Determining the Age of Rocks

Types of Rock and Age Determination

• Sedimentary Rocks
  • Created by sediment depositing on Earth's surface, then compressing and hardening.
  • Layers form on top of each other over time.

Law of Superposition

• Applied to horizontal layers of sedimentary rock.
• Relative Age:
  • Older layers are at the bottom.
  • Newer layers are at the top.
• Provides a relative age, not an absolute one.

Faults in Geological Layers

• A fault is a crack in the Earth’s surface.
• Relative Age of Faults:
  • If layers are shifted by a fault, the fault is newer than the layers.

Igneous Intrusions and Extrusions

• Intrusion:
  • Occurs when molten material (magma) moves into existing rock layers.
  • Igneous intrusion must be younger than the sedimentary layers it cuts through.
  • Law of Superposition does not apply to igneous intrusions.
  • If magma reaches the surface, it becomes lava and forms an extrusion.
• Extrusion:
  • Happens when magma breaks through to the surface.
  • Forms a different kind of igneous rock.

Faults and Igneous Intrusions

• If an igneous intrusion is unaffected by a fault, it indicates:
  • The fault occurred before the intrusion, making the fault older.
• If igneous intrusion is shifted by a fault, it means:
  • The fault occurred after the intrusion, making the intrusion older.

Conclusion

• Analyzing rock layers, faults, intrusions, and extrusions helps determine the relative ages of geological formations.
• The law of superposition is crucial for understanding the sequence of sedimentary layers.