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Engineering Geology
CVL 3315
Chapter 3
Igneous Rock
Dr. Sari Abusharar
University of Palestine
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
2nd Semester 2014-2015
1
Outline of Presentation
Magma: The Parent Material of Igneous Rock
Igneous Textures
Classification of Igneous Rocks
Igneous Compositions
Origin of Magma
Evolution of Magmas
Intrusive Igneous Activity
2
3
General characteristics of magma
Igneous rocks form as molten rock cools
and solidifies
General Characteristic of magma
• Parent material of igneous rocks
• Forms from partial melting of rocks
inside the Earth
• Magma that reaches the surface is called
lava
Magma vs. Lava
5
General characteristics of magma
General Characteristic of magma
• Rocks formed from lava at the surface are
classified as extrusive ‫ سطحية‬, or volcanic
rocks
• Rocks formed from magma that
crystallizes at depth are termed intrusive
‫تحت سطحية‬, or plutonic rocks ‫صخور جوفية‬
General 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)
General characteristics of magma
Crystallization of magma
• Cooling of magma results in the
systematic arrangement of ions into
orderly patterns
• Texture in igneous rocks is determined by
the size and arrangement of mineral
grains
• Igneous rocks are typically classified by
– Texture
– Mineral composition
Igneous textures
Texture is used to describe the overall
appearance of a rock based on the size,
shape, and arrangement of interlocking
minerals
Igneous textures
Factors affecting crystal size
• Rate of cooling
– Slow rate promotes the growth of fewer but
larger crystals
– Fast rate forms many small crystals
– Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases
Among these, the rate of cooling tends to be the
dominant factor.
Igneous textures
Types of igneous textures
• Aphanitic ‫(نسيج دقيق‬fine-grained) texture
– Rapid rate of cooling of lava or magma
– Microscopic crystals
– May contain vesicles ‫( فجوات‬holes from gas
bubbles)
• Phaneritic
‫(نسيج خشن‬coarse-grained)
– Slow cooling
– Large, visible crystals
– Crystals can be identified without a
microscope
texture
Comparison of aphanitic (fine-grained) and phaneritic
(coarse-grained) igneous rock textures.
Felsic Rock ‫فلسي‬
A term used to describe an igneous rock
that has a large percentage of lightcolored minerals such as quartz, feldspar,
and muscovite. Also used in reference to
the magmas from which these rocks
crystallize. Felsic rocks are generally rich
in silicon and aluminum and contain only
small amounts of magnesium and iron.
Granite and rhyolite are examples of felsic
rocks.
Granite
Intermediate Rock
An igneous rock that has an intermediate
silica content. Examples are syenite and
diorite
Diorite
Mafic Rock
A term used to describe an igneous rock
that has a large percentage of darkcolored minerals such as amphibole,
pyroxene and olivine. Also used in
reference to the magmas from which these
rocks crystallize. Mafic rocks are
generally rich in iron and magnesium
Gabbro
Igneous textures
Types of igneous textures
• Porphyritic ‫بورفيري‬texture
– Minerals form at different temperatures as
well as differing rates
– Large crystals, called phenocrysts, are
embedded in a matrix of smaller crystals,
called the groundmass
• Glassy texture
– Very rapid cooling of molten rock
– Resulting rock is called obsidian
Porphyritic texture
Glassy Texture - Obsidian
Igneous textures
Types of igneous textures
• Pyroclastic ‫فتاتي‬texture
– Various fragments ejected during a violent
volcanic eruption
– Textures often appear to more similar to
sedimentary rocks
• Pegmatitic ‫خشن جدا‬texture
– Exceptionally coarse grained
– Form in late stages of crystallization of
granitic magmas
Mineralogy of common igneous rocks and the
magmas from which they form
‫‪Igneous rock textures‬‬
‫نسيج زجاجي‬
‫‪Pumice‬‬
‫نسيج فتاتي‬
‫‪Ignimbrite‬‬
‫نسيج أفانيتي ) دقيق‪/‬خفي التحبب(‬
‫‪Basalt‬‬
‫نسيج بورفيري‬
‫‪Andesite‬‬
‫‪porphyry‬‬
‫نسيج فانيري )خشن‪/‬مرئي(‬
‫‪Granite‬‬
Classification of major igneous rocks based
on mineral composition and texture
Common igneous rocks
Igneous Compositions
Igneous rocks are composed primarily of
silicate minerals
• Dark (or ferromagnesian) silicates
– Olivine, Pyroxene, Amphibole, Biotite mica
• Light (or nonferromagnesian) silicates
– Quartz, Muscovite mica, Feldspars
Igneous compositions
Granitic versus basaltic compositions
• Granitic composition
– Composed of light-colored silicates
– Designated as being felsic ‫( فلسي‬feldspar and
silica) in composition
– Contains high amounts of silica (SiO2)
– Major constituents of continental crust
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
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
Silica content as an indicator of
