Technical University
DCE 2106: GEOTECHNOLOGY I
Session Seven: Petrology
IGNEOUS ROCKS
3.1 INTRODUCTION
Igneous rocks are derived from the molten material of other pre-existing rocks through a
cooling process. The molten material forms at great depth in the interior of the earth and is
referred to as magma. The magma may cool and crystallize below the surface to form intrusives
(plutonics). It may also form extrusive rocks ( volcanics) if ejected into the surface of the earth.
The intrusives can be distinguished from extrusives, on a site, by considering texture and shape
of the rock mass formed. Intrusives are coarse grained. Their mineral constituents are medium to
coarse in grain size and can be recognized in hand specimens. However, extrusives are fine
grained whereby their mineral constituents are not visible in hand specimens. Rapid cooling of
the extruded magma (lava) results into mineral crystals becoming fine grained unlike for
intrusives whereby slow cooling allows migrating ions to form minerals that are bigger in grain
size. A volcanic formation is usually layered and has a thickness which can be determined. An
Intrusion forms a rock mass that is irregular and of undeterminable thickness. It also lacks
vesicles which are prevalent in volcanics.
Typical igneous intrusive rocks are granites, granodiorites, gabbros and
dolerites while basalt, trachytes, rhyolites, phonolites and tuffs are typical
volcanic rocks. In general, lavas represent volcanics that formed from
cooling of molten material which was extruded into the surface through
linear openings such as faults. Faults are narrow opening in stratified rocks
along which stratified rocks were broken and subjected to displacement.
Movement that lead to displacement was caused by magma below the
strata. Molten material that was extruded into the surface at volcanic
centres formed pyroclastics on cooling. Common pyroclastic rocks formed
include tuffs and agglomerates. They are considered to be intermediate
between sedimentary and igneous in their mode of formation. Volcanic
material that formed the tuffs consisted of
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,volcanic ash which was transported by wind and settled elsewhere to solidify
into coherent material.
The volcanic ash that formed agglomerates eroded and incorporated other
solidified rock fragments thereby making it inhomogeneous.
Pumice is solidified bubbling froth that formed on lavas of high silica content. It is light grayish
in colour, of low density, highly vesicular in texture and is abrasive. Vesicular basalt is a
common type of basalt that is generated at volcanic centers. The rock is highly vesicular like
pumice. It is also abrasive but dark grayish to black in colour.
3.2 Classification of Igneous Rocks
3.2.1 Classification based on silica content
This mode of classification has some genetic implication since igneous rocks often evolve
producing subsequent rocks that have varying (siø 2) content (see table 3.1).
Table 3.1: Silica content consideration
Silica content (%) Characteristic rock
Acid (s1ø2>60) Granite, Granochorite, Ryolite
Intermediate(52%<s1ø2>66%) Trachyte, Andesite, Phonolite
Basic(44%<s1ø2>52%) Basalt, Dolerite, Gabbro
Ultrabasic (s1ø2<44) Peridolite,Kimberlites
The silica content include free silica as quartz and the silica that is combined with other ions to
form other type of minerals.
3.2.2 Intrusive Rocks
These occur as distinctive igneous structures. The structures that can be
encountered on a construction site include Batholith, Plutons, Dykes, Sills
etc (See fig.3.1).
(a) Batholith: These are major igneous bodies. They are deep seated and have vertical walls
which extend to great depths. Batholith assimilated fragments of stratified rocks that were in
place during their formation. They are prevalent in ancient geological environments. In Kenya,
batholith are prevalent in Western and Nyanza in which they occur as intrusions in volcanics
that were interstratified with sedimentary formations. Intruded stratified rocks are generally
referred to as country rocks. Table 3.1: Typical Igneous Structures
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, (b) Stock: In geology, a stock is a discordant igneous intrusion having a surface exposure of
less than 40 sq mi (100 km2), differing from batholiths only in being smaller. Circular or
elliptical stocks may have been vents feeding former volcanoes.
(c) Plutons: These are similar to batholiths in mineralogical composition although smaller in
body size. A pluton is a relatively small intrusive body (a few to tens of km across) that seems
to represent one fossilized magma chamber. They are considered to be extensions of batholiths.
Some plutons have circular cross sections while others have irregular cross-sections.
(d) Dykes: A dyke is a narrow igneous body which appears to cut across the bedding planes of
country rocks. The magma that formed dykes was injected into the pre-existing rocks through
week zones such as faults and large fractures. The average thickness of a dyke range from a
fraction of a meter to several kilometers. Rocks that formed dykes are medium or coarse
grained. They have a high concentration of magnesium and iron rich minerals such as
pyroxenes, hornblende and olivines. These are dark coloured minerals which make the rocks to
be dark (mafic) in colour. Chilled zones of amorphous material (obsidian) often occur around
dykes.
(e) Sills: A sill is a narrow igneous intrusion oriented in the same direction as the direction of
country rocks. The sill conforms to the stratification of country rocks unlike a dyke which is
uncomfortable. The magma that forms a sill intrudes the country rocks along bedding planes. A
bedding plane is a weak zone since it is an interface between adjacent formations which are
different in lithology. Magma crystallizes within this weak zone thereby forming the igneous
rock body. The rocks that constitute a sill are also dark coloured. They can be fine grained but are
medium to coarse grained in most cases.
3.3 Textures of igneous rocks
An igneous texture refers to the size, shape and mutual relationship of minerals that constitute
the igneous tock. The texture depends upon factors that contribute to crystallization process that
formed the rock.
