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  1. 1. Igneous Rocks I.G.Kenyon
  2. 2. Definition of Igneous Derived from the latin ‘ignis’ meaning fireFormed by the coolingand solidification of molten lava or magma Comprise an interlocking mosaic of crystals
  3. 3. Extrusive Igneous RocksMolten rock reaching theearth’s surface via volcanoes (lava) is termed extrusive
  4. 4. Intrusive Igneous rocks Molten rock (magma) that solidifies at depth within the lithosphere is intrusive Intrusive rocks may eventually beexposed at the earth’s surface following a long period of uplift and erosion
  5. 5. Crystal Size and Cooling RatesCrystal 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
  6. 6. Crystal Size and Cooling RatesRapid cooling in lava flows at theearth’s surface over a few months results in crystals of <0.5mm in diameter forming (Volcanic)
  7. 7. Crystal Size and Cooling RatesSlower cooling in dykes and sills over hundreds to thousands of years results in crystals 0.5mm to 2mm in diameter (Hypabyssal)
  8. 8. Crystal Size and Cooling RatesSlow cooling in magma chambersdeep underground over millions of years results in larger crystals >2mm in diameter (Plutonic)
  9. 9. Crystal Shape 1- Euhedral Well formed crystals with a regular and recognisable shape. They form when a crystals cangrow freely in a melt and are not impeded by the presence of anysurrounding pre-existing crystals
  10. 10. Euhedral Olivine Six-sided shape 3mmOlivine basalt from Ubekendt Ejland, West Greenland
  11. 11. Crystal Shape 2 - SubhedralPartially formed crystals withsome recognisable shape. They have been partially impeded asthey grew by the surrounding pre-existing crystals
  12. 12. Subhedral Olivine Some faces flat and planar 1mmSome faces curved and embayed Picritic basalt, Ubekendt Ejland, West Greenland
  13. 13. 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
  14. 14. Anhedral OlivineOlivine basalt from Mauritius, Indian Ocean 1mm Irregular outline with no planar faces evident
  15. 15. Phenocrysts Large well formed (euhedral) crystals in an igneous rock In Shap granite the fleshcoloured orthoclase phenocrysts are up to 3cm in diameter
  16. 16. GroundmassThe remainder of the igneousrock made up of smaller crystals In the case of Shap granite, thegroundmass is mainly crystals of biotite mica and quartz
  17. 17. Phenocrysts and GroundmassOrthoclase phenocrysts up to 6cm in diameter Phenocrysts are euhedral and rectangular Implies 2 stage cooling history 1cm Finer groundmass 0.5-1.0 mm in diameter
  18. 18. Equigranular TextureAll the crystals in the rock are roughly the same size Produced by a steady or constant cooling rate
  19. 19. Equigranular Texture 2cmMicrogranite – even cooling, all crystals 0.5 – 1.0mm
  20. 20. Porphyritic TextureLarge crystals (phenocrysts) set in a finer grained groundmassProduced by two-stage cooling
  21. 21. 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
  22. 22. Vesicular Texture Small spherical or ellipsoidal cavities found in lavas Formed by gas bubbles beingtrapped during solidification of the rock. Eg Pumice
  23. 23. Vesicular TextureVesicles 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
  24. 24. Glassy TextureNo crystals visible, rocks are often dark green or black in colour and show conchoidal fracture (like glass) Eg Obsidian formed by theinstantaneous cooling of acid lava
  25. 25. Glassy Texture - Obsidian1cm Shows Conchoidal Fracture
  26. 26. Amygdaloidal Texture The vesicles in a lava are later infilled by secondary minerals precipitated from solution Commonly quartz and calciteAmygdale means ‘almond-shaped’
  27. 27. Amygdaloidal Texture Former vesicles Basalt, volcanic,infilled by quartz crystals <0.5mmEuro coinfor scale
  28. 28. Mineral ContentIgneous rocks are classifiedchemically as Acidic or Basic according to the main constituent minerals present
  29. 29. Felsic Igneous Rocks Quartz, Orthoclase Feldspar,Plagioclase Feldspar, Biotite Mica and Muscovite Mica. Rich in silica >66%
  30. 30. Mafic Igneous RocksPlagioclase Feldspar, Augite and OlivineContain less silica 45 – 55%
  31. 31. Igneous Rock Classification Felsic Mafic Quartz, feldspar Plagioclase feldspar, and mica augite and olivine Volcanic Crystal size<0.5mm in diameter Rhyolite Basalt Hypabyssal Crystal size0.5-2mm in diameter Microgranite Dolerite Plutonic Crystal size >2mm in diameter Granite Gabbro
  32. 32. Cornish Granite All crystals over 2mm Glassy, in diameter-Plutonic colourless quartz1cm Black biotite mica with pearly lustre Subhedral crystal form White/creamy plagioclase feldspar
  33. 33. Shap Granite (Ademallite) Porphyritic texture, large phenocrysts and finer groundmass Finer groundmass ofquartz and biotite mica Feldspar phenocrysts 2-3mm in diameter are euhedral 1cmFlesh-coloured orthoclase feldspar phenocrysts up to 3cm long
  34. 34. Kaolinised Granite Iron oxide staining due to release of Fe ions from biotite mica Biotite mica breaking down Orthoclase feldspar to form chlorite altered to kaolinite by hydrolysis Unaltered grey, glassy quartzGranite is very crumbly and is described as Growan
  35. 35. Micro-GraniteFormed within the crust Mineralogy: quartz, in a sill or dyke feldspar and mica Subhedral crystals Equigranular texture, all crystals 0.5 – 1.5mm in diameter Formed by an even cooling rate over 2 cm thousands of years
  36. 36. Vesicular Rhyolite 1 cm Formed by rapid cooling at the earth’s surface Spherical vesicles up to 3mm in diameterFine grained < 1mm, nocrystals visible, volcanic Mineralogy: quartz,Vesicles represent trapped feldspar and micagas bubbles in a lava flow