composition
• Silica content in crustal rocks exhibits a
considerable range
– A low of 45 percent in ultramafic rocks
– Over 70 percent in felsic rocks
Igneous compositions
Silica content influences a magma’s
behavior
• Granitic magma
– High silica content
– Extremely viscous
– Liquid exists at temperatures as low as 700oC
• Basaltic magma
– Much lower silica content
– Fluid-like behavior
– Crystallizes at higher temperatures
Igneous compositions
Naming igneous rocks – granitic (felsic)
rocks
• Granite
– Phaneritic ‫خشن‬
– Over 25 % quartz, about 65 % or more
feldspar
– May exhibit a porphyritic texture ‫سماقي‬/‫نسيج بورفيري‬
– Very abundant as it is often associated with
mountain building
– The term granite covers a wide range of
mineral compositions
Granite
Igneous compositions
Naming igneous rocks – granitic (felsic)
rocks
• Rhyolite ‫ريوليت‬
– Extrusive equivalent of granite
– May contain glass fragments and vesicles
– Aphanitic texture
– Less common and less voluminous than
granite
Rhyolite
Igneous compositions
Naming igneous rocks – granitic (felsic)
rocks
• Obsidian ‫أوبسيديان‬
– Dark colored
– Glassy texture
• Pumice ‫بيوميس – الحجر الخفاف‬
– Volcanic
– Glassy texture
– Frothy appearance with numerous voids
Obsidian
Pumice
Igneous compositions
Naming igneous rocks – intermediate
rocks
• Andesite ‫أنديزيت‬
– Volcanic origin
– Aphanitic texture
– Often resembles rhyolite
Andesite
Igneous compositions
Naming igneous rocks – intermediate
rocks
• Diorite ‫ديوريت‬
– Plutonic equivalent of andesite
– Coarse grained
– Intrusive
– Composed mainly of intermediate feldspar
and amphibole
Diorite
Igneous compositions
Naming igneous rocks – basaltic (mafic)
rocks
• Basalt
‫بازلت‬
– Volcanic origin
– Aphanitic texture
– Composed mainly of pyroxene and calciumrich plagioclase feldspar
– Most common extrusive igneous rock
Basalt (Scoria ‫)اسكوريا‬
Igneous compositions
Naming igneous rocks – basaltic (mafic)
rocks
• Gabbro ‫جابرو‬
– Intrusive equivalent of basalt
– Phaneritic texture consisting of pyroxene and
calcium-rich plagioclase
– Makes up a significant percentage of the
oceanic crust
Gabbro
Igneous compositions
Naming igneous rocks – pyroclastic rocks
• Composed of fragments ejected during a
volcanic eruption
• Varieties
– Tuff ‫ –طف‬ash-sized fragments (less than 2 mm)
– Volcanic breccia ‫ –بريشيا بركانية‬particles larger than
ash (greater than 2 mm)
Ash and pumice layers
Origin of Magma
Generating magma
from solid rock
• Produced from
partial melting
of rocks in the
crust and upper
mantle
Origin of Magma
• Role of heat
– Temperature increases
within Earth’s upper crust
(called the geothermal
gradient) average 25oC per
kilometer
– Rocks in the lower crust
and upper mantle are near
their melting points.
– Additional heat may induce
melting.
A Typical Geothermal Gradient
‫تدرج حرارة األرض‬
Origin of Magma
• Role of pressure
– An increase in confining
pressure causes an
increase in a rock’s
melting temperature or
conversely, reducing the
pressure lowers the
melting temperature
– When confining pressures
drop, decompression
melting occurs ‫انصھار عند‬
‫تخفيض الضغط‬
A Typical Geothermal Gradient
decompression melting
Role of volatiles
Evolution of magmas
• A single volcano may extrude lavas exhibiting
very different compositions
• Magma become the source of a variety of
igneous rocks.
• The crystallization of magma (basaltic magma)
occurs over a range of at least 200°C.
Evolution of magmas
Bowen’s reaction series
• N.L. Bowen demonstrated that as a
magma cools, minerals crystallize in a
systematic fashion based on their melting
points
• During crystallization, the composition of
the liquid portion of the magma
continually changes.
Evolution of magmas
Bowen’s reaction series
• At the stage when about a third of the
magma has solidified, the melt will be
nearly depleted (‫)استنزف‬of iron, magnesium
and calcium because these elements are
major constituents of the earliest-formed
minerals.
• The removal of these elements causes the
melt to become enriched in sodium and
potassium.
Bowen’s Reaction Series
Bowen’s reaction series shows the sequence in which
minerals crystallize from a magma
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Magmatic differentiation ‫تمايز صھارى‬
• Assimilation ‫تمثل‬
• Magma mixing
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Magmatic differentiation ‫تمايز‬
– Separation of a melt from earlier formed crystals
to form a different composition of magma
‫– أى عملية تسبب انفصال الصھارة أثناء برودتھا إلى طورين سائلين أو أكثر‬
. ‫لھما تركيب مختلف‬
Magmatic differentiation
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Assimilation ‫تمثل‬
– Changing a magma’s composition by the
incorporation of foreign matter (surrounding
rock bodies) into a magma.
