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Lab 5 – Igneous Rocks

Lab 5 – Igneous Rocks. Volcanic hazards Tungurahua, near Banos, Ecuador. Igneous rocks. Igneous rocks – formed from cooled molten rock Intrusive (magma) – coarse grained Extrusive (lava) – fine grained Two stages – fine grained matrix with larger crystals (phenocrysts).

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Lab 5 – Igneous Rocks

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  1. Lab 5 – Igneous Rocks

  2. Volcanic hazards Tungurahua, near Banos, Ecuador

  3. Igneous rocks • Igneous rocks – formed from cooled molten rock • Intrusive (magma) – coarse grained • Extrusive (lava) – fine grained • Two stages – fine grained matrix with larger crystals (phenocrysts)

  4. Igneous Textures • Glassy texture – very rapid cooling of viscous lava  volcanic glass • Aphanitic texture – rapid cooling, ign. rocks with crystals too small to see (fine grained) • Phaneritic texture – slow cooling, ign. rocks with crystals that are visible (coarse grained 1-10mm)

  5. Igneous Textures • Pegmatitic texture – last to cool, very course grained crystals (>1cm) • Porphyritic texture – represents two periods of cooling, composed of large crystals (phenocrysts) in a fine grained matrix • Vesicular texture – ign. rocks with vesicles in them  cause by gas bubbles being trapped in the cooling lava.

  6. Aphanitic Phaneritic Glassy Porphyritic Vesicular Pegmatitic

  7. Felsic v. Mafic • Felsic • Quartz, plagioclase feldspar, K-spar, and muscovite • Light in color (generally) • Mafic • Biotite, amphibole, pyroxene, and olivine • Dark in color (generally)

  8. Igneous Rock Classification

  9. How to Identify • Identify color (light/dark), but be careful! • Identify the minerals in the rock (if it’s not too fine grained) • Identify textures • Classify the rock using the flow chart in Fig. 5.2 and Fig. 5.3

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