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Igneous Rocks

Igneous Rocks. Igneous rocks. Form from “ magma ” (molten rock) “ Lava ” is magma that reaches the surface and looses gas (mostly water and CO 2 ) Magma that crystallizes before reaching the surface forms an intrusive (plutonic) rock

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Igneous Rocks

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  1. Igneous Rocks

  2. Igneous rocks • Form from “magma” (molten rock) • “Lava” is magma that reaches the surface and looses gas (mostly water and CO2) • Magma that crystallizes before reaching the surface forms an intrusive (plutonic) rock • Lava crystallizes to form an extrusive (volcanic) rock

  3. Intrusive igneous rocks • Cool slowly • Crystals grow larger • Texture is phaneritic (lit. “visible grains”) • Must give a grain size: • Visible but less than 1 mm = fine grained (f.g.) • Between 1 and 5 mm = medium grained (m.g.) • Greater then 5 mm = coarse grained (c.g.) • Greater than 2 cm = pegmatitic (see below) • Don’t have to give “phaneritic”

  4. Extrusive igneous rocks • Cool rapidly • Crystals are small (generally too small to see) • Texture is aphanitic (lit. not visible grains) • Where lava cools very quickly (quenches) the rock will be a glass (non-crystalline) • Textural term is glassy

  5. Cooling rates and texture

  6. Aphanitic texture

  7. Phaneritic texture

  8. Porphyritic rocks • When magma cooling rate changes the rock will commonly have two different grain sizes • Term used is porphyritic • The larger grains are phenocrysts which sit in a groundmass • Must indicate the grain size of the groundmass (aphanitic, phaneritic or (rarely) glassy) to denote complete texture • e.g., f.g. porphyritic, aphanitic porphyritic, etc.

  9. Origin of porphyritic texture

  10. Porphyritic texture

  11. Groundmass (aphanitic) Phenocrysts Porphyritic textures

  12. Groundmass (phaneritic) Phenocrysts Porphyritic textures

  13. Other textural terms • Volcanic rocks commonly have bubble holes (vesicles) caused by escaping gas. Texture is vesicular • Use different terms for light or dark coloured vesicular rocks. • Light coloured vesicular rocks generally have many tiny holes. Texture is pumaceous, rock is pumice. • Dark coloured vesicular rocks have fewer and larger holes. Texture is scoriaceous, rock is scoria.

  14. Pumice Scoria

  15. Other special (textural) terms • Amygdaloidal: textural term used when vesicles have became filled with minerals deposited from solutions percolating through the rock • Tuff is a pyroclastic rock formed from volcanic fragments (ash) • Obsidian is a volcanic glass • Pegmatite is an intrusive rock with very large grains (cm size) – typically due to crystallisation from water-rich magma. Texture is pegmatitic

  16. Determining texture

  17. Determining texture

  18. 1 cm Determining texture

  19. Composition • Rocks are named according to the composition (proportion of minerals). The proportion of dark Fe-Mg (ferromagnesian) minerals is an important criterion • Felsic rocks contain less than 10% Fe-Mg mins. • Intermediate rocks 10-30, 30-40% Fe-Mg mins. • Mafic rocks have 40-90% Fe-Mg mins. • Ultramafic rocks have >90% Fe-Mg mins.

  20. Dacite Rhyolite Granodiorite Granite Type of magma Felsic Intermediate Composition Amphibole

  21. F(elsic), I(intermediate), M(afic), U(ltramafic) • Is the rock felsic, intermediate, mafic, or ultramafic? • Determined on the basis of percentage Fe-Mg minerals • Chart gives ranges of % Fe-Mg minerals. • In general one can associate % Fe-Mg mins with colour. The darker the rock, the higher the % Fe-Mg minerals. • Relatively easy to determine for phaneritic rocks • With aphanitic rocks must go entirely on colour: felsic rocks are buff, pink or red (felsic glass, obsidian, is black); intermediate rocks vary from shades of grey to green; mafic rocks are dark green or grey to black.

  22. Name of the feldspar • Name the feldspar. Two choices: • K-feldspar (pink, cream) • Plagioclase (white, grey or blue), striations on cleavage surfaces • Note that in aphanitic rocks you may not be able to see any feldspar (too fine grained). Therefore report feldspar as n.d. = not determined. This is not the same as saying there is none. • Glassy rocks have no minerals, i.e. no feldspar

  23. Fe-Mg mineral • Name the Fe-Mg mineral(s). • There are four choices: • Biotite (usually in felsic rocks) • Amphibole (predominant in intermediate rocks) • Pyroxene (in mafic and ultramafic rocks) • Olivine (in mafic and ultramafic rocks) • Note: In the aphanitic rocks you may not be able to see the Fe-Mg mineral. Report n.d. (not determined) • Glassy rocks have no Fe-Mg minerals.

  24. Identifying the Fe-Mg minerals • Biotite: Black, shiny, flakey • Amphibole: Black/dark green, shiny (visible cleavage surfaces), not flakey • Pyroxene: Black/dark green, dull (cleavage not readily visible), not flakey • Olivine, apple green, glassy

  25. % quartz • Give the percentage quartz • In phaneritic rocks this is relatively easy; quartz is the grey vitreous (glassy-looking) mineral • Note that % quartz varies inversely with the amount of Fe-Mg mineral. • Felsic rock contain significant amount of quartz and a little Fe-Mg mineral (biotite usually) • Mafic or ultramafic rocks little or no quartz and lots of Fe-Mg mineral (pyroxene and/olivine) • In aphanitic rocks report “n.d.” not 0% • Obsidian has no minerals (i.e. 0% quartz).

  26. I(ntrusive) or E(xtrusive) • Is the rock intrusive (I) or extrusive (E)? • Rule of thumb: phaneritic rocks are intrusive, aphanitic or glassy rocks are extrusive • Except, basalts (mafic extrusive) are commonly fine grained (phaneritic)

  27. Name • Use: • the charts (on handout), • examples (at back of lab), • poster (back of lab, this powerpoint file), • book. • Note that the name is, in some respects, the least important column of this lab. • This lab is your first introduction to igneous rocks and it is far more important to become familiar with them than to name them.

  28. Microscopes and pumice • Look through microscopes at slides • Answer questions. • DON’T MOVE THE SLIDES • Don’t confuse colourless and transparent… • With pumice “experiment” think of density – as a whole.

  29. Questions?

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