‫ مثل أجزاء‬، ‫– ابتالع الصھارة ألجسام صلبة أو سائلة غريبة وھضمھا‬
‫ وتعرف الصھارة أو الصخر الناتج عنھا بعد‬. ‫من الصخر المضيف‬
. ‫التمثيل بالصھارة أو الصخر الھجين‬
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Magma mixing
– Two chemically distinct magmas may
produce a composition quite different from
either original magma.
Magmatic differentiation, assimilation, and magma
mixing
Evolution of magmas
Partial melting and magma formation
• Incomplete melting of rocks is known as
partial melting
• Formation of basaltic magmas
– Most originate from partial melting of
ultramafic rock in the mantle
– Basaltic magmas form at mid-ocean ridges
(‫ )حيد وسط المحيط‬by decompression melting or at
subduction zones
Evolution of magmas
Partial melting and magma formation
• Formation of basaltic magmas
– As basaltic magmas migrate upward,
confining pressure decreases which reduces
the melting temperature
– Large outpourings (‫ )تدفقات‬of basaltic magma
are common at Earth’s surface
Formation of basaltic magmas
Basaltic magmas
Evolution of magmas
Partial melting and magma formation
• Formation of andesitic magmas ‫الماجما األنديزيتية‬
– Interactions between mantle-derived basaltic
magmas and more silica-rich rocks in the
crust generate magma of andesitic
composition
– Andesitic magma may also evolve by
magmatic differentiation
SiO2 ‫ وتتميز بأن نسبة السيليكا‬، ‫– أحد ثالثة أنواع معروفة من الصھارة‬
. ‫ من وزنھا‬%٦٠ ‫بھا تكون حوالى‬
Formation of andesitic magmas
andesitic magmas
Evolution of magmas
Partial melting and magma formation
• Formation of granitic magmas
– Most likely form as the end product of
crystallization of andesitic magma
– Granitic magmas are higher in silica and
therefore more viscous than other magmas
– Because of their viscosity, they lose their
mobility before reaching the surface
– Tend to produce large plutonic structures
Formation of granitic magmas
granitic magmas
Intrusive Igneous Activity
Most magma is emplaced at depth in the
Earth.
• An underground igneous body, once cooled
and solidified, is called a pluton ‫سحيقي‬.
Classification of plutons
• Shape
– Tabular (sheet-like) ‫صفيحي‬
– Massive ‫كتلي‬
Intrusive Igneous Activity
Classification of plutons
• Orientation with respect to the host
(surrounding) rock
– Discordant ‫—غير متفق‬cuts across
sedimentary rock units
– Concordant ‫— متطابق‬parallel to
sedimentary rock units
Intrusive Igneous Activity
Types of intrusive igneous features
• Dike(‫ —)قاطع‬a tabular(‫)مسطح‬, discordant
pluton
‫‪Dike — a‬‬
‫‪tabular,‬‬
‫‪discordant‬‬
‫‪pluton‬‬
‫صخر ناري متداخل‬
‫متوسط التحبب فريشى‬
‫الشكل عادة ‪ ،‬له امتداد‬
‫أفقى واسع ومحدود‬
‫السمك ويقطع أسطح‬
‫طباقية الصخور الحاوية‬
‫له‬
Intrusive Igneous Activity
Types of intrusive igneous features
• Sill ‫سدود موازية‬
—a tabular, concordant pluton (e.g.,
Palisades Sill in New York)
.‫ حقن موازيا ألسطح الطباقية‬، ‫• محقون ناري نضيدى‬
Intrusive Igneous Activity
Types of intrusive igneous features
• Laccolith ‫الكوليث‬
– Similar to a sill
– Lens or mushroom-shaped mass
– Arches overlying strata upward
‫ تشبه القبة أو نبات عيش‬، ‫• كتلة متداخلة من الصخر النارى‬
‫ ويكون‬، ‫ تسبب تقوس الصخور الرسوبية فوقھا‬، ‫الغراب‬
. ً ‫مقطعھا دائرى وقاعھا مسطح تقريبا‬
‫‪Intrusive Igneous Activity‬‬
‫‪Types of intrusive igneous features‬‬
‫باثوليث ‪• Batholith‬‬
‫‪– Largest intrusive body‬‬
‫‪– Surface exposure of 100+ square kilometers‬‬
‫)‪(smaller bodies are termed stocks‬‬
‫‪– Frequently form the cores of mountains‬‬
‫• كتلة صخرية ضخمة غير منتظمة الشكل تقطع صخور اإلقليم ‪ ،‬ويظھر‬
‫منھا على السطح ‪ ١٠٠‬كم‪ ٢‬على األقل دون مالحظة قاع تلك الكتلة ‪.‬‬
‫تتكون عادة من صخور نارية متداخلة )غالبا الصخور الجرانيتية( ‪ ،‬ولكن‬
‫قد تتكون أحيانا من صخور اإلقليم تحت تأثير حرارة وضغط مرتفع‪.‬‬
Granitic batholiths that
occur along the western
margin of North
America. These gigantic,
elongated bodies
consist of numerous
plutons that were
emplaced during the
last 150 million years of
Earth’s history
End of Chapter 3