Igneous Rocks I.G.Kenyon
Dec 16, 2015
Igneous Rocks
I.G.Kenyon
Definition of IgneousDerived 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 is determined by the rate of cooling of
the magma or lava
Crystal Size and Cooling Rates
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)
Phenocrysts
Large well formed (euhedral) crystals in an igneous rock
In Shap granite the flesh coloured orthoclase phenocrysts
are up to 3cm in diameter
Groundmass
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 GroundmassOrthoclase phenocrysts up to 6cm in diameter
Finer groundmass 0.5-1.0 mm in diameter
1cm
Phenocrysts are euhedral and rectangular
Implies 2 stage cooling history
Equigranular Texture
All the crystals in the rock are roughly the same size
Produced by a steady or constant cooling rate
Equigranular Texture
2cm
Microgranite – even cooling, all crystals 0.5 – 1.0mm
Porphyritic Texture
Large crystals (phenocrysts) set in a finer grained
groundmass
Produced by two-stage cooling
Porphyritic Texture-Giant Feldspar Porphyry
Phenocrysts up to 5cm long
Groundmass 0.5-1.0mm
Long axes of phenocrysts aligned parallel implies
flow of magma
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
Car key for scale
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
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
1cm
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
Felsic
Quartz, feldspar and mica
Mafic
Plagioclase feldspar, augite and olivine
Volcanic
Crystal size
<0.5mm in diameterRhyolite Basalt
Hypabyssal
Crystal size
0.5-2mm in diameterMicrogranite Dolerite
Plutonic
Crystal size
>2mm in diameterGranite Gabbro
1cm
All crystals over 2mm in diameter-Plutonic
Glassy, colourless quartz
Black biotite mica with pearly lustre
White/creamy plagioclase feldspar
Subhedral crystal form
Cornish Granite
1cm
Flesh-coloured orthoclase feldspar phenocrysts up to 3cm long
Finer groundmass of quartz and biotite mica
2-3mm in diameter
Porphyritic texture, large phenocrysts and finer groundmass
Feldspar phenocrysts are euhedral
Shap Granite (Ademallite)
Kaolinised Granite
Unaltered grey, glassy quartz
Iron oxide staining due to release of Fe ions from biotite mica
Orthoclase feldspar altered to kaolinite
by hydrolysis
Biotite mica breaking down
to form chlorite
Granite is very crumbly and is described as Growan
2 cm
Mineralogy: quartz, feldspar and mica
Equigranular texture, all crystals 0.5 – 1.5mm
in diameter
Formed within the crust in a sill or dyke
Formed by an even cooling rate over
thousands of years
Subhedral crystals
Micro-Granite
1 cm
Fine grained < 1mm, no crystals visible, volcanic
Spherical vesicles up to 3mm in diameter
Vesicles represent trapped gas bubbles in a lava flow
Mineralogy: quartz, feldspar and mica
Formed by rapid cooling at the earth’s surface
Vesicular Rhyolite
2cm
Coarse grained, crystals over 2mm in diameter,
suggesting slow cooling Grey/creamy plagioclase feldspar, variety calcium
rich anorthite
Greenish-black augiteEquigranular texture, all crystals roughly similar in size
Formed deep underground by very
slow cooling over millions of years
Gabbro
Hypabyssal, crystal size mainly 1-2mm
Mineralogy: plagioclase feldspar, augite and olivine
Subhedral phenocrysts of plagioclase feldspar up to 3mm in diameter
Groundmass constitutes over 75% of the rock
Two-stage cooling, finally forming an intrusive dyke or sill
1 cm
Porphyritic Dolerite (Micro-gabbro)
1 cm
Crystal size well under 0.25mm, volcanic
Mineralogy: plagioclase feldspar, augite and olivine
Formed by rapid cooling at the earth’s
surface over a few weeks or months
Chilled margin, very fine grained
almost glassy
Basalt
The End