Notes on Igneous Rocks Classification

Key Learning Objectives

• Determine if a rock was formed near Earth's surface or deep underground (volcanic vs plutonic).
• Classify rocks based on chemical composition.
• Identify and name common types of igneous rocks.

Types of Igneous Rocks

• Volcanic Igneous Rocks: Form on or near Earth's surface (e.g., lava flows).
• Plutonic Igneous Rocks: Form underground when magma cools and solidifies.

Formation and Cooling

• Volcanic Rocks:
  • Rapid cooling at Earth's surface.
  • Results in tiny crystals observable only with a microscope.
• Plutonic Rocks:
  • Slow cooling underground.
  • Larger, visible crystals.

Texture as an Indicator

• Texture: Size of crystals or minerals within the rock.
  • Visible Grains: Plutonic origin.
  • Non-visible Grains: Volcanic origin.
• Examples:
  • No visible minerals: Volcanic rock.
  • Visible light/dark minerals: Plutonic rock.

Chemical Composition

• Silica Content as a Classification Tool:
  • Light-Colored Rocks: Silica-rich (felsic).
  • Dark-Colored Rocks: Silica-poor (mafic).
  • Intermediate Rocks: Mix of light and dark minerals.
• Categories:
  • Felsic: High silica, light-colored.
  • Mafic: Low silica, dark-colored.
  • Intermediate: Balanced light and dark minerals.

Naming Igneous Rocks

• Felsic Rocks:
  • Volcanic: Rhyolite
  • Plutonic: Granite
• Intermediate Rocks:
  • Volcanic: Andesite
  • Plutonic: Diorite
• Mafic Rocks:
  • Volcanic: Basalt
  • Plutonic: Gabbro

Applications

• Use texture to determine formation environment.
• Use color as a proxy for chemical composition.
• Combine texture and composition to identify and name rocks.

Future Topics

• Link between composition and tectonic settings of magma origin.
• Use geological classification to understand geologic history.

Conclusion

• Understanding texture and color helps geologists classify igneous rocks.
• Confidently identifying rock types is essential for further geological studies and understanding geologic history.