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)

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)

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.

Glassy

Pegmatitic

PhaneriticAphanitic

Porphyritic Vesicular

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)

Igneous Rock Classification

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