocks

• Diorite– Plutonic equivalent of andesite

– Coarse grained

– Intrusive

– Composed mainly of intermediate feldspar and amphibole

Diorite

Igneous compositions

Silica content influences a magma’s behavior

• Granitic magma– High silica content

– Extremely viscous

– Liquid exists at temperatures as low as 700oC

– Goes Boom when it erupts

Igneous compositions

Silica content influences a magma’s behavior

• Basaltic magma– Much lower silica content

– Fluid-like behavior

– Crystallizes at higher temperatures

Summary - Igneous Rock Classification

Igneous compositions

Naming igneous rocks – granitic (felsic) rocks. Extrusive products can include:

• Obsidian– Dark colored– Glassy texture

• Pumice– Volcanic– Glassy texture– Frothy appearance with numerous voids

(extrusive foam)

Obsidian

Pumice

Igneous compositions

Naming igneous rocks – pyroclastic rocks• Composed of fragments ejected during a

volcanic eruption• Varieties

– Tuff – ash-sized fragments (less than 2 mm)

– Volcanic breccia – particles larger than ash (greater than 2 mm)

Ash and pumice layers

Summary - Igneous Rock Texture and Composition

FORMS OF INTRUSIVE IGNEOUS ROCKS( النارية الصخور (اشكال

• Commonly observed forms of Plutonic (Subsurface igneous bodies) rocks observed in the field are: dykes, sills, laccoliths, batholiths and Stock.

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

• Concordant: Oriented parallel to surrounding layered rocks. Formed from magma that is injected between the layers of the surrounding rock.

• Discordant:Oriented at an angle to surrounding layered rocks. Magmas cut through the existing layering

FORMS OF INTRUSIVE IGNEOUS ROCKS

Dykes

• They are discordant• Cut across the bedding of the rocks in which

they intrude• Vertical to steeply inclined and sheetlike body• Thickness vary widely from less than 1 cm to 1

km • Injected through fractures, and joints

Mafic dyke

Dykes

SILLS

Sills are relatively thin tabular sheetlike body that penetrates parallel to the bedding planes.

Laterally it may extends for 100s of km and up to 10 km in width.

Mostly sills are made up of gabbros, dolerites and basalts

mudstone

sandstone

limestone

Sills

LACCOLITHS: It is a concordant body, with flat bottom and convex upward. It is dome shaped.

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.

Their occurrence is commonly associated with the mountain-building process

More than 100 km2

These are either granites or granodiorites in composition

BATHOLITHS: are the largest kind of plutons, irregular in shape and occupies large area.

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