Rock identification igneous kc

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Help for A-level Geology students in identifying the main igneous rocks - interactive so you can test yourself as you go...

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Rock identification igneous kc

  1. 1. Rock Identification Modified from a PowerPoint presentation prepared by J. Crelling, Southern Illinois University
  2. 2. Characterizing Rocks There are 3 major types of rocks IGNEOUS – formed from molten magma SEDIMENTARY – formed from sediment (mud, sand, shells, etc,) METAMORPHIC – formed by applying heat and pressure to other existing rocks
  3. 3. The Rock Cycle –a representation of the interrelationship between different types of rocks. NOTE: How do rocks formed at depth (i.e. all except extrusive igneous) get exposed on the surface???
  4. 4. Characterizing Rocks The three major characterizing features of rocks are: • Colour • Composition (Mineralogy/Chemistry) • Texture Note: Even the most sophisticated geological classification schemes are based on these features
  5. 5. Characterising Rocks Classification by Colour Colour Index (used mainly for igneous rocks) • Leucocratic – light colour • Mesocratic – intermediate colour • Melanocratic – dark colour and/or • Felsic – rich in light coloured minerals • Mafic – rich in dark coloured minerals
  6. 6. Igneous Rocks Textures of Igneous Rocks • Aphanitic – fine-grained. Individual grains can’t be seen with naked eye • Phaneritic - grains easily seen with the naked eye • Porphyritic – larger grains in finer grains • Inclusions > Xenoliths > Xenocrysts
  7. 7. Vesicular & Glassy Textures
  8. 8. Xenoliths
  9. 9. 1 9 5 2 4 8 3 7 6 10 11 12 13 14 15
  10. 10. What % Si02?? 66 52 45 1 3 2 4
  11. 11. Colour? Leucocratic (light)? i.e. silicic v mafic  Melanocratic (dark)? Mesocratic (medium)? Grain size / Texture? Phaneritic (coarse)  Aphanitic (fine)? Medium? Black Biotite shiny? mica K feldspar Pink? Mineralogy? Granite Quartz Grey vitreous?
  12. 12. A quicker way? – pink crystals, coarse grained – granite (then check for other minerals to be sure)
  13. 13. Granodiorite …plenty of quartz… but no / not much K feldspar…
  14. 14. Colour? i.e. silicic v mafic Leucocratic (light)? Melanocratic (dark)? Mesocratic (medium)?  Grain size / Texture? Phaneritic (coarse)  Aphanitic (fine)? Medium? Gabbro Not Mafic easy!! – are these green or grey? Dark? (pyroxene or I think pale green!! – so… Plagioclase white? amphibole) Olivine feldspar …but if grey  Mineralogy? Quartz
  15. 15. Colour? Leucocratic (light)? i.e. silicic v mafic  Melanocratic (dark)? Mesocratic (medium)? Grain size / Texture? Phaneritic (coarse) Aphanitic (fine)?  Medium? So what is it? Rhyolite
  16. 16. Flow banding texture – distinctive  rhyolite
  17. 17. Colour? Grain size / Texture? Leucocratic (light)? Melanocratic (dark)?  Mesocratic (medium)? Phaneritic (coarse) Aphanitic (fine)? Larger crystals called?? phenocrysts …white, so…  ?? Plagioclase feldspar NB – story?... Think Bowen’s Reaction series… These aren’t crystals (round – bubble-shaped!) amygdales (infilled vesicles) Medium? So what is it? Basalt 
  18. 18. What are the holes?? Vesicles Colour? mesocratic so…  ?? andesite BUT… What are the GREEN phenocrysts? Olivine!! So… Thinking of magma differentiation, what’s the story here, maybe? Mafic magma… Erupted before olivine could be reabsorbed into melt… Melt will be relatively depleted in Mg,Fe  so… So can’t be andesite lighter coloured than usual?? So what is it? Basalt (vesicular basalt)
  19. 19. Grain size / Texture? Colour? Leucocratic (light)? Melanocratic (dark)?  Mesocratic (medium)? Phaneritic (coarse) Aphanitic (fine)? Medium? So what is it? Dolerite 
  20. 20. Looks pretty light-coloured – and obviously course-grained So might think ‘granite’… BUT… What minerals would you expect? K feldspar and quartz for a start…. So what is it? Diorite Whereas what’s here??? Plagioclase feldspar and mafics…
  21. 21. So what is it? Andesite Colour? Leucocratic (light)? Melanocratic (dark)? Mesocratic (medium)?  Grain size / Texture? Phaneritic (coarse) Aphanitic (fine)? Medium? 
  22. 22. So what is it? Andesite Colour? Leucocratic (light)? Melanocratic (dark)? Mesocratic (medium)?  Grain size / Texture? Phaneritic (coarse) Aphanitic (fine)? Medium? 
  23. 23. So what is it? Andesite Colour? Leucocratic (light)? Melanocratic (dark)? Mesocratic (medium)?  Grain size / Texture? Phaneritic (coarse) Aphanitic (fine)? Medium? 
  24. 24. Yep, they’re all andesite!!
  25. 25. Glassy texture, black coloured… so… How formed? Super-cooled – so no time for crystals to form – usually when erupted into water obsidian
  26. 26. Volcanic glass is chemically unstable – out of equilibrium with surface temperatures – so begins to crystallise (de-vitrify) – snowflake obsidian
  27. 27. If you see this rock, just note the large green crystals  ?? olivine …so it’s peridotite (ultramafic) – not dark black as you might
  28. 28. What’s this? Mantle xenoliths in basalt
  29. 29. What’s this? (look at the massive crystals) Pegmatite Forms when late-stage volatiles are injected into country rock when granite batholith is mostly crystallised
  30. 30. With many thanks to: •The original author – someone from University of Massachusetts! •Additional photos from Roger Weller, Conchise College, Arizona: http://skywalker.cochise.edu/wellerr/rocks/igrocksL.htm

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