Lecture Notes: Rock Identification and the Rock Cycle

Introduction

• Objective: Transition from mineral identification to rock identification.
• Focus: Understanding and identifying different types of rocks using mineral knowledge.

The Rock Cycle

• Definition: Relates different varieties of rocks.
• Three Main Types of Rocks:
  • Igneous Rocks: Formed from cooled, melted rock (starts underground, might form at surface as volcanic activity).
  • Sedimentary Rocks: Formed when igneous rocks are eroded, weathered, and lithified into solid form.
  • Metamorphic Rocks: Formed when sedimentary rocks are subjected to pressure and temperature changes, altering their mineral chemistry.
• Cycle Pathways:
  • Igneous to Sedimentary and vice versa.
  • Metamorphic can erode into sedimentary.
  • Igneous can become metamorphic.

Classification of Rocks

• Methods: Texture and Composition.

Texture

• Crystallization Location:
  • Plutonic: Cooled slowly underground (large mineral grains).
  • Volcanic: Cooled quickly at the surface (small grains or glassy).
• Types of Texture:
  • Phaneritic: Coarse-grained, visible individual minerals.
  • Aphanitic: Fine-grained, minerals not easily visible.
  • Porphyritic: Large crystals in finer matrix.
  • Pegmatitic: Very large grains, usually due to cool with water present.
  • Glassy Textures: Volcanic glass (e.g., obsidian).
  • Vesicular Textures: Bubble holes (vesicles) in rock.
  • Pyroclastic: Made from volcanic ash and debris, e.g., tuff.

Composition

• Color as a Guide:
  • Felsic: Light color; rich in feldspar and quartz.
  • Mafic: Dark color; rich in magnesium and iron.
  • Intermediate: Between light and dark.
  • Ultramafic: No quartz/feldspar, varies in color.

Identifying Rocks

• Process: Use texture and color to initially classify.
• Confirmation: Use a hand lens and apply mineral identification techniques.
  • Important Minerals:
    • Felsic Rocks: Quartz, feldspar (biotite as a modifier).
    • Diorite: Plagioclase feldspar, hornblende, pyroxene.
    • Ultramafic: No plagioclase.
• Naming Rocks: Confirm with mineral content and add any significant modifiers (e.g., biotite granite).

Practical Exercise

• Sample identification using texture and visible minerals.
• Example: Identifying a phaneritic biotite granite based on grain visibility and mineral content.

Conclusion

• Summary: Understand the rock cycle, classify rocks by texture and composition, confirm identity using mineral tests.
• Preparation: Practice for midterms by reviewing these techniques without notes.