lab 5 – igneous rocks
DESCRIPTION
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). - PowerPoint PPT PresentationTRANSCRIPT
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Lab 5 – Igneous Rocks
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Volcanic hazards Tungurahua, near Banos, Ecuador
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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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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)
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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.
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Glassy
Pegmatitic
PhaneriticAphanitic
Porphyritic Vesicular
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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)
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Igneous Rock Classification
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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