Igneous Rocks and Classification

Igneous Rocks

• Igneous rocks form from the cooling and solidification of magma or lava.
• Factors affecting crystal size include:
  • Amount of dissolved gases
  • Cooling rate
• Texture refers to the shape, size, and appearance of mineral grains.
• Different textures include:
  • Coarse-grained (plutonic): Slow cooling, large crystals. Example: Granite
  • Fine-grained (volcanic): Rapid cooling, small crystals. Example: Basalt
  • Porphyritic texture: Two distinct crystal sizes (large and small) formed by different cooling rates.
  • Vesicular texture: Formed by volcanic gases being trapped in the rock. Example: Pumice
  • Amygdaloidal texture: Minerals fill voids or vesicles.
  • Glassy texture: Very rapid cooling, no visible crystals. Example: Obsidian

Igneous Rock Classification

• Classified by mineral composition.

• Felsic (granitic): Light-colored, high silica content, low density. Example: Granite

• Intermediate: Medium-colored, intermediate silica content. Example: Andesite

• Mafic (basaltic): Dark-colored, low silica content, high density. Example: Basalt

• Ultramafic: Very dark-colored, very low silica content, very high density Example: Peridotite

• Composition also influences viscosity, density, and color.

• Intermediate and felsic rocks are often lighter in color compared to mafic and ultramafic rocks.

  • felsic rocks = approximately 65-76% silica (SiO2)
  • mafic rocks = approximately 45-52% silica (SiO2)

• Crystallization, evaporation, and chemical compositions result in various textures.