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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 and
crust 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, uplifting
and 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:
Olivine
Pyroxene
Amphibole
Biotite
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 feldspars
Plagioclase feldspars consists of two end members:
Albite or Na plagioclase, the sodium end member
Anorthite 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
Granite
Hand Sample
Order of
Crystallization
Granite
Thin Section
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
‫)العمليات المسؤولة عن تغير تركيب الماجما ( ‪composition‬‬
‫)ترسيب البلورات( ‪Crystal Settling‬‬
‫)ادخال مواد خارجية في الحشوة النارية( ‪Assimilation‬‬
‫)اختالط الماجما( ‪Magma Mixing‬‬
Magmatic Differentiation
(‫)تكوين عدة انواع من الصخور‬
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 Differentiation
Crystal 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 Differentiation
Assimilation: Magma reacts with the “country rock”
(‫ )صخر مضيف‬which is adjacent to the magma chamber
(‫)غرفة الصهارة‬
Magma composition is altered according to the composition
of the assimilated country rock
Inclusions are rocks
incompletely melted
chunks of country
rock
Magmatic Differentiation
Magma 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 (‫)تبلور الماجما‬
• Texture in igneous rocks is determined by the
size and arrangement of mineral grains
• Igneous rocks are typically classified by both
(‫)تصنيف الصخور النارية يعتمد على‬:
– Texture ( ‫)النسيج‬
– Mineral composition (‫)التركيب المعدني‬
Igneous textures
(‫) انسجة الصخور النارية‬
Texture is used to describe the overall
appearance of a rock based on the size, shape,
and arrangement of interlocking minerals
(‫)معادن متداخلة‬
Factors affecting crystal size ( ‫العوامل التي تؤثر‬
‫)على حجم البلورات‬
• Rate of cooling (‫)معدل التبريد‬
– Slow rate promotes the growth of fewer but larger
crystals (thousands to millions of years)
Igneous textures
(‫) انسجة الصخور النارية‬
Factors affecting crystal size
• Rate of cooling (‫)معدل التبريد‬
– 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 (‫)كمية الغازات‬
Types of Igneous textures
(‫)انواع انسجة الصخور النارية‬
Types of igneous textures
• Aphanitic (fine-grained) texture (‫)صخور نارية دقيقة النسيج‬
– Rapid rate of cooling of lava or magma
– Microscopic crystals
– May contain vesicles (‫( )الفتحات الناتجة عن تسرب الغازات‬holes from gas
bubbles in the lava or magma)
– Very porous; may resemble a sponge; low density; may float on
water
• Phaneritic (coarse-grained) texture (‫)صخور نارية خشنة النسيج‬
– Slow cooling
– Crystals can be identified without a microscope
Aphanitic texture
Phaneritic texture
Igneous textures
Types of igneous textures
• Porphyritic texture (‫)صخور نارية بورفيرية النسيج‬
– Minerals form at different temperatures as well as
differing cooling rates
– Large crystals, called phenocrysts (‫)حبيبات كبيرة‬, are
embedded in a matrix of smaller crystals, called
the groundmass (‫)حبيبات ناعمة من الصخور النارية‬
• Glassy texture (‫)صخور نارية زجاجية النسيج‬
– 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 (‫)صخور نارية فتاتيه نارية النسيج‬
– Various fragments ejected during a violent volcanic
eruption
– Textures often appear to more similar to sedimentary
rocks
• Pegmatitic texture (‫)صخور نارية متباينة 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
(‫)معادن سليكاتية داكنة اللون‬
– Olivine
– Pyroxene
– Amphibole
– Biotite mica
Igneous Compositions
(‫)التركيب المدني‬
Igneous rocks are composed primarily of
light silicate minerals that include:
(‫)معادن سليكاتية ناصعة اللون‬
– Quartz
– Muscovite mica
– Feldspars
Igneous Compositions
(‫)التركيب المدني‬
Igneous compositions
(‫)التركيب المدني‬
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 calciumrich plagioclase feldspar
– Most common extrusive igneous rock
Basalt
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 (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 mafic
Igneous 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
(‫)اشكال الصخور النارية‬
• 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
Dykes
Mafic
dyke
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
Sills
limestone
sandstone
mudstone
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.
BATHOLITHS: are the largest kind of plutons, irregular in
shape and occupies large area.
Their occurrence is commonly associated with the
mountain-building process
More than 100 km2
These are either granites or granodiorites in composition
Stocks: Are smaller irregular
bodies with 10 km in maximum
dimension, and are associated
with batholiths.