igneous rocks i.g.kenyon. definition of igneous derived from the latin ‘ignis’ meaning fire...
TRANSCRIPT
![Page 1: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/1.jpg)
Igneous Rocks
I.G.Kenyon
![Page 2: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/2.jpg)
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
![Page 3: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/3.jpg)
Extrusive Igneous Rocks
Molten rock reaching the earth’s surface via volcanoes
(lava) is termed extrusive
![Page 4: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/4.jpg)
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
![Page 5: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/5.jpg)
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
![Page 6: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/6.jpg)
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)
![Page 7: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/7.jpg)
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)
![Page 8: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/8.jpg)
Crystal Size and Cooling Rates
Slow cooling in magma chambers deep underground over millions of years results in larger crystals
>2mm in diameter (Plutonic)
![Page 9: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/9.jpg)
Phenocrysts
Large well formed (euhedral) crystals in an igneous rock
In Shap granite the flesh coloured orthoclase phenocrysts
are up to 3cm in diameter
![Page 10: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/10.jpg)
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
![Page 11: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/11.jpg)
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
![Page 12: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/12.jpg)
Equigranular Texture
All the crystals in the rock are roughly the same size
Produced by a steady or constant cooling rate
![Page 13: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/13.jpg)
Equigranular Texture
2cm
Microgranite – even cooling, all crystals 0.5 – 1.0mm
![Page 14: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/14.jpg)
Porphyritic Texture
Large crystals (phenocrysts) set in a finer grained
groundmass
Produced by two-stage cooling
![Page 15: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/15.jpg)
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
![Page 16: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/16.jpg)
Vesicular Texture
Small spherical or ellipsoidal cavities found in lavas
Formed by gas bubbles being trapped during solidification
of the rock. Eg Pumice
![Page 17: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/17.jpg)
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
![Page 18: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/18.jpg)
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
![Page 19: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/19.jpg)
Glassy Texture - Obsidian
1cm
Shows Conchoidal Fracture
![Page 20: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/20.jpg)
Amygdaloidal Texture
The vesicles in a lava are later infilled by secondary minerals
precipitated from solution
Commonly quartz and calcite Amygdale means ‘almond-shaped’
![Page 21: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/21.jpg)
Amygdaloidal Texture
Former vesicles infilled by quartz
Euro coin for scale
Basalt, volcanic, crystals <0.5mm
![Page 22: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/22.jpg)
Mineral Content
Igneous rocks are classified chemically as Felsic or Mafic
according to the main constituent minerals present
![Page 23: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/23.jpg)
Felsic Igneous Rocks
Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite
Mica and Muscovite Mica.
Rich in silica >66%
![Page 24: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/24.jpg)
Mafic Igneous Rocks
Plagioclase Feldspar, Augite and Olivine
Contain less silica 45 – 55%
![Page 25: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/25.jpg)
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
![Page 26: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/26.jpg)
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
![Page 27: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/27.jpg)
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)
![Page 28: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/28.jpg)
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
![Page 29: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/29.jpg)
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
![Page 30: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/30.jpg)
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
![Page 31: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/31.jpg)
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
![Page 32: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/32.jpg)
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)
![Page 33: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/33.jpg)
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
![Page 34: Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or](https://reader035.vdocuments.us/reader035/viewer/2022062421/56649cbb5503460f949833ff/html5/thumbnails/34.jpg)
The End