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Introduction to Igneous Rocks: Formation, Classification, and Properties

Learn about igneous rocks, which are formed from molten lava or magma. Discover the different types of igneous rocks, their crystal sizes, textures, and mineral content. Explore extrusive and intrusive igneous rocks, their cooling rates, and how they are classified.

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Introduction to Igneous Rocks: Formation, Classification, and Properties

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  1. GCSE Geology Igneous Rocks Introduction Click anywhere on the screen to move on.

  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, with no crystals

  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 intrusions-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. Phenocrysts Large well formed crystals in an igneous rock In Shap Granite the pinkish coloured orthoclase feldspar phenocrysts are up to 3cm in length

  11. 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

  12. Phenocrysts and Groundmass Orthoclase phenocrysts up to 6cm in length Phenocrysts are rectangular Implies 2 stage cooling history 1cm Finer groundmass 0.5-1.0 mm in diameter

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

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

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

  16. Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5cm long Long axes of phenocrysts aligned parallel implies flow of magma Groundmass1.0 -1.5 mm

  17. Vesicular Texture Spherical or ellipsoidal cavities found in lavas Vesicles represent gas bubbles trapped in lava or magma

  18. 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

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

  20. Mineral Content Igneous rocks are classified chemically as Silicic or Mafic according to the main constituent minerals present

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

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

  23. The Classification of Igneous Rocks

  24. Cornish Granite Glassy, colourless quartz All crystals over 3.0 mm in diameter 1cm Black biotite mica with pearly lustre White/creamy plagioclase feldspar

  25. Shap Granite Porphyritic texture, large phenocrysts and finer groundmass Finer groundmass of quartz and biotite mica 2-3mm in diameter Feldspar phenocrysts are well shaped 1cm Pink coloured orthoclase feldspar phenocrysts up to 3cm long

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

  27. 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

  28. Porphyritic Dolerite Mineralogy: plagioclase feldspar, augite and olivine Medium crystal size mainly 1.0 – 2.0 mm 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

  29. Basalt 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)

  30. Peridotite Coarse grained >3.0mm Ultramafic Low silica content Predominantly olivine Small amount of augite Tiny amount of feldspar Mantle rock 1 cm

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