  37. 37. Gabbro Greenish-black augite Equigranular texture, allcrystals roughly similar in size Formed deepunderground by very slow cooling over millions of years 2cm Coarse grained, crystals over 2mm in diameter, suggesting slow cooling Grey/creamy plagioclase feldspar, variety calcium rich anorthite
  38. 38. Porphyritic Dolerite (Micro-gabbro) Hypabyssal, crystal Mineralogy: plagioclase size mainly 1-2mm feldspar, augite and olivine Subhedral phenocrysts of plagioclase feldspar up to 3mm in diameter Groundmass constitutes over 75% of the rock 1 cm Two-stage cooling, finally forming an intrusive dyke or sill
  39. 39. Basalt 1 cm Chilled margin, very fine grained almost glassy Formed by rapidcooling at the earth’s Mineralogy: plagioclase surface over a few feldspar, augite and olivine weeks or months Crystal size well under 0.25mm, volcanic
  40. 40. Pyroclastic RocksConsist of fragmental volcanic material blown into the atmosphere by explosive activityMainly associated with andesitic and acidic volcanoes
  41. 41. Pyroclastic Rocks – 2 Main GroupsMaterial ejected from the volcano asliquid globules which solidifies in theair and is deposited as solid particlesMaterial ejected from the volcano as solid fragments, this solid material has been fractured by the explosive activity
  42. 42. Materials Ejected in a Liquid State Pelées Hair Volcanic Bombs Pumice Scoriae
  43. 43. Pelées Hair A fine mass of hair-like glass Formed by lava beingexuded through a small orifice and blown about by the windResembles candy floss in 1cm appearance
  44. 44. Volcanic Bombs Larger masses of liquid lava thrown into the air They rotate and take on characteristic shapes Spindle-bombs and breadcrust bombs are most common-usually vesicularVary in size from small droplets to several cubic metres
  45. 45. Volcanic Bombs Some bombs have a Volcanic bombs are large characteristic breadcrust fragments of molten lava up surface, others resemble to 1m in diameter expelled cauliflowers or cowpats during an eruption. depending on the way they land and solidify.Bombs develop a rounded or almondshape as they are twirled through the air.
  46. 46. Section through a Volcanic Bomb Highly vesicular interior Breadcrust exterior, finer 5cm grained and less vesicular due to more rapid cooling
  47. 47. Pumice Highly vesicular material derived from acid lavasVery high porosity and low densitySo light that it may float on water
  48. 48. PumiceSpecimen from Mt. Teide, Tenerife Mineralogy: quartz, feldspar and mica Volcanic, felsic igneous rock Low density, high Vesicles up toporosity, floats on water 3cm in diameter Microscopic grain size, very rapid cooling at the earth’s surface 2cm
  49. 49. Scoriae Associated with basic lavasVesicular but denser than pumiceGlobules of lava are ejected and the exterior chills and solidifies Interior is still hot and moltenUpon landing they are still soft andare flattened into pancake shapes
  50. 50. Section through Strombolian Scoriae Cone Bedding dips 32°SE The cone has been half excavated for use in the construction industry Some layers rich in volcanic bombs 2m Strombolian refers to the style of pyroclastic eruption in which fragments of incandescent, vesiculating basaltic magma are ejected to a moderate height, landing as solid scoria to form a cone
  51. 51. Strombolian Scoriae ConeVolcanic bombs occurup to 50cm in diameter The structure is very friable and has an unstable surface Scoriae clasts range in size from 3 to 15cm
  52. 52. Material Ejected in a Solid State Agglomerate-fragments >64mm in diameter Lapilli – fragments 64mm - 2mm in diameterAsh, Tuff & Dust – fragments <2mm in diameter
  53. 53. Agglomerate – Volcanic Breccia Derived from agglomero meaning ‘gather into a heap’Formed of volcanic or country rock in the vent or as part of the coneProduced by explosive activity which often shatters the top of the coneCoarse material is ejected a relatively short distance before settling back to earth Comprises angular fragments >32mm surrounded by finer tuff and lapilli
  54. 54. Agglomerate – Volcanic Breccia Large angular fragments up to 10cm in diameter5cm VentAgglomerate Large fragments surrounded by material of ash and lapilli size
  55. 55. Agglomerate–Volcanic Breccia, Arico, Tenerife Chaotic mixture of boulders over 2m to ash <2mm in diameter
  56. 56. Lapilli – Particles 2 – 64mm Derived from lapillus meaning ‘a little stone’Most commonly small pea to walnut sized
  57. 57. Tuff – Particles <2mm The lithified equivalent of volcanic ashClassified according to the nature of the pyroclastic fragmentsCrystal Tuffs – composed of mainly crystalsLithic Tuffs – composed of fragments of rockVitric Tuffs – composed of glassy fragments Welded Tuffs (Ignimbrites) – hot fragmentswelded together in Nuées Ardentes eruptions
  58. 58. The Formation of IgnimbritesAssociated with Pyroclastic Flows Nueés Ardentes style eruptions Glowing fireclouds 300-1000°CParticles weld together on settling Activity on Augustine, Alaska, photograph by M.Krafft
  59. 59. The Chimiche Ignimbrite, Arico, Tenerife It is unwelded as the particles were cool by the 15m time they had fallen 10-15km through the atmosphere back to earth This deposit covers more than 150 km2 of the Chimiche- Arico part of the island. It is thought to represent the collapse of a 10-15 km high plinian eruptive column
  60. 60. Volcanic Ash – Unconsolidated material <2mm in diameter Road cutting in the Guimar Valley, Tenerife
  61. 61. The End