1. Igneous Rocks Introduction
AS-A level Geology
Igneous Rocks Introduction
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2. Igneous Rocks

3. Definition of Igneous Derived from the latin ‘ignis’ meaning fire
Formed by the cooling, crystallization and solidification of molten lava or magma
Comprise an interlocking mosaic of crystals

4. Extrusive Igneous Rocks
Molten rock reaching the Earth’s surface via volcanoes (lava) is termed extrusive

5. 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

6. 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

7. Crystal Size and Cooling Rates
Rapid cooling in at the Earth’s surface over a few months results in crystals of <1.0 mm in diameter forming lava flows (volcanic)

8. Crystal Size and Cooling Rates
Minor intrusives-dykes and sills cool slower over thousands of years to form crystals 1.0 mm-3.0 mm in diameter

9. Crystal Size and Cooling Rates
Slow cooling in magma chambers deep underground over millions of years results in larger crystals >3.0 mm in diameter (plutonic)

10. Crystal Shape 1- Euhedral
Well formed crystals with a regular and recognisable shape. They form when crystals can grow freely in a melt and are not impeded by the presence of any surrounding pre-existing crystals

11. Olivine basalt from Ubekendt Ejland, West Greenland
Euhedral Olivine
Six-sided shape
3mm
Olivine basalt from Ubekendt Ejland, West Greenland

12. Crystal Shape 2 - Subhedral
Partially formed crystals with some recognisable shape. They have been partially impeded as they grew by the surrounding pre-existing crystals

13. Subhedral Olivine Some faces flat and planar
1mm
Some faces curved and embayed
Picritic basalt, Ubekendt Ejland, West Greenland

14. Crystal Shape 3 - Anhedral
Anhedral – no regular crystalline shape visible. The shape of the growing crystal is controlled by the arrangement and orientation of the surrounding pre-existing crystals

15. Irregular outline with no planar faces evident
Anhedral Olivine
Irregular outline with no planar faces evident
Olivine basalt from Mauritius, Indian Ocean
1mm

16. Large well formed crystals in an igneous rock
Phenocrysts
Large well formed crystals in an igneous rock
In Shap Granite the pink coloured orthoclase phenocrysts are up to 3cm in length

17. The remainder of the igneous rock made up of smaller crystals
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

18. Phenocrysts and Groundmass
Orthoclase phenocrysts up to 6cm in length
Phenocrysts are euhedral and rectangular
Implies 2 stage cooling history
1cm
Finer groundmass mm in diameter

19. Equicrystalline Texture
All the crystals in the rock are roughly the same size
Produced by a steady or constant cooling rate

20. Equicrystalline Texture
2cm
Microgranite – even cooling, all crystals 1.0 – 1.5 mm

21. Formed by two-stage cooling
Porphyritic Texture
Large crystals (phenocrysts) set in a finer grained groundmass
Formed by two-stage cooling

22. Porphyritic Texture-Giant Feldspar Porphyry
Phenocrysts up to 5cm long
Long axes of phenocrysts aligned parallel implies flow of magma
Groundmass mm

23. Spherical or ellipsoidal cavities found in lavas
Vesicular Texture
Spherical or ellipsoidal cavities found in lavas
As magma moves up towards the surface and the pressure is reduced, gases are exsolved

24. Vesicular Texture
Vesicles represent trapped gas bubbles in this 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

25. Glassy Texture e.g. in Obsidian
No crystals visible due to very rapid cooling
1cm
Shows Conchoidal Fracture like glass

26. Mineral Content
Igneous rocks are classified chemically as Silicic, Intermediate, Mafic or Ultramafic according to the main constituent minerals present and their overall silica content

27. Silicic Igneous Rocks Rich in silica >66%
Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite Mica and Muscovite Mica.
Rich in silica >66%

28. Intermediate Igneous Rocks
Up to 10% Quartz, Plagioclase Feldspar, Hornblende, Augite
Lower silica content 52%-66%

29. Contain less silica 45%-52%
Mafic Igneous Rocks
Plagioclase Feldspar, Augite and Olivine
Contain less silica 45%-52%

30. Ultra-Mafic Igneous Rocks
Mainly olivine, small amounts of augite and plagioclase
Contain the least silica <45%

31. The Classification of Igneous Rocks
% Silica
Content
Chemical Group
% Dark Minerals
% Light Minerals
Rock Names
Over 66%
Silicic
40% or below
60% or over
Rhyolite/Obsidian/Pumice (Fine)
Micro-granite (Medium)
Granite (Coarse)
52% to 66%
Intermediate
40%-70%
30%-60%
Andesite (Fine)
Micro-diorite (Medium)
Diorite (Coarse)
45% to 52%
Mafic
70%- 90%
10%-30%
Basalt (Fine)
Dolerite (Medium)
Gabbro (Coarse)
Below
45%
Ultramafic
90% or over
10% or below -
Peridotite (Coarse)

32. Cornish Granite Glassy, colourless quartz
All crystals over 3.0 mm in diameter-Plutonic
1cm
Black biotite mica with pearly lustre
Subhedral crystal form
White/creamy plagioclase feldspar

33. Shap Granite
Porphyritic texture, large phenocrysts and finer groundmass
Finer groundmass of quartz and biotite mica typically 3mm in diameter
Feldspar phenocrysts are euhedral
1cm
Pink-coloured orthoclase feldspar phenocrysts up to 3cm long

34. 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

35. Micro-Granite Formed within the crust in a sill or dyke
Mineralogy: quartz, feldspar and mica
Subhedral crystals
Equigranular texture, all crystals 1.0 – 1.5mm in diameter
Formed by an even cooling rate over thousands of years
2 cm

36. 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
Mineralogy mainly quartz, feldspar and mica
Vesicles represent trapped gas bubbles in a lava flow

37. Gabbro Equicrystalline texture, all crystals roughly similar in size
Greenish-black augite
Formed deep underground by very slow cooling over millions of years
2cm
Coarse grained, crystals over 3.0 mm in diameter, suggesting slow cooling
Grey/creamy plagioclase feldspar

38. Porphyritic Dolerite
Medium crystal size mainly 1.0 – 2.0 mm
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

39. Basalt Chilled margin, very fine grained almost glassy
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 is fine, well under 1.0 mm, (volcanic)

40. Predominantly Olivine Tiny Amount of Feldspar
Peridotite
Coarse Grained >3.0mm
Ultra-Mafic
Low Silica Content
Predominantly Olivine
Small Amount of Augite
Tiny Amount of Feldspar
Mantle Rock
1 cm