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DCE 2106: GEOTECHNOLOGY I
Session Seven: Petrology
IGNEOUS ROCKS
3.1 INTRODUCTION
Igneous rocks are derived from the molten material of other pre-existing rocks through a
cooling process. The molten material forms at great depth in the interior of the earth and is
referred to as magma. The magma may cool and crystallize below the surface to form intrusives
(plutonics). It may also form extrusive rocks ( volcanics) if ejected into the surface of the earth.
The intrusives can be distinguished from extrusives, on a site, by considering texture and shape
of the rock mass formed. Intrusives are coarse grained. Their mineral constituents are medium to
coarse in grain size and can be recognized in hand specimens. However, extrusives are fine
grained whereby their mineral constituents are not visible in hand specimens. Rapid cooling of
the extruded magma (lava) results into mineral crystals becoming fine grained unlike for
intrusives whereby slow cooling allows migrating ions to form minerals that are bigger in grain
size. A volcanic formation is usually layered and has a thickness which can be determined. An
Intrusion forms a rock mass that is irregular and of undeterminable thickness. It also lacks
vesicles which are prevalent in volcanics.
Typical igneous intrusive rocks are granites, granodiorites, gabbros and
dolerites while basalt, trachytes, rhyolites, phonolites and tuffs are typical
volcanic rocks. In general, lavas represent volcanics that formed from
cooling of molten material which was extruded into the surface through
linear openings such as faults. Faults are narrow opening in stratified rocks
along which stratified rocks were broken and subjected to displacement.
Movement that lead to displacement was caused by magma below the
strata. Molten material that was extruded into the surface at volcanic
centres formed pyroclastics on cooling. Common pyroclastic rocks formed
include tuffs and agglomerates. They are considered to be intermediate
between sedimentary and igneous in their mode of formation. Volcanic
material that formed the tuffs consisted of
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1
,volcanic ash which was transported by wind and settled elsewhere to solidify
into coherent material.
The volcanic ash that formed agglomerates eroded and incorporated other
solidified rock fragments thereby making it inhomogeneous.
Pumice is solidified bubbling froth that formed on lavas of high silica content. It is light grayish
in colour, of low density, highly vesicular in texture and is abrasive. Vesicular basalt is a
common type of basalt that is generated at volcanic centers. The rock is highly vesicular like
pumice. It is also abrasive but dark grayish to black in colour.
3.2 Classification of Igneous Rocks
3.2.1 Classification based on silica content
This mode of classification has some genetic implication since igneous rocks often evolve
producing subsequent rocks that have varying (siø 2) content (see table 3.1).
Table 3.1: Silica content consideration
Silica content (%) Characteristic rock
Acid (s1ø2>60) Granite, Granochorite, Ryolite
Intermediate(52%<s1ø2>66%) Trachyte, Andesite, Phonolite
Basic(44%<s1ø2>52%) Basalt, Dolerite, Gabbro
Ultrabasic (s1ø2<44) Peridolite,Kimberlites
The silica content include free silica as quartz and the silica that is combined with other ions to
form other type of minerals.
3.2.2 Intrusive Rocks
These occur as distinctive igneous structures. The structures that can be
encountered on a construction site include Batholith, Plutons, Dykes, Sills
etc (See fig.3.1).
(a) Batholith: These are major igneous bodies. They are deep seated and have vertical walls
which extend to great depths. Batholith assimilated fragments of stratified rocks that were in
place during their formation. They are prevalent in ancient geological environments. In Kenya,
batholith are prevalent in Western and Nyanza in which they occur as intrusions in volcanics
that were interstratified with sedimentary formations. Intruded stratified rocks are generally
referred to as country rocks. Table 3.1: Typical Igneous Structures
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, (b) Stock: In geology, a stock is a discordant igneous intrusion having a surface exposure of
less than 40 sq mi (100 km2), differing from batholiths only in being smaller. Circular or
elliptical stocks may have been vents feeding former volcanoes.
(c) Plutons: These are similar to batholiths in mineralogical composition although smaller in
body size. A pluton is a relatively small intrusive body (a few to tens of km across) that seems
to represent one fossilized magma chamber. They are considered to be extensions of batholiths.
Some plutons have circular cross sections while others have irregular cross-sections.
(d) Dykes: A dyke is a narrow igneous body which appears to cut across the bedding planes of
country rocks. The magma that formed dykes was injected into the pre-existing rocks through
week zones such as faults and large fractures. The average thickness of a dyke range from a
fraction of a meter to several kilometers. Rocks that formed dykes are medium or coarse
grained. They have a high concentration of magnesium and iron rich minerals such as
pyroxenes, hornblende and olivines. These are dark coloured minerals which make the rocks to
be dark (mafic) in colour. Chilled zones of amorphous material (obsidian) often occur around
dykes.
(e) Sills: A sill is a narrow igneous intrusion oriented in the same direction as the direction of
country rocks. The sill conforms to the stratification of country rocks unlike a dyke which is
uncomfortable. The magma that forms a sill intrudes the country rocks along bedding planes. A
bedding plane is a weak zone since it is an interface between adjacent formations which are
different in lithology. Magma crystallizes within this weak zone thereby forming the igneous
rock body. The rocks that constitute a sill are also dark coloured. They can be fine grained but are
medium to coarse grained in most cases.
3.3 Textures of igneous rocks
An igneous texture refers to the size, shape and mutual relationship of minerals that constitute
the igneous tock. The texture depends upon factors that contribute to crystallization process that
formed the rock.
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