Igneous Rocks
Definition of Igneous
Derived from the latin
‘ignis’ meaning fire
Formed by the cooling
and solidification of molten
lava or magma
Comprise an interlocking
mosaic of crystals
Extrusive Igneous Rocks
Molten rock reaching the
earth’s surface via volcanoes
(lava) is termed extrusive
Intrusive Igneous rocks
Molten rock (magma) that solidifies at
depth within the lithosphere is intrusive
Intrusive rocks may eventually be
exposed at the earth’s surface following
a long period of uplift and erosion
Crystal Size and Cooling Rates
Crystal size is determined
by the rate of cooling of
the magma or lava
Instantaneous cooling of lava
erupted under water as pillow
lavas results in a glassy texture,
devoid of any crystalline form
Crystal Size and Cooling Rates
Rapid cooling in lava flows at the
earth’s surface over a few months
results in crystals of <0.5mm in
diameter forming (Volcanic)
Crystal Size and Cooling Rates
Slower cooling in dykes
and sills over hundreds to
thousands of years results in
crystals 0.5mm to 2mm in
diameter (Hypabyssal)
Crystal Size and Cooling Rates
Slow cooling in magma chambers
deep underground over millions of
years results in larger crystals
>2mm in diameter (Plutonic)
Large well formed (euhedral)
crystals in an igneous rock
In Shap granite the flesh
coloured orthoclase phenocrysts
are up to 3cm in diameter
The remainder of the igneous
rock made up of smaller crystals
In the case of Shap granite, the
groundmass is mainly crystals
of biotite mica and quartz
Phenocrysts and Groundmass
Orthoclase phenocrysts
up to 6cm in diameter
Phenocrysts are euhedral
and rectangular
Implies 2 stage
cooling history
Finer groundmass 0.51.0 mm in diameter
Equigranular Texture
All the crystals in the rock
are roughly the same size
Produced by a steady or
constant cooling rate
Equigranular Texture
Microgranite – even cooling, all crystals 0.5 – 1.0mm
Porphyritic Texture
Large crystals (phenocrysts)
set in a finer grained
Produced by two-stage cooling
Porphyritic Texture-Giant Feldspar Porphyry
Phenocrysts up
to 5cm long
Long axes of phenocrysts
aligned parallel implies
flow of magma
Groundmass 0.5-1.0mm
Vesicular Texture
Small spherical or ellipsoidal
cavities found in lavas
Formed by gas bubbles being
trapped during solidification
of the rock. Eg Pumice
Vesicular Texture
Vesicles represent trapped gas bubbles within a lava flow
Vesicles range from 2mm
to 1.5cm in diameter
Vesicles are stretched and
curved indicating flow of the lava
Car key for scale
Glassy Texture
No crystals visible, rocks are
often dark green or black in
colour and show conchoidal
fracture (like glass)
Eg Obsidian formed by the
instantaneous cooling of acid lava
Glassy Texture - Obsidian
Shows Conchoidal Fracture
Amygdaloidal Texture
The vesicles in a lava are later
infilled by secondary minerals
precipitated from solution
Commonly quartz and calcite
Amygdale means ‘almond-shaped’
Amygdaloidal Texture
Former vesicles
infilled by quartz
Euro coin
for scale
Basalt, volcanic,
crystals <0.5mm
Mineral Content
Igneous rocks are classified
chemically as Felsic or Mafic
according to the main
constituent minerals present
Felsic Igneous Rocks
Quartz, Orthoclase Feldspar,
Plagioclase Feldspar, Biotite
Mica and Muscovite Mica.
Rich in silica >66%
Mafic Igneous Rocks
Plagioclase Feldspar,
Augite and Olivine
Contain less silica 45 – 55%
Igneous Rock Classification
Crystal size
<0.5mm in diameter
Crystal size
0.5-2mm in diameter
Crystal size
>2mm in diameter
Quartz, feldspar
and mica
Plagioclase feldspar,
augite and olivine
Cornish Granite
colourless quartz
All crystals over 2mm
in diameter-Plutonic
Black biotite mica
with pearly lustre
crystal form
plagioclase feldspar
Shap Granite (Ademallite)
Porphyritic texture, large
phenocrysts and finer groundmass
Finer groundmass of
quartz and biotite mica
2-3mm in diameter
Feldspar phenocrysts
are euhedral
Flesh-coloured orthoclase feldspar
phenocrysts up to 3cm long
Kaolinised Granite
Iron oxide staining due to release
of Fe ions from biotite mica
Biotite mica
breaking down
to form chlorite
Orthoclase feldspar
altered to kaolinite
by hydrolysis
Unaltered grey, glassy quartz
Granite is very crumbly and
is described as Growan
Formed within the crust
in a sill or dyke
Mineralogy: quartz,
feldspar and mica
Equigranular texture,
all crystals 0.5 – 1.5mm
in diameter
Formed by an even
cooling rate over
thousands of years
2 cm
Vesicular Rhyolite
1 cm
Formed by rapid cooling
at the earth’s surface
Spherical vesicles up
to 3mm in diameter
Fine grained < 1mm, no
crystals visible, volcanic
Vesicles represent trapped
gas bubbles in a lava flow
Mineralogy: quartz,
feldspar and mica
Equigranular texture, all
crystals roughly similar in size
Greenish-black augite
Formed deep
underground by very
slow cooling over
millions of years
Coarse grained, crystals
over 2mm in diameter,
suggesting slow cooling
Grey/creamy plagioclase
feldspar, variety calcium
rich anorthite
Porphyritic Dolerite (Micro-gabbro)
Hypabyssal, crystal
size mainly 1-2mm
Mineralogy: plagioclase
feldspar, augite and olivine
Subhedral phenocrysts
of plagioclase feldspar
up to 3mm in diameter
constitutes over
75% of the rock
1 cm
Two-stage cooling, finally
forming an intrusive dyke or sill
1 cm
Chilled margin,
very fine grained
almost glassy
Formed by rapid
cooling at the earth’s
surface over a few
weeks or months
Mineralogy: plagioclase
feldspar, augite and olivine
Crystal size well under
0.25mm, volcanic
The End

AS Guide To Igneous Rocks

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