4.2 Igneous rock classification and textures

Igneous rocks form from cooling magma or lava, creating diverse textures and compositions. The IUGS classification scheme uses mineral content and texture to categorize these rocks, ranging from felsic to ultramafic based on silica levels.

Textures in igneous rocks reveal their cooling history. Rapid cooling produces fine-grained or glassy textures, while slow cooling forms coarse-grained rocks. Common examples include basalt, granite, obsidian, and pumice, each with unique properties and uses.

Igneous Rock Classification

Classification of igneous rocks

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• IUGS classification scheme employs mineral composition and texture as key parameters focuses on quartz content and alkali feldspar to plagioclase ratio (QAP diagram)
• Felsic rocks contain high silica content (>63% SiO2) appear light-colored due to abundance of quartz and feldspars (granite, rhyolite)
• Intermediate rocks possess moderate silica content (52-63% SiO2) blend light and dark minerals (diorite, andesite)
• Mafic rocks have low silica content (45-52% SiO2) appear dark-colored due to abundance of ferromagnesian minerals (gabbro, basalt)
• Ultramafic rocks contain very low silica content (<45% SiO2) appear very dark-colored composed mainly of olivine and pyroxene (peridotite, dunite)

Textures in igneous rocks

• Aphanitic texture exhibits fine-grained crystals not visible to naked eye results from rapid cooling of magma (basalt, rhyolite)
• Phaneritic texture displays coarse-grained crystals visible to naked eye forms through slow cooling of magma (granite, gabbro)
• Porphyritic texture shows mixed grain sizes with large crystals (phenocrysts) in fine-grained matrix (groundmass) indicates two-stage cooling process (porphyritic andesite)
• Vesicular texture contains small rounded cavities (vesicles) formed by trapped gas bubbles during rapid cooling common in volcanic rocks (pumice, scoria)

Formation of igneous textures

• Cooling rate effects determine crystal size rapid cooling produces fine-grained or glassy textures slow cooling forms coarse-grained textures
• Magma composition influences crystal formation felsic magmas tend to form larger crystals due to higher viscosity mafic magmas often produce smaller crystals due to lower viscosity
• Volcanic environments promote rapid cooling resulting in fine-grained or glassy textures (obsidian, basalt)
• Plutonic environments allow slow cooling leading to coarse-grained textures (granite, diorite)
• Crystal growth factors include:
  1. Nucleation rate: frequency of crystal seed formation
  2. Growth rate: speed of crystal enlargement
  3. Available space: room for crystals to develop without interference

Identification of common igneous rocks

• Basalt exhibits mafic composition aphanitic texture dark color often contains vesicles used in construction (pavement, buildings)
• Granite displays felsic composition phaneritic texture light color contains quartz, feldspar, and mafic minerals widely used in countertops and building facades
• Obsidian shows felsic composition glassy texture black or dark color formed by rapid cooling of lava historically used for tools and weapons
• Pumice presents felsic composition vesicular texture light color low density formed by rapid cooling of gas-rich lava used in abrasives and lightweight concrete